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The basics

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Will_D:
First of all thanks to the mods for creating this new area.

I have been maching metal as a hobby for many many years and am quite computer literate.

Now all this talk of CNC has got me interested in spending more money on the hobby.

I have a Sieg SX2 Plus from Arc fitted with 3 axis magnetic DROs and am getting tired of twiddling the X- axis on long cuts.

So some form of stepper motor control may be needed for X & Y and maybe Z.

First major question is:

1. What about backlash in the feed screws? Does CNC and steppers etc aleviate the problem or do I first need to get rid of the backlash? Looking at the price of some ball screws it would be cheaper to just buy a new Mill!

sparky961:
Backlash is a bitc beast.

When machining by hand, you generally know which way the backlash is going and can compensate for it.  Most software allows for a backlash compensation but all that does is turn the crank enough to take up the backlash when you change directions.  You can't reliably cut circles and arcs with this because the cutting force will either push/pull the cutter into the backlash either before or after the compensation move happens.

That said, if you're taking light enough cuts that a bit of pressure on the gibs/locks can prevent the cutter from self-feeding, it can work ok.  Just don't get too excited about complex curvy shapes if you don't eliminate physical backlash first.  You can also use "CNC Assist" for some jobs, where you insert stops and comments like "LOCK THE Y AXIS" (do some machining with X) "UNLOCK THE Y AXIS" and so forth.

My own strategy will be to try to plan my machining cuts so that backlash adjustments aren't necessary while in a cut.  Instead, I'd take the cutter away from the part, change directions and then do another cut.  If this is unsatisfactory then I'll try anti-backlash nuts.  If I'm not too old to be machining by that point, I may install ball screws or just scrap the entire works and get a real machine.

Thanks for suggesting this area.

PK:
Whilst there are many strategies for backlash compensation, its a bit like owning an old car. You can prop it up and keep it going for years, but when you eventually buy a new one you realise you should have just replaced it much sooner.
In CNC that means replacing leadscrews with decent ballscrews. There really just isn't an alternative.

awemawson:
I do agree that ball screws are the way to go for a CNC conversion, however they can be a bit of a menace if you also want the machine to be able to be used by hand.

The free movement that they provide meaning that the carriage can more freely be moved by cutting forces, so locking a stationary axis becomes even more important.

SwarfnStuff:
For what it's worth Will.
       Some with these small machines make and use Delrin / Teflon nuts moulded to the actual lead screw. Here is one link that will give you the idea.
http://www.instructables.com/id/DIY-Anti-Backlash-Nuts-Super-Cheap-and-Simple/

Naturally, there was another rather well written  build log but alas, no can find.

Possibly not for the purists among us but if finances are limited they seem to work for those who have used them. Ball screws are great though. I still turn handles too. Champagne taste on a beer budget is yet another of my difficulties in life.

    If you don't want to go the CNC route there are others that just motorize the x axis using a car windshield wiper motor and voltage control.

John B

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