Spindle? deflection

This topic has 44 replies, 12 voices, and was last updated 19 September 2016 at 15:35 by Neil Wyatt.

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16 September 2016 at 19:47 #24738
Dan Carter
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@dancarter89683
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16 September 2016 at 19:47 #256251
Dan Carter
Participant
@dancarter89683
Hi All,

Was just indicating something in the chuck of my Warco 290VF and noticed the needle moving when I lent on the bar. After a bit of testing I get about .02mm of deflection 1 cm from the chuck, by pushing not terribly hard on the chuck itself (more if I push on the end of the bar)

I tightened up the spindle bearings with no effect, and then noticed I can get the same effect by pushing or pulling on the headstock. If I mount the indicator on the headstock, there is almost no movement pushing the chuck, which seems to imply the movement is either between the headstock and the bed, or the bed itself twisting. (I can still see movement between headstock and chuck if I pull the bar end).

Is this amount of deflection to be expected ?

Dan
16 September 2016 at 20:33 #256263
mechman48
Participant
@mechman48
Hi Dan

When you got your machine did you get a 'Test certificate' which should show you what their inspection tolerances checked out as. You could always Google 'Schlesinger' & see what inspection certification for machines he devised… in fact in this months MEW… # 246, there is an article on him & his testing procedures/methods & standards he devised.

You could also check in your manual for bearing identity & check with bearing suppliers SKF / FAG / NSK et al, as to what their tolerances are. If as you say you are only getting .02mm movement… that's only .0007 thou, that may well be within manufacturers acceptable tolerances. I wouldn't be unduly concerned with that minuscule amount.

George.
16 September 2016 at 22:30 #256299
not done it yet
Participant
@notdoneityet
Headstock to bed(?) deflection does not seem promising, but the rest need qualifying. How long was this bar, its diameter, and material? All bars will deflect given sufficient force.
16 September 2016 at 23:04 #256305
MW
Participant
@mw27036
"Was just indicating something in the chuck of my Warco 290VF and noticed the needle moving when I lent on the bar. After a bit of testing I get about .02mm of deflection 1 cm from the chuck, by pushing not terribly hard on the chuck itself (more if I push on the end of the bar)"

Can you replicate this movement on another piece of bar? Have you tried using a big piece of stock as smaller ones will be more flexi.

M W
16 September 2016 at 23:19 #256310
Dan Carter
Participant
@dancarter89683
The 0.2mm deflection was measured on the bar about 10mm from the chuck, but by pushing on the chuck, not the bar, so not likely to be the bar bending.

Fwiw the bar was 40mm en3b maybe 200mm long. Measured deflection at the same point was a lot higher tugging on the end of the bar, but I don’t remember what.

I understand something is going to move with enough force applied, it was the fact that it did so with a relatively small force applied at the chuck that surprised me
16 September 2016 at 23:25 #256311
Mark C
Participant
@markc
Not done it, MW, Unless I am mistaken, if you read Dan's post he answered those questions already, the movement is not in the bar.

Dan, I expect you will find that you are measuring the castings distorting (probably too strong a word as the movement will minute). Your machine is a little larger than my Boxfords but not massively so and my machines will move if you load the head or bed with any external force. The head may be loose and you should be able to check it is tight but 20 microns is a small amount, I would just get on with using the machine (and avoid leaning on it when chucking work).

Mark
16 September 2016 at 23:31 #256314
Dan Carter
Participant
@dancarter89683
Mark, that was my suspicion – I did check the headstock casting was bolted down tight.

A Google for spindle deflection had thrown up some American results suggesting that essentially zero deflection (measured in tenths) should be expected, but maybe the southbends etc are just built more heavily
16 September 2016 at 23:41 #256317
Mark C
Participant
@markc
Dan, the spindle deflection will be deflection of the spindle relative to the head casting – you might expect to have difficulty measuring it there (but it will move I am certain – just not very much).

Boxfords are essentially the same as Southbends and I guarantee that all machines move if you push and shove them. The degree of movement will be relative to the size and mass of the machine along with the specific design. What I am saying is, unless you have a machine that has used up half the years steel production or has a nameplate with DSG on it, it is going to be possible to measure some movement with ease.

