CNC Plaster Sculpting

Wow, where have I been?!? It's been 2 years since I got the mill fully running CNC programs. Almost a year ago I made the KFM logo piece. My mill has sat mostly idle for the last year. A basement remodeling project took a ton of time. I've also been working on converting an acoustic snare drum to electronic. I probably should have posted some articles about that...

Last fall I joined Generator, a new maker space in Burlington, VT. I'm super exited about it. I took a metal working class in November so I can do some basic welding now. They've got a rapid prototype area including 3D printer and laster cutter plus a Roland CNC milling machine.

I used the 3D printer to make some mounting brackets for the snare drum project and haven't thought of a good project for the laser cutter yet. My main interest is the milling machine. Of course I built my own, but the Roland is quite a bit more capable. My machine is quite good for working aluminum and making "machine shop" type functional parts. The Roland mill looks to be intended for prototyping industrial design type things.

I'm exploring the idea of using the Roland to make some plaster sculptures. Many years ago 3D modeling and animation was part of my full-time job. I was pretty good with Lightwave and Blender. Blender is an open source program which has come a tremendously long way since I used it last. I rediscovered it in mid November and have spent quite a lot of time with it.

I have some ideas for several sculptures that are sort of "relief" in nature, so a rectangular brick of material with one side carved out. The Roland mill supports 2-sided milling via a 4th rotary axis. I'll get to more sophisticated designs later. I created a model of a "bug" that was inspired by an origami cicada. I cut the piece on the Roland mill. Here it is:

The material is a mixture of plaster of Paris and glue (and water). The glue helps keep the dust down quite a bit. I'd even say there's no air-borne dust at all but I suppose there are trace amounts. Roughing the piece with a 1/4" bit and fairly high feed rate produces some nice "chips". High-speed finishing with the smaller bits produces powder that pretty much falls close to the bit, nothing getting flung against the side of the enclosure.

As you can see from the photo I'm getting air bubbles in the plaster. I'd like to push the machine to get some very fine detail out of it, but the bubbles ruin it. I've looked in to pressure chambers and vacuum chambers for eliminating bubbles but I don't really want to spend that much on it. I originally started using the plaster as a practice material because it's cheap.

I made a simple form for casting the plaster bricks and built in a "vibration" rod on the underside. I attach a drill to the rod and run it for a few minutes after pouring the cast. I'm also trying different mixing strategies.

The plaster is Dap - basic stuff from the hardware store. I'm told it's often "stale" whatever that means. I've been studying plaster and it turns out there is quite a variety of formulas. I was tooling around ebay looking for a good price when I found a seller that had several formulas. I contacted him through ebay and then ended up talking about it on the phone with him. He's going to send me some samples so I can mess around a bit to figure out what might be better. A more "pure" gypsum with less "filler" material may not bubble as easily. I'm also going to experiment with latex paint instead of the glue.

KFM Logo Sculpture Finished

My Kung Fu Machinist sculpture is done! I'm very happy with it. It's not perfect, but it was largely an experiment. All three elements, red sun (painted aluminum), black bird (painted plastic) and the base (polished aluminum) proved effective.

My photography isn't fantastic. I don't have proper lights and studio props. I did quite a bit of scrambling trying to find stuff to use to give it a proper backdrop. Here's one on a white background:

I set it up on a large sheet of paper to create a seamless background. I placed a piece of glass on the paper to create the reflection. It was a good try, I'm not sure why it doesn't look better. I think if the room had black walls and ceilings the base would look a lot better. It looks kind of washed out.

Here's another shot on a piece of grey paper.

I used two back lights, one of them red, and used the flash. There are a couple of nice specular highlights.

Finally, this shot was done on the back of a floor mat I stand on in my shop:

It's heavily color-corrected. It's kind of cold looking, but probably looks the most like how I see the real object.

I am know planning my next sculpture which will use the same surface treatments. I think I can do better with the red. If I brush the aluminum or otherwise give it some texture, I think the transparent paint will show through some of the specular highlights better and it will look more like anodized aluminum instead of just something painted red.

One Step Forward, Two Steps Back

The work on the KFM 3D logo is coming along well. The "bird" and the "sun" are done and assembled. It looks really nice. I don't want to post any pictures of it now, I don't want to spoil the impact. It looks really nice. Did I mention that?

