Adventures in Cutting Aluminum and Acrylic via CNC!
Two little jobs that just popped up recently challenged us to learn how to cut some types of materials we hadn’t before with our Shopbot. Nothing like work to get you to try new stuff! I’d thought I’d blog what we did, and how it worked, for others that are getting into cutting these materials, or for those out there that might be interested in having us make something for them out of these materials.
First off, we carved a custom bar top for a friend of ours out of 1″ thick acrylic. We think it came out great:
To do this job we used a 3/4″ ‘straight plunge double flute’ cutter with a 1/2″ Shank from Amana Tool. If you know about bits, skip this next part, but here’s what this means: It’s that this bit cuts a slot or groove (called a ‘kerf’) 3/4″ of an inch wide, it can ‘straight plunge’ into the material (i.e. drill, some bits can only cut sideways), it’s got two cutting edges (‘flutes’) that are straight (if it was a spiral, like a drill bit, it would normally say so here), and it’s got a shaft (‘shank’) of 1/2″ (that’s the part that the router holds onto). Normally, when cutting wood, we use a bit that’s both thinner (1/4″ diameter), that’s a spiral (downcut, so that the cutting action of the spirals hold the wood down to the table as they cut), and that’s not such a large shank (again, 1/4″).
Why the big, straight bit? Well, the thicker the bit the less deflection and vibration it’s going to have. The bits, being carbide steel, can actually bend very slightly when being pushed around by the Shopbot’s powerful arm, and especially so when cutting 1″ plastic. So a big, thick bit really helps. So does the straight flute part. When cutting plastic, a straight flute bit so far has given us the best edge. Also, seeing that this was one big part, with a carved out element to it, we didn’t have to worry about such a large ‘kerf’ making it a problem to fit everything on the sheet. Using a smaller bit gives you a smaller kerf, and therefore lets you fit parts closer together on a sheet of material.
So we cut this out with many passes, so that the bit was only cutting out about 3/8″ of material at most. We’ve found taking many, faster passes to cut and/or carve something out works a whole lot better than taking one deep, slow pass. Taking multiple passes lets us have a better edge that requires much less sanding, and it actually makes things faster and safer to boot.
This was a test bit that we cut. As you can see, the edges aren’t bad, and if we polish them, it looks great.
Which brings us to the next job, which was cutting some 6160 Aluminum sheet for a friend’s robot project. Now, I know using one Robot to make another Robot isn’t a wise idea, for it leads down the slippery slope of the Robots taking over, but in this case the other Robot is an R2-D2, so I figure it’s ok THIS TIME, for R2-D2’s help save the galaxy and people and stuff. Anyways. The aluminum was 1/4″ thick, and he got a whole sheet of it. This was a very challenging thing to learn, for cutting aluminum isn’t that easy with a CNC tool like a Shopbot. Speed and depth are very, very important. If the bit gets hot at all, the Aluminum melts just enough to stick to the bit, wrap it in soft metal, and pretty much render it completely dull. You’ve got to stop the Shopbot before it hurts itself, take the bit out, clean off all the loaded up aluminum (not easy!), and try again. It took many tries before we got it right, but once we did, it went pretty well. What you want is nice, big chips flying off of the bit, and for the bit to keep cool, which we did with a little squirt bottle of soapy water that we sprayed the bit with as it cut.
Sadly, those same chips are sharp as hell and fly everywhere. If we do a lot more of this, we’ll have to figure something out to deal with that. Thank goodness for the new Shopvac we just got.
As you can see, we were able to get a pretty clean edge once we figured it out. For these cuts we used a 1/4″ two-flute upward spiral bit. Each pass cut only 7/64″ deep, and the Shopbot speed was set at about 0.25″/sec (with a plunge of 0.10″/sec). If we tried to go any faster, it quickly ran into problems, but with these settings it did cut quickly enough to still be reasonable. To contrast, when we cut wood, we run about four times this speed.
One thing about Aluminum: it vibrates a lot. You really gotta hold it down. But because it’s so stiff, you can place the parts really, really close together without worry.
And you can get some pretty clean edges.
Sadly, we don’t have any pictures of these parts we cut, for it was a real rush to get the parts cut and over to my friend in time. But you can see the ‘aftermath’ here, and we’re now scheming and dreaming up all sorts of Aluminum projects, so you should see something great real soon…