Why can't I flatten the back of my mortice chisel?

[--Tom--]

People use flattening and polishing interchangeably.
Flatness is easy to start thinking in terms of engineering concepts rather than woodworking- flat enough is good enough as rarely will you use the whole back of a chisel as a reference plane.
A chisel so out of flat to be unusable would be an unusual occurrence.

Polishing gives a refined surface finish which is beneficial to taking and holding an edge.
Only high end chisels will come polished - with cheaper chisels often being cheaper as the additional finishing steps will have been shortcutted. Them grinding the surfaces through the different grits to achieve a fine finish costs money which the customer pays for.

Now if it were possible to just polish the mm at the cutting edge that would be an efficient process - a la the ruler trick for plane irons - though that divides camps. The ruler trick for a chisel would shift the edge from being out of line with the back of the chisel and so isn’t advisable for anything you may use for paring or where you reference off the back.
So polishing the back inevitably gives you about 10mm or so as you need an amount wide enough to locate as a flat on an abrasive.

Doing the whole back doesn’t add much to functionality other than it being a one and done operation- and when you have all the stuff set up and are working through the process you kinda may as well. If you start and the back is very uneven then just focusing on the part behind the edge is efficient and quicker, but anything in reasonable shape I feel you may as well just do the whole thing.

It’s worth knowing a bit about what you’re doing and why - so as a beginner crack on and make stuff until you feel you’re more impeded by your tools than your skills- and also til you feel it’s worth the effort tool fettling.

Oddly I quite enjoy fettling as I can do it in the odd 20 mins I get in the workshop whereas woodwork projects need longer time blocks to make progress in.

 

[Jacob]

......

Now if it were possible to just polish the mm at the cutting edge that would be an efficient process....

That's what you get with a typical normal new chisel as mentioned above. The slight hollow makes sharpening easy. In fact trad Japanese chisels have a hollow deliberately ground in the face

- a la the ruler trick for plane irons - though that divides camps. The ruler trick for a chisel would shift the edge from being out of line with the back of the chisel and so isn’t advisable for anything you may use for paring or where you reference off the back.

But in fact many people use the "ruler trick", without a ruler because it's easier, and without giving it a thought; just by putting more pressure near the edge as they struggle away to remove the burr from chisel or plane blade. Over time the face becomes very slightly bevelled or convex, but who cares nobody notices!
PS come to think - the flattening craze couldn't have happened with trad sharpening on oil stones as they themselves are generally not perfectly flat, unless brand new.