If you look back a post, there's a 3D rendering of the design. The base will be milled from 1/2" aluminum plate. For all three pieces I'm using a basic "profile" cut: the drawings are simple 2D shapes and the cutter runs around the shape with an allowance for the width of the cutting bit. Depending on the material and thickness, multiple passes at increasing depth may be used.

You have to account for the cylindrical shape of the cutter when designing your parts. The "sun" is just a circle, there are no inside corners. For something like an L-shaped piece, there's one inside corner. You can't make a perfect square corner (if you are cutting from the top), it's going to be curved like the cutting bit. So the smaller the radius of the cut, the closer you can conform to the actual shape.

In designing the base, I planned for a 1/8" diameter cutter, so the inside corners already have a 1/16" radius. I have a 1/8" cutting bit with a 3/8" depth of cut. This worked well for the sun and the bird, but the base is the full 1/2" of aluminum. I looked around a for bit with 5/8" DOC, but couldn't find one. I bought a Dremel bit at Lowe's that has pretty long cutting flutes. It doesn't really look like a milling bit, it's got like 8 flutes or something. Suitable for very high-speed work perhaps, finishing but not so good for roughing. It should do the trick but I will have to be careful and make multiple shallow cuts.

For the sun and bird, I worked with a large rectangular piece of stock held down to my tooling plate with some simple 90 degree clamps (designed to hold my vice) with a piece of sacrificial stock underneath. The NC program just cuts the part out as you might do for balsa wood sheet parts for an airplane. Since the base piece is 1/2" aluminum and the Dremel bit won't plunge cut, I'd have to rough cut with a much larger bit. I don't even have a suitable 1/4" bit that will do 1/2"+ DOC. I decided I didn't want to waste that much material. Better to start with a piece just a little larger than needed and trim off what you don't need.

With profile cutting, you eventually remove all the material between the piece you want to keep and the part you don't. The clamps are holding down the part you don't want leaving the good material to fly free just as you are trying to finish the cut. I used two techniques to hold down the good part. First I glued the aluminum sun to the sacrificial backing with Gorilla glue. This worked really well, but required power sanding to remove the glue. For larger pieces this is probably a good approach. With the bird (plastic) I used the "tabs" setting in the CAM program to leave material in a few spots to hold the good to the waste. This also works well, but requires extra manual work to remove the tabs. I need to get better files for this technique to provide a good finish.

For the base, I need to hold the part down from one side, cut the shape on the other side, move the clamps from one side to the other (without ever leaving the piece free to move) and then cut the second side. I don't really have good clamps for this. I considered buying a clamping set but decided my shop philosophy would be to make whatever I can and buy what I can't. I need the experience making things. I thought I'd make this clamp. It's a similar part to my base piece, meaning I don't have clamps to make the clamps. So I came up with this design for a simple strap clamp suitable for the size of my mill:

This is a bottom view, the shoulder in the upper right is designed to provide some lateral resistance. There could be some slippage between the clamp and bolts, we'll see. Here's the first one I made:

The bolt on the left runs through a threaded hole and pushes the clamp up, the bolt on the right runs in the slot and pulls the clamp down (via the threaded hole in the plate). I'm using the Popsicle stick to keep it from scratching the plate. I gave it a moderate tightening and it's quite strong, just the one! Four should be plenty strong unless I get super aggressive hogging material off with the 3/8" cutter.

I cut the stock for this clamp with a hand hacksaw in my bench vice. It was quite a bit of effort. I haven't done a ton of aluminum work over the years. I expect this to change now with CNC power. Manual stock rough cutting is going to get old pretty quick. A floor-based horizontal bandsaw is the common tool for rough-cutting stock, but I don't really have the space for one. Also I've gone many years without a reciprocating saw ("Sawsall"). So I bought one. A reciprocating saw is great for cutting 2x4's and such, things you can press the front plate against. Small parts in a small vise are not so easy.

I interrupted my clamp making to make a couple jaw inserts for my vice:

This provides a nice "fence" to run the saw against, cutting horizontally. Now I can get back to cutting the stock for the other clamps. Then I can get back to making the base of the KFM logo.

Logo rework, prep for 3D

For my next project I'm working on a 3D version of the Kung Fu Machinist logo. I've added the typography to the sun and bird image. I bounced some ideas off a friend who's a much better designer than me. He helped me focus my ideas for integrating the type with the form. Here's the color 2D design with the type:

Here, kungfumachinist is all one word. My blog title is Kung Fu Machinist, but I seem to have grown attached to the idea of it being all one word. I definitely like it in all caps. My goal was to have it look like drafting lettering.

