Rockwell mill conversion- acme or ball?

[lkasdorf]

I want to do a CNC on my Rockwell knee mill. The acme screws have very little wear. Do you think that with the installation of anti-backlash nuts, that I could do a successful cnc conversion using the existing acme screws?

I guess what I'm asking is- if you have essentially zero-wear acme screws, is it always going to be wanting? I know that there is more drag on an acme system, and that a ball screw it designed for zero lash (right?).

Anyway, I'm just wondering if I should even bother, or just go right to ball screws. If I'm going to go to the trouble, I want something with .001 repeatability.

I only have a about $100 in the mill (part of an excellent package deal), so if I spent a few hundred more, it would not be a huge deal.

This will be my first CNC experience, by the way.

Thanks
Lynn Kasdorf

[Rally]

Are you for real?? I would start by doing some research!
I would not put a lead screw into anything period! You are taking of doing a CNC conversion for a couple of Hundred?? That is funny!
It would be in your best interest to just use the Mill as is. Sorry if I’m sounding like an As…….. but judging by your question that was the only thing I could say.
However, for what it’s worth, there are books on how to do such conversions.

[Rally]

Ok! I will take some time to explain. First off, it is possible to use standard lead screws but the outcome will not be satisfactory.
When you are saying CNC Conversion are you taking about real CNC results or is your intent just to be able to move a table up and down? Anything is possible!
You need to start by getting an Idea of what it is that you want to do, once you know reply and I will point you in the rite direction.
PS: Expect a quality conversion to be something like $2000 to about $5000.

[lkasdorf]

Originally Posted by Rally it is possible to use standard lead screws but the outcome will not be satisfactory.

I have been doing nothing but research. And I've heard some people say that you can use acme screws and get rid of the lash, but the big disadvantage is the extra drag.

So- your conclusion is to not bother with the Acme screws. Pretty much my conclusion as well, but some folks say you can do it.

The few hundred remark was meant to be the delta between acme screws and rolled ball screws. Probably closer to $1k delta.

I just spotted on this site a glowing review of Roton screw products and the statement that they are way cheaper than McMaster Carr. I'll investigate these folks.

[InspirationTool]

Originally Posted by lkasdorf And I've heard some people say that you can use acme screws and get rid of the lash, but the big disadvantage is the extra drag.

For me the worse problem is that the anti-backlash adjustment requires more frequent adjustment due to the wear of the screws. The ballscrews shouldn't need adjustment very often at all.

-Jeff

[Rally]

I may be wrong but I don't think that Roton has precision ground screws all they carry is rolled. If you are OK with the lesser Quality they will do.
High quality rolled screws usually will be in the 1 to 2 thousand accuracy range whereas a precision ground screw will be more like0.0005 per foot.
If you really want CNC results that that is the way to go. Just Imagine an offset of .002 per foot that would be .004 by the time you run the X to the other side.

[Pretorien]

Originally Posted by Rally I may be wrong but I don't think that Roton has precision ground screws all they carry is rolled. If you are OK with the lesser Quality they will do. High quality rolled screws usually will be in the 1 to 2 thousand accuracy range whereas a precision ground screw will be more like0.0005 per foot. If you really want CNC results that that is the way to go. Just Imagine an offset of .002 per foot that would be .004 by the time you run the X to the other side.

Is this error cumulative along the length or random within the part?

EM

[Rally]

I don't exactly know, from what I been told the screws will sometimes go out in the process of manufacture. Maybe someone else will help answer that question.
If I was you I would go with ground.

[phantomcow2]

What do you plan on doing with this mill? Let this guide you as far as what kind of ballscrew you want. Rolled is what 95% of hobbyist use. I recall a thread a while back here on hte zone where somebody tested the 5/8-.2 ballscrew and found it was far better than .004" per foot. Not surprising

[lkasdorf]

Originally Posted by phantomcow2 What do you plan on doing with this mill? Let this guide you as far as what kind of ballscrew you want. Rolled is what 95% of hobbyist use. I recall a thread a while back here on hte zone where somebody tested the 5/8-.2 ballscrew and found it was far better than .004" per foot. Not surprising

That seems entirely plausible. I'm sure that roton or other rolled ballscrews would be just fine. It will be used for misc work around the shop, and to make parts for a large CNC router, hopefully. I'm really more concerned with lash than absolute accuracy, i suspect. I might like to use this to cut prototype printed circuit boards- if a trace is off by .004, no big deal. However, if the lash is bad enough to prevent it from making good circles, that would be a problem.

I mostly want to gain experience with a CNC machine and end up with a decent mill in the process, perhaps that I can sell at some point for the next great deal I find

[phantomcow2]

I can almost guarantee Roton's stuff is better than .004" per foot. I called them a while ago for pricing info, I recall it was pretty good. I do remember they charged 10 dollars extra if you wanted a preloaded nut. If it was me, I would just get the two non preloaded nuts and mount them properly to make an antibacklash setup.
It would definitely be in your best interest to get ballscrews though. THat way you can use smaller (cheaper) motors.

[phantomcow2]

Remember to take pics of the process