What TPI for new cnc machine?

[Thys]

Hi All, a question posted to a good cnc friend Tweakie, but he advised me to rather hear what the forum had to say, so your input would be greatly appreciated.

I am busy designing a new machine and want to use it primarily for cutting wood on a flat table. I am thinking of a cutting area around 1.5m square.

I am able to have my own lead screws (ACME) cut to whatever pitch I want. I wanted to ask you advice on this. What tpi would you recommend me going for based on the application. I would like to cut in the range of about 1000 – 2500 mm per minute and accuracy, although important, is not critical to the 0.01 mm. Or do you think that is a little fast for cutting wood?

If the lead screw option in the wrong one, what other method would be advised for moving the axis (plural)

Any comments would be appreciated
Thys

[RICH]

I'll leave this one to the cnc router folks. No harm in reviewing or comparing the spec's of pro model machines since that will surely give you  a sound basis to question your deisgn intentions and what others may recommend. Do it right the first time ....it's cheaper.

RICH

[ostie01]

You will have to take into consideration how you'll drive the motors and what kind of motor you'll use.

Acme screw do not run as easy as the ball screw do so maybe more powerful motor will be needed.

Don't be too loose on accuracy, you will find out very soon.

Will you use reduction gear or timming belt as a ratio for the lead screw.

Jeff

[ger21]

1m/min is really much too slow for cutting wood. On our big commercial router at work, we cut at 20m/min. For a home built machine, I'd shoot for 3-5m/min minimum.

When using steppers, you have to trade resolution for speed, since they lose torque as rpm increases. So you can go very fast and have lower resolution, or very slow and have really good resolution.

With your screw length approaching 2m, I'd recommend 2 turns/inch (12mm lead). 4 will get you a little more resolution, but you'll probably get whipping at higher speeds.



I have 60" 4tpi screws on my router (1/2" acme), and screw whip limits my maximum speed to 150ipm (3.8m/min). My shorter Y axis can get to 4.8m/min, and it's still really a lot slower than I'd like.

I really have to question the need for .01mm accuracy. Most mid grade acme leadscrews here in the US have a tolerance of  ±.009"/ft, which is .2mm/300mm.
Unless your screws will be precision ground to much better than .01mm tolerance, you'll never see that.
.01mm is .00004". Don't you need a temperature controlled machine and room to maintain those kinds of tolerances?
Also, wood will easily move more than 25-50x your target accuracy with changes in humidity.

I'm hoping you meant to say .1mm, in which case the 2tpi screws will be more than precise enough.

[alenz]

Gerry, I interpreted Thys’s comment “accuracy, although important, is not critical to the 0.01 mm.“ as meaning that he did NOT need accuracy to the nearest hundredth of a millimeter. Those are watchmaking units.
Al

[Thys]

WOW, some excellent comments, thanks thanks thanks!!
yes, Gerry, I don't need that accuracy as stated. I am happy with what you recommend at 0.1 or 0.2mm
thanks all for taking time to help me. I REALLY appreciate it.
Thys