Hand Tools

Subject:
My take
Response To:
Unicorn summary requested ()

Winston
Here's my take on all this. I know others have a their own perspectives.

David found that buffing a chisel can greatly increase its edge durability compared to normal honing on stones. This result has been duplicated by me, Bill, Wiley, and Derek. In my testing, when chopping through maple, we're talking about preserving a shaving level of sharpness at least 10x longer than a normal flat-honed chisel.

Many of us believe that the bevel angle very near the edge (like within .002") is the key to the durability. For one chisel, I honed it to 30 degrees on stones, then buffed it, and in a microscope photo of the profile, I saw that the angle very near the edge was as high as 50 degrees, but that was only within .001" of the edge. David took a similar photo of his buffed chisels, in which the angle was slightly shallower than mine, and the microbevel was somewhat longer.

The durability also seems to not come at the cost of cutting ability. The chisels I've buffed are sharper than any other method that I've used. (Normally I finish my chisels at 30 degrees on a Shapton Pro 12000, using a honing guide.)

My theory on this is that ease of cut initiation depends on the size of the apex, much more than the angle at the apex, and that buffing results in a very small, uniform apex. I also think that the majority of of the resistance from cutting comes from wedging from the contact area further away from the edge, simply because there's so little total contact area within .002" of the edge. What buffing does is create a profile that's strong only where it needs to be -- right at the edge -- and everywhere else it allows a shallow angle that provides for low cutting resistance.

I made a video about how I do it here: https://vimeo.com/440866741 . However, that it turns out that I didn't buff that chisel enough, which I found when I went to use the chisel. Returning it to the buffing wheel for a few more seconds fixed that. (I have a bit more info about this in the video's description.)

Others have been using bench buffers and other types of compound. We're still trying to figure out the best kind of wheel and compounds to use -- not necessarily for maximum performance, but primarily for ease and reliability of sharpening.

Regarding stropping: It seems likely that it's the high angle near the edge that really matters for durability, and that buffing is not the only way to achieve this. Bill has stropped a chisel at a higher angle, with good success in a dovetailing project. I just tried stropping at a high angle, and some quick informal testing suggests that it also results in a very durable edge. In my case, I stropped it on green compound at about 40 degrees, until I saw a micro bevel that was around .003".

At any rate, there are still many things that we're trying to figure out about the unicorn profile, hence all the posts about it. As we learn more, we'll be able to distill that knowledge into something that's easier for people to digest.

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