[erikjgreen]

I'm currently shopping for ballscrews and supports for my CNC lathe project (which will come shortly after I finish wiring my mill) and I have the opportunity to buy some modular NSK ball screw supports, model WBK35DFF-31.

These are heavy units made for 35mm precision rolled ballscrews, and each unit has 4 of these bearings:

http://www.consbrgs.com/details/108/108_1_16.html

already mounted and preloaded inside. It almost seems like the bearings would be worth the price ($50 a piece).

What I'm wondering is if I should get a couple of these, which are oversize for my use, and either get ball screws to match or put a bushing on the screw end to make them fit? These would surely be more accurate and solid than any support I could make myself, if a bit large and clunky for my use.

Alternatively I could buy one of them and use the bearing pairs for making my own supports (or something else, they're nice bearings) although I'd have trouble establishing the preload they seem to need.

Suggestions?

Erik

[RICHARD ZASTROW]

Those bearings have a Fafnir number. If they are new Fafnir bearings they're worth more than $50.

I wouldn't do the bushing thing. You will probably introduce bearing run-out and then have far lower grade than a P4/ABEC7.

If you bush the ball-screw will you still have a shoulder to locate the inner race of the first bearing? A shoulder on the bushing may not locate the bearing properly.

Dick Z

[erikjgreen]

Well, the actual bearings are that linked spec, but they're the ones made by NSK as "thrust angular contact" (35 TAC 72B).

I probably could get a bushing accurately made (by having it cylindrically ground) with a shoulder but you're right, it'd be better to use the right size ball screw. I'm looking into the cost of getting a pair of 35mm screws now.

If I get those screws and these supports, I should have quite a good ballscrew setup, even if it's much bigger than I planned. I suppose I wouldn't have to worry about screw accuracy or loading, although I'd need to handle the extra inertial mass of the screws plus resistance from the preload.

If I got an extra one of these, what could I use the bearings for? I'm guessing if I pulled them out I'd have trouble getting them preloaded in a new application properly, although maybe I could use a couple of the bearings to support the "free" end of the 35mm screws?

I'm actually considering buying a couple extra of the support unit assemblies to see if I could sell them online for a bit more than $50.. they're new in the box.

Erik

[jacek.jerzy]

You should consider using plain ball thrust bearings if you do not need high rapid travel - eg. a pair of 51104 and a radial ball bearing 6004/6104/6204. No machine tool manufacturer in the whole world would use P4 class bearing for supporting ball screws - not necessary and too expensive. I know at least two machine tool manufacturers (one of them makes horizontal boring bars with 130mm spindle dia.) who use these types of bearings, even on pretensioned ball screws. Modern bearings from reputable makers are all very accurate - in mass production of such bearings there is no time for selecting bearings into different accuracy classes - it is cheaper to make them all as accurate as possible (in normal production environment) and label the differently according to demand.

[RICHARD ZASTROW]

Ref: post #4 All REPUTABLE machine tool builders use ABEC7/P4 angular contact ball screw support bearings with ABEC9/P2 running tolerances. Also, they apply heavy preload.

Fafnir and FAG/Barden lowest precision level for ball screw support bearings is ABEC7/P4.

However, you may not require that level of accuracy/precision. If not, go for the lower priced option that meets your needs.

We can't go low on price. Our product is very fast and accurate in a specialized niche. Our customers can't afford less than what is necessarily quite expensive.

No self respecting machine tool builder would try to compromise quality for cheapness.

Dick Z

[jacek.jerzy]

ref: post #5; "All REPUTABLE machine tool builders" use highest accuracy bearings only for the most critical applications. Also, they do not "apply heavy preload" but they apply preload which is appropriate for the application. Otherwise bearing manufacturers would not also supply bearing sets with light and medium preload. Heavy preload shortens bearing life and increases ball screw temperature which in turn leads to ballscrew expansion, which in turn ... Most of "All REPUTABLE machine tool builders" rather use roller/needle (INA ZARN) bearings for heavy preload and not angular contact ball bearings.
Lowest accuracy class for FAG/INA balscrew support bearings is P5 and not P4.
Your statement that "No self respecting machine tool builder would try to compromise quality for cheapness" makes no sense - all machine tool builders (even the self respecting ones) have to compromise between quality and cost.
Please see post #59 in http://www.cnczone.com/forums/linear..._basics-4.html.

[RICHARD ZASTROW]

I didn't say "cost", I said cheapness. Yes, some builders of what I call cheap "economical imports" build with lower quality bearings etc. Those are cheap copies of quality machines you read about here being modified (corrected) to become usable.

FAG/Barden, Fafnir/Timken, SKF all state in their literature they use ABEC7/P4 with ABEC9/P2 running tolerance as the minimum quality level in their ball screw support bearings. They also supply the bearings with a heavy preload as standard.

I do agree that the bearing processes produce ABEC7/P4 as normal. They just charge more when a customer asks for higher level bearings.

I wouldn't bother putting ABEC7/P4 bearings on a cheap rolled ball screw. That's just putting lipstick on a pig.LOL

Dick Z