OMAX 2626 Waterjet Cutter (and a Banner Image for this Blog)

OMAX 2626 WATERJECT CUTTER

The learning perks of being a student at Georgia Tech aren’t stopping–I recently described the opportunity I had to learn how carbon fiber parts are made, and now I’m getting to learn how to use a waterjet cutter!!  The design program in the Woodruff School of Mechanical Engineering just opened a new design studio and it has a brand new OMAX brand 2626 JetMachining Center. 

A waterjet machine is a device which combines highly pressurized water with an abrasive sand-like material (garnet) and forces the water slurry out through a small-diameter nozzle to cut intricate, computer-controlled paths into parts.  We were told that this machine has the ability to cut through 8″ thick steel (whoah).  The material which is being cut is supported in what I call the “machining tub” on top of vertical steel plates.  The plates get cut up by the water after the nozzle makes many passes over them–they eventually have to be replaced when they get too degraded and nothing is left to support the work material on a flat plane.  The diameter of the water stream is only 0.030″, so very thin material path cross sections can be cut:

The software (a two-part package: “OMAX Layout” for constructing your cut-path, and “OMAX Make” for running the machine according to the path you generated) is very flexible and easy to pick up through playing around with the GUI, and is even more readily picked up if you are familiar with 2-D C.A.D. drawing.  The software can even auto-generate toolpaths, which usually only require minimal cleanup.  The software also has many included fonts for cutting letters into material (which I will be exploiting for a project, described shortly).  Here’s the standard-issue military stencil writing done in some aluminum (this part only took 30 seconds to create after hitting the go button!):

The software also gives the user the ability to import images and clean them up into paths which the waterjet can follow.  Pretty cool!  Here’s a screenshot from the OMAX Layout software:

 

THINGS I LIKE ABOUT THIS MACHINE

1. Brittle materials?  No problem.  Conventional machine tools put a hard edge to the part to remove material.  Brittle materials chip and ugly surface finishes are the result when brittle materials are machined conventionally.  Brittle materials can be easily machined by an abrasive waterjet with no macroscopic cracking or chipped surfaces on the part.  Great examples of this capability are shown at OMAX’s website where people have done work with stone.
2. Temperature sensitive material?  No problem.  Conventional machining, especially when done at high speeds, can generate a lot of heat as material is removed.  Many automated CAM tools have nozzles which deliver coolant to the machine part surface to defray this drawback to machining, but waterjets avoid this problem and eliminate the “Heat Affected Zone” which can be detrimental if a metal’s microstructure is changed unfavorably.

THINGS I DON’T LIKE ABOUT THIS MACHINE

1. Somewhat limited in its ability to machine parts with complex shapes.  OMAX does make nozzle heads which can pivot, which gives the machine much more flexibility.
2. Constant replacement of parts which get worn out quickly.  If I remember correctly, the ruby jewel which focuses the water stream is $50.00 and has to be replaced every 40 hours.  Garnet has to be purchased (and can only be filtered and recycled a couple times before it too becomes too worn out to be reused).  The nozzles and tubes exposed to the waterjet / garnet also wear out.  The vertical metal slats which support the parts inside the machining basin also get cut up and must be replaced:
3. Erosion backsplash occurs when the waterjet encounters the vertical metal slats supporting the part…  When the stream of water is cutting through the part and exits the back of the material into the pool of water below it, then the back surface of the part you are machining will have good surface quality…  But when the nozzle traverses to a point in the material which is backed up by one of the vertical steel slats supporting the material…  Then you get what I call “erosion backsplash”.  To counteract this, a cheap sacrificial material can be placed under your part to bear the brunt of this caustic backsplash effect (I’ve heard drywall / gypsum board is a cheap material that works well for this purpose).   More of it is evident here:
4. The focus of the stream degrades with distance from the nozzle which can result in unintended chamfering of the part edge…  Again, this can be eliminated if the machine is equipped with a head that can pivot slightly to eliminate this drawback.  Even without this feature on our machine, I witnessed no visible undercut on a 1″ thick piece of aluminum.

 

A BANNER IMAGE FOR THIS BLOG 

I have an idea for a “banner image” for this blog.  This is part of the reason I changed the layout of the blog to the “Vigilance” WordPress theme–it allows me to easily upload a banner image.  Most blogs just have the blog title heading the webpage in one of two ways.  It may simply be large text above the posts below it, or included as text inside a “banner image” spanning the homepage of the blog.

