Thread: Removing the convexed area ("belly") from a badly convexed chisel

Thank you.

Gee the ease really makes it seem like all you need is to pull your finger out, stop worrying, and drag it along a white wheel aiming for the middle and and she'll be right.

I dread to think what would happen if I tried. I do notice in that video that the sides are still very large and give them a good reference. In most bevel edge chisels you wouldn't be able to avoid rocking.

I was thinking of a jig involving the chisel in a cross-slide vice, travelling underneath a white wheel dressed into a round. But can't see how you'd enough get clearance on your average grinder. (Plus even in my head each method I've had in mind seemed really, really dangerous in terms of having a chisel fixed into a vice with hard grinding pressure hitting it.)

I did pick up some grinding stones for my die grinder and might play around with those.

Originally Posted by woodPixel

and here is a video of a Jaoanese chisel maker putting a hollow in the backs:

Japanese Tool making Part 2 - YouTube ,.... at the 16'20" mark

Thanks for the post on flattening nasty backs.

For really nasty ones that I'm determined to use... I settled on using a Dremel. I follow a technique sort of like precision scraping. I run the dremel slow - speed 1, and touch thr back, making shallow dots on the highest spot in the hump, then take 1 or 2 passes on the paper. Lather, rinse, repeat.

Key points:
1. Sticky back (PSA) sandpaper on a granite surface plate. Loose sandpaper is the devil.
2. Sandpaper cuts really aggressively at first, then drops off. I buy it in 25-yard rolls. Replace it as soon as it slows down. You will absolutely wear yourself out on dull sandpaper, and get nothing except frustrated.
3. Finger pressure and position makes a tremendous difference. People often find problems with the chisel back side opposite their finger hold. They don't realize that the finger pressure causes them to bear down harder under their fingers - they're just trying to avoid skinning their fingerprints off on sandpaper... Move your fingers nearer to the side that's not getting as good contact and it will even out.

Very shallow gouges from the low speed dremel sand out very quickly, but speed up the flattening process immensely. In this case, more of a good thing is not a good thing. Don't speed up the dremel or try to cut deeper to make it go faster.

Last of all, I made a point to quit fooling with badly humped backs. There's probably a reason that "Jewel" of a chisel ended up in a pile like that. Nobody else wanted to fool with it after somebody's kid "sharpened" it on a belt sander... So, thank you for bringing the dead back to life.

I'm with the rest of you in the truth about chisel back prep in ye olde days... I've received antique chisels with backs in 2-conditions:
More or less original factory condition.
Humped somewhere between "meh" and "banana."

I've never received an old one that showed evidence of the back being carefully maintained flat, but I've got plenty that were flat enough to be useful.

Thanks - interesting thoughts.

I tried the Dremel. It was just too slow for me. When you spend more than 20 minutes on an operation like that I get thinking I could just drive an Uber and earn enough money to buy a premium chisel, or look up a engineering shop that might surface grind.

One point I would add to the below:

You're right but I think I misunderstood this for a long time. I assumed that finger pressure might be flexing the steel slightly.

I think that's wrong. I think it's moreso that you reach a point where, when there is so much metal contacting on the abrasive plate, you stop cutting because the pressure being brought to bear means the abrasive particles cease cutting well. It's not flexing the steel but engaging the abrasive.

This I think is explained well in this video: PRECISION GROUND TOOLROOM STONES - YouTube

I think this really does explain why it is that you slow down so disproportionately if you just try and lap the whole back of a wide chisel. When have twice the contact area hitting abrasive particles you need twice the pressure to cause the abrasive particles to cut. Once the contact area is very large, well unless you're jumping on the chisel you can't bear down enough to cut as much as you did when you were only knocking off high spots.

Put another way, once the flat area becomes 10 times as big as when you started, you are not removing material at 1/10th of the rate but a much smaller fraction.

I think this is why I found so much difficulty in just lapping away and why and even people who know what they're doing (refer the blog of David Charlesworth in first post) reach a point of giving up. It's not that lapping becomes very slow going, it's that once you lap to a certain point, the lapping will cease cutting and you are just burnishing or rubbing the object, and cease appreciably removing material.

Originally Posted by truckjohn

Thanks for the post on flattening nasty backs.
...3. Finger pressure and position makes a tremendous difference. People often find problems with the chisel back side opposite their finger hold. They don't realize that the finger pressure causes them to bear down harder under their fingers - they're just trying to avoid skinning their fingerprints off on sandpaper... Move your fingers nearer to the side that's not getting as good contact and it will even out....

