Best way to flatten a chisel with a concave back?

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Fred85

New User
Josh
So i'm really just beginning to get into hand tools; got my first set of chisels for christmas and i've only now begun to sharpen/hone them.

Anyways, i received a set of Stanley chisels (this line: http://www.stanleytools.com/default...c=3+Piece+FatMax®+Short+Blade+Chisel+Set). I realize they are low-end, but i figured i'd attempt to make them all they can be :icon_thum

I got the back of the 1/4" chisel to a mirror polish, but i realized the larger sizes are actually a bit concave on the backs (the 1" chisel is quite noticeable). My question is what is the best way to fix this? Or should i even bother with them? I spent a good bit of time with some mdf and sandpaper on this one-incher but stopped when the edges were beginning to round over...

Thanks in advance
 

froglips

New User
Jim Campbell
Ah, the joys of flattening backs......

In many ways, concave can be good. Assuming it is concave side to side. You win the chisel lottery. Concave means less work for you! It is the equivalent of hollow ground Japanese chisels.

What you are after is to have a shiny flatness along the cutting edge and both sides. Over time (future sharpening), the concavity should hone away. Were the bottom convex (bump), then you are in for fun......

Sandpaper is likely the cause of the round over due to the flex in any paper backed abrasive.

I find it best to use sandpaper for rough flattening. Keep an eye on the dubbing (rolling) and don't let it get too wide.

To overcome the dubbing caused by both stones and sandpaper, I like to go off the edge of the stone/paper by about 3/8ths of the width of a blade. I've found that creates an imperceptible hollow grind making for much more even wear of the stone/paper.

I work on the abrasive side to side while working front to back. I.E. start with the edge a little off the long side of the abrasive (say 1/4" for sake of sake). Work right to left while slowly bringing the edge onto the abrasive. Work onto the abrasive about 3/8 of the way and then work back off. This motion also creates a sort of front to back hollow grind.

To help with scratch pattern reading I will skew the angle just a bit so I can get diagonal scratches (rather than perpendicular to the sides). Say you skew one way at 80 grit, skew the other way at 120 and you'll see a contrast in the scratches. Helps you know when you are done with the 120 and ready for 220 etc etc.

After several passed right/left left/right drifting on and off, I switch to a second pattern. We can call this front to back. Or cutting edge to handle. I start again 1/4" off the edge and pull the edge back onto the stone 3/8 the width. The push it from handle to cutting edge back off the edge of the stone/paper.

But instead of a side to side motion, its front to back. I do this in almost a V pattern so I can progress left and right. Hope that makes sense, tis a bit hard to explain. One motion is left right, the other is front back.

If you work the tool in full contact with the abrasive you end up conforming the metal to the shape of the surface and in the case of papers, drift into fresh grit. As grit break down, they make finer scratches. But in sandpapers, if you've worn down a section then drift into fresh, you get a whole world of new coarser scratches. Often, you end up drifting your cutting edge, effectively dubbing the cutting edge and wasting your effort.

I'd also suggest starting and learning on the 1/2" chisel. Back flattening can be a lot of work, and 1" is a very large surface to learn on.

Dang, I've given this way too much thought and spent way too much time figure'n to do this :)

Jim
 

Fred85

New User
Josh
Using a disc sander crossed my mind, and then i realized that i don't have one!

^^Unfortunately they are concave front to back :thumbs_do and now i'm thinking i should alter my technique and perhaps retire the sandpaper for this sort of thing. I was using a 3" sanding belt cut to lay flat on a similarly sized piece of mdf, but it was just clamped down rather than adhered together...perhaps a source of problems?

I'm attempted to purchase new chisels, perhaps Narex? This is a good learning experience...
 

froglips

New User
Jim Campbell
I'm partial to clamping paper myself. It breaks down faster, so gluing it can be a real time chewer.

I clamp off the edge of my granite floor tile so the clamps tension the paper taut.

I view the use of paper for coarse shaping work. 80 and 120 grits. Then switch to stones for polish and sharpening.

One other perk in clamping is you can use water or WD-40 or any other lubricant without it breaking the glue bond. Our club did 4 months of sandpaper on glass and we exhausted nearly every combination of paper adhesion. In the end, clamping it down was just easier.

Concave front to back is ok. A good 1/4" flat on the cutting edge is enough for most uses of a bench chisel. In this case, just be sure to keep more downward pressure on the cutting edge.

When I've dealt with convex front to back, I've used a 1" belt or slipped Popsicle sticks under the belt to focus the abrasive in the convex area, effectively creating the concave front to back you have.

I'm partial to learning on lower quality chisels. Mistakes and trial-n-error can be great teachers. I just fear doing that kind of learn'n on good quality tools.... (and I'm cheap!).

Stanley for the most part are good steel. They often are not surface ground as flat as higher end tools.

But, its totally your call and how you wish to spend your time/money budget :)

Do you have stones or are you considering them?

There are many systems. Having gone down too many, I regret not heeding Chris Schwarz advise. Pick one and stick with it. They all have good and bad, but in the end they all work.

My only comment on water vs. oil is your shop. If you have an unheated shop and plan to store your stones, skip most water and go oil. I had a 4000 water stone crack due to ambient moisture and a long cold winter. There are ceramic stones and a few water stones that can handle being frozen, they just cost a bit more.

Oh, I'm partial to stones. There are lots of great machine approaches. I just prefer (Read: obsessed) with waterstones......

Wow, you got me going! I sure do enjoy this whole sharpening thing :)

Jim
 
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