Slotting wood with a laser

My experiments with using the Roland CNC SRP didn't turn out too well. Although the GCode program worked well, it turned out to be very difficult to get consistent heights for the parts. Out of every run of eight parts, I would have to throw away three on average for being too short. I suspect that this was because the end mill exerts an upward force on the parts as it cuts. Normally this isn't a problem, but if you have eight slabs of acrylic in a vise, the acrylic-on-acrylic friction may not be enough to hold it in place no matter how much you dare tighten the vice.

I did manage to get the GCode program down to an hour, but the other problem was drilling the holes. The drill bit kept heating up and melting the plastic, and I had to flood the coolant onto the part and bit to get it cool enough. The Roland doesn't have flood cooling, so I ended up slathering coolant constantly onto the bit. And even then, sometimes the heat was just too much.

So in the end, my hypothetical cost to produce a single module went down only to 56% of the original contracted metal parts price instead of the 39% I originally estimated.

But, I had a new idea where I might be able to cut an entire module side at once using the Trotec laser cutter. My estimate for the cost using this method is back to 39%.

Part of the method involves cutting 1/8"-deep slots, and I figured out a way to do this with the laser. I just engrave at various speeds until I find the speed that cuts a slot as deep as I need.

Next week I should be able to cut the sides for an entire module, and then we'll see how it goes together.

Design of an Elevated Fixture for a Roland MDX-540SA

Behold! This is the standard table on the Roland MDX-540SA CNC milling machine, costing nearly $37,000. I took a picture of the table and then used Photoshop to eliminate distortion due to perspective and pincushioning. Then I grabbed some guides so that I could measure the positions of the holes.

Roland table

The Roland specifications show that the holes, which are metric 8mm (M8) tapped, are 110 mm apart in the X axis (horizontal in the picture) and 55 mm apart in the Y axis. There are no specifications whatsoever about the positions of any other holes. This, in case you weren't paying attention, is a $37,000 machine.

Take a look at the cluster of four holes in the lower left that don't fit the pattern. Also look at the similar cluster in the lower right, next to the Z-height post. The lower leftmost hole is almost covered by the post. Two out of each of these four holes is a locating hole. But the Roland manuals don't tell you where they are, or what size they are. I had to figure it out myself, This, by the way, is a $37,000 machine.

Using this photo, and taking a few measurements physically, I was able to determine the exact positions and sizes of these holes. All of the holes are M8 except for four -- count 'em -- four locating holes for 6 mm dowels. Four locating holes, two stuffed in the lower left, and two in the lower right. These locating holes are for a 4th axis attachment. So, four locating holes off in the corner of a table on a $37,000 machine.

Did Roland cheap out? Are they acting corporatist? Honestly, how hard and costly could it possibly be to add a bunch of usefully-placed locating holes? I can understand not adding a hole to a part that you make millions per month of, if a hole costs a penny. That's a savings of $10,000 a month right there.

No wait, I take that back. I never understood that kind of race-to-the-bottom accounting. Does anyone have any idea why Roland would not put locating holes on their table? Please hypothesize in the comments.

So behold, a Google Sketchup drawing of the table (minus the Z-height post, which I didn't measure). Click here for the Sketchup file. You're welcome. (Update, 14 March 2011: In fact, the two locator holes on the right aren't even locator holes. They are slots, and so they are useless for locating. Also see the update at the bottom of this article).

Roland table

So, we can now begin to design an elevated platform where we can add fixtures. Why do I need an elevated platform? Because the toolhead will only go down to 99 mm (3.9 inches) above the table. This means that if you really, really wanted to crash your tool into the table, you would need to have a tool that stuck out of the toolhead by 3.9 inches. If you're milling with a 3.9 inch long tool, you must be cutting foam or something. Any harder material and such a long tool would deflect, making a precision CNC machine kind of pointless.

So the end result is, if I want to create a decent platform for my workpiece, assuming about an inch of tool stick-out, I need to raise the table 2.9 inches or so. Furthermore, since my platform is going to be truly useful, unlike Roland's standard table, it will have many locator holes and many bolt holes that have to be fixed in the same position no matter how many times you take the elevated platform on and off. So the four locator holes on the table are crucial.

Finally, I need to be miserly with the material that goes into the fixture. We have a 1/2" tooling plate that will serve as the platform top, and it weighs 16.5 pounds. The Roland's capacity is 44 pounds, leaving 31.5 pounds for the rest of the platform, the workpiece fixture, and the workpiece itself.

The first step is to make two plates, each of which is located by the locating holes on the table and secured with M8 bolts:

Roland platform 1

The plates are 1/2" aluminum, which I calculate will weight around 3.8 pounds. I'll press-fit 6 mm dowel pins into the platform, which will slip-fit into the locator holes on the table. Ideally the locator holes would be more spread out, but thanks to Roland, this is what I'm stuck with (NextFab preferred that I not remove the table, put holes in, and put the table back on).

Next, I will be holding the platform up with pillars made from aluminum tube. So I need to add holes to the plates for attaching the tubes:

Roland platform 2

Each tube is held in place with two 3/16" dowel pins press-fit into the plate and the tube, and two screws to make sure the tubes don't come out.

Next, the tubes themselves go in. How tall should the pillars be? If I want the end of a tool with one-inch stick-out to reach the top of the platform so that I can make fixtures using the Roland itself, the top of the platform must be one inch below the bottom range of the toolhead, which means 2.9 inches above the table surface. Since we have a 1/2" plate on the bottom, and a 1/2" tooling plate on the top, that means the pillars must be 1.9 inches tall.

Roland platform 3

There are two locating pins, thus the need for press-fit holes. Also each pillar has a tapped hole to secure the top plate to.

And that's it! The tooling plate fits on top, located by the pins, screwed in with four screws.

I've ordered the 1/2" aluminum plate for the bottom, and the tubing, plus various hardware, and some reamers to create the press-fit and slip-fit holes for the pins. Hopefully they will all arrive by Saturday so that I can start building this thing.

Update, 14 March 2011: In fact, the Roland documentation says that origin repeatability when the machine is turned off and on again is +/- 0.0008". So you're not guaranteed to find the origin if you need accuracy of 0.001". Also, over twelve inches, the thing may be off by +/- 0.004".

The conclusion is that the Roland MDX is unsuitable for CNC. It is marketed as a subtractive rapid prototyping machine, not for CNC manufacturing.