Hi,

I am wanting to build a small router for doing pcb's etc and am trying to decide between ballscrews and backlash compensation. I have priced ballscrews locally to be just short of $1k aud for screws and nuts - this is prohibitively expensive. Is a square or acme thread with software compensation just as accurate? I am hoping to be accurate to +-0.1mm is this an achievable goal with square threads and software?

Thanks

Ben

Acme screws with Delrin nuts works good. There are quite a few ideas floating around these pages for DIY antibacklash nuts.

BTW, I would avoid software compensation if possible. I think it's better to get your hardware tight rather then trying to compensate in software.

:) :) :)

Hey, thanks for the help. I think I will use Delrin and a good quality ground acme leadscrew. The steppers I have are a good size already, and if it just means I will be going slower, then thats ok as I am not doing production or anything. At the moment all I see the router doing it PCB's, brass plates, and foam contours for either casting or making wings etc.

I am actually planning on a cnc hotwire cutter also.

Regards

Hey, don't think using acme screw means it won't go fast! I swear by the 1/2" 10TPI 5 start screws I have on my machine (X and Y axis) I get 300IPM+

On the Z axis I have 1/2" 10TPI 2 start screws and get about 120 IPM

/.../ I am wanting to build a small router for doing pcb's etc and am trying to decide between ballscrews and backlash compensation. /.../ Is a square or acme thread with software compensation just as accurate? I am hoping to be accurate to +-0.1mm is this an achievable goal with square threads and software?

The choice is not between ballscrews and backlash compensation. Ball screws will also have backlash if they aren't preloaded.

If you want your machine to have a repeatability better than +-0.1 mm, you should be fine with precision ACME with tight or preloaded nuts.

There's good and bad ACME, just as there's good and bad ball screws. Look at the specifications of the particular screw you are interested in to see if it's good enough.

Make sure you can get the repeatability you are after without using software backlash compensation!

Arvid

Okay, I have found some good quality stainless V threaded rod and some delrin at good prices. I have attached a drawing of how I see preload being applied - can anyone say if this is a suitable method?

I am thinking of using M6x1.0 rod which will give a resolution of 0.005mm per step.

Thanks again for your help.

Ben

You'd be better off using ACME or Trapezoidal (metric acme) instead of the V thread, if you can get it easily.

Gerry, What happened to the image link that was originally posted? And why does the forum replace some web addresses with ***********'s?

I was thinking of the V thread from an accuracy point of view - I can buy ground threaded rod and a tap to cut the delrin. To do this as an Acme thread I would have to cut the thread on both the lead and the nut. This would also mean having to grind tools to do the job - surely a V thread and tapped nut would be more accurate than a lathe cut thread?

Gerry, What happened to the image link that was originally posted? And why does the forum replace some web addresses with ***********'s?

I was thinking of the V thread from an accuracy point of view - I can buy ground threaded rod and a tap to cut the delrin. To do this as an Acme thread I would have to cut the thread on both the lead and the nut. This would also mean having to grind tools to do the job - surely a V thread and tapped nut would be more accurate than a lathe cut thread?

Please read this-
http://www.cnczone.com/forums/faq.php?faq=vb_read_and_post#faq_vb_censor_explain
Their are certain websites that I don't allow linking to for personal reasons, sorry for the trouble.

*snip*I was thinking of the V thread from an accuracy point of view - I can buy ground threaded rod and a tap to cut the delrin. To do this as an Acme thread I would have to cut the thread on both the lead and the nut. This would also mean having to grind tools to do the job - surely a V thread and tapped nut would be more accurate than a lathe cut thread?

Why can't you buy ACME/trapezoidal screws?

Where is ground V-threaded rod used? I thought V-thread is intended for fastening and then I can't see what use it would be to (precision) grind it?

BTW, V-thread is a lot less efficient at transfering motion than trapezoidal. That makes sense if you think about that it is meant for fastening things (preventing motion).

Arvid

Gerry, What happened to the image link that was originally posted? And why does the forum replace some web addresses with ***********'s?


There is a link in this post:

http://groups.yahoo.com/group/DIY-CNC/message/5516

Arvid - that makes sense really.... Maybe the guy I asked meant rolled not ground (when I phoned I asked for ground acme thread). I have not been able to find anything other than rolled acme threaded rod - the quality of which I wouldn't even put onto a vice...

I think I'll just get a length of EN25 and cut it...

Maybe this would do: http://www.motion-solutions.com.au/ ?

They sell "Ball Screws & Actuators" lead screws, with anti-backlash nuts and lead error of 0.010 inch/feet (about 0.75 mm/m I think). There's also a precision version with 0.003 inch/feet.

PS. Maybe standard V thread is cut (turned)? No idea really...

Arvid