runout after facing on the lathe, how can this be possible??

[thanvg]

Is your saddle locked?
Joe

yeap, saddle always tightly locked

 

[thanvg]

I've never actually run a lathe with a bent spindle, but it seems logical to me that you cannot face a flat plane with a bent spindle. This is because of precession of the axis. To face a plane, there must be a fixed axis for that plane to rotate on.

Well, I also don't know what exactly a bent spindle means. If we assume that it is bent on the piece protruding from the headstock, I think facing would not be an issue. If it is bent inside the headstock and this means run out on the protruding piece, still the same. I think that if there is no axial play facing should be flat.

 

[Monarchist]

You are right, this is the detective work...if pressure from the TS solves it, my thurst bearings need some attention.

Thrust & preload you can find with a length of wood for a lever and a DI, power OFF.

Wobbly pulley sez you'll need to dig deeper.

If you come up empty and the spindle & bearings are righteous, get that wimp-ass QCTP and holder out of the equation temporarily.

Or borrow a TP grinder.

 

[Johnny Larue]

Hi Monarchist,

thanks for the reply.

show it worse towards lower-RPM center of the work than at outer periphery".
Thanos

see this....that means the problem is the bearing and the force the tool has and the depth of feed and the speed.....so if you want it flat...you have to maybe double the speed and cut the feed in half since the tool is lifting up and then releasing and dropping down and as it gets more SFPM on the outside it gets better....so and I am guessing as to speed and feed but try this....1200 RPMs and .0015 per rev....that should come out within .0005 ...if not...another trick to eliminate this is to feed in to center and then feed out to large diameter and off part....then see how flat...the radius on yer tool can be too big also causeing this...good luck...let us know if it helped ya

 

[Johnny Larue]

see this....that means the problem is the bearing and the force the tool has and the depth of feed and the speed.....so if you want it flat...you have to maybe double the speed and cut the feed in half since the tool is lifting up and then releasing and dropping down and as it gets more SFPM on the outside it gets better....so and I am guessing as to speed and feed but try this....1200 RPMs and .0015 per rev....that should come out within .0005 ...if not...another trick to eliminate this is to feed in to center and then feed out to large diameter and off part....then see how flat...the radius on yer tool can be too big also causeing this...good luck...let us know if it helped ya

PS....on hard stuff If its doing this I feed at .0007 or lower to eliminate this also...its similar to endmill deflection

 

[iwananew10K]

a while back you said you were going to replace the spindle bearings.

What were they and what did you replace them with?

 

[James Lederer]

Have you advanced the compound dial to remove any backlash?

 

[easymike299]

You are indicating the face at 90 degrees from the cutting line. If you put the indicator in the right place you would probably see 0 runout. If so this would explain the apparent mystery but would not address the cause.

 

[magneticanomaly]

PERHAPS the axial runout you measured on the spindle shoulder. Means spindle is bent. Bent spindle, as long as bearings rotate true, should not cause the problem you observe.

BUT if the thrust bearing face on your spindle is near the nose, perhaps it has axial wobble, too, from the bend.

But wobbly thrust face on spindle, as long as its counterface in the bearing is flat, will have reduced thrust capacity, but will not make the spindle hop axially.

BUT if this lathe has a plain bearing, and when the spindle was bent the thrust face indented the bearing, THEN you have an axial face-cam, pushing spindle in and out.

Too many "IF"'s, but worth eliminating. I vote for hard spot in your cast iron.

 

[metalmagpie]

Something is moving. That's why it isn't facing flat.

If you perform the diagnostics described in (brain fart) that popular book on scraping you can find out if your cross-slide travel is still perpendicular to the spindle axis. It may need to get scraped in.

It is unfortunate that a lot of guys' first project is a critical backplate. It'd be better to start with rubber washers for your basement sink or something.

Problems like these are exactly what machine shop training classes are for. You can talk them over in detail with a knowledgeable instructor, plus you have 10 or 20 other students who can help you think it through. And you aren't trashing a critical part for your own shop.

metalmagpie

 

[Red James]

If the power cross feed screw is bent, and you power feed the facing cut you can generate strange things.

 

[Kallam]

Check the belt does it have a steel splice in it.
Do the pulleys run true. Disengage the belt an see if the spindle itself still has run out.
Mark the high spot does it repeat at the same place every time.

 

[Rasmillion]

Possibly sag in that indicator, its a long one, might be some backlash, (or a loose indicator tip)

EDIT: sorry i missed the surface plate tests. so this idea is out.

 

[Porschefix]

PERHAPS the axial runout you measured on the spindle shoulder. Means spindle is bent. Bent spindle, as long as bearings rotate true, should not cause the problem you observe.

BUT if the thrust bearing face on your spindle is near the nose, perhaps it has axial wobble, too, from the bend.

But wobbly thrust face on spindle, as long as its counterface in the bearing is flat, will have reduced thrust capacity, but will not make the spindle hop axially.

BUT if this lathe has a plain bearing, and when the spindle was bent the thrust face indented the bearing, THEN you have an axial face-cam, pushing spindle in and out.

Too many "IF"'s, but worth eliminating. I vote for hard spot in your cast iron.

I like the explanation, it accounts for why the error in the cut as it occurs when tool pressure is applied to the face plate, and then when there is no pressure applied to thrust why the error reveals itself. I propose a test, make a cut wide enough to measure on the BACK side face of the face plate and see if an error is noted while applying thrust the opposite direction, (toward the tailstock instead of the headstock) . Even If the spindle is bent there will be no error using the condition that the spindle had dented (or whatever), damaged the front of the axial thrust bearing causing the spindle to oscillate in and out under pressure applied from the front.

 

[thanvg]

Hi to all,

can't thank enough for all the help and the advice. Usefull tips that one should keep in mind in any case.

So, returning to the workshop I setup for measuring axial play, like this:

(making sure all operations were safe of course, levers did not touch something sensitive on the lathe)

Got quite some play, 0.05 mm or more (0.002"), here it is:

So, after some adjusting on the spindle thurst bearings (and the plain tapered-OD straight-ID spindle bushings) I got it to behave.

Took another test cut, got less than 0.01 mm, which is ok both for my practical needs bur, more, as a proof of concept on the root cause of the whole issue. Please find the video on the next post, due to PM limitations.

So, continued with facing the actual backplate-chuck mating surfaces and remeasured (ok again) so that I can complete my build.

Of course, extra investigation would not be bad, there are lot to be done on this little old lathe. But I considered the riddle solved, and am happy for this and grateful for all your help.

BR,
Thanos

 

[thanvg]

Here is the video of the runout on the face after the thrust bearing adjustment

 

[thanvg]

a while back you said you were going to replace the spindle bearings.

What were they and what did you replace them with?

Hi iwananew10K, thanks for the interest. I think you might be referring to an older discussion on my mill's spindle bearings. Well, this was solved by using sharp cutters and paying more attention on the speed/feed configuration. Will update the respective thread if I have not already.

BR,
Thanos

 

[Limy Sami]

Here is the video of the runout on the face after the thrust bearing adjustment

Looks pretty good to me - AKA sorted

Nice job

 

[thanvg]

Thanks for the forum that pointed out the axial play which, I admitt, had not crossed my mind.
Other suggestions were also valid, I'll keep them in mind for future reference.

Thanks a lot guys!