Learning CNC - threaded rods and lead screws

[Spikes]

Dear all,

per my other thread, newbie here trying to build a first CNC for learning purposes, which means I'm actually inclined to build something less good and then upgrade it to get a sense for the improvements. However money is tight so I'd rather avoid doing something completely stupid. Build is for a cheap cnc 3"x2" cutting some wood and plastic, maybe aluminum but from other posts this seems maybe unreasonable. Speed isn't too much of an issue.

I've read quite a bit about threaded rods and lead screws. First off, I admit I'm still confused about all the different attributes of lead screws, I've seen prices all over the place. I understand it's down to tolerances and precisions, but I can't quite understand how that affects a case like mine of a simple CNC to cut some wood and plastic at slow speeds to learn.

So far I got that:
- threaded rods aren't designed for linear motion and with much higher TPI the motors need to work a lot harder and move the load far slower
- this means that for example when cutting wood I may not get enough SFM and the wood burns
- also of course doing a cut takes ages
- motors may overheat?
- the rod's threads will wear a lot faster
- even a home-made anti backlash nut I'm not going to get a lot of precision (but again, maybe enough to make a simple sign?)
- rod is more likely to not be perfectly straight, maybe not noticeable by the eye, but enough to cause further imprecisions

anything else that's bad about using a threaded rod?

When it comes to leadscrews, I'm at a bit of a loss of what matters. I got the general idea that it solves the above problems, however:
- for such a simple machine, do I need more than a multiple starts? it seems that more starts = more precision, but also more load on the motor, is that the case?
- the pitch seems to be 2mm altho some links talk about 8mm, does such a thing exist or did they mean the lead (ie 2mm pitch x 4 starts)?
- is an 8mm lead screw good enough for this machine or would going bigger make a difference that's worth the added cost? say a 12mm lead screw.

I understand for many of these questions I could "do the maths", however the maths seems pretty complex and despite being willing to study I'm not going to take a degree in mechanical engineering so I'm hoping to get a ballpark/baseline from experienced people on the forum that have gone through this stage already. In fact, given the context, unless it's in the category of stupid mistakes really nor worth making, I'm kind of inclined to try a threaded rod and then upgrade, but I'll wait for comments, should be around $10 wasted.

thanks in advance for all the input,

Spike

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[ger21]

If money is really tight, use 1/2-10 single start acme, which is about $10 for 3ft. And much better than plain threaded rod.

[Spikes]

thanks for the quick reply Gerry. If you don't mind and for my own erudition... I've seen threaded rods with the same TPI and pitch as acme leadscrews, would it then be correct to assume that those two would have the same advancement speed in theory? At which point I'm guessing it's still not worth it because of precision/backlash and wear, correct? thanks

[ger21]

Acme is more efficient than standard threaded rod. So less power is wasted due to friction with the nut. With standard threaded rod, you'll lose up to 70% of your power in friction alone.
And acme screws won't wear out nuts nearly as fast.