Does belt grinder hurt temper?

A

AttilaTheHun

Joined
Apr 10, 2011
Messages
124
Howdy,
I'd like to change a scandi grind to convex and from what I read a belt sander is the easiest way to go. It seems every bladesmith uses one on youtube. Disclaimer: I'm not one, that's why I ask here.

However I recall from visiting a knife making museum that in the old days the grind was ground with a rotating water stone, so as not to overheat the blade while grinding and damage the temper.

So my question is: why in the world do people use a belt grinder on a tempered blade? It seems like a sure way to ruin the temper, especially on the edge region which is narrow and will overheat real fast.
So so far I'm planning on doing the convex edge manually with a file and an angle jig.

But back to my questions: Doesn't using a belt sander damage the temper?
 
It could if you let the blade heat up. If it gets to hot to touch then you are too hot and dip it in water to cool it off.

Do one pass and dunk in water.
 
Like Frank said, it can if you get the blade too hot. I sometimes grind hard (after the blade has been heat treated and tempered). When I do, I grind bare handed and when I feel the blade heating up, I dunk it in water. Usually that's one or two passes depending on your belt and the grit you're using. One reason that some makers will grind hard is to lessen the chances of a blade warping during heat treat. The more meat left on the blade, the less chances of it warping. I mostly grind hard when using 1084.

Don't try and grind stainless steel hard. Like me, you'll regret it pretty quick!
 
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Allrite then, I'll get a belt grinder.
My next question is about the progression from scandi to convex. What's the best/most foolproof way to do it?
I'm thinking of starting almost tangent to the scandi grind, doing a run on both side, then increase the angle by 2 degrees, pass on each side, repeat until angle is tangent to blade itself. That should be 3 passes for a 7 degree scandi.

Then use buffing compound to merge the transition lines of all 3 passes.
Thoughts?
 
BTW the procedure above will likely create a circular convex since the angle changes by a constant with every step. It doesn't have to be this way though. Which shape is considered the best convex grind? arc of a circle, oval, ellipse?
 
I believe that you will want to finish your transition using a slack belt. You remove the platen from the grinder. That lets the belt conform to the contour and remove the ridges. Just don't take it down to the edge until you're almost done.
 
I suspect that to make a convex grind all the way to the edge you might have to loose a small amount of blade height - i guess it depends of edge angles. I think elliptical describes the typical convex grind the best. The spine though is still the widest part.

It will take more than 1 pass per angle change (Many or use slack belt for most of it). If blending, I think you will want stronger grit depending on what you are looking for. Even with a jig, there is a learning curve to using a belt grinder, you will want to practice on some cheap knives. I don't know how files and diamond sharpening plates would work on your hardened steel, but it may be easier than a 1 speed belt grinder.
David

Thought of one more thing, the edge will be hyper sensitive to heat, stay away from it as best as possible. There are belts to wet grind or I would hit the edge with diamond or stone, either with water.
 
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Welcome to BF, please fill out your profile a bit so we know what city you are in (there may be someone local to you that can help you)

You do run the risk of overheating your edge if you are trying to reprofile an already hardened blade. My first question is simple. Why?
my second question, what are you starting with? does it actually have enough material in the blade for this to produce your desired end result?

Third, if you are looking to do a convex grind, and you are going to get a belt grinder, why not start with a piece of steel and make your own blade from scratch?

-Page
 
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