This is the typical two-dimensional way of titling a blog: putting the blog title over the first post, or right on top of an image of some sort. 

What I want to do is break the blog title “Valuable Mechanisms” out of the plane of the blog page, while also including it in the banner image.  I will do this by skewing the blog title lettering within the banner image.  This will convey three dimensionality and the “out of plane” effect which I’m after.  Skewing the words in the banner image could be done in image editing software.  But, since this blog is decidedly focusing on the design of physical products and objects, I’m choosing to achieve the 3-D lettering effect by cutting my blog title into a block of clear acrylic with the waterjet, and then photographing the acrylic block for the banner image.  I plan to fill the letters in the block with colored two-part epoxy.  With colored letters suspended inside the clear plastic block, I will photograph the block on a white background.  By maintaining a white background on my blog, the blog title is effectively broken out of the plane of the blog webpage.  This is an interesting twist on the ho-hum “blog title on solid color background.”  This project will also give me plenty of practice with the waterjet, it will allow me to learn the quirks of epoxy and acrylic material first-hand, and probably most importantly: it will emphasize, as soon as the webpage loads on a viewer’s computer screen, that this is a blog about physical products and objects.

(Update 04-19-09:  Ha!  While searching for jobs on the web today, I came across fuseproject’s Spacescent Perfume Bottle.  It employs a similar “solid suspension” concept, which I’m applying here!)

Here is the 12″ x 12″ x 2″ block of acyrlic, ordered from the ever-helpful McMaster-Carr:

Notice that the cast surface of the acrylic is crystal clear, but the sawed edge is rough, and therefore opaque:

I suspect a key problem to overcome with this project will be achieving good surface finishes on the acrylic.  Being able to achieve crystal clarity will probably require quite a bit of experimentation with sanding and polishing methods, especially on the insides of the waterjet machined letters–that could be quite a trick!

I am currently working on lettering layout, as well as creating a practice piece using the font I’ve selected.  This practice piece also incorporates all of the letters which have tight tolerances and what I suspect could be “problem areas” in sanding and polishing.  I will soon be testing the rough-edge of the acrylic block shown above for sanding techniques which will render it clear again.  I have some sets of sandpaper going all the way up to 1500 grit from test specimen polishing for my research…  If that level of smoothness doesn’t get the acrylic back to crystal, I don’t know what will!

I’ll provide update posts as this project progresses.

Update 04-05-09

Today I took a few minutes to sand the sawn-off edge of the stock acrylic block to see what kind of clarity I could get with the sandpaper I already had available to me (used 320, 400, 600, then 1500 grit).

The results of the sanding were not perfect in terms of achieving optical clarity, but are very promising for my project.  I sanded a small circular “window” into the corner of the 12″x12″ block, as shown.

There are some important things which I learned about acrylic optics from this sanding process (as well as some things which can’t be directly observed in the pictures I’ll show here):

1.  Viewing something straight through two non-parallel surfaces of the acrylic block is not possible–the index of refraction for acrylic is such that only something pressed directly up against the surface of the second non-parallel surface will be visible to a viewer looking through the first.  Note how the two “peel lines” are visible at such different areas on the two surfaces in the image above; this is due to the material’s refraction angle.  I dug up another image on a computer modding website which illustrates this:

Note how the rest of the letter “C” cannot be seen through the cube of acrylic.  This is due to the refraction angle of acrylic (which is actually illustrated at Wikipedia).  It renders viewing things through non-planar surfaces difficult at best.

2.  A second important thing I learned about acrylic and its clarity is that, even if one acrylic surface is very uneven, when a material is pressed close up against it, it is still readily visible through another parallel acrylic surface which is already polished.  To show this, I set the previously shown  “JUSTIN” machined block of aluminum on my desk…  Observe that the “looking glass” window which I polished in the block is at the top of the upright block, and that consequently, there is a rough-cut sawn-off face opposite it and visible through the block:

Now, what happens when we set this “rough-cut sawn-off surface” down on top of that JUSTIN cut-out, and then “look through the looking glass”?

Now, while that does look a little hazy, keep in mind that the second surface it is being viewed through a very rough-machined surface, absent any polishing.  That’s promising!  What happens when I lift the acrylic block about an inch off of the machined JUSTIN piece?

The rest of “JUSTIN” is all but invisible–unless it is pressed right up against the rough-cut surface.

