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Robin Hewitt
23-03-2009, 05:42 AM
Not easy when your centre drill loses it's tip and you don't want to lose position :confused:

Bottom left hole in the piccy, looks a bit chewed but hey, it's there.

I attacked the remains of the drill tip with an ordinary masonary bit and smashed it to dust, then an undersized drill to make sure it was all out.

By now the hole was clear but way off centre. I switched the drill bit to a 5mm milling cutter and put it back on centre before drilling it through and threading it M6.

Amazed I got away with it :beer:

John S
23-03-2009, 04:25 PM
First off nice recovery but unless you are putting a centre hole in a shaft for running in the lathe forget centre drills.

I know most books and tomes of learning tell you to centre drill first but all these were written years ago before CNC. Industry hardy ever uses centre drills now, they use spotting drills very short fluted and stiff drills with a different angle on them and it's well worth buying a couple of the smaller sizes to replace centre drills.

If you do need to centre drill then it also pays to regrind the drill and loose half the pilot length, they are far too long to do what they need to achieve.
years ago it was to form a reservoir for the white lead that lubricated the dead centre. Nowadays most people use a revolting centre and this doesn't apply. All the pilot has to do is stop the point touching the end and this can also be achieved by having a small flat on the very nose of the revolving centre, this also protects the centre to a degree as if it doesn't have a sharp point it can't get damaged.

Now someone will say grinding the pilot down will loose half it's life, but snapping it will loose all it's life, make the choice.

JS.

Smiler
23-03-2009, 08:19 PM
years ago it was to form a reservoir for the white lead that lubricated the dead centre. Nowadays most people use a revolting centre and this doesn't apply.

JS.

Please say that is a typo and not another new machining term I need to learn :)

Jeff.

Robin Hewitt
23-03-2009, 09:40 PM
I know most books and tomes of learning tell you to centre drill first but all these were written years ago before CNC.


I was written years before CNC to :D

OTOH it's never too late to try something new, I will get some split point spotting drills and give them a whirl.

Still need my centre drills for the lathe though because they cut 60 to match the centre.

Robin Hewitt
29-03-2009, 11:12 PM
Industry hardy ever uses centre drills now, they use spotting drills very short fluted and stiff drills with a different angle on them and it's well worth buying a couple of the smaller sizes to replace centre drills.

Bought some TiN coated 4.8mm stub drill bits in cobalt HSS.

I tried starting a hole edgeways through the teeth on an aluminium timing pulley and watched the bit bend like a banana :eek:

I went up and down a few times to see if it could straighten itself up but it didn't, then I reached for one of my trusty old Slocombe's which put me back on true instantly.

Did I buy the wrong thing? :confused:

Gilly
16-05-2009, 02:14 PM
here's a bit of advice
john S Advice is spot on (forgive the pun) centre drills old hat
when you have broken the end of your c/drill into the the job this then becomes embeded
into the material ,so if you try to remove it the tool will push off toolsteel on toolsteel
hence the banana
so heres what to do get a carbide tool (stronger then toolsteel) low revs and just feed in
slowly until embed toolsteel is removed then you can drill it after
hope this helps
best regards
Gilly

John S
16-05-2009, 02:22 PM
Bought some TiN coated 4.8mm stub drill bits in cobalt HSS.

I tried starting a hole edgeways through the teeth on an aluminium timing pulley and watched the bit bend like a banana :eek:

I went up and down a few times to see if it could straighten itself up but it didn't, then I reached for one of my trusty old Slocombe's which put me back on true instantly.

Did I buy the wrong thing? :confused:

Here's a spotting drill

http://cgi.ebay.co.uk/3MM-NC-SPOTTING-DRILL-90-DEGREE-COBALT-SPOT-DRILL-*NEW*_W0QQitemZ140320534140

No connection with the sale just came up in a search. Note the very short flute length to give rigidity.

John S.

Robin Hewitt
16-05-2009, 02:42 PM
Here's a spotting drill

Hi John

I saw them, bought one in 8mm, seems okay but I was not instantly converted. Still like my Slocombes :beer:

Hi Gilly

I have tried that in the past with varying degrees of success. If the broken end starts to turn it usually prepared to drop out. If it flips and suddenly becames a lot taller, being edgeways on, it is goodbye carbide cutter. That's why I favour attacking them with a cheap masonary bit if I can't be fagged to set up and spark it out :biggrin:

best

Robin

themachinist
17-05-2009, 02:17 PM
Yeah spotting drills everytime i'd say they are much more robust than centre drills, never had one break on me yet...touch wood!

Pmac
25-01-2010, 10:53 PM
Hi ,
Being new here and having read your post just thought i would add what i do.
I have a set of carbide centre drills for slowly peckdrilling out a broken hss c/drill.
Also i have found that going a few sizes up on the c/drill and just using the point to spot the position helps centre the drill

Regards Phill