Drill bit sharpening jig

[challenger]

I have a thread going on about a question regarding using a lathe to sharpen drill bits. I wasn't sure if I should put this in that thread or start a new one. If the mods feel that starting a new thread is bad form I'll happily combine this in that thread.

I recently discovered a fixture for my KO LEE T&C grinder that, I think, will be adaptable for grinding drill bits. I also remembered that I had this collet holder and collets. It's an MT taper and the fixture is a 3/4 bore so I'll make an adapter for it to fit. The part I'm adapting the collet holder to is part of a pair of centers fwiw. The "nut" on the top of the bore will index the collet holder 180* when I make the adapter and mill two grooves into it on opposing sides. Any input is greatly appreciated.
Thanks

[Mcgyver]

depends on diameter and how long the gripping surface is. The challenge of holding drill bits is you're gripping the land only. Its thin and at a very small helix angle. Unless the gripping surface of the collet is quite long, so that its gripping a fair bit of the land as it makes its way around, it won't be stable. Most collets do a poor job unless unless the drill bit is small compared to the collet

[old mart]

You would need to make very long collets in relation to the size so they can hold a decent length of drill. Also the collets would only hold drills over a very small range. This is the reason 6 jawed self centring chucks are used for holding drills for sharpening.

[cijuanni]

Even if you can hold the bits consistently, will that jig rotate via a cam or offset to properly grind a bit?

If you can find an decent description of how a Potts type grinder achieves the correct geometry it might help.

[BCRider]

You would need to make very long collets in relation to the size so they can hold a decent length of drill. Also the collets would only hold drills over a very small range. This is the reason 6 jawed self centring chucks are used for holding drills for sharpening.

With ER collets being so cheap these days and having the wider range of holding I was thinking at one point that something based on those would be one option to cover a wide range of sizes. Something like ER32 for up to around 3/8 to 7/16 because they are long enough to catch enough of the spirals for proper support then go to a bigger ER size collet for the added length to catch enough of the spirals for stability in the larger size drills.

To get enough support for good centering we need to be able to grab somewhat more than 1/4 turn or more of both side spirals I figure. A half turn or more would be ideal of course but I think we could get by with maybe a 1/3 of a turn on each side?

[Alan Smith]

ER collets are fine for drill holding. I use ER20 collets in a Dbit grinder for 4 and 6 facet drill grinding and they work very well.

[BCRider]

Thanks for the confirmation Alan. What are your thoughts on the larger sizes and the need for holding enough of the spiral?

[old mart]

Make sure the er collets have the same bore right through, I have some nasty ones which only have the correct size halfway. They should hold drill bits reasonably well as long as the drill diameter is less than 1/5 of the collet length.

[djc]

Make sure the ER collets have the same bore right through, I have some nasty ones which only have the correct size halfway. They should hold drill bits reasonably well as long as the drill diameter is less than 1/5 of the collet length.

I believe the smaller sizes in all ER ranges all have this feature. It is part of the design. Have a look at Regofix' site as they show cross sections of the collets.

Apropos other posts: the Vertex drill grinders use ER collets, but backwards with the drill point against a stop so the projection from the holder can be controlled. A non-dead length holding system for drill grinding will produce inconsistent results.

Another one to look at is the Tormek system (4 facet only) which uses a system of interlocking Vee-jaws to overcome the problem.

[challenger]

The collets are 1-3/4" in length. This isn't long enough to grip a drill bit?