What this behavior allows me to know is that the insides of the letters (hard to reach) may not have to be mirror polished (a difficult task anyways),  while the outsides of the blocks (easy to sand and buff) should be mirror polished to achieve this effect I observed.  The “polishing issues” which I had earlier feared would be the greatest challenge in this project may not be an issue at all!

3.  During the wet-sanding process of 600 and 1500 grit, I noticed that the water, while it was on the sanded surface, rendered it temporarily more clear than when it was dry.  This is a good sign: a liquid can fill the small interstices left by the sanding process, essentially allowing “optical clarity” while the liquid is in contact with that surface.  This bodes well for the letters which I plan to fill with pigmented epoxy.  Due to the fact that the “hardest to polish” areas will be inside the letters, and those same volumes are going to be filled up with liquid epoxy (of low viscosity–a criteria in my search for the “right epoxy”), the liquid epoxy will not only achieve the “pressed up against the surface” effect noted in the previous point, but will hopefully allow the “optical clarity through liquid-on-surface” effect which I observed during sanding.  This is doubly reassuring.

All of these positive developments likely render my worries about polishing and optical transparency irrelevant, but I’ve also heard that flame treating the surface of acrylic can render the surface optically clear again…  But this probably requires quite a bit of manual skill, and exposing the inside of intricate machined letters to uniform heating to achieve this effect would be darn near impossible given what I’ve got (although…  a heated wire jumps into my mind as the perfect instrument configuration for heating the internal surfaces of a part which has been cut on a waterjet…  hmmm…  yes…  a heated wire could be easily traced around the inside of each letter cut in the block, to heat those internal surfaces to transparent smoothness.  another project, if necessary.).  Based on what I’ve seen here today, it looks like something this elaborate won’t even be necessary.  Also, I used 1500 grit as the peak smoothness for the sanding process and could still see scratches left by the 1500 grit in my ‘polished’ surface–higher grit will improve this even further.

Today’s investigation has increased my confidence in the blocks turning out as I envisioned them.  Questions remain about the epoxy, which I have not bought yet, let alone tested…  Acrylic/epoxy adhesion issues?  Epoxy shrinkage issues?  More later…

 

Update 04-07-09

I’ve selected the fonts which I think I’ll be able to “get away with” in terms of being able to sand inside the lettering, and created the part files and toolpaths for both the practice piece and what I think will work for the final “Blog Blocks.”  For the practice part, I chose two different “edge-cut qualities” (tradeoffs between speed of cut and edge quality), the two different fonts I want to use, and used the words “TESTING PIECEB” because this phrase contains most of the “problem letters” which might give me a difficult time during sanding:

I will also be experimenting with “no sanding” / “light sanding” / “meticulous sanding” on this test part to see how the dyed epoxy handles the different surface finishes.  Here is the part file for what I will most likely be using as the final “Blog Blocks”:

The green lines which are zig-zagging up the lettering are “traversing lines”–paths which the water nozzle will follow but will not cut on.  It just provides a line for the machine to follow in between cutting.  If you click on the images to see them close-up, you can also tell that there are “Lead-in/out” lines which are internal to the letters.  This is where the waterjet initially pierces the part to get the cut started for that specific letter…  It’s best to get this “rough-cut” process started out in the middle of waste-material as opposed to right on the letter’s edge, which you want to look nice. 

I was going to cut up the test part this evening while I was in the shop, but the machine was locked down.  Apparently they only want students using it during the day.  That’s reasonable.  If you have a little experience, I think there’s minimal chance that you could injure yourself when using machine, but you never know.  I’ll cut the part up in two days when another one of my fellow TA’s is working the “open hours” for this lab.  Tomorrow, I will probably order epoxy and dye.

 

Update 04-17-09

Much progress has been made after a couple stumbles. First, I put the “TESTING PIECEB” layout in the OMAX Make simulator to determine how long it would take to machine these words into a 2.0” thick block of acrylic. It would take TWO HOURS. This is unreasonable for a personal project. I returned the 2.0” thick block of acrylic (Thanks, McMaster-Carr-Awesome-Return-Policy!), and purchased 0.5” thick acrylic instead.  This thickness will still convey the 3-D lettering effect that I want to achieve, as well as allowing the blocks with the letters machined in them to stand vertically on edge.  I had seen 1.0” thick aluminum machined fairly quickly, but I think because this part required many more piercings (13 letters, 13 piericings–the aluminum part I saw machined required only two piercings), it demanded much more time.

