Grinding urasuki and a DIY wood radius platen

[Cushing H.]

My guess is this is mostly directed at Stacy....

There is a couple-year old thread related to this, which does not quite answer my questions. For reference (but not to necropost) it is here: https://www.bladeforums.com/threads/usuaba-grind-w-radius-platen.1514532/

So .... questions are as follows: in that thread, stacy refers to creating a (I think) radiused sanding backer by creating a certain length fulcrum, attaching it to a piece of wood, then sweeping back and forth on the grinder to create a certain radius. would that technique work well enough to create a wood radius platen for use on the actual grinder?? (obviously, this would be a seldom used thing .... otherwise a metal radius platen would be better. Second - would such a wood platen work well enough if just secured directly onto the grinders brackets - OR should it really be placed on top of a flat metal platen (with appropriate counter sunk screws to hold it in place)??

Then .... when it comes to actually grinding the urasuki, what do you do..... start with the outer radius of the radius platen basically centered on the back of the blade .... then work the grind until the concave grind just touches the edge???? or do you take another approach?? (if you start at the center, here is where the parallelogram grinding setup Stacy mentions would be really useful .... unless once you get the hollow started gives you the same "feel" as does freehand grinding a flat bevel.......)

 

[Cushing H.]

OH .... and is it really necessary to make the platen out of hardwood? I think all of the wood I have laying around is pine.....

 

[Joshua Fisher]

I’d imagine pine would not work well, imagine how much hit and friction you get on those radius platens and the pressure of grinding would most likely deform pine very quickly, there’s a reason a lot of people run a cooling system on them. If I were going to make one out from wood lignum vitae would be my choice as it’s been used for bearings due to the oil content and hardness. I would try to start centered then once established you’ll feel the hollow and can rock it either toward the edge or spine to move it where you want.

 

[Hubert S.]

This video showed up in my recommendations recently. It shows how to make a radius platen by hammering a piece of steel into shape and then welding it to a piece of angle. I think you recently mentioned you wanted to get into welding, might be another good reason to start...

 

[Cushing H.]

I mentioned wood because that idea came up 6 months or so ago as a way to make a one-off, limited use radius platen. I sencerely doubt I will be doing many grinds like that... but wanted to try it on a yanagiba I have in process. I know Stacy mentioned fudging it with a contact wheel... but that seemed like it was too tricky...

 

[Mitchell Knives]

I don't think a wooden radiused platen would last very long. Quite a bit of friction will be created; even if you were using a metal radiused platen.

I've ground a few urasuki in the past. I'll typically take a smaller contact wheel (8" - 14") and grind a shallow hollow across the center of the blade, then move to my 36" radiused platen. Starting with the smaller wheel makes it a bit easier to center the grind and moves metal slightly faster. I imagine you could do the entire operation with a reasonably sized contact wheel by simply rocking it up or down to broaden the grind if you're careful.

 

[Cushing H.]

I don't think a wooden radiused platen would last very long. Quite a bit of friction will be created; even if you were using a metal radiused platen.

I've ground a few urasuki in the past. I'll typically take a smaller contact wheel (8" - 14") and grind a shallow hollow across the center of the blade, then move to my 36" radiused platen. Starting with the smaller wheel makes it a bit easier to center the grind and moves metal slightly faster. I imagine you could do the entire operation with a reasonably sized contact wheel by simply rocking it up or down to broaden the grind if you're careful.

The trick with the contact wheel is what Stacy recommended. Sigh .... I guess I need to go get some mild steel and practice before I turn to my hardened blade, huh?

 

[Mitchell Knives]

The trick with the contact wheel is what Stacy recommended. Sigh .... I guess I need to go get some mild steel and practice before I turn to my hardened blade, huh?

You could also try grinding it with a smaller contact wheel, then finishing up the hollow grind via hand sanding. Making a 36" radius sanding block is pretty easy.

 

[Cushing H.]

You could also try grinding it with a smaller contact wheel, then finishing up the hollow grind via hand sanding. Making a 36" radius sanding block is pretty easy.

Ugh ... doing that kind of removal on a 64 Rockwell hardened blade seems ... painful

 

[Mitchell Knives]

Do it before HT.

 

[Cushing H.]

Do it before HT.

sorry .... too late!

 

[Stacy E. Apelt - Bladesmith]

The urasuki is very shallow. With a 36" wheel (or radius platen) and a 2" wide blade, it is only .025" deep. On a 48" wheel it is .020" deep. It doesn't take long to do it or to sand it smooth after HT. On the omote, the main bevels should be left for post-HT grinding.


There are plenty of other ways to get it in pretty close without a big wheel or radius platen.
1) Use a radiused wheel. On an old wheel, or a lower price polymer wheel. grind the surface into the desired radius. Remember, it is very shallow. Put a J-flex belt on and grind the urasuki with the blade point up or down. This will make the desired hollow good enough to finish up by hand sanding.

