Broken centre drill recovery

[Robin Hewitt]

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

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]

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]

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

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-SPOTTIN...mZ140320534140

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

John S.

[Robin Hewitt]

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

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

best

Robin