This information made me wonder how different materials and different material thicknesses compare against each other in a waterjet machining-time comparison.  To do this comparison, I plotted the machining times against material thicknesses for four different materials in two different plots.  Two brittle materials (plate glass and acrylic) and two ductile materials (steel and aluminum).  The first plot was of relatively thin material thicknesses (0.01” to 0.1” thickness, in 0.01” increments:

And for “thicker” part thicknesses:

What can be learned from these plots? First, metals (and I suspect: ductile materials in general) machine faster than brittle materials at very thin thicknesses. This is most likely due to the extra time required to pierce brittle materials: a special, low-pressure setting must be used when initially piercing brittle materials such as glass and acrylic (and as you’ll see later—even with the low-pressure setting, this piercing can cause fracture in acrylic). Ductile materials with greater thicknesses will eventually overtake equivalently thick brittle materials, and require more time to machine… My hypothesis regarding this is that once the initial piercing has been made, brittle materials machine easily; the garnet is able to easily microscopically fracture the brittle material ahead of the jet stream, once the stream is completely through the part. But ductile materials, with their ability to absorb energy through (relatively) extensive plastic deformation, require the abrasive jet to work at advancing the path for longer periods of time. This is reflected in the second graph, where aluminum and especially steel outrun acrylic and glass in machining time. Another thing that can be learned is that machining time is not a linear process. Time to machine increases exponentially, a second order polynomial fits this data far more accurately than a linear fit (a bad assumption in my observation of the 1” thick aluminum conclusion: “oh, time-to machine will just–approximately–double for acrylic that is twice as thick.”). With this data, it is not possible to tell if this exponential machining time behavior is due to more time spent time in cutting the path, or making the initial pierce.

So, with 0.5” thick acrylic, the “TESTING PIECEB” path will only take 10 minutes to cut.  I was now ready to cut the test piece. First, the acrylic must be positioned in the machining tub so that it won’t be moved when it is being machined. This is accomplished by either using bar clamps to brace it against the sides of the tub, or by weighting the part down on the vertical metal slats.

Since my last post, I thought of a more novel fixturing method which would prevent any of that “erosion backsplash” from occurring. My original plan was to support the acrylic block directly by placing it on top of a tile which I had purchased. This was what Ted, the OMAX technician, had suggested I do, to prevent the erosion backsplash. This plan still made me nervous though. It raised questions in my mind:

What happens when a 40 ksi water jet stream exits acrylic and impacts tile? Will the tile erode less readily than the acrylic, and case erosion of the acrylic anyways? More likely, the water jet will cause the acrylic block to “float”—imagine the water hitting the tile, being projected sideways and lifting the acrylic block up off the tile. If this were to happen, the stream would then haphazardly cut the acrylic and ruin the piece.

My first thought to preempt these potential problems was to machine the lettering into the tile first, then set the acrylic on top of the tile and cut the letters into it… This would allow “free passage” of the waterjet after it exited the acrylic—with the letters cut into the tile, nothing but a free pool of water would resist the water exiting the backside of the acrylic. This would prevent any erosion backsplash. However, I knew that this would effectively double the time required to machine parts, since I would have to cut two brittle materials (acrylic AND the tile) instead of only one piece.

On further reflection, I recognized that the areas which I would be machining were relatively small portions of the overall plate of acrylic.  By supporting the fringe edges of the plate of acrylic which would not be machined, I could lift the whole area to be machined above the vertical metal slats, allow the entire area to be machined to be suspended above nothing but water. This is shown here, in the fixturing I implemented:

The clear acrylic strip visible in the picture above  is lifted approximately 0.3″ above the slats below it.  Here, I placed two small steel plates vertically on the aluminum support plate, to make sure it would not get shifted during the machining:

Care was taken to ensure that the edges of the acrylic exactly match the edges of the L-shaped plate which is bolted in place inside the machining tub to mark the “home corner.” This precise positioning, relative to the L-shaped plate, enables accurate machining from the corner of the acrylic plate.  To ensure this accuracy, the “machine and path home” must be reset so that X = 0 and Y = 0 along the edges of the L-shaped plate.