Only the tip of the back of the chisel needs to be flat for sharpening purposes if it is concaved, but flatting the chisel much further down is more important if it is convexed, however your pic's in your first post seem to show a concaved chisel, where it is dished in the middle, but you mention "belly" (assuming this to mean sticking out) which supports your convexed description of your thread.

Originally Posted by Cgcc

Thanks - interesting thoughts.
.

I think this is why I found so much difficulty in just lapping away and why and even people who know what they're doing (refer the blog of David Charlesworth in first post) reach a point of giving up. .

David has fairly limited experience with this and tends to only want to show things that can be done easily the first time (that's his market, beginners).

The answer is that if you're lapping a wider area, you start with a grit that's more coarse, unless you have something useful like a contact wheel or a belt sander with a precise platen and a whole lot of tension (few do).

In toolmaking, there are two grits that I typically use - 80 grit, and 220. This isn't a hard and fast rule, but the gist of it is that 80, unless it's spent, will cut anything off of a linisher fast enough, and 220 will prepare an area for a medium stone. Both are adhesive backed so that they are affixed to glass and won't lift.

I think some of these things are pretty easy to do, but what becomes complicated is when someone wants to do them for the first time (on 80 grit, you can't lift the back end of a tool, for example, or you've just created a lot of extra work by accident), or when the suggestion of a couple of rolls (perhaps $50) is too much just to flatten something. If the hobby lasts, it's not - those rolls will find a use somewhere, even if it's woodworking.

If a back is too out of flat for 80 grit paper (al-ox paper, by the way, not silicon carbide), then it can probably be spot ground or maybe the tool set aside if it's too far gone (in the past, I've used a linear sander to remove the belly from a very poorly treated laminated iron and a third of the thickness was gone by the time it was ready to be hand lapped. If that's the case, then it's not worth using in a plane).

I learned to sharpen from david's first video on plane iron sharpening. In that, he used a king 800 to sharpen a new iron. I copied what he did - it was fine for one time (I followed it exactly, it worked - but if one limits themselves to that or even paper with short runs of flat area, like a 12" reference surface with no tool holding aid), then the outcome will be predictable. with a holder and a longer run, and not being stingy with paper, pretty much anything can be flattened.

(the idler of a cheap belt sander - like the cheap bench top ones - can be used, but with some discretion. same with a bench grinder drawing the chisel across the wheel rather than in line, but it may take some time for someone to master that stuff).

royflatmate - I'm not sure what your picture request is for. Like for pictures of a method, or pictures of the chisels?

20211003_104543_copy_1516x980.jpg

20210914_111118.jpg
(no handles on the parers - i made them for a toolmaker and he'll make his own)

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Another set of parers

20210731_164019_copy_1254x980.jpg
And a large set of fatter bench chisels (shorter and fatter), same thing as above, the recipient wanted to make their own handles.

This kind of making is a lot like preparing old tools, but on steroids (plus the bits about heat treatment, that's a little further on to do it well).

There's no machine tool use for any of the above, it's freehand ground and the backs are finished by hand lapping after using a good but cheap 4x36 belt sander to establish crude flatness (it's not one of the over/under type with a cogged belt from the motor - it's a better machine that can actually tension a belt and that has a long flat platen with a graphite layer).

as I'm talking about using a belt sander here, it may be worth nothing that the belts used are referred to as "ceramic", which is just a name (all of the alumina belts, even the old ones, are a type of ceramic).

"ceramic" belts for knife making and tool making, though, instead of having big tough particles that dull but are very durable (good for wood), they have particles made up of clumps of tiny micron-sized bits. So the "grits" wear, but it's really just shedding the tiny micron sized particles. In doing that, they remain fresh, but also create far less heat, so with some discretion, I can grind the bevels on all of the chisels as well as finish grind the thickness of the chisels (as in, there's a *lot* of grinding that happens after the chisels are already hard and overheating is a legitimate threat) with these belts and a bucket of water and nothing turns brown or blue quickly.

Those belts are fairly expensive, but if they have a purpose (making chisels or kitchen knives, etc), then the price isn't so bad.

It's far less easy on a power sander to use something like a red alumina belt, or gold or white or gray (far more heat). But the ceramic belts also need speed to work right, so they can't just be put on a $75 flea market sander, and since they're friable, they're no good for hand use - so no need to waste money trying one cut apart and stretched out. The cheap older alumina (white, red, gold) is far better at that - the particles get dull, but they're tough and they don't just break away.