Should a linoleum knife have a concave bevel?

[Another Shepherd]

My wife uses a $9 linoleum knife to cut bananas off the tree. She asked if I could sharpen it. Foolishly, I said, "Sure!"

Half an hour later, I am not making a lot of progress, and it is apparent that the bevel along the edge is concave. There is no secondary grind that I can detect. The goniometer suggested that the edge was about 10 degrees per side but with a lot slop like you would expect from a dull convex bevel, but now that I think about it, a dull concave bevel would also give you a lot of slop in the angle measurement.

I am using the standard method of coloring the bevel with a marker and trying to file off the color. I am using the same diamond rod that I use for a recurved machete. As you can see from the photo, the rod is taking off color near the edge and at the top of the bevel but not much in between. There is no burr yet that I can feel. So the bevel must be concave

I am wondering if there is a reason that a linoleum knife should have a concave bevel? If so, what is a good way to sharpen it? I do not have any grinding wheels.

If there is no reason for a concave bevel, I assume that I will eventually get it flattened out. But reprofiling S90V on a TSProf is easier. :-(

 

[777 Edge]

Interesting. I'm not an expert on linoleum knives, but I assume that one like this was likely ground on a small and narrow grinding wheel to be able to follow the recurve shape of the blade when it was made. The small diameter of the grinding wheel (possibly even worn down to a small size) would have been what caused a concave edge when it was made. I would personally increase the angle, 10 degrees per side seems excessively low and probably not necessary.

 

[Obsessed with Edges]

+1 to the above: it was likely ground on (or with) a wheel.

I wouldn't even worry about trying to flatten that bevel. In fact, a concave grind, or 'hollow grind' by another name, has advantages of its own. It reduces the contact area behind the apex, which minimizes friction in cutting tough material like cardboard & linoleum & other plastics. I'd just tune up the apex of the edge and leave it at that. That's all it really needs. And the hollow behind the edge also means the resharpening need only remove a minimum of metal very near to the edge - sharpens up much faster that way.

 

[Another Shepherd]

Thanks for the responses!

Yes, it makes sense that the blade was originally ground with a small wheel.

The problem is that there is still some color from the marker on the apex and there is no burr. The diamond rod I am using is rounded and abrasive on one end, so I might be able to use that end to get into the concavity to reach the apex.

 

[Obsessed with Edges]

Thanks for the responses!

Yes, it makes sense that the blade was originally ground with a small wheel.

The problem is that there is still some color from the marker on the apex and there is no burr. The diamond rod I am using is rounded and abrasive on one end, so I might be able to use that end to get into the concavity to reach the apex.

Just go a tad wider in angle, in the portion where the apex isn't yet burred, so the hone is only contacting the apex and nothing behind it. Effectively, you'll be putting a microbevel in that portion, to get the apex thinned out and sharp. No need to get into the concavity at all, or to touch anything behind it.

 

[catspa]

I sharpen my flooring knives (I doubt anybody has installed linoleum since the 60s, it’s all sheet vinyl now) with a fine chainsaw file. No concave profile, just beveled at about 12-15ish degrees.

When I was much younger, I learned how sharp a flooring knife was by accidentally sticking one about 2” deep in my right thigh. Ever since, I’ve been careful with them.

Parker

 

[Another Shepherd]

I sharpen my flooring knives (I doubt anybody has installed linoleum since the 60s, it’s all sheet vinyl now) with a fine chainsaw file. No concave profile, just beveled at about 12-15ish degrees.

Yes, I'm thinking I should try to keep the bevel under 15 DPS

When I was much younger, I learned how sharp a flooring knife was by accidentally sticking one about 2” deep in my right thigh. Ever since, I’ve been careful with them.

Ouch! I hope I can get this knife almost that sharp. So far, it's not even sharp enough to cut skin.

 

[catspa]

If you’re going to only use it on vinyl flooring, you could probably go 10 and still be okay. If like me, you’re going to sometimes use it on harder materials, 12 is probably a safer bet.

Parker

 

[Alberta Ed]

A round file like a chainsaw file should work.