I am considering building a high performance CNC router, something in 9’x5’ size area. Something that can be run all day and go real fast. Sort of a gift to myself.

I am looking to rapid in the 1800 IPM range (maybe a little slower) so I’m looking for suggestions insofar as the drive design.

Right off the bat I know that ballscrews will not work.

So I am down to Rack and Pinion or Belt.

If I go with a standard rack design what is the tooth/pressure angle that is favored?

If I go with a belt design I’m looking at using 8mm x 20mm HTD, fixed on both ends with the X axis drive motors mounted on the gantry. There will be two motors for the X axis. I considered using a loop configuration with the X axis drive motors mounted on the table but I would need a belt 5.5 meters around. That is one big belt. So I am forced to put the motors on the gantry and deal with the extra mass.

As you can tell I like the belt idea a little more, I like belts, I understand them. A Rack and Pinion I’m not too sure about.

Now of course it’s the X axis I’m worried about. Moving the whole gantry is the hard part.

I know I am going to use 1000 in/oz peak servos, and some quick math gives me the following.

To get things down in the useful area the motors are reduced 4.1 to 1, so that give a useful torque of 4000 in/oz. The HTD pulley I like has a pitch diameter of 2.5 (or r 1.25).

4000in-oz /1.25” (radius) = 3200 oz of force = 200 lbs of force.

200 lbs of force x 2 motor for the X axis = 400 lbs of force.

I figure the gantry will weigh about 100 lbs.

After a little acceleration math I get an acceleration of 1544.35 inch/second^2

Peak velocity is 1800 IPM.

1800 IPM = 30 IPS

30 IPS (max speed) / 1544 (acceleration) = .02 seconds to reach max speed. (This is not the right way to do this section of the equation but it gives an idea).

Does anyone have any comments?

I am looking to rapid in the 1800 IPM range (maybe a little slower) so I’m looking for suggestions insofar as the drive design.

Right off the bat I know that ballscrews will not work.


Is that due to budget reasons? Virtually all serious commercial routers use ballscrews.

3400ipm ballscrew router http://www.shodausa.com/platinum.htm

3150ipm cutting feedrate http://www.komo.com/CNC%20Routers/Machines/vr_mach_iii.htm

A 9 foot long screw will whip a lot, but I think one of those mentioned spinning nut design; I am not in the mood to design a spinning nut. Also, this is MY machine, but it is going to live at my cousin’s house, who is a wood worker. So ease of maintenance is a must. I think ballscrews require more “love” than a belt.

given the speeds you think you want to go and the minimum weight a gantry ends up being after you start adding things on (router motor servo motors and such) a belt would stretch to much over an area of 9'.

While it might seem lighter and such I'd say belt stretch would make for some unhappy performance..

Also I don't see how your gantry could weigh less then 100lbs..

Figure 10-15 for your router motor
I know the 1100oz motors are at least 10 and are probably more.. And you would have one hanging off for z and at least 1 other attatched to the gantry some where else for Y.. so right there you have 30-40lbs and you have not even started adding structural stuff..

All 3 of my motors move with my gantry but I have rack/pinion on my long 100in axis.. Ball screws on my short 50in..

Also I would not suggest belt for Z.. Something with some mecanical resistance (ie ball screw) is best for that in my oppinion. As that would help stop the tool from falling through the work when the system faults out. How ever I like having the rack on X as I can manually move my gantry around if I need to when the motors are off where I could not do that with a screw.. Belt would give the same benifit I figure.

But I doubt your gantry will be ~100.. To be stiff it will be more..

b./

IndHobby, take a look at the thread I just posted. I thought this was a very interesting belt drive system.

http://www.cnczone.com/forums/showthread.php?t=30320

I was figuring a screw for the Z axis, that didn’t scare me.

The Y axis I was on the fence about.

It was the X, I was worried about the most.

What sort of rack do you use for the X axis, what is the tooth and pitch?

Insofar as the 100 lbs for the gantry, I might end up a little more than that but that will just affect the acceleration rate, so that is not a worry either.

I used the 14.5 pitch racks mcmaster car sells. Seems to work just fine..

I can fast jog around 800ipm ish if I want..

Although my screw tops out around 210ipm..

Plenty fast for me..

100lbs for the gantry is to light for the size you are talking about though.. Know way it can be that light and be ridgid enough... I'd have to look it up but I'm sure my 60x1" hardened steel bars (2 of them) on my Y are pretty heavy just by themselves.. plus my servo motor + thompson 10:1 planetary isn't light either..hehe

b.