Question: C7 ballscrews vs acme+delrin nut+screw mapping

[psykhon]

Hi: Im in Argentina, getting the rigth parts for a relatively accurate router in here is a tad difficult and the local money rate to the dollar does not make things simpler. I m in the process o upgrading mi home made cnc wich i use to drill and depanelize pcb.
The main problem currently is the screws, and the options there i have to improve accuracy are:

*-Hiwin C7 rolled ballscrews, with 1 nut per axis (the nuts are very expensive here)
*- Chai's (linearmotionbearing2008) C7 with 2 nuts and some preload betwen them
*- Standard acme with delrin nut (tensioned) and to buy a hiwin magnetic linear scale with 5um accuracy to map the screws. My steppers (3.1nm) have more than enough power to overcome the lower efficiency

How much accuracy do i need? I can live with 0.002in in repetitivity and maybe 0.001 backlash.


The X axis is made out ot rexroth (8020) aluminum profiles with 15mm hiwin rails. The Y and Z are the remains of my first cnc and they have a skate bearings guide, very smoth and suprisingly accurate moves.

the X and Y total length is 80 cms (+-31.5 inches) and 60cms of travel (+-23.6 inches)
the Z has 15 cms travel(almost 6').
Steppers are 3.1 nm driven by a gecko 540. Using EMC2.


Ill try to post some recent pictures tomorrow.

I have spent a LOT of hours reading post over and over but still can decide, any help-guidance will be very much apreciated.

[Bear5k]

If you need a fair bit of speed, then go with Chai. Otherwise, an ACME screw should deliver what you need given that size. You should also check eBay more generally, though. You can often find full screw assemblies (ballscrew, nut, mounting blocks) in that size range for between $100 and $200 in C5 accuracy. Delivery costs to Argentina are something that has to be factored in, as well.

[psykhon]

Hi Bear: thank you for your answer.

The main reason there im only looking at Chai's products and not other deals is because I would like a trusted source of replacementes, if a buy a C5 deal I migth not be able to get a replacement in ebay or have to pay too much. Also Chai have a good reputation in here.

Does a magnetic linear scale with a resolution of 5-10 um be of any good if i use it to map the C7 ballscrews?

there inst much info on screw mapping, at least without using an external service with laser interferometry, Im stating to think that nobody or very few people does this and they are good with standard C7 accuracy

[Bear5k]

Hi Bear: thank you for your answer.

The main reason there im only looking at Chai's products and not other deals is because I would like a trusted source of replacementes, if a buy a C5 deal I migth not be able to get a replacement in ebay or have to pay too much. Also Chai have a good reputation in here.

Does a magnetic linear scale with a resolution of 5-10 um be of any good if i use it to map the C7 ballscrews?

there inst much info on screw mapping, at least without using an external service with laser interferometry, Im stating to think that nobody or very few people does this and they are good with standard C7 accuracy

I'm not sure that I'd worry too much about parts availability. The stuff from THK, NSK, etc., all seem to have stuff pop-up on eBay all the time. Conversely, I would expect Chai's stuff to largely be "disposable" - uneconomical to fix vs. replace.

As for mapping a C7 screw: it's not worth it unless you already own the equipment and can DIY. The same goes for things like high-precision linear encoders. By the time you added on the cost of buying and correctly installing one, you are probably well over the cost of upgrading to a better screw and with probably no better accuracy (e.g., I paid $350 delivered for a 1400mm travel C5 NSK screw with both bearing blocks already installed; I doubt you can get a C7 screw to do better for anywhere close to the $200 - $250 price differential).

If you are trying to hit 0.003in accuracy with a bit of a tolerance for backlash, then a simple ACME screw is a great way to go if you don't need speed. If you want speed, then go with a T7 screw. If you want a bit of leeway in your build quality for accuracy and you are paranoid about backlash (like me), then buy up to a C5, but look for one that has the appropriate bearing blocks already installed (they aren't cheap if buying new).

If you need a lot more speed than a screw can provide, then look into a belt drive or rack-and-pinion system if you are, again, less concerned about backlash. For ordinary cabinet work and most furniture, I suspect that R&P is really the right way to go. If you are looking for lab-grade accuracy in a small form-factor, I think you are probably looking at C5 screws, and going up from there.