Chisel backs getting out of flat with sharpening over time

[Fitzhugh Freeman]

I'm sure this has been covered but I failed to find it in searching.
I have a problem with convex areas just behind the edge of chisels. I think it's from my technique in flattening the backs and exasperated by how I sharpen. I want to fix the ones I've already done this to, and I just received some new chisels and don't want to mess them up too.

I just got a new mortise chisel and a few new to me bevel edge chisels and I want to avoid causing the same problem with them

I've read arguments against having to fully flatten the backs of any but pairing chisels and don't want to get into that. I run into problems with the backs not being flat since I often use the backs as registration surfaces, same reason I saw given for pairing chisels. I only own a few and they all get used for pairing at times. I don't own bullnose, shoulder etc. planes so I use chisels for pairing in cases where I might not otherwise, so even my super cheap orange handled chisels get a lot of use pairing.

All this applies whether first sharpening or just removing wire edge, and the effect is small but there. I read that you only have to flatten the last section (some say half inch, others 2 inches and so forth). Unless I slide the blade side to side the problem occurs. I find it much easier to slide forward and back - perhaps that is the source of my problem? The drawings are obviously highly exagerated...

chisel_bevel_1s.png
When I flatten the back I move the chisel forward and back. This means the part that gets the most work is the front area, as marked. It ends up making a bit of a concavity.



chisel_bevel_2s.png
Only, as I move it forward and back the rear section also ends up on the flat stone (glass w/ paper in my case).




chisel_bevel_3s.png
This leads to a rounding of the front, as seen above. Lines show how flat surface as blade moves forward and back. The blade itself is tipping, not glass, of course.




chisel_bevel_4s.png
Which causes the above dreaded convexity just behind the tip.





So, two questions: How to prevent this? And how to fix it?

I think removing the wire edge should involve removing little to no material from the back itself so I suspect my chisels have been like this since I first flattened them and I've only become aware over time.

I think I should invest in a steel plate and some carborundum powder. Are the plates milled or ground super flat and what type/hardness steel should they be? Seems getting some flat ground annealed steel plate would be much cheaper than a similar piece of metal sold for this purpose. For the prices I've seen I could get three pieces and use them to flatten each other. Heck, always wanted to try learning hand scraping... (too many other projects on the list already).

Thanks!

[Steve Voigt]

Fitz, 2 thoughts:
1) It really is not possible, IMO, to create the concavity you describe using sandpaper on a flat plate. Given something that starts flat, lapping will at best keep it flat, and often will just make it convex. I suspect your problem is really just the convexity you show in fig.4.
2) You should move the chisel (or plane blade, whatever) from side to side, not back and forth. The back and forth is the source of your problem.

To fix your chisels, put some 100 (or 120, or 180, depending on how bad the problem is) grit on your platen. put the whole back, or as much as possible, on the sandpaper, and move side to side. Scale up in grits after your initial flattening.
Joshua Pierce has posted several times about using a small grinding wheel in a hand drill or dremel to create slight concavities, then flattening these out on something flat (sandpaper, stone, whatevs). I've used this strategy and it works, but it's probably too drastic for what you're doing.
I would not attempt to learn the art of scraping on the back of a chisel. A piece of very mild steel is a better place to start.
You don't need any additional tools; you just need to work on your lapping technique, IMO.

-Steve

[george wilson]

You can't scrape hardened tool steel anyway.

[Jussi Auvinen]

Assuming you're using this method http://www.youtube.com/watch?v=9aDPZzMvVTA (Lie-Nielsen chisel sharpening advice -> lapping/honing the back with forward and back strokes)
...I cant think of anything going wrong.

You just need to focus the pressure on the tip of the chisel and make sure you keep the balance. Make sure your stones/media is flat.

[Chris Vandiver]

Side to side or figure 8 is a better way to flatten your chisel and plane iron backs. Either way it's a skill that is learned and doesn't happen automatically. Practice makes perfect.

[Ryan Baker]

Make sure your paper really is glued down flat everywhere (all the way to the edge). Are you sure the glass is flat and that it is not flexing on whatever surface it is on?

Make sure you are keeping the chisel down flat all the time. This is easier to do with a side-to-side motion. If the bottom of the chisel flares out near the tang/handle, make sure you are not riding up on this.

Make sure you are not lifting the chisel at all when removing the burr (thus only removing material from the tip).

[Fitzhugh Freeman]

So, the solution seems to be a side to side motion (now it seems obvious) and make sure the glass on a really flat surface. The glass itself is flat but I checked today and it does flex enough to take on slight concavities and the counter I sharpen on is not nearly as flat as I assumed.

To clarify - I didn't mean hand scraping a chisel! I meant only that I've always found the concept interesting and like the thought of scraping my own surface plate - but know I'm not likely to do so, too many other things to try first.

However, I'm still mystified. I doubt LN would suggest doing it this way if they had the results I have so I'm not saying it has to happen with forward and back, just that in my case it did and this is my analysis. So I'm still missing something. I'm certainly over thinking it, but that's how I am.

As I see it and as I tried to show above, the very concavity that I always hear is not nearly as bad as a convexity will lead to a smaller convexity at the tip: exactly what I'm running into. Please take another look at 3. above as it is at the heart of my (mis)understanding. The blue line shows the stone surface when the chisel is fully on the stone. See how it necessarily will grind the edge so that it lines up with the level of the back that is on the stone but farthest away, nearest the handle? Then the other (red, or is it green?) line shows the angle the stone makes when the chisel is not fully on the stone. See how it grinds a different angle a bit back from the tip? It is this varying angle that is causing my problems.

