Use quality centre drills!

I bought some cheap ones, broke every time!

Jim

If a facing cut leaves a pip in the middle of the work, the tool is off centre. The tool will ,not cut properly, and you will have this problem.

Also, if the Tailstock is off centre, this will help to break centre drills., becaus when the drill contacts the work, the work tries to move it around in a circle, rather than ju8st being cut.

To bring a tailstock on centre will need an alignment bar, a DTI "Clock) and a magnetic base, or some means of holding the clock on centre height in the toolpost.

And obviously, given the small diameter of the tip, don't force it!

HTH

Howard

In my first few days when I started my apprenticeship the old hands taught me to shorten the tip of centre drills to prevent breakage. The tip is only needed as a grease reservoir when using dead centres, not required with live (Running) centres. Last 30 or more years I have only used spotting drills for starting drilled holes.

Three jaw chuck? Some can be lousy, regarding run out. So….

Make a centre by cutting it in situ. Use that centre to check the alignment of the tailstock (with a centre fitted to the tailstock). If that is good, follow above advice. If a long way off, correct the tailstock position.

Suggestions:

1. buy good name brand drills

2. use a bigger /biggest centredrill to get the cone, then change to a smaller one to finish, if needed.

3. use a drop of good cutting oil.

4. get the tailstock aligned on centre before any of the above

There might be a clue in the OP's post in that the tailstock does not seem to maintain alignment when slackened. Needs investigation?

+1 – it's the first thing I'd check. Huge sideways forces are applied if the tailstock isn't aligned with the spindle axis. Twist drills can bend enough to cope with moderate misalignment, but centre drills are designed not to bend! If the drill and axis are misaligned the tool will bore rather than drill and is likely to break.

Possibly the tail-stock has been deliberately offset to cut a taper, or wasn't put back accurately after a taper job, or has wandered over time. These are all fixed by resetting the tailstock. (How this is done depends on the design, but tailstock's usually have pairs of adjusting set-screws that allow the ram part of the tailstock body to be swivelled slightly on it's base.) Likely to be the cause is the alignment error is right-left.

More difficult if the misalignment is due to a worn bed. Likely to be the cause is the error is up-down and varies depending on where the tailstock is on the bed. Check the tailstock alignment carefully over the the full length of the bed. Most likely cause of variation is bed wear, which can be confirmed with a steel-rule and torch.

Again depending on the design, the tailstock may not be mounted on the lathe correctly. My Far Eastern tailstock is unlikely to lose alignment because it runs on a prismatic bed , but setting the bolt and cam lever correctly is fussy. I've become averse to taper cutting with Far Eastern tailstocks because getting them spot on is fiddly.

If the tailstock is misaligned, the error is made easier to see by extending the ram some distance before starting the cut. Carefully watch what happens to the tip of the centre drill as it approaches the end of the job. The tip may be obviously misaligned with centre of the job. Might help to blue the end of the job and see if the drill tip scribes circles in the blue. If not obviously wrong, very gently wind the drill tip into the surface: if the tailstock is off-centre, the tip will wander noticeably as it starts to cut.

Another possibility: is the hole being made in the end of a long job so that it can be supported by a fixed or live centre? If so, the job may droop enough below the spindle axis to cause a serious misalignment. Being self-taught I don't know if this is the best way to fix the problem, but I centre-pop the physical centre before putting long or floppy jobs in the lathe, then make sure the centre-pop and drill align before cutting, and then support the end whilst drilling.

I've not had a particular problem with inexpensive centre-drills, but being hard, brittle, and designed not to bend makes them prone to snap. Might be possible to fix the problem with a better-made centre-drill, but I think it's better to identify the root cause if I can. Check everything! Material, operator, cutting tool, and machine are all suspects.

Dave

Yes definitely check tails stock alignment, these days I only use centre drills if a centre is needed, much prefer spotting drills.

Tony

The tip was fine, it was the cone that let go.

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?

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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