Sharpening the HSS bits

[volker-rat]

I bought some HSS bits directly from Woodrat and tried to sharpen it with the diamond stone in the same way Henry showed in the Woodrat video.
No problem to get my chisels and my Japanese kitchen knife very sharp, but with the HSS bits i made something wrong.

Does anybody have an idea how to make it correct?

Volker, Germany

[extiger]

It is important that the diamond abrasive be held perfectly flat against the inside edge. This is the edge which is flat. It is also called the 'cutting edge.'

The outside edge, which is curved, must never be touched. It you do try to sharpen it, you will change the diameter of the cut.

Gary Curtis - California

[snickgrr]

Most of the time you don't touch the outside. Sorta long story so I'll make it short. I bought the real big dovetail bit quite a few years ago and have never used it. Bought so I could have it if or when I needed it. It looked a little funky when it first came, random weird grind marks. Didn't think too much of it.

So I'm in the middle of building a new workbench for myself and thought I would use big sliding dovetails to hold the end caps. I hogged out 99% of the cut first on the tablesaw, the router bit would only have to cut the little triangles on one side at a time of the groove and do the final 1/64" depth cut. I start with only the depthing cut and it proceeds well until I hit a 1/4" little bump. The router stops and smoke starts to come out. I think WTF!. Try it again, same result.

I pull the bit out of the router and look at the blackened tip of it and pull out my calipers. Now my calipers aren't the best but it appeared to me that the cutting portion of the bit was a smaller diameter than the flat grind spot on the outside. So in my exaggerated drawing A-A is larger than B-B. The gray just represents a circle so it's easier to understand. I took a diamond plate and spent an hour trying to take down that edge. Put it back into the router and tried again. 100% better except for the top portion of the cut.
I'm going to have to take out the bit again and grind it again.
A little agitated at this point that the bit left the shop like it did.

[extiger]

It would seem you are referring to the 28mm Dovetail bit from England. The length of line B-B should be 28mm. It makes no sense that A-A would protrude beyond that. There would be the source of your burning.

Gary Curtis

[snickgrr]

There would be the source of your burning.

Gary Curtis

Correct and that is what I ground down. It's much easier to take down the excess metal towards the bottom of the bit and I did a pretty good job of it but it's much harder to do it where the bit narrows.
I've moved on at the moment to other parts of the bench I'm building but will shortly need to come back to the dovetail bit and address it once again.

[pierre]

The best advice is to let Woodrat know about this problem, the bit should be perfect when leaving Woodrat-office. So let Martin or Henry know.

[snickgrr]

Will do. In the meantime. Took the bit to the Tormek, the water cooled sharpener/grinder and worked the entire circumference behind the cutting edge and the bit cuts like butter now. It's like night and day, thin shavings rather than blackened wood and smoke.

[arep3]

sorry for the late reply but i thought i would add my 2c.

if A-A is bigger then the actual cut then that defenately explains the smoke and poor cut. you guys hit the nail on the head with that one.
Yet what id suggest is if the cutter is rather large in size, to use a sharpening machine to remove the extra material from the stock of the cutter or to take it to a tool sharperer/maker and have it professionally done. This is to make sure that there is an even amount of material is removed from each side to keep the cutter in balance. theres nothing worse then a large cutter spinning upto 22,000RPM and being off balance....

Cheers, Ron

[Cincinnati]

I'm assuming that little "+" is the center of rotation of the bit. AA and BB do not pass thru the center. If the center is designated point "C", you need to look at the lines CA and CB. Wood burning could be caused by a few reasons. I'd start with what I consider the most common - cutting speed at the bit tip. This is a relational combination of Rotational speed and feed rate into the bit plus depth of cut. Linear feed rate is also a function of the wood you are cutting. Rock Maple will burn before Cedar.

Back to the bit geometry. Unless someone has filed down the flat AB, I doubt CA is greater than CB. It's hard to measure if it doesn't pass through the bit center of rotation.

Runout is also a likely factor. Have you checked the bit runout? Remove the bit from the collet and re-chuck it. If a few foreign particles are in the collet it will have opportunity to dislodge.You can check this with a steel rule by turning the bit in reverse by hand and observe if the cutting edge stays in contact with the steel at various locations around the cutting circumference.

Greg