What is the best recipe for very nice speed?

[gearsoup]

Hello all,

I am designing a 4'x4' CNC router (overall size is 5'x5'). It will be the standard Thomson linear shafting, rolled ball screw deal. The X axis will be using 2 ball screws. Machined components will be out of billet aluminum.

I had intended on going with 3/4" ball screw with a lead of .203" per turn. The ball nuts for this size were nicely priced at $58 a piece. Seeing that I needed 4 of them, it seemed the way to go.

The problem I ran into though, is that I was thinking about getting the 1015 oz. kit from DMM http://www.dmm-tech.com/Products.html

These motors have a max rpm of 1080. This will only give me a peak of 200 ipm. This is not fast enough. I run CNC machining centers all day that move along at 1000 IPM. Granted, they also cost $250,000. I think seeing my machine plod along at a measly 200 ipm is going to get annoying.

I was hoping for somewhere around 400-600 IPM. I realize that motor size isn't everything, but on the other hand, I also want it to ramp up pretty quickly.

So, do I go to a lead screw with a much faster pitch (say, .500" to 1" per turn), or find a different motor setup? One note here: I am not opposed to doing all my wiring. That is what I did with my last CNC router. I was looking at the DMM kit simply because it was saving me all the work of sourcing all I needed.

I really would like to stay with servos, mainly for their efficiency, less noise, etc. etc.

I also would like to go with a commonly available ball screw, seeing as I would like to make a couple of thse (my father-in-law wants one)

Any comments?


thanks for your replies,

Seth

[Pplug]

At my house I have the two routers listed below in my signature. The converted CMM router routes consistently faster do to its acceleration rate. It used to have some custom leadscrews at about a .25" pitch. I am in the process of replacing the leadscrews with ballscrews and a .3 something pitch (10mm). I'm also saving up for a motor, powersupply and controller replacement. All the details can be found in this thread:

Granite base CMM to Router Conversion

I can maintain just about the same acceleration with the same motors and can run the mill at higher speeds! I admit that my mill is under powered for 1/4"+ bits at medium to high speeds. The benefit is, I can carve 3d signs about twice as fast as my much larger Techno LC4896! The top speed might be lower on my CMM mill but due to the acceleration, it can finish 3d work much faster! With the new ballscrews, motors, and controller I think I can go much faster!

[rowbare]

Do you need that much torque? Perhaps a DMM kit using their 400 watt motors would be better. That gives you about 600 ipm.

bob

[Pplug]

What about a rack and pinion drive system?

[ger21]

You might get some whipping of the screws at 2000rpm. A .5 pitch or higher screw would be a better option, but they're usually much more expensive.
An alternative method is to use a belt drive to spin the screws faster, say 1:2.

[gearsoup]

I thought about going with a rack and pinion, but I think I am going to end up cutting aluminum on this machine, in small quintities.

I have been looking at the larger pitch screws, but I was worried about the resolution of the motors affecting the accuracy, seeing as a small movement of the screw is now producing a larger axial movement.

[gearsoup]

You might get some whipping of the screws at 2000rpm. A .5 pitch or higher screw would be a better option, but they're usually much more expensive.
An alternative method is to use a belt drive to spin the screws faster, say 1:2.

How would a machine behave with 1" dia ball screw with a lead of 1" per turn? Granted, it would probably move along quickly, but would the lead be too great for the motors to move? I'm no mechanical engineer, so a lot of this is speculation.

In regards to spinning the screws faster with belts, that would bring them up to the 2000 rpm valuem and run the risk of whipping. I am aware of this issue. Seeing as the screw is almost 5 feet long, I think it's a valid concern

[dertsap]

, but would the lead be too great for the motors to move?

the screws shouldn't have any problem turning with those size of motors , i use 2"/rev screws with 1340 oz on the x and 770 oz on the y , the accel decel are set quite high and theres plenty of torque and speed

[Pplug]

What about a spinning ball nut? That provides 0 whip on the screw because it does not move, and the nut can be spun faster with lower inertia. My techno 4896 does that on the y axis. They have models with the same ballnut design that run at 600 IPM.