Lathe alignment and cross slide play issues

[Ex contributorParticipant @mgnbuk]

"However I am still curios as to how one determines the most likely source of the misalignment."

Alignment of the spindle axis to the bed is usually checked with a precision test bar in the spindle internal taper. It is usual for the spindle to be set pointing slightly high towards the tailstock and to slightly towards the operator – "slightly" being in the order of 0.001" per foot (0.035mm/ 300mm) – published standards usually specify the acceptable amount.

HTH

Nigel B.

[Harold Hall 1Participant @haroldhall1]

You are correct Michael ( W) that you cannot correct major errors in a lathe by twisting the bed. However, if the lathe bed is set using a precision level the lathe may still not turn parallel due to minute errors in the alignment of the headstock spindle. In this case a very very small twist can be incorporated so as to compensate for such an error. To my knowledge, very few lathes have a built in means of aligning the headstock directly.

It occurs to me that some reading this thread will be unaware why I am making what I call, Workshop Grade Cylindrical Squares. They are for use, normally in pairs, for supporting items on the milling machine. This was originally to make two accurate angle plates, the result being much better than my purchased angle plate.

I use a pair to support one face whilst machining the other, then doing the same with the second face. For the ends I mount the angle plate diagonally between the two squares with one supporting one face and the other supporting the second face and the ends machined. This ensures that all four edges are at right angles with both faces.

Harold

[Harold Hall 1Participant @haroldhall1]

Sorry Nigel but I had not fully taken note of your comments when I posted the above else I would have added the following.

The figures that you quote are interesting and would seem logically, being a means to compensate for the forces existing whilst in use The weight of the chuck, and more so a large workpiece, would compensate for one and the force on the workpiece when being machined would tend to diminish the other.

Incidentally, Myford when machining the top of the bed had a means of slightly lowering it in the centre. When machined and then removed the bed would return to its normal state and would be very slightly high in the middle, left to right that is. The weight of the saddle, cross slide and top slide when eventually fitted would then return it to nominally level, Of course all such measures are compromises as they do not achieve perfection.

Harold

[Ex contributorParticipant @mgnbuk]

Harold,

Your reasoning for the alignments was the explanation I was given by the fitters I was working with the first time I had this shown to me. I had a look at the alignment reports for a couple of the lathes at work (2 Harrisons & a Gildemeister) – all showed a maximum permissible vertical error of 0.02mm/300mm (high only) and 0.01mm/300mm horizontal error (facing towards toolpost only) on a test bar mounted in the spindle taper.

The "bending the bed" method you mention for Myfords was also employed at Boxfords on the later 1020/1130 machines – but the bend was applied before the pass under the induction hardening device. This equpiment (Radyne IIRC) was mounted at one end of a Snow surface grinder. In use, the bed was put on a fixture & a bolt put through the middle of the bed & torqued to a particular setting. The grinder table was passed slowly under the heating coil of the hardening device (which was shaped to have an even clearance over the bed formation) and heated a narrow area across the width of the bed. A coolant jet followed the heating element to quench the surface. After hardening, the tensioning bolt was removed & the bad was ground in one setting with a form-ground wheel (a diamond roll dresser was mounted on the end of the table opposite the hardening device). The basic formation had been gang-milled on the bed casting prior to hardening & grinding.

My recollection is that the "bend" in this instance was introduced to control distortion during the hardening pass – the heating coil had to remain at a set clearance & it was found that the bed distorted during heating, causing the heating coil to touch. This burned holes in the heating coil.

There may also have been issues with the hardened bed being "humped", which required more grinding to clean up – putting the "bend" in the bed before the hardening pass was a solution to both issues. This all happend around early 1981 – I spent 10 months or so at Boxfords finishing my apprenticeship (after the company I originally worked for closed) and was involved in putting the supply in to the new grinder/hardener installation & provided assitance as required during the initial "working up" of the installation.

Nigel

[Ex contributorParticipant @mgnbuk]

Duplicate post

Edited By Nigel Barraclough on 19/02/2013 20:22:02

[Harold Hall 1Participant @haroldhall1]

Thanks Nigel Interesting!

Harold

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