backlash

[MIKE JEFFERS]

hello
iv'e built 3 cnc's in the past few months all for little money ,no geckos,no
ball screws & running free ware, and its been a good learning curve
but now its time to spend some of the cash earned to better things.
however..... all my machines have backlash of less than 0.03mm
bear in mind that this is 1.a gantry router using stainless m10 threaded rod
and plastic (green stuff maybe delrin, a skip find) nuts ,used for engraving.
2. micro mills x1 type using standard lead screws and a bit of tweaking on the gibbs.etc ( one of which has been run nearly every day since october)
am i fooling myself to think that ball screws are a wish list thing when i do an x3?
your thoughts are apprecated on this , by the way i make custom parts for
bikes & cars so apperance is more important than silly tolerances.

[MIKE JEFFERS]

cmon
someone must have an opinion on this !

[JFettig]

I prefer ballscrews if you have the money to drop on them, overall you will have a faster moving system and possibly more acurate. If you go with an x3, youll want to get some preloaded ballnuts.

I am running a regular screw on my lathes z-axis which is 16tpi like the stock screws on the mini mills, I only get about 30ipm out of it with 272oz-in motors, on my mini mill with much larger motors, I amm currently running 210ipm (400-500oz-in motors) and gecko drives on both. I wouldnt expect that kind of speeds out of a machine typically but I guess I got lucky

lol, silly tollerances

I do some cutting on my mill faster than my lathe would ever move(I have a 8000rpm spindle so a normal 2000rpm spindle wouldnt cut quite as fast).

Jon

[fyffe555]

Mike,

my 2p's worth; Theres no easy answer and its up to you and what you want to do with it.

If you use ballscrews you will not get good results if you ever try machining by hand. The ballscrews can be backdriven and easily move under cutting loads.

You have to drop a load of dough on ballscrews and antibacklash nuts to get below the backlash typical on good acme or similar (regular, square, trapizoidal, machinescrew, lathe drive screw) profile. If you can live with some backlash then Acme should be significantly cheaper. Precision acme at +/- 003" is actually more precise than some regular rolled ballscrew.

Ballscrews *need* wipers and a clean screw. Acme can get away without it.

Ballscrews are hugely efficient ~90% compared to regular screws ~30% and so you can get away with less in the way of motors/drives/psu's and save cash or have a lot of power in reserve.

Ballscrews can usually provide faster motion than regular screws - but theres a huge caveat that the rest of the machine has to be up to it too. Can the mill handle cutting in your materials at the speeds ballscrews are capable of.

Ballscrews are usually hardened so end machining for bearings and drive needs some thought. Acme is usually a lot softer and easily machined.

For the drive nuts you have to use ballscrew nuts. For acme you can easily make your own of various std and antibacklash configurations and the taps are readily available or make one from a length of screw.

As a Disclaimer; dispite all the above I'm putting ballscrews on my minimill and not quite sure why!

hth

Andrew

[JFettig]

Another thing, If you take a look at the performance curve of most stepper motors, you have very little torque at high cutting speeds with stepper motors where youll have higher torque at the motor with high tpi ballscrews. If you do a gear reduction, especially on leadscrews, you will not only run slower, but have possiblity of less torque overall.

This is one good reason to use ballscrews, your still in the higher torque range of the motor.

Jon