2) Hold the blade horizontal to the contact wheel on your biggest wheel. Raise or lower the tip so the blade is at an angle to the horizontal plane. This will make the wheel grind a wider hollow. Play with this on some scrap metal to get the right amount of tilt needed. Clean it up by hand with a radius sanding block.

3) Use your biggest wheel and start a hollow grind down the center of the ura. Move this grind up and down to walk it to the top and bottom of the desired urasuki. Finish by hand as above.

4) The purchase of a radius platen is a good investment. Used properly it will last a long time. You can make your own from A-2 or D-2 by hand pretty easily, too. It isn't necessary for it to be absolutely perfect arc as long as it has a smooth curvature.

TIP:
Grind the urasuki all the way to the edge at the mune and ha. Once done and sanded smooth. place the blade on the flat platen or a surface plate with sandpaper taped on it (doing it by hand on a surface plate is the best way). As you thin the blade (it is a very small amount), it forms the narrow flat surfaces at the mune and ha. Go slow until it is the amount you desire. If you need more in any one part of the blade, apply more finger pressure there.

 

[Cushing H.]

The urasuki is very shallow. With a 36" wheel (or radius platen) and a 2" wide blade, it is only .025" deep. On a 48" wheel it is .020" deep. It doesn't take long to do it or to sand it smooth after HT. On the omote, the main bevels should be left for post-HT grinding.


There are plenty of other ways to get it in pretty close without a big wheel or radius platen.

2) Hold the blade horizontal to the contact wheel on your biggest wheel. Raise or lower the tip so the blade is at an angle to the horizontal plane. This will make the wheel grind a wider hollow. Play with this on some scrap metal to get the right amount of tilt needed. Clean it up by hand with a radius sanding block.

4) You can make your own from A-2 or D-2 by hand pretty easily, too. It isn't necessary for it to be absolutely perfect arc as long as it has a smooth curvature..

Thanks Stacy. I was actually sitting here trying to find a way to make my own out of steel. We did talk several months ago about the trick of using a contact wheel with the blade at an angle .... but I think then you commented that it was a little tricky doing it that way .... and was easy to get the urasaki too deep....

If I were to to to give it a go with A2 or D2, would your trick of creating a pivot of suitable length and using it against your chest to pivot the steel block against the grinder work well enough???? I guess with annealed steel and a coarse belt you could hog a lot of metal pretty darned fast????

Final question re doing this out of wood: right now my need is to just give this a try on a single yanagiba that I am making more or less on a trial basis (for my own use). If I were to make something out of say, maple or oak, do you think it would be sufficient for just a single grind? (I guess though the other concern might be fire if the wood backer gets too hot????????). If I am pleased with the way this works and think I might want to make more knives with this geometry, I might well invest in a better made radiused platen....

(right now online I am really only seeing a single option for a domestically made radius platen.....)

 

[Cushing H.]

slight update. Just spoke with Bill Behnke (who sells CNC machined radius platens our of MI), and he actually said that he had a friend who make a radius platen out of hardwood, and that it actually worked ok (though not for major manufacturing). I think I will give that a go .... and if I like the result and think I will do more, I will give Bill a call and order one of his more precise more robust radius platens....

 

[Stacy E. Apelt - Bladesmith]

You can take a piece of 1/2" A-2/D-2 and do it by hand and eye on the flat platen. Go slow and try and get an even curve. On an 8" platen with a 18" radius (36" wheel) the curve will come right to the back edge on the ends (.05" left). On a simulated 48" wheel, the curve on 1/2" steel 8" long will come to within .15" of the back.

 

[Cushing H.]

You can take a piece of 1/2" A-2/D-2 and do it by hand and eye on the flat platen. Go slow and try and get an even curve. On an 8" platen with a 18" radius (36" wheel) the curve will come right to the back edge on the ends (.05" left). On a simulated 48" wheel, the curve on 1/2" steel 8" long will come to within .15" of the back.

But .... my reading is that I want a 36" radius (simulating a 72" diameter wheel) .... right??

 

[Cushing H.]

The other thing I am thinking is to take your 36" fulcrum .... but it on the back of the thing to be ground .... then take a like and pen/scribe and scribe the arc onto the metal block (or wood), then remove the fulcrum, and go by hand as you describe above (grinding down to the scribed line)....

 

[Stacy E. Apelt - Bladesmith]

That will work. Just epoxy a 2X4 to ten platen and remove it when done.
Don't forget to drill the platen mounting holes before you start.

 

[Cushing H.]

That will work. Just epoxy a 2X4 to ten platen and remove it when done.
Don't forget to drill the platen mounting holes before you start.

Lol ... yeah, I did think about that. I happen to have some 2” wide black walnut I originally cut for scales. I’ll start there ... and will post pictures

 

[FredyCro]

Another idea I read somewhere is to take an old stainless steel pot and cut a piece of the wall, you could epoxy it or ca it on your hardwood platen to achieve the radius. Shouldn't take long. Two bolts through your steel platen and some metal thread inserts in the hardwood and grind away. I bet it would also last for more then one urasuki.