securing ballscrew bearing / pictures of my homebuilt cnc

[guru_florida]

Here is some pictures of my progress on my homebuilt cnc machine. My goals are for a precision/accurate machine for small robotic parts, pcb's, etc.

I am trying to decide how I am going to secure the fixed-end bearing (dual-row thrust/axial bearing) to the ballscrew. Unfortunately, there is no thread on either end to screw a nut that will *squeeze* the bearing. I have a good interference fit already. I chilled the ballscrew and heated the bearing to fit the bearing on and still had to bang it on. I probably couldnt bang it back off now that its cooled...but is this enough strength? (probably not.) You can see the bearing I am talking about in the pic. I have a closeup too.

btw: you can see more detailed pictures of my machine so far at:
http://photos.colinmackenzie.net:81/cnc/

Thanks,
Colin

[RICHARD ZASTROW]

Colin, If you "banged" on the bearing(s), odds are they're already damaged. Is it possible to have threads installed outside of the bearing seat area? I don't know what your "dual-row thrust/axial bearing" type of construction is or what level of precision/accuracy you require. Personally, I'd opt for 60 degree angular contact (AKA ballscrew support) bearings for accuracy and axial support. As NC Cams explained to me some time back, double row angular contact bearings have internal clearances and therefore some end play. I do believe there are some with seperate inner races that can be preloaded but I've never used them as yet.

Good luck

Dick Z

[guru_florida]

Maybe I shouldnt have said banged. The bearing still runs very smoothly. I only tapped on the inner race with a some pliers.

How can I apply some threads to the shaft? I have taps, but this would be the opposite. Have to get a metal shop to do it?

I am not sure the angle of the dual bearing. It doesnt seem to have any play at all. It seems a good quality. It's a high speed bearing if that means anything.

I am still learning a lot about designing cnc machines. Learning cnc is like peeling away an onion, each layer exposes more details/issues!

Colin

[RICHARD ZASTROW]

Colin, Looks like you got a little ahead of yourself. lol Seriously, you might want to stop and plan/design ahead. I have no idea of where you are in preparing your next move or what is already completed. Your bearing must be mounted in some sort of support; the nut also; are you going to support the opposite end? if so, with a "floating" bearing?

ref. metal shop to do it? probably, choose a good one. Assumptions are a bad idea but here we go. If your shaft end is case hardened (assumption 1) it may be a simple grind thru the case hardening, turn and thread for a bearing locknut. May require a spacer/adapter to clamp the bearing against a shoulder. or?

There's a lot of good info here. Peruse the project logs and other forums, most of youre future questions have already been answered. If you don't find it, ask specific questions and you'll get more answers than you need (or even want).

Another load of info is at the websites of the bearing, ballscrew and linear way systems companies, especially the engineering and technical support areas. They'll give you all the shaft shoulder and housing shoulder dimensions etc. Read them, good stuff.

Hope that helps for now, adios

Dick Z

[guru_florida]

I am designing the entire thing in solidworks. Here are some pics from SW for the fixed end of the bearing mount. (no threads or screws are shown in the SW models.) The mount is made from two pieces of rectangular aluminum.

Yes, the other end is floating too. The ballnut carriage mount is done and is what is holding the ballscrew in place in the pictures. I am relieved that it lines up with the two end holes perfectly (where the bearing mounts go).

I am getting a lot of design ideas from 5bears.com (my overall inspiration) and what I have seen from people on here. I am a competent engineer but I lack a lot of the proper tools.

C

[RICHARD ZASTROW]

Cory, Obviously you're further on than I "ASSumed". My bad. Judging from your latest post, you're using direct coupling to the axis motor. It appears you have included an opening to access the coupling. That looks more sturdy than 4 round rod/spacers and supports the bearing as well.

Are there any labels on your bearing? Maybe one of us can identify it for you.

What tolerances are you requiring? I tend to go for extremes. That often leads to expensive over specifying components.

Dick Z

[djr76]

With as little as that machine is, you can just make that housing 1 pc. It would be way more accurate.

[guru_florida]

Oh yes, the 4 round spacers was definately a "temp" thing! I will be using flex couplers to attach the motor and screw shaft, but yes, direct drive. The top hole is to access the flex coupler.

I wish I could make this out of one piece right now, but I have very limited tools. A dremel tool, portable drill and access to a drill press 40mins away! I am suprised things have worked out this well so far! I would buy some tools but I live in a small apt. My neighbors are going to hate me when I finish this machine!

I am designing my machine as though I will get .0001 precision. And yet, only hoping I get anywhere near that!!! (few tools and all.) I am counting on the systemic process of tightening the bolts to makup a lot of the accuracy. There is some clearance in most of my bolt holes. The bearing holes I cut small and trim with my dremel for a tight fit.

I am sure I will end up with a machine that can make some nice robotic parts and do a lot of the things I want. In the process I will learn a lot more about cnc, the pitfalls and traps, and continue to upgrade the machine in the process...using the machine to create itself - so to speak. Going back to the onion analogy, I think making your first machine is a right of passage...then you truly understand the problems. Here I am making a machine that I have never actually seen in real life much less used before! (Well, once I got a tour of the cnc lab at my university 10 years ago.)

Colin

[guru_florida]

Here is the link to the bearing on ebay:
http://cgi.ebay.com/ws/eBayISAPI.dll...e=STRK:MEWN:IT

[RICHARD ZASTROW]

That link indicates a standard (normal axial clearance value .0002"-.0008") double-row angular contact bearing with (2) shields. (not seals). Do you have any other markings (C2 C3 or?) on the bearing? Any on the screw or nut? We might be able to determine tolerance stack potential before assembly hopefully avoiding obvious excess. Actual total compliance will have to be measured after assembly, as there are many possible contributors to "problems".

Dick Z

[RICHARD ZASTROW]

Colin, Ref. your above post (#8) by flex coupling I hope you mean bellows or similar. The type of flex coupling that permit radial "torqueing" will introduce lost motion between the axis motor (and encoder/resolver) and the ballscrew. Not good for .0001" stuff. The bellows type coupling compensates for limited angular and axial misalignment without changing rotational displacement, as long as appropriately sized.

Dick Z

[guru_florida]

The flex couplers are these:
http://cgi.ebay.com/ws/eBayISAPI.dll...MEWN:IT&ih=011

I think they will work without contributing to error. As for the bearing. I played around with it more and it probably isnt the greatest. I think I can detect a little play, mostly by applying a moment to the bearing perpendicular to the rotation axis. Its not visually apparent, but I can feel it. It will have to be replaced later I'm sure...along with a lot of the other parts.

I really appreciate your time and help so far...I am learning with your feedback.

Colin