Sharpening round blades

[jghm]

If any of you have a member of the family that sews, or especially quits, you may have seen one of these. A rotary cutter. It's used to cut fabric, think ultra sharp pizza cutter. (She also has a circle cutter model that works great on gasket material.) My wife uses it to cut large pieces of fabric into smaller ones for quilting, only to sew them back into larger ones again...??? Anyway, the blades are tungsten steel and rather expensive. Trying to show my shop can be useful, I said those could probably be sharpened. Mounted it on a mandrel in the lathe and tried to put a edge back on it. Everything I tried failed. Stones, wetdry paper, even diamond stones. All the above just held by hand on the rotating blade. Once in while it may have improved some, but nothing close to what a new one is. Any ideas?
Thanks,
John

 

[Amigo]

JGHM, I don't know for sure, but I suspect the very fine edge is "rolling" over on you as a stone or lap is held against the spinning blade edge.
Now, I don't know if this is the case on this fabric blade, but many blades of this nature are edged by stroking radially rather than circumferentially (sp?). Ever take a close look at a scalpel?
If it were me, I'd try a very fine black Arkansas stone stroking very lightly on both sides. The idea is to stand the super fine "burr" straight up, radially. Not unlike the way a chef's knife edge is "stood up" with a steel by stroking away from the cutting edge.

The blade on a mandrel makes for a relative safe way to handle it and have access to the entire circumference edge. However, I would rotate the lathe, by hand, a few degrees at a time, stop, stroke radially both sides, rotate, stop, stroke radially etc.....
An even better way would be to make a jig that held the blade at the desired angle to the stone while stroking the blade the length of the stone.
This is labor and time intensive, so by the time you're are done, next time, you might want to just run out & buy a replacement.

Good Luck!

 

[Tony Wells]

Direction of the stone stroke should be perpendicular to the edge, not tangential. A fine rotary disk on an air motor with a light touch would do it.

 

[jghm]

Looking very closely with a magnifier I see the original grind was radial or perpendicular. So I'm must be going about this wrong. This could be a little harder than I first thought. She has dozens of old ones so I have plenty to try to do something with. Thanks for the replies!
John

 

[jghm]

I already tried what 1200rpm suggested but with an grinding wheel. It seemed to put a sharp edge on it but not smooth enough. It needs to cut every thread and almost any invisible roughness will miss a thread or in my case many threads. Maybe with a diamond wheel. Thanks for the ideas.
John

 

[pdentrem]

Lee Valley had a good fact sheet on sharpening. I do not see it now on their website.

First use your stone etc to sharpen and then finish with honing the blade to finish sharpening with leather and their green compound. Their had pictures showing that regular grinding and stones left a jagged edge and honing would imprive the edge a bunch. I do know that when I take may saw blades to the local shop to get the carbide teeth resharpened that he does use a hone that gives that final mirror finish to the teeth and that blade cuts like a hot knife thru butter.

 

[jghm]

The project had to be put on the back burner for a bit, but thanks for more ideas! I'll keep you posted.
John