Sharpening hollow punches?

[JakeS]

I know this isn't really a woodwork question per se, so apologies if it's inappropriate, but I figure that someone on here probably has a better idea than I do, and I know the hand tools forum loves a good sharpening thread...! ;-)

I have a set of hollow punches, which unfortunately arrived in the kind of state one could politely describe as "completely blunt". I'm fine with my chisel-sharpening technique, fairly happy with my approach on plane irons, but I have little idea how best to go about sharpening a circular cutter... does anyone here have any insight?

Since it's a punch and will never be used for paring, I imagine it doesn't have to be quite so sharp as I'd prefer my chisels, so I was wondering about just grinding or filing the sides at an angle (the set is the kind that cut the outside of the circle), but I'm wondering if there's any good way to keep the removal of steel even all the way around, or whether I just have to eyeball it...

 

[Jacob]

Called wad punches if they are these:

Trad plumbers kit for leather washers and also used by other leather workers.

Eyeball it. They don't have to be perfect as you'd expect to use them on an end grain block.

 

[Dee J]

Either by eye, or if you want to get really complicated, then toolpost grinder on a lathe....

Dee

 

[thick_mike]

These things are available for cork borers (used for boring holes in corks and bungs for lab work). Maybe there is something similar available for punches, or maybe you could make one yourself.

 

[bugbear]

Either by eye, or if you want to get really complicated, then toolpost grinder on a lathe....

Dee

Looks to me as if spinning the tool in a lathe (or drill press, but less convenient) and presenting a sharpening stone (or SiC paper) would serve pretty well.

BugBear

 

[Richard T]

If by eye, stone on the outside, fine emery wrapped dowel on the inside.

In my experience once sharp they tend to stay sharp as long as you don't try to cut anything that is beyond it - and also use a safe anvil like, as Jacob says, end grain wood.

 

[Jacob]

Either by eye, or if you want to get really complicated, then toolpost grinder on a lathe....

Dee

Looks to me as if spinning the tool in a lathe (or drill press, but less convenient) and presenting a sharpening stone (or SiC paper) would serve pretty well.

BugBear

God no it'd be impossible to hold concentrically for starters. You'd effectively be machining a new edge each time and losing a lot of wad punch. And the new profile would be as per a 50p piece!

 

[Alf]

'nother vote for the Mk1 eyeball. I'm sure there is a way to jig it, but I suspect a case of the jig time/sharpening time ratio makes it not worth the bother except for the really keen jig-maker.

 

[bugbear]

Either by eye, or if you want to get really complicated, then toolpost grinder on a lathe....

Dee

Looks to me as if spinning the tool in a lathe (or drill press, but less convenient) and presenting a sharpening stone (or SiC paper) would serve pretty well.

BugBear

God no it'd be impossible to hold concentrically for starters. You'd effectively be machining a new edge each time and losing a lot of wad punch. And the new profile would be as per a 50p piece!

I was thinking of low RPM and hand held stone - not some kind of mounted stone. Just like sandpapering the rim of a bowl on a lathe.

BugBear

 

[Richard T]

If the thing was turned to shape in the first place, it should still turn true to sharpen - but I wouldn't bother meself unless there was an awful lot to take off.

 

[JakeS]

If the thing was turned to shape in the first place, it should still turn true to sharpen - but I wouldn't bother meself unless there was an awful lot to take off.

To be honest, on a couple of them I'm not entirely convinced they've been ground at all! In places up to a millimetre-wide blunt end remains... but I don't have a lathe, so there's that risk out of the window anyway. ;-)

Thanks for all the advice, guys - I guess I'll give it a go by hand/eye on one of the sizes I don't need to see how it goes... and thanks for the tip about the end-grain, it hadn't occurred to me at all; I was expecting to just use the long side of a bit of scrap softwood. I'll make myself a little miniature butcher's block, I guess!



(And yes, Jacob, a similar thing to that tool - I've seen those ones called 'Arch Punches' before, I'm sure, this one is more like a cylinder of steel that has had a hole bored out the middle that curves up to a release hole on the side. I expect it's probably cast rather than bored, but I'm sure you get the idea.)