Here's the 3D concept:

The bird will be black and the base raw aluminum. The sun will be aluminum, too, painted in a transparent cherry red acrylic. I did a test of the painting and engraving of the lettering. When it first came off the mill, I wasn't terribly impressed. There seemed to be some rough edges in the path. I carefully cleaned off the chips and cutting fluid and looked at it with a magnifying glass. The technique I used to generate the lettering had the paths all doubled so each was cut twice. Also the paths didn't seem to be linked in the most efficient way. I suspected these 2 things contributed to the apparent rough points in the path. But then... I grabbed a tooth brush and scrubbed the lettering and all that ugly stuff disappeared!

I think the actual part looks better than this photo. Obviously I didn't paint the whole region. I left room on the scrap part for another practice run. You can see some "balls" at the ends of some of the lines. I'm not sure if these are entry or exit wounds, so to speak. I'm going to take a shot at removing the double-stroking of the paths. I think that will contribute to an overall cleaner cut.

Feed Speed

I've implemented feed speed. In machining, "speed" usually refers to the rotation speed (RPM) of the spindle/cutter. Feed rate refers to the speed that the cutter moves through the material - the speed of the table. Here, I'll just use speed for the latter.

Up until now my gcode interpreter has simply been building a 3D path and ignoring the speed information in the file. I've been "babysitting" the program runs, moving the speed override ("throttle control") as I need to. I've added support for the speed information in the gcode.

For the acceleration development I've been rendering little boxes around each location point and coloring it according to the speed through the point. I thought it would be cool to change the color of the line as the speed changes. I thought such a thing would be pretty challenging and I'd give it a try later on. Using the point-coloring method or printing speed values to the console isn't very readable. I came up with a clever idea for coloring the line and it turned out to be pretty easy. Here's a shot:

I created a list of colors like the ones used on weather maps to indicate storm intensity. The vertical traversals in white in the center look a little funny because the up speed is different than the down speed and the colors kind of jump back and forth as the scene is rotated. Having the color blending in the lines really helped develop the support for speed changes.

I've calculated the maximum feed rate of my mill to be 18.75 IPS (inches per minute). Obviously since this is the first time I've had IPS support, I've never considered the speed in any sort of objective way. I just run the mill and position the throttle "by feel". So now I need to calibrate my brain and learn what speeds work for what materials and processes. I don't think there will be too many values I'll actually use. I'll probably use something like 1, 5, and 10 for nearly everything (and 18.75 for rapids, of course).

First Real Project

My friend Bob is a rock star. Well, he's the bass player in a band, I don't know if that counts. Anyway, the band is called Phoenix Rising and Bob did the logo design. Probably more than a year ago he did a graphic design job for me and I suggested I could make a key chain for him with the band's logo on it. So right on schedule, I finally got it done. It's not really a key chain, though, it's 3/16" thick aluminum and over 2" x 3". I decided to call it a "plaque" and put a couple holes for screw mounting to a guitar case or something.

It's a simple design, an etching with a profile cut out. I really didn't have a suitable bit to do the etching with. I bought a Dremel engraving kit with a mix of grinding and engraving bits in it. I assumed the bits all had a 1/8" shank as most do. I've got a 1/8" collet just for Dremel bits. Unfortunately the engraving bits were 3/32" shanks. So I went ahead and used my drill chuck to hold the bits. This is considered bad form, drill chucks aren't designed to take side loads, but the engraving was only 0.015" deep and went slow. I'm not terribly happy with the cuts, but after breaking a small bit doing PCB milling a year ago, I was reluctant to invest in another high-quality engraving bit without being sure what I wanted.

I did a couple of tests in a vinyl sheet material. I mounted the stock over a sacrificial plexiglass piece and held it with the same hold-down clamps I use for my vise. I ran the engraving program and everything went fine.

Then I switched to the 1/4" collet and mounted a 1/8" bit to cut the profile. I immediately realized my first error when I brought the head down. With the stock mounted so close to the surface of the table, the head with the short 1/8" end mill won't come down far enough to cut through the stock. So ends practice run number one.

One of the greatest challenges with this mill is the work envelope. You've got to plan ahead to make sure the bit can get every where it needs to go without running into the limits of the machine. I took some measurements and figured I needed to raise the work surface 1/2". I even took off the lower Z-axis limit switch to get a little more room. Fortunately, I had in the works a large tooling plate I mentioned in my last post. I didn't have any holes in it, so I had to take a quick detour from this project.

