High accuracy, small CNC

[PaulMakesThings]

Edit: I placed this in woodworking because it is for plastic, so wood is closer than metal. If there is a better place for it I would appreciate if it is moved there.

I'm working on a graduate project with a group. Our goal is to make a system to rapidly custom cut dental abutments. I am a mechanical engineer and well versed in electronics but this will be my first CNC build. For now I'm looking for advice on what movement parts use, or which ready made machine to buy, we can get into motors, drivers and controllers from there. Instead of going on and on, I'll just explain the project requirements and hope some helpful people will give me their input. If the constraints sound impossible, tell me that as well, just explain why and how I should try to change them.

Goal: Develop a system to manufacture on demand custom formed dental abutments (plugs to cap off jaw screws, shaped like a tooth on the part that contacts the gum)
Responsibility:Machine, electronic hardware and driver software. Another group and budget will handle CAD/CAM software.
Budget: $2500
Time: Six months
Required precision:+/-0.001"
Material to cut: Polymers
-Density 1320 kg/m3
-Young's modulus (E) 3.6 GPa
-Tensile strength (σt) 90-100 MPa
Work piece size: less than 0.5"x0.5"x0.25" (little)
Work piece direction: highly detailed top, convex curves around the sides, back requires no machining (think of the bottom half of an extracted tooth)
Speed: It is desired that the part can be ready in 2 hours or less

I doubt they have ready made CNC's this tiny, but not knowing is why I'm asking. If you were going to design this machine, where would you start? I'm always interested to hear about solutions and considerations I didn't know about. Thanks.

[judleroy]

Look into the taig or sherline Cnc mills. You should be able to get what you need with your budget.
Judleroy

[acondit]

Take a look at

this video

this video

to see what is possible. Cradek turned a small mill into a 5-axis machine which could easily handle your needs. Ubuntu Linux with EMC2 is free so all of your funds could be put into the machine and electronics.

Alan

[PaulMakesThings]

That is encouraging. Even a mini mill is still far larger than we need for these parts, but I realize the savings of making a machine closer to the size we need would probably not make up for the cost of customizing it. I looked up both products, the sherline CNC mill is $880, that is CNC ready, no drivers or motors. The tiag one looks like i may be complete, it comes with the machine and software, but it's hard to tell if it comes with everything in between, such as drivers boards, if all that were included its a decent deal at $2295.00, and the resolution looks great.

acondit, we posted at the same time so I didn't see your post. Thanks for that video, I am considering a home made or modified mill. Since we have a machine shop at the school and I am decent at using it making our own CNC machine with some ready made parts is an option too.

[Walky]

If you decide to go DIY, I think that a good combination for that kind of job would be Hiwin guides (or better), 12mm ballscrews with two nuts each (back to back), a Wolfgang Engineering spindle and a Gecko G540.

[acondit]

Paul,

Take a look at 5Bears website. He built a minimill mostly from scratch. You can see what is involved in getting an accurate machine. I am building a PCB-Mill (small router) and I would be concerned about the time required for a complete build for a student project.

Alan

[Drools]

One of my favorites in the small router category is Don's.
http://www.cnczone.com/forums/diy-cn...ry_router.html
Going with 80/20 can make the build a little easier for people mechanically challenged such as myself.

[PaulMakesThings]

Drools, that is a nice build. I was thinking if I made one it would be something like that, possibly made from thick PVC board. For such small work it wouldn't take very much.

acondit, thanks. The 5 bears website looks like a good resource. His earlier discoveries kind of mirror mine, so I'm sure what he talks about later will come in handy. Like he said I am considering buying parts on ebay, and they do offer some very small parts. He seems to provide a good rundown of all the little parts involved, I'll show this to my team. You are right to cite time as a concern. I have successfully completed several builds on time in my research, and the main thing I can credit for it is not biting off more than I can chew. Still, a built machine is worth considering. It's such a small part we have the opportunity to make something that would look nice in a dental office, like a kitchen appliance, I would be missing an opportunity to not at least consider building it with my team, and we do have some history of lab machine builds. If interested, here's a picture of one of my latest, it's a fatigue tester for bike forks.

Walky, thanks for the list. That's more help that I was expecting. That gecko drive in particular looks interesting. Do they make a 5 axis version? We aren't set on 5 axis, but it's a consideration. I'm guessing they don't since I see talk of using a G251 to add a 5th axis. I also appreciate the movement hardware suggestions.

[Walky]

Are you really considering 5 axis CAM software? those are really, REALLY expensive AFAIK.

BTW, there's no 5-axis version of the G540, it's 4-axis only.

[acondit]

Originally Posted by PaulMakesThings Do they make a 5 axis version? We aren't set on 5 axis, but it's a consideration. I'm guessing they don't since I see talk of using a G251 to add a 5th axis. I also appreciate the movement hardware suggestions.

Dan Mauch (Camtronics) built a 5 axis controller for a customer of his. He talks about the difficulties of integrating a G251 with a G540 4 axis controller to create a 5 axis controller.

By the way, in case you don't know, the G540 is basically a fancy BOB in a special metal housing with (4) G250 drives attached to the BOB. It is probably one of the best buys in the market today for a small machine controller.

You might get away with a 4 axis system, but the more I thought about it the more it seemed to me that you would need 5 axis. Given the speciallized nature of the machining, maybe you could fake it, if you mounted the 4th axis at a 45° angle to the spindle. It might actually make the programming more difficult than true 5-axis though.

Alan

[PaulMakesThings]

That's something I'm trying to figure out. I'm going to meet with the dentist sponsoring the project next week. It seems like since the form should only expand as you move down from the top it may be doable with 3 axis, which would make this way easier, but maybe not.

[RomanLini]

3 axis should be ok. You can also use reduced-shank ball end cutters and even keyseat cutters and rounded keyseat cutters, to produce a certain amount of undercut even on a 3D machine.

There's a lot of small dental CNC machines on the market although they seem expensive!

You could try a sherline and put some motors on it (as someone said) or a small X1 mill, but really for a working area of just a couple of inches (and really light cutting loads in plastic) you have a LOT of options to build it yourself.

If you do, please start a build thread as I would love to see what you come up with, there are not many tiny precision machine on this forum!

I made a 370x260x65mm "semi precision" plastics mill (I work in robotics and specialist machinery design, like yourself), you can see it in my build thread;
http://www.cnczone.com/forums/cnc_wo...ll_router.html

I used the plastic Igus brand linear bearings and I can highly recommend the plastic bearings for small low speed high precision linear movement.

Good luck with the project!