I drew the last image wrong, btw: it shows the angle at the very tip rounded past horizontal. It will actually only point to the farthest part of the back that has been worked, which happens to be the far end of the concavity that spans the worked portion of the back.

The back starts out flat, gets a little concave, then the tip becomes convex. In reality the latter two happen concurrently.

I'd like any pointers on fixing my thinking or what I'm missing in how they do it and get proper results, but in the real world I'll switch to side to side and have flat backs.

Thanks!

[Sam Takeuchi]

How wide is your abrasive sheet?

[David Wong]

If the back of your chisel was concave to begin with, I can see how a back and forth motion would create the rounded belly near the tip. As someone mentioned before, not all chisels are flat all the way back to the socket or handle. To save time, you may want to only flatten the first inch or two of the back (depending on the type of chisel).

I formerly used a kanaban and silicon carbide powder to flatten chisel and plane backs, but I switched to waterstones, because the kanaban process is so messy. It is difficult to casually gauge intermediate progress because the SiC forms a black paste that sticks to the blade.

[Jim Koepke]

Fitzhugh,

This is where having your location in your profile might help. You may have once mentioned where you live, but my memory isn't as good as it used to be. There may be a member close to you who would be willing to spend some one on one time with you to figure out why this is happening to your chisel backs.

If you are in the southern Washington - Portland, OR area send me a PM and we can figure out how to get together to work on this.

Others have commented on many ways of working on the back side of blades. There are caveats involved in every way of flattening a back.

For many the side to side method works fine. One does have to be very careful to keep the pressure even or you may end up with a rounded back. Back and forth works great as long as the blade stays flat. If it is rocked as in the Peter Sellers method it can become convex.

There may be a few causes in your situation.

The edge of the paper may be lifting and causing your blade to rise.

There could be swarf building up and lifting your blade.

Your abrasive sheet could be buckling (bubbling) under your blade.

There are likely a dozen more possibilities causing problems.

jtk

[Fitzhugh Freeman]

The sheets are 3.5 by 9.

I wonder, does diamond paste create the obscuring gunk as well? I assume it would. In searching for info before starting this thread I saw a post saying how very much faster it was than paper. I don't have that many blades total, but I suspect I'll find many of them could use re-flattening after creating the problem I described. Could be looking at quite a process.

I have some 1084 sheet arriving Monday. I'm tempted to dedicate a portion to that use, at least temporarily.

Jim, thanks for the offer. I'm down in Berkeley CA. I'd filled out the profile but didn't notice you have to hit save after each field. Just fixed it. Anyone nearby will be rewarded with pie or similar, even if they just show me their shop.
I was using a small piece of glass in the past and didn't have the paper glued down. I wet it underneath (mineral oil) so it mostly stuck and awkwardly held it with my left hand while moving the chisel or other blade with my right. I now have a larger piece the glass place let me have. I think I'll have them cut it into pieces that fit a single sheet with room for a jig to slide/roll on bare glass. I do think having it not glued down causes a very small rounded bevel right at the edge.

I'll give it a good try today and see what I learn. Thanks for all the suggestions!

[David Wong]

Fitzhugh,

I use Scotch (3M) Spray Mount Repositionable Adhesive, with sandpaper on a granite plate. This is a lot less aggressive than the 3M 77 spray. You can usually find it in an artist supply place. If you remove the sandpaper in a day or two, it comes right off without scraping. I clean the residue with a little mineral spirits. Be sure to wear a dust mask when spraying so you will not breath in any glue. I just spray the back of the paper. If you are using glass, be sure to mount the glass on a flat surface like a piece of MDF.

[Fitzhugh Freeman]

Thanks! I can't stand 77, which is all I have. Nasty stuff.


Well, I managed to flatten the backs. I spent a LONG time working on them. They're the super cheap ones I have so I spent the time trying different approaches and learning what worked and what didn't. I found Side to side hard as I really tended to have a convexity down the length of the blade. Seemed flattening the full length but holding the chisel at about a 45 or 60 degree angle for a while, then with small circles for a while, then back... kept my hands from locking up and got them nice and flat. Remember I had to undo a lot of damage I'd previously done and normally it shouldn't take so long. These orange plastic no brand chisels are rather hard and take a long time to flatten, or even sharpen.

I'll finish tomorrow by sharpening up the new chisels. It is nice to finally have fixed those problem cases. Thanks for the help. I didn't realize I could make something so simple so complicated.

[Steve Friedman]

Fitzhugh,

Before I started understanding what I was doing (and I still have lots to learn), I managed to put a belly on quite a few blades. As you're finding, removing the belly is one of the hardest things to do by hand. If you try to flatten the back before removing the convexity, you will just make it worse because your blade is hitting the abrasive at two spots - the apex of the belly and the edge - just making the belly bigger.

The solution is to make that belly area concave - sort of like a Japanse chisel. You can try to grind the back with the edge hanging off the abrasive, so that you are only grinding the belly, but that's very difficult to do because it's extremely challenging to exert pressure as you balance perfectly on the convex belly. I think the most realistic solution is to grind away the belly with a powered grinder or belt sander and actually make that area slightly concave. Once the belly is gone and there is a slight concave area in that spot, you can flatten the back. You only neat to flatten a small strip along the edge.

Just my opinion.

Steve

[Steve Voigt]

Now that you've got them flat, I'd suggest you try to avoid being overly aggressive when chasing the burr, so that you keep the backs in good shape. I rarely show my chisel/plane backs to anything coarser than 600 grit, and it shouldn't take more than a half-dozen to a dozen strokes to chase the burr, then on to a finer grit.
And don't worry about those cheap orange-handled chisels. If they're as hard as you say, you can do great work with them; don't let anyone tell you different.