general questions about machine speed

[neeboy]

I belong to a community workshop where we have a home-brew CNC router. I've been using it to machine very detailed architectural reliefs using pink insulation foam. It's been working great....but it's too slow. I've tried increasing the speeds/velocities in Mach 3, but I've experienced things like skipped lines of code, axes not moving (machine is traditional screws), etc.

A typical piece I'm doing is about 2.5 x 3.5 inches, using a 1/16" bit with .5" of cutting depth and 10% step-over. I tried first with 40% stepover and it was awful, very jagged. So if I have to, I'll run a slow machine, period. The quality of the cut was not affected after increasing velocities/speed (using same stepover), but after about 5 minutes the whole process went out of whack and the piece did not machine correctly.

So...I'm considering getting/building my own machine. How are motors sized to allow for greater speeds and velocities? (I'd like to have at least a 4'x4' cutting area) I've also seen chain-drive machines that claim much higher speeds and velocities, but I'm skeptical.

Any helpful advice? Thanks for any informed replies!

[Al_The_Man]

So...I'm considering getting/building my own machine. How are motors sized to allow for greater speeds and velocities? (I'd like to have at least a 4'x4' cutting area) I've also seen chain-drive machines that claim much higher speeds and velocities, but I'm skeptical.

Any helpful advice? Thanks for any informed replies!

Use suitably sized Servo's may be one answer.
I assume you are using steppers right now?
Al.

[neeboy]

Use suitably sized Servo's may be one answer.
I assume you are using steppers right now?
Al.

Yes they're steppers....not sure what size.

[KOC62]

Have a look at Joe's 4 X 4.
http://www.joescnc.com/themachines-hybrid.php

[ger21]

Multiple factors influence speed.
Method of motion (screws, rack and pinion, ...)
Motors
Drives
Power supply Voltage.

The combination that's proven to provide the best performance, is the Gecko G540, 380oz motors, with a 48V power supply. You can save $200 going with cheaper drives, but you'll usually only get half the performance of the G540. This combination works well up to a 4x4 size machine, and many use it up to a 4x8 machine. This assumes wood or aluminum extrusion construction. A heavy all steel machine may need more power.



So, assuming the components above, here's some ballpark numbers:

1/2" acme screws are good for up to about 36" of travel. Longer than that, and whipping will limit performance.

1/2-10 single start - good for about 100ipm,
1/2-8 2 start - 200-250ipm
1/2-8 4 start or 1/2-10 5 start - 350-500ipm

For 4 ft and above, rack and pinion is probably the best choice. Depending on how fast you want to accelerate, you can get up to 1000ipm rapids or more. Cutting speed will probably be closer to 400-500ipm max, depending on depth of cut.

Personally, I'd stay away from chains, but I've seen a few people that are happy with them. I've also seen others replace them because they weren't happy with them.

When I cut hardwoods with a 1/16" ballnose, I use a stepover of .002", which gives me a glass smooth finish with no tool marks.