After homing the X and Y axes of the machine at the corner of the acrylic, the Z-height must be set at the correct standoff—0.060” off the surface to be pierced. This is conveniently achieved by lowering the nozzle until a quarter can barely fit between the nozzle tip and the part surface:

 

The nozzle must then be traversed around the surface of the part to make sure that the height is approximately the same throughout the entire cutting path. This is easily checked by doing a “dry run” of the cutting path. This runs the nozzle on the path you created in “OMAX Layout,” but without water cutting into the part. The nozzle can be run at various speeds in a dry run and can even be paused to observe whether there are any problems with path interference in areas with tight tolerances.

After doing the dry run, it’s time to rock! The water level is raised to about 0.5” above the surface of your part (so that the waterjet will exit the nozzle with the nozzle submersed under water; this prevents entrainment of air into the stream), and the anti-splash foam muff is placed over the nozzle.  The nozzle is then “homed” to the X, Y, Z = 0 position. One hand is placed on the “pause” button to stop the machine if there are any problems, and then the machine is allowed to cut the path!

You can see the reddish cloud of garnet developing around the part as the machining begins. Here’s the part after it finished machining and the water was lowered:

There is red garnet all over the part which can be washed away with the garden hose on the tub. You can see that some of the cut-out letters floated up out of the cutouts in the picture above. After the garnet was washed away:

Next, a simple “saw ” operation was performed to remove the 2” wide testing piece from the main acrylic sheet. Here’s what it looks like after the saw cut:

How did it turn out? Actually, pretty awful! Even with that low pressure initial pierce, you can see in the picture above where a number of “scalloped” fracture surfaces emanated from the peircing point, in the bottom of the letters. Here are some close-ups of those scallop shaped fracture sufaces:

More:

And in the following two pictures, you can see where the reddish garnet was actually forced in between the newly fractured surfaces:

And again:

I later observed that the garnet could be blown out of these areas by separating the cracked areas gingerly with a knife tip.

Well then, how am I to deal with this issue? I did this “testing piece” precisely to discover what issues and problems I would face in waterjetting acrylic, and now I’ve discovered those issues; I really can’t have an aesthetically pleasing banner image for my blog if the letter blocks are fractured to the point of uber-ugliness. My approach to resolving this will be two-pronged, to ensure that these scallop-shaped fractures won’t affect the final aesthetic appeal of my work.

First, I will drill pilot holes at the exact location where the waterjet will initially pierce the acrylic. In so doing, I will remove the fracture-inducing piercing event as best as I am able to… If a pilot hole is located where the initial pierice is performed, the high-impact energy will be removed from the machining process altogether.  The piercing event is essentially a high impulse, 40,000 p.s.i. jet of water impacting the brittle acrylic surface.  This action is analagous to taking an ice pick, placing the tip on a plate of glass, and then smashing the ice pick handle with a hammer. Of course it will fracture unevenly! The hole diameter I will drill will necessarily be small to fit inside the lettering…  But these holes, in order to be effective, must be exactly aligned with where the nozzle head begins the pierce. Undoubtedly my accuracy in drilling these holes with a drill press (sorry, no CAM hole drilling available to me) will be slightly out of alignment in some instances. Where this occurs, these scalloped fracture surfaces will likely appear. To resolve this, I’m aware that acrylic is, fortunately, able to be “glued” together; acrylic cement, if applied to two surfaces which are pressed close together, can be bonded to an optically clear joint.  Because the fracture surfaces are very close to each other and are mirror images of each other, I suspect that TAP Plastics acrylic cement will allow me to repair my workpiece, if this fracturing does occur. So, to drill the pilot holes, I laid out a grid on the workpiece with a couple sheets of composition notebook paper that had 0.2” grid spacing, and marked where the piercing points were (these locations could be measured from the drawing in the OMAX Layout software part file):

I then drilled the holes using a drill press:

After carefully positioning the piece in the home corner, I’m again ready to machine… A “dry run” was executed to ensure that the nozzle’s pause at the piercing point in the cut path was centered over as many of the pilot holes as possible:

I then let the waterjet cut the part. This “final piece” took about 30 minutes to cut. After it was cut out, I just had to perform a saw operation (straight line water jet cut) to remove these two word blocks from the parent piece. First, I cut out the “Valuable Mechanisms” block with the saw feature in OMAX Make:

WHAT?!?! Reallly? REALLY? Did I really just do that? Oh man. Well, that’s what happens when you forget to weigh the part down and fixture it securely before beginning machining. If it’s not weighted down, the water jet will force the part around and cut a horrifically haphazard line into the piece. Gah. Frustrating.

