Instructables

Sharpening Forstner Bits

Picture of Sharpening Forstner Bits
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INTRO:
There a huge number of websites out there sharing all types of information about sharpening forstner bits, I have tried a few and this is what I find works best.

NOTE:
I bought cheap forstner bits that were titanium coated for extra edge durability.  As the bits didn't arrive very sharp there is no way of confirming if the titanium coating helps with edge durability.

After the first use the bits were dull, burnt and chipped. I was drilling in to Iroko at a sensible rpm but for a new bit that shouldn't have be a problem.

HOW TO:
To sharpen I use a rotary tool such as a Dremel or pictured is a black and Decker one. A cut off disc used for the flat area's and a cone shaped stone for the rounded area (see picture). The important thing is remove the same amount of material from each surface so the bit still works as intended.

This only takes a few moments and removes the titanium coating, but the bits cut better than new.

There are a few variations in forstner bit design but it you use the disc of flats and the cone for curves you should be able to do them all.

Once grinding is done a quick de-burr with a honing stone or mop maybe needed to take off any wire edges.
pfred21 year ago
If your bit burnt then your RPM was not as sensible as you may think it was. I've drilled some pretty tough stuff where I had to air cool while I was drilling. I mean I was running as slow as I could go and I was still cooking bits. My mill goes down to 160 RPM too! But air cooling, using a blow gun with an air compressor makes a huge difference in materials you cannot flood with a liquid coolant.

But the work I've done where that has been a factor is a bit extreme. I think I may have a picture of one thing I made where I had to do this

http://i.imgur.com/dHlY6.jpg

That is about a 3 inch in diameter hole I cut into inch thick phenolic with an adjustable paddle bit. Hey, I wanted a perfect circle. But yeah, it is pretty nuts.
rimar20001 year ago
Interesting and useful, but I don't agree in a point. An image worth more than some words.
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pudtiny (author)  rimar20001 year ago
That is one of the many debated issues I discovered when researching. By cleaning this face you take the wire edge of the top face and also maintain the slight bevel it has. Some guides say only do this face and not the top edge, I guess it may come down to the quality of the original bit.

Thanks for taking the time to constructively comment.
Maybe it is as you read, but if you touch that face, you should touch evenly ALL IT. Otherwise you modify the angle.
pudtiny (author)  rimar20001 year ago
Fair point