Dear Chris: I have a couple of NEMA 14 steppers that I want to mount directly to the WorkBench base. While the screw holes line up perfectly, the large holes are slightly too small. I don’t want to break the plastic, so what is the best way to enlarge them?

Would a CNC router work? Or would that shatter it?

I don’t have a laser strong enough to cut it, so I’m not quite sure what I can use. Any suggestions?

Thanks again, Steve in Montana

 

Dear Steve in Montana:

To your question about enlarging the holes on the Base:

  • How much bigger are you going? The existing small holes are about 0.175″, and they are designed to accept an M4 or an Imperial #8 screw.
  • I took an old test Base we have and tried drilling it out.
    • I found that going to just over 0.200″ on them worked quite well. I just used a hand drill and a sharp twist drill and opened them up.
    • I did get a little bit of chipping, but I didn’t shatter the Plexiglas.
    • An M5 is about 0.197, so if that’s what you need, that should be okay.
  • However, I then tried going much larger, to just under a quarter inch (0.242″). That didn’t go so well.
    • I got a lot of chipping, and if I did a bunch more of them, I’d almost certainly create a crack in the Plexiglas.
    • Having said that, you can crack Plexiglas and it will still be serviceable, as long as it doesn’t explode/shatter into a thousand bits.
    • Some people use acetone to weld acrylic back together but I’m not really a fan of that. I use Plast-i-Weld from Amazon. It’s everything you need. You can buy it at Hobby Lobby too if you need it the same day.
    • Here’s another option from Amazon, but I haven’t tried this one.
    • You really need some type of large syringe applicator like this.
    • You just want the solvent to wick into the crack; it literally dissolves the two sides and makes them one again.
  • Whichever you use, WORK IN A WELL VENTILATED AREA. Those are some nasty chemicals, and you don’t want them in your lungs.

Having said that, I haven’t tried routing, but that should be a very friendly way to do this. Here’s a thread that may help.

  • Holding the workpiece down is IMPERATIVE; if it catches and lifts, it’s going to come after you and hurt you. Be sure to wear safety glasses. I’m pretty careful whenever I cut acrylic sheet for that reason.
  • Speed of the spindle is key; looks like it wants to be high, which feels right to me. Like about 20K RPM or so…
  • Cutter choice is also key; I’m pretty sure you should use carbide, and it’s likely to be a small cutter for the holes you want. Those can be hard to find.

Lastly, the laser doesn’t have to be that big to work. We use an 80W, but a 30W should work just fine. Just go slower. What do you have?

Laser is the best choice, if you can swing it.

Do you have any scrap Plexiglas you can experiment on?

Hope that helps! Let me know how it’s going.  C

 

Dear Steve in Montana,

Barbara just pointed out that I totally misread your question–you want to enlarge the BIG holes. Sorry about that!!

Most of my comments still apply, but obviously, I missed the boat. Will be happy to advise more when I understand your intent a bit better. Maybe a picture or a napkin sketch to clarify?

Thanks!  C

 

Dear Chris:

Ha ha! Yeah, I could tell you were thinking the little holes. That’s ok. I think your info still applies.

My laser is a small 5W laser diode thing mostly good for engraving wood. I’m thinking the CNC router, but I wonder about the speeds & feeds on it and whether to use a climb or conventional cut. I don’t think I have any acrylic like this to experiment with.

I guess I could just go for it, but I hope you could add some insight since you use acrylic more than I have.

Thanks, Steve

 

Dear Steve in Montana:

Yup–I totally whiffed on that one!

Couple of follow-on thoughts:

  • [Chris starts by suggesting another method to mount the Steppers that does not involve cutting the acrylic. Watch for the next “Dear Chris” post! We’ll have photos of the alternate method.]
  • If that doesn’t float your boat, I’d probably go with the CNC router:
    • Keep the speed up if you can, around 20K on the RPMs.
    • Feed in a spiral from the center out, so you don’t cut everything at once. Use a cutter no bigger than 3/8 to plunge right into the center of the existing hole, and then spiral gently outwards.
    • I’d climb feed, pretty sure. [Note: see below for an explanation of climb feed vs. conventional feed.]
    • Honestly, I’m not sure what feed to use, so I looked it up here in a handy guide from Tap Plastics. They recommend about .004 to 0.015 per tooth chipload; which apparently works out to 100 to 300 IPM. 300 seems fast, and of course, it depends on how many flutes your cutter has. You’ll probably use a 2-flute spiral, so it seems like 100ish IPM seems right.
    • People route acrylic sheet all the time, so it should be just fine.

Hey, if you get into trouble, we’ll figure something out.

Let me know what you decide to do, if you need anything else, and how it turns out, okay?

Thanks!  C

 

How will it turn out? Which method will Steve choose? Will it work? Have you tried hacking the WorkBench Base? Let us know!

Watch for more on Steve’s exciting project in the next blog post.

 

Note on “climb” vs. “conventional” feed:

There are two ways to cut material using a milling machine or a router:  conventional feed or climb feed (also called “conventional milling” vs “climb milling”).

A full discussion of the two was beyond the scope of this exchange, and Steve had already made it clear that he understood the difference, so I didn’t feel the need to explain.  But for readers less familiar with machining and routing techniques, it’s an important distinction.

When you are using a milling machine or a router (a machine with a rotating cutter), “conventional vs. climb” refers to the direction of the cutting path relative to the rotation of the cutter. Climb vs conventional feed

If your cutter is rotating clockwise when viewed from above, and you feed the router against the rotation of the cutter, that is conventional milling or feed. In conventional cutting, the cutter resists being drawn into the material, because the cutter is pushing the workpiece away from you as you feed the material.

Alternatively, if you feed the router with the rotation of the cut (in effect, the cutter is now pulling the router into the material), that is climb milling.

Typically, you can only do climb milling on a machine that controls the rate of feed, because when feeding by hand, climb milling can get away from you quite dangerously, say when you’re taking a heavy cut with a large router. So that is NOT recommended.

But with a milling machine or a CNC router, the machine can control the rate of feed and resists this “runaway condition,” so this is safe. Climb milling usually gives you a better finish (see this online article for more than you wanted to know about the topic), so it is generally used whenever a machine is doing the hard work.

Hence my recommendation to Steve to climb feed this cut if using a CNC router on acrylic.  Assuming the acrylic workpiece is safely and securely held down, and the feed rate is not too fast, climb milling will give you a much better finish in acrylic.