Flattening chisel backs

[bglenden]

I am flattening some medium quality chisel backs on my DMT diamond stones. The small ones (through 1/2") went reasonably quickly, but my 5/8" one is taking forever (I am more than 3 hours into it). My theory is that even my extra coarse (220) grit stone is not coarse enough. Should I just try a coarser sandpaper on something flat? (Maybe if I do it at midnight I could use our granite countertops...). Maybe 120 grit wet/dry sandpaper? Advice very welcome - I'm scared of what might happen as I get to the really wide chisels!

Cheers,

Brian

[flairwoodworks]

I am flattening some medium quality chisel backs on my DMT diamond stones. The small ones (through 1/2") went reasonably quickly, but my 5/8" one is taking forever (I am more than 3 hours into it). My theory is that even my extra coarse (220) grit stone is not coarse enough. Should I just try a coarser sandpaper on something flat? (Maybe if I do it at midnight I could use our granite countertops...). Maybe 120 grit wet/dry sandpaper? Advice very welcome - I'm scared of what might happen as I get to the really wide chisels!

Cheers,

Brian

Hi Brian,

If you are spending over 3 hours flattening a 5/8" chisel... yikes! First of all, when I flatten a chisel, I don't bother to flatten the entire back. While I do lap the entire back, I stop when I have the inch closest to the tip flat. In ten years when I sharpen the bevel past the lapped back, I will relap to establish a flat 1". Not everyone realises that only light pressure is needed when working on diamond stones. Bearing down only wears the stone faster and is actually less effective at removing metal. You could try a coarser grit, but I don't think it's necessary unless your chisels are really ugly.

[bglenden]

Maybe indeed I'll just give up on flattening the entire back!

Cheers,

Brian

[Bob Rozaieski]

Has anyone ever really stopped and asked why chisel backs have to be flat? I mean really thought about it, and not just repeat what has already been said thousands of times over? Maybe even challenge what has been said thousands of times over?

I'm sorry to hear you've been caught up in the hype Brian . Don't feel bad. You're a victim of the system. This whole chisel back flattening thing has been going on for...well...I don't know...too long.

Believe it or not, I've actually found it to be more beneficial to my style of work if my chisel backs aren't flat. I'll even work with a tiny little back bevel if I have to in order to get the chisel sharp where it matters (i.e. only at the very edge). I have chisels that are only polished for about 1/32" up the back; maybe less. They're still sharp and they still work fine.

I think the whole flattening the back of chisels thing is pretty unimportant. I actually prefer concave backs the best. But even if I have a convex back on one of my chisels, I don't worry about it. I'll sometimes even lift the chisel slightly when chasing the burr and intentionally add a tiny (unmeasurable) back bevel. In reality, small back bevels on chisels aren't the super bad thing they have been made out to be. In fact, I kind of like them. I think they help allow me to steer the chisel better. Blasphemy I know. But I'd rather spend time working wood, not lapping steel.

Here's some food for thought...carvers use double bevel chisels all the time, and they can still make flat surfaces with them. Why is that?

[bglenden]

Thanks Bob.

I couldn't think of any reason for the conventional wisdom, but know that I don't know enough to challenge it. (Naively it seems like the drift errors from the human control/uneven splitting/... must be an order of magnitude greater than any errors coming from the nonflat back). I'll stop lapping and start paring.

Cheers,

Brian

[jon003]

A blade's edge is the junction of 2 planes. If one of the planes is is not flat, the corresponding blade edge will not be straight. Other than that, the only importance of a flag back is if you intend to register your chisel against a surface to use. Back bevelling a little bit has the same effect, you're bringing the second cutting plane into a straight alignment with the first.

Like Bob, I tend to only flatten a very small area on the back of my chisels. I can't manage to flatten only 1/32nd by hand against my stones, so I usually have a half inch or so. (You must have great balance to hold 1/32nd on a stone!)

[outofstepper]

Although Bob just managed to undo a century of woodworking history in 4 paragraphs

(and he's probably right!), to answer your original question -- a little out of the box --

if you have a machine shop near you, they could probably gang 6-10 chisels on a surface

grinder for you -- might cost you a 6 pack.

-Tony

[mdhills]

I can get not needing a flat back if you are constantly adjusting for your progress.

Seems like having the flat back would help for operations that rely on registration more -- paring an angled mortise is the case that springs to my mind.

Matt

[Jonathryn]

I seem to be doing a lot of initial chisel sharpening lately. My procedure is as follows:

1. Run the back against a fairly high grit stone of approximately 1000. That polishes things up so you can see if you have a convex, flat, or concave back and roughly how much work it's going to be.

2. Cut a very coarse 40 - 60 grit belt sander belt and glue with contact cement it to a flat surface like mdf. The belt I'm using now lays out to about 3 feet in length, five inches in width.

3. Run the chisel back against this three or four times, check progress, dip it in some water if it's getting warm. Once the chisel back has a consistent grind from this grit along the area in question, go on to your 220 DMT. It usually goes very quickly, wider chisels take longer. I flatten an inch or two from the edge--whatever I can get registered without chewing up my fingers too badly.

Important to note: If your chisels are flat from the edge to about 1/4-inch back, maybe even a little less, and concave along the back from there like a Japanese chisel, there's no reason to flatten that whole area. You're just removing more surface area, and more metal, without getting the chisel any sharper at the business end.

I'll have to disagree with Bob on dubbed chisels. If you're paring something, dubbed chisels defeat the purpose of an acute (<25º) bevel angle. It makes them more like 30º. For chopping, I think it depends on whether you're trying to reference it to something. Oh well, to each their own. There's more than one way to accomplish the same tasks.

[lwllms]

I am flattening some medium quality chisel backs on my DMT diamond stones. The small ones (through 1/2") went reasonably quickly, but my 5/8" one is taking forever (I am more than 3 hours into it). My theory is that even my extra coarse (220) grit stone is not coarse enough. Should I just try a coarser sandpaper on something flat? (Maybe if I do it at midnight I could use our granite countertops...). Maybe 120 grit wet/dry sandpaper? Advice very welcome - I'm scared of what might happen as I get to the really wide chisels!

Cheers,

Brian

Brian,

Diamonds are an amazing abrasive, they'll cut anything. Their big drawback is that they're slow cutting because of their faceted soccer ball kind of shape. I think you'll find something like 80 grit aluminum oxide sand paper glued to a flat surface a lot faster. The sandpaper will dub the edge so stop with it when the dubbing starts to be bigger than what remains to be flattened.

If one was to use only one sharpening stone, then it probably wouldn't matter much if you actually get the back flat as long as you get the flat face of your tool to match the stone. If you're using multiple stones to speed honing then flat becomes important because it's repeatable. While there are times that a flat back is very helpful with a chisel, I agree that it usually doesn't make much difference in the use of a chisel. When it comes to sharpening, however, a flat back and flat stones facilitate repeatable and predictable results while adding considerable ease and speed to the process.