[tinmancarving]

Hello,
I am new to CNC routing and in the process of sourcing a machine. I have looked at several machine in my budget and have noticed that some come with ball screws on all 3 axis and some only on Z with gear racks on X&Y.

Is there a performance difference? How about accuracy.

I come from the aerospace manufacturing industry and all of our machine are ball screw on all axis. Because of this I have not experience with a rack system.

All comments welcome.

Thanks,
Rob

[ger21]

A lot more than just rack vs ballscrew will dictate performance, as well as accuracy.
On cheaper machines, rack and pinion will tend to be faster. Especially on stepper driven machines.
But keep in mind that with a stepper machine, generally, the faster the machine, the lower the resolution it will have. This is due to the fact that steppers have a fixed number of steps per resolution.

Most larger machines under $30,000 dollars will tend to be rack and pinion, as it's easier to implement over long distances, and cheaper than long ballscrews.
I would say on 4x4 and larger machines, about 95% or more of the machines I see on the forums I visit are rack and pinion, and I've never seen anyone complain about it.

[louieatienza]

Many high end portal style machines use rack and pinion and they can attain very high accuracy. The higher end machines use helical racks and pinions, and use two motors, master and slave, that remove backlash by preloading against each other during accel and decel, and move in tandem during rapids.

On smaller machines springs or gas struts help keep the pinion against the rack.

[hanermo]

Well..
its really a question of what the machine is meant to do.

HIgher accuracy machines, as well as mechanical processing in metals, tend to be ballscrews.
So high end lasers, machining centers == ballscrews.
Wood routers - all sorts.
Plasma - r&p seems to work well.

[louieatienza]

Look at the Zimmermann FZ100 with its helical racks x and y and abc head... designed to mill large aerospace parts - very large parts!

[1Jumper10]

I considered the same thing when i built my cnc router. I went with R&P primarily based on cost. My router is slightly under 4x4 and it was easier to use rack and pinion than to have parts machined to mount ball screws. Plus, the ball screws in my price range, Chinese suppliers on eBay, had some backlash. Zero backlash ground screws aren't cheap! I am very pleased with the performance of the rack and pinion drive on my machine. My backlash compensation is set at zero and it works very well. Going with rack and pinion over screws was one design decision I am very happy with.