Well, fast forward a few days and a couple dollars for a new sheet of acrylic, and I did end up getting the “Valuable Mechanisms” block cut out properly:

Unfortunately, the “The Blog of Justin Ketterer” word block in this second piece had a “B” in “Blog” which was destroyed… This second block of smaller words was more fractured in general (you can see that in the janky saw cut picture above); you can see that those scalloped fracture surfaces were much more prevalaent in this group of lettering, but were absent in the “Valuable Mechanisms” block shown immediately above. Destroyed letter “B”:

Fortunately, this same set of words (“The Blog of Justin Ketterer) from the first (screwed up saw cut) block was not cut so poorly. Though the scalloped fracture surfaces are still present in that first attempt, I will try cementing the cracked features back together to optical clarity with the acrylic cement; I will hopefully be able to recover this piece (circled in green):

And I will use the “Valuable Mechanisms” which turned out nicely in machining the second acrylic block (circled in green):

 

I am now waiting for the acrylic cement to arrive from TAP Plastics. I will test it on the “TESTING PIECEB” part to ensure that the fractured surfaces can be bonded back to optical clarity again. If this bonding experiment works well, I will then apply it to the scalloped fractured surfaces in the letters of the final piece: “The Blog of Justin Ketterer.” I have sectioned the testing piece into several pieces on which I will test for good adhesion between the acrylic and epoxy, as well as epoxy and dye appearance, and various sanding techniques:

And, finally, the two-part epoxy and dye have arrived.  I purchased a low viscosity, optically clear two-part epoxy with good adhesion properties (from Epoxies, etc, the resin/hardener was their 20-3302 LV (low viscosity) model).  The low viscosity will hopefully allow it to blow into any surface flaws I’m not able to remove, as well as easily allowing any bubbles to rise to the surface so they can be popped before hardening in the epoxy, otherwise an unsightly void would be imbedded in the colored lettering.  Its optical clarity will not interfere with the pigment or discolor it in any way.  And the good adhesion properties will hopefully allow it to tenaciously grip the acrylic, even if it’s sanded smooth.  I bought opaque gray and red pigments from U.S. Composites.  Pics of the products:

 

 

More to follow!

 

Update 04-24-09:

Now that the epoxy and pigment has arrived, I have to experiment with sanding the inside surfaces of the letters to see if, after sanding, the colored epoxy letters will be more aesthetically appealing, as opposed to the epoxy just bonding to the machine-cut surface of the letters.  I had originally planned on sanding both of the letters in two of the cut up pieces in the “TESTING PIECEB” block, but realized that I could get away with only sanding one surface on the inside of one letter and still be able to see whether the sanded surface looks better after epoxy is applied.  Accordingly, I only need to sand two blocks; one for the gray pigment and one for the red pigment.  The two pieces I’ll be sanding, the strips of sandpaper, and a bottle of water to perform the wet sanding:

 

 

I used the progressively finer grit sanding process mentioned earlier, when I had sanded the 2.0″ thick acrylic block.  But, to get inside the small letters, I had to wrap the sandpaper around the tip of a small standard screwdriver:

 

The parts of the two pieces which were shown above which I’ll be sanding are the top inside surface of the “T”s.  Wet sanding:

After sanding, the “visible through a sanded edge effect” is evident:

 

 

After sanding, I now have to see if those splits can be repaired with the cement, and then the letters can be filled with epoxy.  The acrylic cement arrived from TAP Plastics:

 After using a knife-tip to separate the acrylic cracks carefully to blow out as much garnet as possible, I employed a syringe with a fine needle on it to squirt the acrylic cement into the splits.  How did it turn out?  Did it seal the splits and return optical clarity to an otherise ugly, shattered part?

Not really.  I think that A) not enough of the garnet was able to be removed, causing a gap and thus not allowing a good bond and B) the cement was not able to flow into the farthest reaches of the split before setting up (the cement is very quick-setting).  This left the far fringes of the splits still separated, and not sealed back to optical clarity.  The cement did NOT work in this application, however, where it WAS able to act on surfaces which were able to be pressed together with the cement between them, it did a good job!  Some pictures:

Before cementing:

 

 

After cementing:

 

 

And  the other part:, before cementing:

 

 

After cementing:

 

 

What this means is that I will be restricting my “word blocks” to only the title block itself, the “VALUABLE MECHANISMS” block.  This block, fortunately, had no splits in it since all of the pilot holes lined up with the piercing point of the water nozzle.  The “BLOG OF JUSTIN KETTERER” block was not as fortunate, and the pilot hole misalignment led to many shattered areas in the lettering.  Since these splits cannot be repaired to a nice enough appearance to merit inclusion in the banner image, I won’t bother filling them with epoxy.