Mark
16 September 2016 at 23:47 #256321
Dan Carter
Participant
@dancarter89683
Thanks Mark (and other responders). Will attempt to stop obsessing now

Dan
17 September 2016 at 06:32 #256337
Hopper
Participant
@hopper
Posted by Dan Carter on 16/09/2016 23:19:34:
The 0.2mm deflection was measured on the bar about 10mm from the chuck, but by pushing on the chuck, not the bar, so not likely to be the bar bending.

This could include any movement of the chuck backing plate mounting to the spindle or the chuck mounting to the backing plate. Better to try to get the dial indicator reading on the actual spindle (or at least the back plate), then grab the end of the bar and see how much movement of the actual spindle you have.

Depends too on what type of headstock bearings you have. Ball bearings, even with preload, will always have a bit of clearance and thus a bit of movement. If you want to really tighten it up, fit opposed taper roller bearings in place of the ball bearings, which should give you movement of zero.

But if your lathe has been turning well, to size and parallel and not chattering like an old fishwife (sexism alert!) then probably not worth worrying about.
17 September 2016 at 07:46 #256342
Raymond Anderson
Participant
@raymondanderson34407
Dan, Don't be unduly concerned about that small deflection. Every lathe will deflect by a certain amount. I am fortunate enough to own a DSG and when the brother tested the machine before I bought it when he pushed down on the 12" chuck it showed a very slight movement [ If I remember correctly it was something like 0.003mm] I took a lot of weight to show that figure,but movement none the less.

My other lathe a Warco GH 750 also shows movement when pushing down on the chuck even more than the DSG. which I would expect, although it is less the 0.02 mm you are seeing. So as Mark C suggested, If it turns parallel and accurate then use it, and don't concern you're self with it..

As far as the yanks go with their Southbends ect and essentially "zero deflection " no chance, every machine tool will deflect some more than others. Rigidity, Mass, and Design have the most say in the matter. So go ahead and enjoy your lathe.

cheers.
17 September 2016 at 16:09 #256447
MW
Participant
@mw27036
Ray is correct, Deflection is simply part of physics, if something pushes hard enough it will repel whatever is in the way.

If you put a long bar into the lathe and clock up the deflection on the end of the rod, it will be greater and greater the further away it gets from the chuck. What you're observing is completely natural. Put a center on the end of it or a steady, and the deflection will be lessened. This physically demonstrates the advantages of turning with support on two points when using material over a given length.

There is no such thing as zero deflection or perfection for that matter. You are always turning or making something within limits of error from forces beyond practical control. Even if you put a heavy piece of stock in a vice, i'm sure with enough leverage you could get it to move a bit.

If your machine is doing this under the slightest touch, something hasn't been secured properly. I take it this chuck has a backplate mounting, are those bolts firmly holding the chuck and seated in its register?

Without actually seeing this machine i could spout a boatload of advice that has nothing to do with your particular problem, but if it seems something is integrally wrong with the machine, such as movement of the bearings in the headstock, you need to talk to warco about it.

Michael W
17 September 2016 at 19:43 #256492
Mark C
Participant
@markc
Michael, did you read the original post and the subsequent messages regarding the deflection measurements?

It is not the chuck or anything else hanging out the end of the spindle, it is the head and bed that is moving.

Mark
17 September 2016 at 20:09 #256504
JasonB
Moderator
@jasonb
Just put a dti on my 8yr old WM280 (6yr old FAG bearings) and could not measure any movement with dti on the head.

On edge of chuck pulling on a 5" long bar with all I could muster I got maybe 0.0003" and with the dti on the bar about 5mm out of the chuck 0.0005-6". Which would suggest some movement in the chuck jaws, its a fairly new Bison not the supplied chuck.
17 September 2016 at 20:24 #256508
Mark C
Participant
@markc
Jason, try mounting the dti on in the toolpost or on the saddle with the stylus on the side of the head casting or spindle etc (anything convenient will do) and give the head a shove – you will get some movement. The 20 microns would be about typical from my experience.

Mark
17 September 2016 at 20:33 #256509
JasonB
Moderator
@jasonb
Mark, that was with the mag stand on the cross slide.

As I said no movement of the headstock and that was pulling the bar sticking out of teh chuck which should have given me more leverage than pulling just the headstock.
17 September 2016 at 20:34 #256511
Dan Carter
Participant
@dancarter89683
Clearly some movement is inevitable – but the actual amount is the key issue. What I was hoping to learn from this posting was whether what I was seeing was reasonable or not. Consensus seemed to be yes, BUT ……

Thanks for your test Jason – I will have to wait till tomorrow to check, but I am certain I will get much more deflection than you under the same conditions – I fear it might be 10 times as much.

