Spindle? deflection

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

I should have added that I am pushing the top of the head casting - so no direct contact with the spindle assembly..

Mark

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

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

Something definitely wrong there. Must be something loose where the headstock casting is anchored to the bed. Loose bolts, v-ways not sitting on the flats but riding on the crests or on burrs left from manufacturing. I would take the headstock off and run a very fine file all over the V ways on the bed and in the headstock then check with bearing blue to see just where it is seating.

Make sure when you are taking this measurement that the dial indicator stand is clamped to the bed and not to the carriage or cross slide, which could introduce movement there.

But again, if your lathe is turning OK, parallel, to size, no chatter etc, probably not worth worrying about.

Guessing wildly, is it possible that the lathe has been lifted by the headstock at some time? There's usually a warning in the small print not to do it.

I don't know what the effect would be. Can anyone explain what would happen to a lathe if you did lift it by the headstock?

Dave

Just tried a lever type dti and with it mounted on the cross slide and bearing against the spindle flange ig get about 0.0003" when pushing the headstock backwards with a lot of force. Suspect the measurement in teh video will be a bit more too with this dti.

Hopper, there is no problem as such. We are talking tiny amounts of movement here. If the bolts do need tightening, they will not take much - they certainly won't be "loose". The heads were both fitted very carefully and the mating surfaces stoned to ensure no debris present.

It never fails to amaze me how people get hung up on a few microns of movement or size discrepancy etc. We live in a world full of elastic materials - our machines being made from elastic materials, they are just very stiff elastic materials!

leaning on the side of the head stock will result in differential thermal expansion and a change in spindle position - it just won't be very much and you would be hard pushed to measure it (probably a couple of microns at most, more likely nanometers ). You might be able to measure it with strain gauges by loading one gauge on the spindle instead of a dti and having a reference cell close to it to provide temperature compensation (which would be required for this degree of measurement).

Mark

Jason, I was pushing pretty hard and had my other hand on the tail stock to give some purchase! I appreciate that "a good shove" is a little open to interpretation and I am not exactly a "slip of a thing"...

Mark

.

Quoted, for emphasis ^^^

Well-said, Mark

MichaelG.

I should be out cutting grass but if I dither about enough here it might rain and I won't have to do it.

I have a WM280, which is slightly lighter than a 290 and has a smaller headstock bore (26 rather than 38mm).

My test set-up in the picture. The brass bar is ½" diameter, and the dial indicator is 140mm away from the chuck jaws.

Results:

Pressing firmly with one hand on the dial-indicator end of the brass bar causes a 0.41mm deflection.

90kg sitting on the headstock - no measurable deflection.

Standing on the floor and attempting to twist the headstock - - no measurable deflection.

Leaning to press down as hard as possible on the chuck with my feet on the ground causes a 0.01 deflection.

Conclusions:

• ½" brass bar is obviously elastic
• My headstock doesn't move relative to the bed when sat on
• It's possible to get a deflection by pressing on the chuck that is probably a mix of spindle and chuck movement.
• 90kg! I need to go on a diet.

Possibly there's something odd with Dan's set-up, but we don't know how much force he's using, or what the lathe is stood on. I wouldn't worry too much unless the deflection was causing the lathe to produce poor work.

As it's still not raining I shall have to report for gardening duty, blast it,

Dave

I wouldn't worry S.O.D, i'm pushing 84 and i'm 6ft. I just remember when i was a teenager i was 50kg and filing at college used to nearly do me over.

PS. I loathe garden duty, if it were'nt for the birds. 

M W

Edited By Michael Walters on 18/09/2016 18:28:23

Dave,

You have the dti in the wrong plane. You need it at 90 degrees to that, in the horizontal plane and roughly on center with the axis (but not critically so). Then you will get a reading if you push.

It never rained here all day either although it was cloudy at times. I ended up in my garden - shifted about 4 tonne of hard core from top to bottom (250 ft) and then levelled and whacked into place ready for concrete pump on Wednesday (small mater of 10 cubic meters to lay..... I would happily have mowed a small paddock instead!

Mark

only had time for one test today (will do more tomorrow), so repeated the one in Jason's video - I get maybe .025mm, so about 3 times Jason's result - although I am 100kg, so maybe that explains everything

Dave - Did you try with the indicator horizontal on the bar ? I get nothing either with it vertical and pushing down on the headstock, but I get .03 or so pushing horizontally on the headstock with the indicator horizontal - which probably says something about the fit of the headstock on the bed.

