Lead Screw type and bering type????

[cjluke]

Hi Gurus

I am designing a 3 axis CNC Router with the X and Z axis on a movable gantry that moves in the Y axis. This machine will be capable of working an 18" x 18" x 6" space. I plan on adding a 4th axis later.

In order to get the best precision and accuracy for my dollars, I have chosen Mutli-Start ACME - 1/2" Dia x .200" Lead w/ Bearing and Anti-Backlash Nut for 1/2" Dia x .200" Lead Multi-Start ACME. It seems that the penalty for not going with precision ball screw and anti-backlash ball nut is only the increased friction and a somewhat shorter lifespan. The dollar savings is however, very significant indeed.

Does that seem like a good choice or should I turn to a life of crime in order to build this thing

The other question I have is about lead screw bearings: it seems to me that no one "out there" is offering any kind of thrust bearings/mounting systems that will allow you to pre-load the lead screw to eliminate backlash caused by the lead screw moving in the bearing blocks.

What is the current thought/solution for this? It seems that it is a complete waste of time and money to buy high precision lead screws/anti-backlash nuts only to mount the lead screw on straight up bearings

Any comments or questions on my design would be appreciated/answered.
thanks
cj

[dfmiller]

CJL,
I am no guru.
Have you looked at some of the eBay leadscrew options? I picked up a set of ballscrews at a good price. They have very little backlash. They are the types with internal ball returns. They are so much nicer than the domestic "actuatorGrade" rolled ballscrews. Your machine is fairly small so going with larger motors would overcome the effiency loss without breaking the bank.
For zero lash bearings you can use two deep groove bearings with a spacer and then preload with a nut. Or go with a thrust bearing and a normal bearing. I am working on a project that uses 2 8x22 mm roller blade bearings with a third one used as a spacer. They are so cheap it makes sense to make a precision spacer out of the third by removing the hub and bearings by brute force. VXB bearing is a source for these bearings.
If you are needing more details I can post a drawing of the end details. The best place for ideas on end configuration is download the Nook catalog. I based my details on this resource.

Dave

[DonFrambach]

I'm no guru either. That said, I recently built a 14"x14" router and used single start ACME lead screws. For me, that has worked well. I wonder if you really want to use multi-start lead screws with only 18"x18" travel. Fast rapids are not so important in small routers.

[cjluke]

Thanks for the replies guys...

I don't actually think the multi-start lead screw is a requirement other than in my searches... I have only found high precision/accuracy in the Multi-start offerings. I really want an accurate and precise machine. That brings me to my second question in the original post... it seems to me that you really need to have pre-loaded thrust type bearings on the lead screw if you want to reduce backlash to an absolute minimum. Am I being too optimistic... or just anal I haven't found anyone offering those types of bearings with their drive combos.

Speaking of drive combos... the anti-backlash nut needs to be specifically for the ACME lead screw that you get. They aren't very standard and can vary significantly.

What are some good sources? I will check out ebay (lower cost is good) but I am not opposed to paying more for the performance that I want.
thanks
cj