BV30 Lathe

[Jonathan]

In the cross slide I have 40*30mm gap for a ballnut, has anyone got any suggestions as to a 10-16mm ballnut with pre-load that would fit?

Have you considered putting it on the end of the cross slide, not under? It will stick out at the back, but it means you're not limited by the size of the nut. That's what I think I'll end up doing on my lathe (Colchester student).

You say your spindle bore is MT5, but in the manual it says MT4 so is the manual wrong?

There's more to it than just getting a 'beefier' motor. Even just having a high/low gear with the ratio carefully selected (i.e. 2:1 is not likely to be optimal) will make a big difference. A reasonably approximation for a VFD is the motor will output roughly full power to half rated speed, but below that it drops proportionately to the speed. So with 2:1 you'd get full power all the way down to 25% speed, but much below that you're struggling.

For example lets say it's a 4-pole motor so about 1450rpm. If the ratio is greater, say 2.5:1, then you'll get full power from half way to full speed and full power from 1450/2.5 to half that. So that's rated power from 290 to 580rpm and 725 to 1450rpm. Between there the minimum power will be if you need just over 580rpm on the high gear, that's (725-362.5)/(580-362.5)=60% of rated power. So lets say the minimum power requirement is 500W, that means with 2.5:1 you'd need at least a 500/0.6=833W motor and the operating (>=500W) region would be 174rpm to 1450rpm compared with 580-1450 with direct drive.

Quick sketch:


So in short, plot a graph of the motor speed/power for 1:1 and the other ratio on the same graph and get them to overlap appropriately.
(Note I picked 2.5 pretty much at random, you'll need to work out how much power you need for each speed first.)

[Jon S]

Jonathan,

wrt spindle: evidently you have read the manual for my lathe more recently than me, I have no reason to question the MT4!! Thank you for correcting me :-) I am sure I would have worked that out at some point when I draw up the spindle :-)

my grandiose ideas of using 5 AC precision bearings as per SKF recommendations: I think now are more likely to be tapered rollers as you suggested - they are much tougher and 2 of them should be just fine.

wrt cross slide: putting ballnut at the rear of the slide seems a bit ugly but tis a workable solution, a driven ballnut would be nicer, but I can't find anything appropriate.

wrt motor: As I said, the gearbox has too much slop in it for it to stay for live axis use, I am looking to invest in an Servo to drive the spindle, something slightly smaller (this is a 36Kg motor!!) than http://www.ebay.co.uk/itm/REXROTH-IN...#ht_959wt_1071 :-)

With a 1:2 drive:spindle ratio that would give me 9K RPM :-) I don't see any issue with low speed using a servo motor.

Clearly the inertia ratio here is overkill and I don't want to blow that much on it but I have time for hunting.

cheers,

Jon

[Jonathan]

I was thinking you would use a timing belt as that gets an accurate ratio and negligible backlash. If you can afford a servo that will of course be excellent.

9000 rpm is a bit much isn't it?! Bear in mind that tapered roller bearings have much lower speed ratings than angular contact bearings, so if you want a speed above their rating you'll have to use angular contact. Ratings are here:

http://www.skf.com/skf/productcatalo...e=3&startnum=7

The reference speed is with grease and the limiting speed is with oil and optimal cooling, so best to be below that. Tapered roller bearings is likely the cheapest and strongest option with a pair nearest the chuck and one deep groove bearing at the other end of the spindle and not fixed axially to ensure thermal expansion doesn't increase the pre-load on the other bearings...

Driven ballnut - like I did for my router?

http://www.mycncuk.com/forums/showth...t-design-ideas

When I convert my lathe I'll use a rotating ballnut for the leadscrew so I can still have a handle on the saddle to use it manually. Also saves working out mounts for the ballscrew to fit on the annoying curved surfaces of the bed castings.