Centering With Low Tailstock

[electrosteam]

Hi,
My lathe has a significantly low tailstock, perhaps 0.15 mm close to the spindle with minimum tailstock extension.

I intend to correct it 'one day', but in the interim I would like to progress a few jobs requiring centres (like 5 inch gauge axles).

It seems to me that the orientation of the two cutting edges of the centre drill could have a significant effect on the accuracy of the resultant centre.

The question is, does a horizontal cutting edge orientation reduce the influence of a low tailstock when making shaft centres?

Comments please on how (if possible) to get good shaft centres with a low tailstock.

Note that this subject is important to me because I was trying to clean up the centres in old axles accurate to the existing bearing surfaces so that I could subsequently put the axle/wheel assemblies between centres for final wheel clean-up.

Happy machining,
John.

 

[Lew Hartswick]

So whats 0.15mm something like 0.006 in. Just shim the tailstock. Keeping it
parallel to the bed.
...lew...

 

[bentprop]

I have this very problem,but unless your lathe is a newish one,it's more likely to be caused by wear on the bed near the chuck,as that's the area most used.
This means if you shimmed the tailstock,you would have to remove the shims if you're going to be turning between centres,as that would then make the tailstock to high.
Having said that,I have never done any turning between centres,and don't envisage doing so in the near future.So exactly how DO you shim the tailstock?And how to measure the exact amount of shimming needed?Enquiring minds want to know ;D

 

[abby]

To set up the tailstock you need a piece of silver steel (drill rod) about 3/4" dia if possible , which is centre drilled both ends.
Put the steel between centres , mount a DTI on your tool post or compound slide and "clock" along the length, any discrepancy will show on the DTI. Tailstocks have provision for "setting over" so that long tapers can be turned, use the setting screws to adjust the discrepancy out.
Another method is to place a razor blade between the two centres and adjust until the blade is held at right angles.
Before adjusting the tailstock check that the lathe is bolted down correctly as this can cause discrepancies and twist the bed , also that the headstock is aligned correctly with the bed as they can be easily displaced during a move , slackening the bolts and re-torque-ing is often enough.
Turning between centres is the best way to carry out second ops , a job taken out of a 3 jaw chuck will rarely go back in and be concentric to the first op.