Retro-fit question

[gmac]

My question is on general retrofit of medium to small asian milling machines: Understanding that most machines not originally designed for CNC use leadscrews and seldom use any ballscrews, is there a reliable method in CNC software to make sure the machine approaches the cut from the proper side so that the lash in the leadscrews can be negated, or is it necessary to convert all axes of the machine to ballscrews or replacement zero-lash lead nuts? these type of nuts seem to be standard on some machines ( shoptask?) but either special design and order or user-built on most others. how does one get around this in the general-purpose low-budget CNC conversion ?

I have seen the frog cnc units - what is the present state of the art for low-budget three-axis controls suitable for use on a Homier or Harbor freight mini-mill? (need the setup with all components - this is the entire hardware and software set except the main control computer and monitor.) last I looked was about 9 months back - is there anything new in the conversion-kit market since then??

thanks

[HuFlungDung]

For best results you should get some anti-backlash nuts for your lead screws, or even some inexpensive rolled thread type ballscrews might serve.

The trick to getting good finish and tool life is to "climb mill" as we call it. This is the method when the cutter tends to pull itself along the work, which could cause a lot of problems if you have a lot of backlash in your screw/nut. This is why ballscrews are preferred.

Another nice thing about climb milling is that you don't need a lot of power in your servo or stepper drives, because the cutter tends to pull itself along, so then the servo/stepper acts more like a motion regulator, rather than a brute force applicator.

Precision ballscrews would be highly recommended if you are attempting to produce holes on precision spacing.

Don't forget to take a look at the method that is used to mount the ballscrew to the table. It should be using ball bearings for thrust take-up, not sleeve bushings. This will eliminate one more source of backlash.

[HuFlungDung]

One other important reason to use precision ballscrews is so that you can do nice accurate circles and arcs. Backlash is a real bearcat to deal with as the machine is crossing the quadrant lines (from X+ to X- and Y+ to Y-). You'll be quite limited as to what you can do with your cnc if you're always battling backlash.

Whats that saying? The bitter taste of low quality remains long after the sweet aroma of low price is long forgotten.

Spend enough money to do it right the first time, or don't spend a cent.

[gmac]

On a retrofit using a quality fullsized machine or if there was a specific manufacturing task in mind - by all means I would say the ballscrews are the way to go. I agree with you philosophically, but the practical aspects are a limiting factor in my environment . the trick on a limited hobbyist budget is - how to get enough quality to reach the goal without paying so much that you cant afford to do anything else. if we all had $25K to spend on a new machine there would be no need to ask how to save money!

a number of the 3 -in-ones manufacturers are now advertising "quality leadscrew equipment" that minimizes lash - exactly for the purpose of adapting later ( by the owner) to CNC, or so that they can sell their own CNC retrofit gear. I dont like their patterns for the machines so im not in the market for one of them, but I still like the ideas they come up with sometimes. im not planning on using a development machine to facet gems here - more as a learning and experimentation tool on a hobbyist scale . If plans and budget came along for projects needing better control and if the screws cant provide it, then I would certainly go that route. right now I cant see spending 2-3x as much on the screws as on the machine - kinda defeats the purpose. The other side is that by learning to use the machine for it's inherent capabilities one learns a little more about the limitations of the technology and how to approach the problem in such a way as to avoid them.

[HuFlungDung]

I sympathise with the costs you might be looking at for precision ballscrews.

Check out Reid Tool Supply company. I'm looking at page 384 of the 2002 catalogue, and there are some rolled thread ballscrews in there that are worth serious consideration.

Even if all you want to do is hobby work, you'll still be glad you spent a little extra dough at the start, than to be disappointed later on when your hobby gets serious.

The other factors such as not being able to climb mill is a serious handicap, plus the extra feed power required to "conventional mill" is going to cost you more in terms of more powerful servos and drives. If you decide to go the stepper motor route, the chances of losing steps is significantly higher when conventional milling, unless you are using a closed loop feedback drive.

[HomeCNC]

I used the rolled ball screw stock from reid tool. It works fine in my Drill/mill CNC conversion. After making my pre-loaded nuts I get about .002 backlash.

[CNCadmin]

How did you make your pre-loaded nuts?

[HomeCNC]

This is one of the plans I sell on my web site. You can take two standard ball nuts which are not to expensive and machine a spacer tube with a ring and apply some heavy duty spring washers to get some preload. When you tighten the ring it pushes appart the nuts to give you about 18 pounds pressure. This takes up the backlash of the screw but allows you to over come any difference in pitch of the screw. I have heard of some people welding the nuts in a jam condition, but this is bad for wear on the screw.

[CNCadmin]

Using a pre-loaded nut like you have plans for, will that not cause to much drag for steppers?

[HomeCNC]

If you find that the load is too much you can always back off a little on the preload ring to reduce the pressure.