Beginners question (sorry) - why I am breaking my small centre drills?

I am currently turning down some 3/16" silver steel rod to make some clock arbors onto which I will eventually mill some integral pinions. I need to support the ends with a centre but have succeeded in breaking off both ends of my smallest centre drill. What am I doing wrong? The larger centre drills seem to cut perfectly well. Is my speed too slow perhaps? Should I be using cutting fluid? Luckily I have allowed enough spare length and was able to extract the tips by shortening the rod a little.

Almost certainly too slow a speed, also silver steel can be a bit of a pain to machine so cutting oil would help too, also need to make sure you are not applying too much force with tailstock.

Edited By Alain Foote on 23/05/2019 17:41:04

Nigel,

A couple of possibilities;

The end of a new piece of silver steel is often hard and either needs to be heated and slowly cooled or the first 10mm cut off with a hacksaw before machining. You might already know that because you say that your larger centre drills are fine?

I once bought a couple of duff small centre drills that wouldn't cut - so are yours ok? You could check by using one on something like aluminium or brass - just to check it cuts ok.

Yep, very often, small centre drills are run too slow and they struggle - so you could as you suggest look at the speed. Lubrication helps but a quick dip with a centre drill shouldn't need that much - i.e., a quick squirt of oil of coolant if you have it.

Phil H

Thanks for your quick response! A couple more bits are on order - decent HSS ones this time I hope. The last was was part of a cheap job lot.

A few more possibilities:

• Small centre drills are rather weak and you're heavy handed. (I break them fairly often.)
• The centre drill is too cheap...
• The tailstock is off-centre. Drilling off centre causing it to bore, cutting on one side rather than drilling. Usually fairly obvious when the tip first makes contact, but the other give away is the drill cutting an over-sized hole.
• It's one of those days. (I bored a cylinder exactly 0.1mm too big this afternoon.)

Dave

Check the tailstock is accurately aligned, if it is slightly out it will try to bend the centre, large ones will plough their own furrow but small ones snap easily.

Mike

One of the first things I was shown when I started my toolmaker apprenticeship was how to shorten the end of a centre drill. The old hands would not use one straight out of the box. I still do it to this day 56 years later on all sizes up to BS4. Try it, only a small amount is needed to miss the end of the centre,

Check that the tailstock spindle doesn’t move around, any free play will try to bend the centre drill off centre. Slightly clamp the spindle to stiffen things up. Use a collet chuck rather than a drill chuck to try to get better true position.

Nigel

How about a different approach.

• Turn a 60 degree male centre on the end of the silversteel.
• Grip a piece of brass in your lathe tailstock drill chuck.
• Centre drill the brass using the lathe headstock chuck.
• Blob of grease in the brass centre hole before turning the pinion.

Keep checking the loading of this bearing arrgt as the silver steel will expand as it heats up.

Hope this helps

Regards

Derek

Nigel

As no one else seems to have mentioned it ... The preferred 'horological' approach is to catch the centre with a graver. By all means [assuming that your tailstock is aligned] proceed thereafter with a centre-drill, but it's best to start the job with the graver.

MichaelG.

.

Demonstrated at about 1:33 in this video:

https://m.youtube.com/watch?v=4jTKNLjLrNM

[ a good steel graver should suffice ]

Edited By Michael Gilligan on 23/05/2019 21:05:35

I've found some generic centre drills have overlong and fragile pilots. If you're cute with a bench grinder or patient with a hand stone you can reduce the length, as said above. When I tried paying a bit more for a good brand, I got shorter, more robust pilots that worked better. I still use the cheap ones as well, but don't put hard pressure on them in strong materials.

I've found some generic centre drills have overlong and fragile pilots. If you're cute with a bench grinder or patient with a hand stone you can reduce the length, as said above. When I tried paying a bit more for a good brand, I got shorter, more robust pilots that worked better. I still use the cheap ones as well, but don't put hard pressure on them in strong materials.

Nigel

Any kind of pip after facing will push a small centre drill off centre and snap the pilot.

Emgee

Lots of great ideas and tips - thank you!

Two good quality centre drills arrived in the post this morning. I set up my silver steel rod using a belt and braces approach (collet in the headstock and fixed steady), spent more time than usual checking everything was nicely lined up and tools were at the correct height, increased the speed, applied plenty of lubrication and took things gently. Initially I thought I had had another breakage but it was just the swarf coming off. Hole was successfully drilled.

I think having a pip on the end might have contributed to one of my earlier breakages. I want to learn how to use a graver so I intend to do some practice.

Good result, Nigel

MichaelG.

As has been mentioned before, centre drills are for drilling centres for centres (I.e. holes with an oil reservoir for lathe centres to run in).

Spot drills that are short, stiff and pointy do the job better. I have half a dozen and have never broken one.

Neil

There is no need to use a fixed steady to drill a long protruding shaft. Hold in the collet with a minimum amount sticking out, face and then drill. You can then pull out what length you need and bring up the tailstock.

Neil's comment about spot drills doing a better job should say at making a dimple to start a drill, you could think he is saying they do a better job at producing a recess for a ctr.

The problem with the last piece I needed to centre the end on was that I had already part turned it down to a small diameter, but because I couldn't get close enough to the end because my tool was too large I cut the end of with a saw without thinking ahead that it would need re-centring if I wanted to turn it some more. Back in the lathe the piece was now sticking out too far because of the step I had machined on, at a diameter for which I had no collet. It was at this point the fixed stead was brought into action.

My mistake, I now realise, was not solving the problem of how to turn up to the tailstock centre (for which your reply on this has been most helpful, thank you).

I am glad others have noticed that some of the "Colonial" centre drills are dire. If you put a colonial against a good brand, the differences are striking. Buy cheap and buy twice!

Martin

When I manage to break or wear out the colonial BS-0 one that I have been using for the last 4-5 years I'll bear that in mind