By continuing to use this site, you agree to our use of cookies. Find out more
Forum sponsored by:
Forum sponsored by Allendale July 23rd

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

All Topics | Latest Posts

Search for:  in Thread Title in  
Nigel Watts23/05/2019 17:32:35
26 forum posts

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.

Alain Foote23/05/2019 17:40:42
22 forum posts
2 photos

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

Phil H123/05/2019 17:42:43
177 forum posts
27 photos

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

Nigel Watts23/05/2019 17:53:10
26 forum posts

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.

SillyOldDuffer23/05/2019 17:57:28
4601 forum posts
988 photos

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

Mike Poole23/05/2019 17:58:22
avatar
2048 forum posts
47 photos

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

Chris Evans 623/05/2019 18:08:11
1451 forum posts

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,

HOWARDT23/05/2019 18:27:40
438 forum posts
14 photos

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.

Derek Johnson23/05/2019 20:38:13
4 forum posts

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

Michael Gilligan23/05/2019 21:01:36
avatar
13827 forum posts
603 photos

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

Mick Burmeister23/05/2019 21:24:04
4 forum posts

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.

Mick Burmeister23/05/2019 21:24:05
4 forum posts

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.

Emgee23/05/2019 21:26:36
1157 forum posts
206 photos

Nigel

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

Emgee

Nigel Watts24/05/2019 14:20:02
26 forum posts

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.

Michael Gilligan24/05/2019 17:00:57
avatar
13827 forum posts
603 photos

Good result, Nigel yes

MichaelG.

Neil Wyatt24/05/2019 22:08:51
avatar
Moderator
16446 forum posts
686 photos
74 articles

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

JasonB25/05/2019 07:34:17
avatar
Moderator
16051 forum posts
1687 photos
1 articles

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.

Nigel Watts25/05/2019 07:59:52
26 forum posts

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).

Martin Johnson 125/05/2019 09:41:58
122 forum posts
1 photos

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

JasonB25/05/2019 10:13:06
avatar
Moderator
16051 forum posts
1687 photos
1 articles

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 mindsmiley

All Topics | Latest Posts

Please login to post a reply.

Magazine Locator

Want the latest issue of Model Engineer or Model Engineers' Workshop? Use our magazine locator links to find your nearest stockist!

Find Model Engineer & Model Engineers' Workshop

Support Our Partners
Eccentric Engineering
Warco
Meridienne oct 2019
TRANSWAVE Converters
Eccentric July 5 2018
Advertise With Us
ChesterUK
Ausee.com.au
Allendale Electronics
emcomachinetools
Subscription Offer

Latest "For Sale" Ads
Latest "Wanted" Ads
Get In Touch!

Do you want to contact the Model Engineer and Model Engineers' Workshop team?

You can contact us by phone, mail or email about the magazines including becoming a contributor, submitting reader's letters or making queries about articles. You can also get in touch about this website, advertising or other general issues.

Click THIS LINK for full contact details.

For subscription issues please see THIS LINK.

Digital Back Issues

Social Media online

'Like' us on Facebook
Follow us on Facebook

Follow us on Twitter
 Twitter Logo

Pin us on Pinterest