Breaking centre drills

I repeat - the OP says that the tailstock "has a lot of slack in it" when the lock is released. Twist in a lathe bed isn't going to give enough offset at the tailstock to keep snapping centre drills - but a tailstock that loses alignment when unlocked might! Suggests that there is something that needs looking at - how is the tailstock guided? Has anyone else met a lathe with any pretensions to accuracy where the tailstock does not stay aligned even when unlocked?

Can you tell us what the lathe is, and how the tailstock is located on the bed?

When I lock the tailstock, the alignment shifts 0.02 mm. That is 0.04 mm in turned diameter. I align the tailstock locked so turned diameters will be OK.

On my lathe (Chinese HBM BF290 & DC300), drilling accuracy using the tailstock is less accurate then using a drill in the tool post (CNC drilling).

For the BF290, when the tailstock base is locked, there is absolutely no movement in the tailstock. The alignment after locking is also very repeatable. I have never checked the small DC300 for this.

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May we have a photograph, please ?

… just to make sure we are all ‘on the same page’

MichaelG.

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Before you ask: https://www.model-engineer.co.uk/forums/postings.asp?th=103028&p=1

Edited By Michael Gilligan on 09/09/2022 07:32:47

Assuming that the problem is not operator error in overdoing feed rate, if the workpiece and / or tailstock barrel are moving about.

Again, we assume that there is no pip on the work, and that the end has been faced

And don't forget the drill chuck holding the centre drill. Does that hold the centre drill without movement, and along the axis of the lathe? Might be worth trying a different drill chuck.

Any movement must be eliminated, otherwise broken drills are a possibility.

keep eliminating variables, until the root cause has been found, and can be rectified.

Howard

Assuming that the lathe is the Portass PD5 you mentioned in another thread, when you tighten the tailstock clamping handle, it should pull the tailstock up against the angled dovetail side of the flat bed ways, so it will always clamp in the same position, so no need to worry about that.

But there is adjustment side to side to align the body of the tailstock with the headstock spindle if the centre drill is not drilling right on centre. The body of the tailstock and the base are two separate pieces, with an adjusting dovetail or step slide running sideways for this purpose.

Other than that, use the largest centre drill you can, at the highest RPM you can.

Use a good quality name brand centre drill. Not cheap no-name junk.

Use a pecking motion to keep clearing the chips and to keep the tip cool.

Flood it with whatever oil you have in the nearest oil can, to keep the tip cool and lubricated.

Don't try to drill the chamfered part of the hole to full depth on the centre drill. Only a third to halfway is needed to make enough of an angled face for the tailstock centre to engage with in use.

And as said already, forget centre drills for starting drilled holes. Use a spotting drill instead. Or a very large centre drill and just use the very point to put a small divot in the job to start your first drill proper. Reserve centre drills for when you actually need to use a tailstock centre in the hole afterwards.

Edited By Hopper on 09/09/2022 11:07:07

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Strangely … When I looked back at previous threads, there was a photo : but I cannot find an Album

If it’s the Portass lathe, the underlying problem could be headstock alignment

… I would check that before exploring other avenues.

MichaelG.

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In case it’s not obvious:

Quote from lathes.co.uk

”Offered in two forms, one as the simplest kind cast as one piece, the other with a separate bas that allowed it to be set-over for the turning of slight tapers.”

Edited By Michael Gilligan on 09/09/2022 12:52:28

I rarely use centre drills these days, unless it’s actually for use with a centre. I locate holes with a spotting drill and not broken one yet!

Because of the small diameter of the nose of centre drills, the maximum speed available should be used, together with lubrication. The extended nose is there for a purpose, to make sure the flanks of a centre are making contact without the point touching. For drilling, a spot drill would be preferred, or I use a "centre" drill with a 90 degree flank rather then the 60 degree intended for using with a centre.