Lathe run out

The department I supported at work (RGT then EGT then eventually Siemens) used alignment mandrels for years, we had to make them as large a diameter as possible because of sag. We calculated the effects of diameter and material and wall thickness to get them to have as low a sag as possible but it was always there. We ended up with aluminium alloy mandrels with diameters that reduced along their length, staring off about Ø200 then a section of about Ø150 until we had a mag base type rod in the end. The sag was lowest when the diameter was as large as possible and wall thickness did not need to be very high. As soon a laser alignment became available we bought into it immediately. So I have to say Hopper is 100% correct here, your long and slender mandrel will have measurable sag that will be there regardless of spinning or not. So side to side may be OK but up and down no chance.

Machine tool setting was done with optical systems for years to avoid sag like this and we had one at work that we could not use on most of our setups because of there being no way to mount it. When we got the laser based systems we gave the optical system to the local astronomy society.

Martin C

Well of course I tried it my way first, dismal failure.

Then I fixed the Engineers' level, removed the bodge piece of folded Chinesium, redrilled the scale pivot to square, put it back together again.

Then I levelled the bed. The tiniest tweak on any foot changes everything, took hours

The largest bar I had was about 1.5" diameter. The chatter was decorative at the end but I got 6" of smooth.

Left hand end 1.5712"

Right hand end 1.5709"

Error 0.0003"

Bliss

If you bturn a waist on your bar so you have 2 'collars' at either end you only need to turn those for your test. instead of traversing down the entire length.

regards Martin

He's been told this already

Tony

Well he aint listning then.

;O)

Martin

Hi Martin

I did think about that but any weakening of the bar would bring that chatter closer to the headstock and everyone seems to agree I need 6" for the test. Plus I still have 1.5" or useable diameter for something else, I'm paying over £29 a meter for 1.5" BMS, precious stuff...

Robin

Here's one I made earlier...

Neil

NB further tests made with DTI, no need to cut it again.

Edited By Neil Wyatt on 04/06/2021 21:01:59

Proof of my new notion that all the best lathes are out by 0.0003"

Grumpy people are probably a sign of 0.0004" or greater. We should pity them

Pete Rimmer and Hopper are right on the mark. I suggest making the test cuts with sharp HSS to minimize part deflection due to tool pressure. For aligning the headstock to the bed suggest you look for jacking bolts on the headstock. My Clausing Metosa had jacking bolts on the outboard side of the heastock. My Nardini has the jack bolts directly below the chuck.

There is absolutely no need t purchase a test bar. They come from China or India and are of unknown accuracy. The way you turned the test bar in the chuck is the right way to do it.

I feel your pain getting the lathe level. It involves a lot of deep knee bends. If you ever need to re-level the machine find a helper to turn the leveling screws while you read the level.

Hi John

I measured again, about a zillion times, and I think I got it wrong. Blooming Vernier scales. I think I am 0,0016" out on the diameter.

Best not tell the grown-ups because they get cross.

The bar is still in the chuck and a DTI in the tool post says it is 100% parallel so nothing has moved.

If I drop the front right jack to get 0.0008" error on the DTI would that be cheating?

Robin

If bat first, you don't succeed, try again.

The EXACT diameter does not matter on your test "bobbin", as long as the reading is the same at both ends..

I would advocate reworking until you get Zero deflection on the DTI, or whatever level of error is acceptable to you.

Trying to work to produce an error, may cause confusion.

Howard

Hi Howard

The DTI is following exactly the same erroneous path as the tool, so it reads true.

I reason that I have to make it read wrong before it can cut right.

Misaligned lathes are about as easy to understand as quantum field theory.

Robin

I had assumed that you were using the "bobbin" method of checking the alignment, as per Ian Bradley.

The DTI method is presumably being used with an alignment bar.,

Not sure that you can mix the two methods, (DTI measuring a bobbin, since it would follow the path taken by the tool).

But use whichever one provides the required end result of a twist free bed.

Howard

I unscrewed the front. right foot until the DTI showed about a 0.0008" error.

I repeated the skim cut and measured it twice, using my new found skill for reading Vernier micrometers

Left-hand end 1.5696" or 1.5695"

Right-hand end 1.5698" and 1.5698" again

Will it stay that way?

26 thou on the screw moved it 1.3 thou on the diameter which is 20:1

Looks like you have succeeded.

Good on yer!

Howard