Mark – the difference here seems to be that Jason gets less movement than me "with all I could muster", whereas I wasn't pushing particularly hard.

For the sake of completeness:

Headstock to bed bolts are tight, Backplate to chuck is tight, backplate to spindle is tight, spanner nut on back of spindle is tight, spindle runs in taper roller bearings, spindle is if anything a bit too tight (hard to turn) after adjusting spindle nut.
17 September 2016 at 21:01 #256522
JasonB
Moderator
@jasonb
Her you go, I was not going to bother uploading it but this shows the mag base on the cross slide, indicator against rear of chuck, bit of 3/4" bar in teh chuck and pulling back and forth. Not easy to hold camera still and pull at the same time.

0.01" per div and needle moving about 1/3rd so 0.0003" this would be a total of head/bed flex, spindle play and any chuck/backplate play. This is what the tool will be acting against so more meaningful than just headstock movement which I could not see on teh needle.
17 September 2016 at 21:04 #256524
MW
Participant
@mw27036
You should let us know of any developments Dan, since if it is serious, it may come creeping back, as these problems often do. Just for clarity here, i take it this lathe is a recent purchase and so you've only just noticed this?

Michael W
17 September 2016 at 21:29 #256529
Dan Carter
Participant
@dancarter89683
Michael,

Will update. It is a few days short of a year old (assuming you count from the replacement after the first duff one), but had either never noticed this or just not thought anything of it until now – I have just bought a new indicator holder and was messing about with it (Before anyone asks, the problem is not the holder, once I noticed I checked with several others)

(On a completely unrelated note, the new holder is a noga with the fine adjustment at the bottom and it is really excellent compared to the generic chinese one I have)

Dan
17 September 2016 at 22:14 #256540
Mark C
Participant
@markc
Right, just went down in the workshop and measured both of mine. They both show about 1/2 thou (1 division on my 0.0005 Batty clock) with the stylus pressing on the back of the chuck near the mounting plate and at center height. i have to apply a decent bit of a shove to get movement here but it is clearly moving.

Note to self; check head clamp bolts are tight as there appears to be some hysteresis in the readings….

Mark
17 September 2016 at 22:15 #256541
Mark C
Participant
@markc
I should have added that I am pushing the top of the head casting – so no direct contact with the spindle assembly..

Mark
17 September 2016 at 22:48 #256544
Muzzer
Participant
@muzzer
A really useful tool for this situation and also for measuring the rigidity of machine tools as a whole (from work piece all the way around the machine tool and back through the tool) would apply (and actually measure) a representative force (some 10s of kg) and measure the resulting deflection, hopefully not many thousandths. Ideally it would be able to do so in each of the 3 orthogonal directions.

I suspect this would be quite revealing, not least when we start to dig into the old chestnut that is rear parting tools, upside down and reverse operation etc. Judder / chatter is dependent on stiffness (linear) and backlash (nonlinear).

Something like this, drawn up in Fusion 360:

The grey bar is clamped in the tool post, the cylinder is a ball bearing that pushes up on the workpiece or spindle, the fawn coloured bar is a swing arm and the thing in the pocket under the DTI is a 40kg load cell from a digital luggage scale (available at pound shops etc). Missing is a thumb screw in the threaded hole behind the DTI that applies the required load between the grey bracket and the load cell. The resulting deflection can be read off the DTI. Ideally the DTI would show no movement as the load is applied but in reality it is likely to be quite readable.

I hope to get around to making this soon but my armchair workshop currently lacks the capability.

Murray

Edited By Muzzer on 17/09/2016 22:49:53
17 September 2016 at 23:03 #256546
Mark C
Participant
@markc
Murray, you can get a nice flexible amplifier for load cells (resistive) from RS. I don't have the number to hand but they are very good and easy to use. they are very stable and will work in a number of bridge modes (full, half and quarter if I remember).

If you get to experimenting with load cells, you might find it better to use the thin film elements RS also sell to compliment the amps, you would be surprised at how much output you can get from a lump of 25 x 6 GP with just the lightest touch!

Mark
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