How to sharpen centre drills?

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How to sharpen centre drills?

This topic has 8 replies, 6 voices, and was last updated 28 April 2010 at 12:12 by Ian S C.

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17 March 2010 at 16:07 #15460
marcusj
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@marcusj
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17 March 2010 at 16:07 #49848
marcusj
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@marcusj
I have Harold Hall’s excellent book ‘Tool and Cutter Sharpening’ but I’m left with the question: how do I sharpen my hard working centre drills?
24 April 2010 at 23:12 #51095
Michael Wilde
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@michaelwilde12011
Hi, I also have this book and I’m currently making the grinding rest described within. I’m not sure about sharpening centre drills, maybe if you had one of the common swing type drill grinding attachments that are also described in Harolds book you could rig it up to hold a centre drill? Remember the projection from the front of the attachment is specific to the diameter of drill being sharpened so this would be the pilot diameter of the centre drill. I’m pretty sure the angle of the pilot tip of a centre drill is 60 degrees aswell as the conical, centre forming section, but I could be wrong…
Also, you can’t shorten the pilot section too much as the length is needed as clearance for the tip of your tailstock centre once an impression has been made with the centre drill.
I broke a centre drill today (whilst making the fence for the grinding rest). The pilot of the centre was well bedded in to the material so I marked out for the hole on the opposite side, centre popped, centre drilled and then very gently drilled until the drill felt as though it wasn’t cutting. I placed the material in the bench vice and centre punched the bottom of this blind hole. This punched the broken section of centre drill out the other side! Beats trying to ‘chisel’ it out with old scribers! Sometimes trying a magnet is useful aswell if it’s not stuck in too tight.
24 April 2010 at 23:42 #51096
Michael Wilde
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@michaelwilde12011
Hi, just been googling centre drill geometry. I found this .PDF wich is quite useful. http://www.shis.com.sg/product/100.pdf

with reference to my above post about the pilot angle being 60 degrees: it’s not! It’s 120 degrees. Should make it a bit easier to set in drill grinding attachment. If a length of square material was drilled/reamed as accurately as possible on one face to accept the centre drill body, then a grub screw fitted perpendicular to this to clamp the centre drill. It would be possible to set this up on the drill grinding attachment with the relevant projection and the pilot flutes in a vertical plane. The square material would sit nicely into the ‘V’ of the attachment and would allow it to be turned exactly 180 to grind the second flute to the same setting as the first. I don’t know if this would work in practice, it’s just a thought. There’s probably many other methods but it’s all I can think of right now. Let me know how it’s going if you attempt this!

Cheers, Mike
25 April 2010 at 12:27 #51112
chris stephens
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@chrisstephens63393
Hi Micheal,

Centre drills tend to break their tip when you don’t withdraw the drill often enough, whilst drilling. The tip is very fragile and any flute clogging is likely to snap the end off.

chriStephens
27 April 2010 at 21:28 #51202
Sub Mandrel
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@submandrel
I have an ocassional problem when centre drilling from the tailstock – once in a blue moon* the drill wanders off centre and creates a circuilar groove, and forcing it snaps the drill

Keeping a close eye on things and forcing the drill back on centre can cure it just as it starts, if I am quick. I think the cause is an over-extended, unlocked tailstock.

Neil

*my cussing turns the air blue too…
27 April 2010 at 22:56 #51210
Michael Wilde
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@michaelwilde12011
I used to have the same problem when centre drilling/drilling. I thought it might have been the tailstock out of align but when checked was not so. I gripped a rod in the drill chuck, gripped the other end in the 4 jaw chuck, put a clock on the morse taper of the drill chuck and rotated the lathe spindle by hand. An error of about 3 to 4 thou (0.075mm to 0.1mm) was found. The drill chuck and morse taper was a cheap set-up, I ordered a new chuck and MT and the problem was solved!
28 April 2010 at 08:31 #51213
KWIL
Participant
@kwil
I usually find that by the time the tip needs a regrind the main cone angle also needs attention, so its just a spare bit of HSS now and all I need is a new one. Life is much too short to regrind centre drills.
28 April 2010 at 12:12 #51217
Ian S C
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@iansc
I just sharpen my worn out/broken center drills as stub drills, countersink bits, or use them as tool steel for boring bars. Ian S C
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