I am also fairly sure the ways are twisting a bit, as the amount of deflection varies with how far the mag base is from the headstock.

For those asking whether it matters, does the lathe turn ok etc, yes, mostly. I have had parting problems, which were mostly down to the belt slipping or lack of torque in high range, but may have been worsened by lack of rigidity. Similarly, I don't ask all that much of the lathe - hard to say whether Jason's or Dave's would take a bigger cut with less complaint - and I agree it probably doesn't matter if it mostly does what I need. Still, I think we would all like to get the most out our machines if possible.

Anyhow, video of my version of Jason's setup below:

and Dan:

"Dave - Did you try with the indicator horizontal on the bar ? I get nothing either with it vertical and pushing down on the headstock, but I get .03 or so pushing horizontally on the headstock with the indicator horizontal - which probably says something about the fit of the headstock on the bed. "

Doh! I'll do again it properly and report back.

I made a mess of cutting the grass too: it looks like a drunk did it, which I wasn't honest.

Cheers,

Dave

Ok, repeated the test and confirm that I get about 0.03 horizontal deflection with the DTI 140mm out along the brass bar. I had to push the headstock as hard as I could to get that. My lathe isn't bolted down and I managed to move the lathe and stand slightly doing the test - over 300kg I would guess.

Being lazy and unfit I now feel like I moved Mark's 4 tons of hardcore.

I've not noticed any problems using the lathe and wouldn't have realised that the deflection was possible without trying the experiment.

Dave

Dave, funny what you can find when you go looking - especially in the right place!

Next time you go the shops (a nice big M&S store is ideal) stand between the columns on an upper floor with your feet flat and heels supporting your weight, if you wait for some people (big heavy variety are good for this) you will feel the floor bouncing up and down. You can then use this as an excuse to run out of the store to the nearest ground floor coffee shop and claim you need a strong coffee to calm your nerves (you could also insert pub and choice of drink if you are a brave man).

Mark

If things did not deflect, they may well break or deform permanently - eventually. A fact of life, noted by Hooke, when he postulated his law on spring extension. Resonance is yet another detail - of which the Romans clearly knew (they broke marching step while crossing bridges) but the designers of the infamous tacoma bridge did not expect (it fell down).

Try climbing to the top of a 300' chimney and noting the 'sway'. I can tell you that it is considerable! Again they need to or they would fall down. Flower stalks are often good examples on a breezy day, too.

recently there was a thread where someone was concerned taht a foot-long bar and a load in the tens of KG caused noticeable twist in a lathe bed.

In practice the torque being applied was far greater than would actually be experienced at the tool. It is easy to over-estimate the real world deflection and the best judge is how you actually get on in practice.

We recently published a biography of Georg Schelsinger in MEW but these days industrial machines are more usually tested set up in their final home and after adjustment to ensure their performance is in-spec, rather than the GS approach of measuring the tool on a factory test bed, which doesn't really tell you how it will perform on the bench.

Neil

You might be right Mark but if you pick up a digital luggage scale at a pound shop, you get a 40kg load cell, amplifier and display for....well, £1. Last time we looked, RS was charging a bit more than that.

The other thing you notice is that most of the commercial load cells you buy from the likes of RS are designed to measure a linear force by causing a bending moment in a beam. There is very little room to get one of these inside a portable scale, so in fact the arrangement is quite different. You'll also find that the load cells used within most bathroom scales are the same concept. In this CAD model I made of a typical example, the strain gauge is attached (glued) to the yellow area and the load applied to the blue tit in the middle. There are 2 actual strain gauges within the component and they are at 90 degrees, which makes sense when you think about it. In a "normal" cell they would be parallel, IIRC.

If you buy a set of bathroom scales rather than luggage scales, the amplifier thingy is designed to take 4 inputs and arithmetically sum them to work out the net weight applied but apart from that the arrangement is very similar. You can often find such scales on offer at places like Aldi, B&M etc for £5 or so (eg Salter brand etc).

Years ago (when there was still a company called "ICI" in existence), I developed a strain gauge based system for detecting leaks in the trans-Pennine pipeline that went from Wilton on Teesside down to Runcorn in Cheshire. This carried ethylene at 1000 bar and obviously a leak would be expensive and embarrassing. It was possible to detect a very small leak with this non-invasive method. I expect the pipe is still there but I doubt it is still in use now. That's about 150 miles of high grade stainless steel pipe of ~6" diameter just going to waste.

Murray