I mounted the tooling plate, and found the center rear edge. I then made a number of holes on a 1/2" grid clustering them on either side of center figuring I didn't need too many holes in the middle. Anything small enough to fit in the space in the middle likely could go in the vise just fine. Tapping lots of holes is tiring. I lost focus at one point of the drilling phase. My wife called me while I was in the middle of transiting the spindle from the left to the right side. I took the call and when I finished I assumed the head was at the target location and proceeded to drill the hole. Then I looked at the DRO and realized I was a little more than 1/4" short of the target. I made another hole to pair up with it, so now I've got 2 holes that are off the 1/2" grid, but they're actually in a pretty useful spot for holding the vice.

After completing the drilling and taping of the tooling plate, I mounted the sacrificial piece and another vinyl practice piece. I ran the engraving job again, then did the first roughing pass of the profile with the 1/8" bit. I've been using Cam Bam to generate the gcode. It supports "tabs" to hold the inner material in place while the profile is cut. The size of the tabs I chose turned out to be too small. I switched to the final finishing pass with a 1/16" bit and the tabs broke free. So I doubled the size of the tabs. A valuable practice run...

The blank stock mounted:

Engraving:

Profile cut:

You can see the tabs in the above picture. I had to grind them off and clean up, of course. This was a bit of a drag. The final milling pass was pretty clean so to have to grind and sand was a bummer. If I had to do it again I'd drill the holes first and screw the stock down. This would leave the complete perimeter free so the tabs wouldn't be needed. That's OK, I got a feel for the tabs and I know what the correct size is and what I'm up against for future jobs that don't have any interior holes for securing.

After milling, I painted the face gloss black, let it cure and sanded it with a random orbital sander. This left the black only in the engraving path. I took a shot at polishing the aluminum with my Dremel. I bought some polishing compound and hit the sides and the back. I figured the back isn't going to be seen so it would be good practice. It's not perfect, but it shows promise. Polished aluminum looks really nice.

Bare aluminum in the hands can feel a little dirty, so I sprayed on a clear coat enamel. And here is the final piece:

Way and Screw Cover

I visited a friend of my father over Thanksgiving break. He has a rather extensive machine shop in his basement. This guy is good. He builds his own telescopes. So what, right? You went to a meeting of the local astronomy club and some guy had a plywood stand with a cardboard Sonotube and some optics, and bam! he's built his own telescope, right? You could do that.

No. This was different. This guy grinds his own optics. I didn't see any mirrors he made but I wouldn't be surprised if he could do that, too.

Anyway, he squared up a large piece of stock of mine to be made into a tooling plate. I'll get to that in another post. He also gave me some vinyl from a shower pan liner or something. He suggested I use it to make way and screw covers. The Y-axis is pretty exposed on the Sherline so I thought I'd give it a try.

The front side of the machine is easy. It's "open ended" so easy enough to add mounting material to hold the cover. Here's a shot of the front piece I made:

I used this huge piece of Corian solid-surface counter top for the material. It's plastic. It's machines pretty nicely although it gets a little stinky if it heats up too much. It doesn't look very good, however. It's kind of poor simulation of granite or something. So I just painted it with a flat black and buffed it a bit. It fits right in with the anodized black aluminum of the machine. Ok, the piece I made has the 3 screws on top. Of course I used my hot glue gun to mount it. 

Here's the first of three attempts to design a mount for the rear. The challenge with the rear is that anything attached to the rear reduces the range of motion. Above I added the mounting pieces outside the rear column. Theoretically the cover would fold up on itself once and compress against the column costing only 80 mils of Y-travel. I attached the vinyl sheet so it folded up against the table. This resulted in the vinyl arching down against the screw. As the table moved to the rear, the downward bow became strong enough to stall the stepper motor.

Here's the third attempt:

The second attempt had no support in the middle and the vinyl dipped down creating a path for swarf right down to the screw/way. The third design is just one piece with a thin bridge to provide support. So I'm losing about 1/4" of travel at the rear.

Here it is with both covers in place:

And an engraving operation with the covers doing their job:

(Yes, that's a drill chuck. Yes, I know I'm not supposed to use it for milling, but I bought a Dremel engraving kit assuming the shanks were all 1/8". Turned out half of them are 3/32" and I don't have a collet that size. This is a pretty light engraving cut so it should be OK.)