Lead screw mounting

[HCC]

Hi guys, I've been a long time reader of this forum and gained an invaluable amount of information, and I've finally bit the bullet and decided to build my own machine. I've browsed the forum (maybe not hard enough) looking for a few specifics with regards to how to mount lead screws. I've decided on a 24" 1/2"-10 acme screw for my x and y-axis, but I'm having a tough time figuring out how to mount them.

I know I need angular contact bearings to prevent loading the stepper motor axially and to reduce backlash, but there seems to be two school of thoughts, should I place two angular contact bearings on the end that the motor mounts to and tension that end, and use a free-floating radial bearing on the other side (to allow for expansion and contraction of the lead screw), or should I tension the lead screw by using an angular contact bearing on both sides of the lead screw so the entire length of the lead screw could be tensioned? What are the pros and cons of each type of design? What size angular bearings and contact angles are typically used for this application? Their dimensions seems to be in metric most of the time. What are good places to get them (vxb bearings seems like they have a wide selection), what does the numerical nomenclature mean?

Some people have the ends of the lead screws machined and others don't, so what are the advantages of having the ends machined? Is there any way to get lead screws with pre-machine ends? Anywhere I can get it readily available? If I need a free-floating end, does that end need to be machined as well?

Thanks for your time,
-Howard

[vger]

Howard,

I used 1/2 10 acme for all three axis on my machine with dual bearings at the motor end only. The other end has a bearing, but it just floats on the end. I didn't use AC bearings, just plain deep groove ball bearings. I have a bench top mini lathe and mini mill which made things a bit easier for making parts. You should be able to find a machine shop that can prep the acme end for you.

I took the acme and turned about 2" of the end down to match some 9 mm ID bore bearings with a small step near the threads for the face of the inner race. I then turned about 1" of the end down to 5/16" and threaded it for 5/16 24 nuts. And finally turned 1/2" of the end down to 1/4" to match the coupling to the steppers I used. The other end of the acme was just turned down to the 9 mm for the foating bearing. For bearing mounts I bored a hole through a 3/4" aluminum plate the size of the ID of the outer race of the bearings. Then bored the OD of the bearings in both sides of the plate for the bearings to slip fit into. The bearing mount could be made in three parts (layers) if you don't want to pay for machining work....

I've been using mine for almost 2 years now and the the bearings are still tight and smooth.

Steve

[ger21]

I used 1/2 10 acme for all three axis on my machine with dual bearings at the motor end only. The other end has a bearing, but it just floats on the end. I didn't use AC bearings, just plain deep groove ball bearings

Mine are basically the same, but I used skate bearings. Here's how I did them.


http://www.cnczone.com/forums/diy-cn...out_lathe.html

[SORCHEROR]

i used 1/2 ID bearings so i didnt have to turn the screws down,i used double bearing at the motor end and a single at the other,i used the 1/2 acme nuts on each side of the double bearings,i drilled and tapped holes for set screws in the nuts,works fine,the other end doesnt need any nuts,also had to turn the outside of the nuts down a little for clearance,also made my own couplings with 1/2 bore one end 1/4 other end

[RotarySMP]

If you use one preloaded AC bearing block and the other end floating, you have (hopefully) rigidly located your screw axially.

If you use two set of preloaded bearings, on each end of the screw, you can pretension the screw, and increase it critical bending rotational speed. Forget this for your current plans.

What do you mean by not having the ends of the leadscrew machined? Do you mean putting bearings straight onto the thread OD?

If you don't have a lathe to do this:
5 Bears - Y axis ballscrew
then you are probably wasting your money with AC bearings.

[SORCHEROR]

yes,i put the bearing on the OD of the threads,worked fine,its not perfect but works