I then masked off the back of these two practice pieces with duct tape, and surrounded the edges with duct tape which held them down against some scrap 0.008″ thick PVC type I sheeting which I used for another project.  This duct tape ‘fixturing’ would help to form a seal which would hold in the liquid epoxy and prevent it from flowing out the back of the lettering, preventing a mess:

OK.  YES.  I’m aware it’s camo duct tape.  It was all the store had available, at the time I was there.  I then set out the resin, the hardener, and the gray and red pigments in their proper ratios using syinges that had volumetric graduations on them (purchased from, yep, McMaster-Carr).

 

 

 

Mixed the pigment, resin, and hardener in paper cups using a paint stirrer.

 

 

Filled the letters by pulling the newly mixed and pigmented (and curing) two-part epoxy into the same syringes used to measure the resin and hardener (these syringes will now have to be thrown away since they now have curing epoxy in them.  i have spares, and they’re meant to be disposable):

 

 

Filled the letters up to beyond the “brim”; the excess can be machined or sanded off later:

 

 

 

 

 

In the next post, I’ll be sanding or maching off the excess epoxy, sanding and polishing the edges, and seeing whether it’s worthwhile to sand the insides of the lettering based on what I see with these two practice parts.  The project’s getting much closer to being done now!

 

Update 04-30-09:

After the epoxy was done setting, I removed the duct tape.  Interesting points to note: though a little bit of the epoxy leaked onto the PVC sheet, the epoxy did NOT adhere to the PVC; it peeled off cleanly. And good news: the epoxy appears to be adhering very well to the acrylic itself.  I bought this acrylic partly because it was low shrinkage, but the question of epoxy-adherence-to-acyrlic was still a big question mark.  Pulling off the duct tape was also a fairly simple task; the fibers of the duct tape left cross-hatch “dimples” on the bottom surface of the epoxy:

 

 

As shown in the last update, I had sanded the inner top surface of the letter “T” in these practice pieces and left all the other inner surfaces unsanded–left at the same level of roughness which resulted from the waterjetting operation.  How did the two surfaces compare?  On the left is the sanded surface of the letter “T,” on the right, the waterjet machined surface:

When observed this close up, the difference in roughness is clear…  The inner sanded surface of the “T” is nice and smooth, while the shadows on the upper surface of the “S” show that roughness is visibly more rough.  However, for my purposes (creating a “word block” for my banner image) this roughness is not unreasonable.  Given the magnification of the word block in the final photo for the banner image, this roughness is irrelevant (not to mention that sanding all the inner surfaces of the letters in the “VALUABLE MECHANISMS” block would be a tedious and time-consuming process).

Between the gray and red pigmented epoxy, I like the look of the red pigmentation better.  It will *pop* more in the banner image, when it is on a white background.  Thus, I will continue forward with this investigation using the practice piece which has been dyed red.  There was significant overflow of red pigemented epoxy on the upper surface:

 

 

Note also that the fractures in the sides of this piece allowed some epoxy to leak out of the duct-tape fixture and the level of epoxy in the letters had sunk below the upper surface; this is the reason for the “sunken” look which the letters possess above.  I sanded all the surfaces of this practice piece to flatness using a belt/disc sander machine in my research lab’s machine shop.  I was careful to sand the surfaces with the piece pressed flat on the support plate; I wanted to ensure all the surfaces were as perpendicular as possible.  Here is the upper surface after all six surfaces had been rough-sanded with the belt sander:

 

 

Here is a picture of the backside of the piece; note the small void here…  An air bubble which did not rise to the surface of the liquid epoxy before it had cured (other than this, the epoxy was remarkably void free–good thing I chose the low-viscosity epoxy!):

 

 

 

I then proceeded to perform the progressive sanding process mentioned earlier; working my way up to 1500 grit.  The setup, showing the strip of sandpaper taped down to a piece of Aluminum.  Water has been applied from the bottle shown, to perform the wet-sanding process:

 

 

 

Sanding:

 

 

 

