Breaking centre drills

[Rowan Sylvester-BradleyParticipant @rowansylvester-bradley37244]

Another beginner's question: when I centre-drill something, I almost always end up breaking off the small centre bit of the centre drill. Why is this happening? Am I using too small a size centre drill? Or have I not got the centre drill properly centred on the axis of the lathe? Or both? Or something else? If it's not centred properly, how do I get this centred correctly? The tailstock on my lathe has a lot of slack in it when the nut is loosened, but I always try to get it centred correctly before tightening the nut.

All hints welcome.

Thank you – Rowan

[Rowan Sylvester-BradleyParticipant @rowansylvester-bradley37244]

[peak4Participant @peak4]

There's a good chance that the tailstock isn't centred correctly when you start to drill; this could be height, as well as front to back. What sort of lathe do you have?

Also, is there a pip left in the middle of the workpiece after facing it, but prior to centre drilling.

Lastly, unless you're turning between centres, it's often a better idea to use a spotting drill, as they are more robust and easier to sharpen.
They work fine for starting drilling axial holes etc.

Until you work out what's going wrong, have a play with a spotting drill, as they will likely produce a decent central hole for you to continue with a centre drill if need be.

Bill

[jimmy bParticipant @jimmyb]

Use quality centre drills!

I bought some cheap ones, broke every time!

Jim

[Nigel McBurney 1Participant @nigelmcburney1]

Before drilling,face off the end of the bar or work being turned,turning tool should be at centre height so that no central pip is formed,a centre pip will force the drill off centre, when centre drilling with small centre drills use a lubricant,a drop of ordinary lubricating oil will do ,though on tougher material a Rocol type lubricant is better,the problem with a centre drill,is that the small drill flutes are straight ,which do not remove swarf compared to a twist drill, with a very small centre drill ie 1/8 or 3/16 inch dia I run the S7 on top speed ie 2000 rpm ,and a bit slower ,say 1000 rpm when drilling a tough material like silver steel ,feed in gently and wind the drill back 3 or 4 times to clear the swarf. My lathe is in good condition,on a worn lathe if the drill runs off centre it will break,the only way to overcome this problem would be to start drilling with a lot larger centre drill to achieve a shallow dimple which runs true,then use the small drill to achieve the small centre you require,then face off a few thou to remove any traces of the large dimple.

[Howard LewisParticipant @howardlewis46836]

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

[Dave HalfordParticipant @davehalford22513]

I had that with some from a well-known supplier, fine on EN3 but anything tougher and bang.

[Chris Evans 6Participant @chrisevans6]

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.

[Martin KyteParticipant @martinkyte99762]

Slow down and feed gently and use some cutting oil.. Also check uour tailstock alignment.

regards Martin

[not done it yetParticipant @notdoneityet]

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.

[HuubParticipant @huub]

I use a 4 mm (thickest part) centre drill in a collet holder (have a spare) to get the most accurate centre hole. The centre drill is made of HSS Co 5%. These are a bit more expensive than plain HSS but last longer.

A common HSS centre drill or short spot drill will also do well.

Checkout this video for Tailstok Alignment

[Jeff DaymanParticipant @jeffdayman43397]

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

[Martin ConnellyParticipant @martinconnelly55370]

Do make sure there is no twist in the bed otherwise setting the tailstock on centre near the chuck may mean it is off centre away from the chuck.

Martin C

[NealebParticipant @nealeb]

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

[colin hawesParticipant @colinhawes85982]

Make sure the tailstock is aligned near correctly ,use a high speed because it's a small drill, peck and withdraw the drill several times adding a bit of lick, oil or coolant each time; works well for me! Colin

[SillyOldDufferModerator @sillyoldduffer]

Posted by Nealeb on 08/09/2022 08:16:46:

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

[Grindstone CowboyParticipant @grindstonecowboy]

Posted by SillyOldDuffer on 08/09/2022 10:09:07:.

…pairs of adjusting set-screws that allow the ram part of the tailstock body to be swivelled slightly on it's base…

Eek! Surely not swivelled?

Rob

[Tony Pratt 1Participant @tonypratt1]

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

Tony

[not done it yetParticipant @notdoneityet]

Posted by Nealeb on 08/09/2022 08:16:46:

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

Most surely, the bed should be aligned (twist removed) before even thinking about centre drilling?

I think the OP possibly needs to start at the basics, if the bed is not aligned properly. One cannot miss out basic alignment procedures.

’Seems’ is not a word I like to see. Measurement, to provide evidence one way or the other, is far better than a guess.

[Dave HalfordParticipant @davehalford22513]

Posted by Dave Halford on 07/09/2022 19:28:21:

I had that with some from a well-known supplier, fine on EN3 but anything tougher and bang.

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

[Bo’sunParticipant @bosun58570]

Yes, certainly check the tailstock off-set clamping screws. The screws on my Warco WM250 were found loose. Presumably from new.

[NealebParticipant @nealeb]

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?

[HuubParticipant @huub]

Posted by Nealeb on 08/09/2022 22:48:44:

Has anyone else met a lathe with any pretensions to accuracy where the tailstock does not stay aligned even when unlocked?

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.

.

[Michael GilliganParticipant @michaelgilligan61133]

Posted by Rowan Sylvester-Bradley on 07/09/2022 18:57:20:

… The tailstock on my lathe has a lot of slack in it when the nut is loosened

.

May we have a photograph, please ?

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

MichaelG.

.

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

[Howard LewisParticipant @howardlewis46836]

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

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