After sanding, a surface results which is similar to the semi-opaque surface witnessed in the earlier pieces which had been sanded up to 1500 grit smoothness.  Also similar to earlier results, when water was applied, the surface which was wetted became MUCH more clear.  Contrasted here…  Upper surface not wetted (semi-transparent):

 

 

 

Upper surface wetted (quite transparent!):

 

 

 

Lower surface not wetted (semi-transparent):

 

 

 

Lower surface wetted (quite transparent!):

 

 

 

Thus, as mentioned earlier, water fills the small scratches formed by the sanding and renders the sanded acrylic surface optically clear again.  The opacity of the sanded (but not wetted) images above is not sufficient for the aesthetic goals of my banner image–optical transparency must be returned to this piece of acrylic.  A final polishing process will achieve this.  I’ve caved in to my desire for a more versatile set of personal tools and ordered the full-bells-and-whistles Dremel 400 XPR toolkit, which comes with buffing wheels and polishing paste.  Soon I will be graduating from Georgia Tech.  At that point, I won’t have access to all the machine tools and hardware which being a student gives you free access to.  This new Dremel will have to stopgap me until I get my own machine tools.  Looks like the size of my personal projects will have to be on the order of “Dremel scale” until that point 😦 …

Having decided on using red pigment for the “VALUABLE MECHANISMS” block, I now filled the letters of that block with epoxy.  Again, I masked off the back of the workpiece with duct tape.  In the next picture, you can see (to the right) where I’ve firmly pressed the duct tape down on the back of the acrylic, forming a good seal which won’t leak much epoxy.  To the left, the duct tape has not yet been fully pressed down.

 

 

I then duct taped the piece down to another piece of the PVC sheet used earlier with the practice piece, mixed the epoxy and pigment, and filled the letters:

 

  

I calcula-estimated how much red epoxy I would need, but had quite a bit leftover.  I’ll be using this excess red epoxy for another project I have in mind :-).  After this block’s letters had been filled:

 

 

Next step will be to belt-sand and hand-sand the exterior surfaces of this piece to 1500 grit smoothness, just as I did with the outer surfaces of the red-epoxy test-piece.  Once the Dremel arrives, I’ll practice-polish the red-epoxy test piece first.  If that turns out looking nice, I’ll do the same to the “VALUABLE MECHANISMS” block.  Then I’ll be ready to snap the banner image photograph!

 

 

Update 08-21-09

 

Damn, this post takes a loooong time to load with all the images in it–will have to parse up my longer blog posts in the future.  I’ll keep this short so my computer doesn’t crash.  After removing the duct tape from the “Valuable Mechanisms” block, I used a belt sander in my research lab at Georgia Tech to remove the excess epoxy off the front:

 

 

After this rough sanding, I did the stepped hand-sanding process up to 1500 grit sandpaper smoothness.  The Dremel 400 XPR arrived and is packaged in a nice case that is overflowing with a fantastic number of dremel bits which can perform a wide variety of operations.

For this project, I found that a four-step polishing process worked best.  There was a lot of experimentation which went into figuring this out, and polishing the entire block took a couple hours of tedious work after that.  One thing I learned is that using acrylic cement to “smooth out the surface” isn’t a hot idea; you just end up having to buff out the rough surface which it leaves behind.  The four step polishing process was:

1. Coarse felt dremel wheel (Dremel item #429) with brown polishing rouge (Dremel item #421).
2. 7/8″ string buffing wheel (bought here) with blue rouge (meant for polishing plastic, bought here).
3. Stitched muslin buffing wheel (Dremel item #423, but I used a generic brand), no rouge.
4. 7/8″ string buffing wheel, no rouge.

The finished, polished product is visible above.  It is not as smooth as the as-recieved, cast surface was–viewing things at a distance through it renders your view a little hazy.  However, for the purposes of making a banner image out of it, the clarity is sufficient.  I then took a picture of the block on a white paper background (visible below).  I actually ended up just lying it flat on its back, as in the image above, and taking the picture with the camera at an oblique angle.  Using lamps with incandescent bulbs to illuminate the subject for this application was less than ideal–if I find someone with access to a proper photo studio, or who has a camera with a detachable flash, I’m sure he or she would be able to make the final image in the banner look much better (or, similarly: someone with more photoshop skills than me)!

Fun project!  Learned a lot: how acrylic/pigment/epoxy interact, how brittle materials behave with a waterjet, the process of polishing plastic, how acrylic can be glued, and feed rates for different materials in a waterjet cutter.  Good times.