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

UKworkshop.co.uk

Help Support UKworkshop.co.uk:

This site may earn a commission from merchant affiliate links, including eBay, Amazon, and others.

donturner

Established Member
UKW Supporter
Joined
27 Feb 2023
Messages
36
Reaction score
37
Location
Leatherhead
The back of my mortice chisel is not completely flat. Specifically, the corners of the cutting edge are both slightly shallower than the rest of the back face, meaning they aren't honed to form a sharp edge. Here you can see the remnants of permanent marker left after dragging it across a fine diamond stone.

1709401697747.png


Here's my process for getting the back flat (it is essentially this method from Paul Sellers but with plate glass instead of a granite block):

- Stick wet and dry sandpaper of 120, 180, 240 and 320 grit to a piece of plate glass
- Wet the sandpaper using glass cleaner (it's just what I usually have to hand instead of water)

1709401738128.png


- Drag the chisel backwards over each grit of sandpaper a few hundreds times, working my way up through the grits
- Move to fine, then extra fine, DMT diamond stones

1709401788088.png


No matter how hard I try, I cannot get those corners to be flat with the rest of the back. What am I doing wrong?

About the plate glass
I have checked that the glass is flat using a straight edge (manufacturer claims tolerance is 0.042mm over 100mm). Perhaps the glass isn't truly flat and my straight edge tolerance isn't high enough to pick it up.

It is supported from below with rubber strips glued to almost all of the underside

About the chisel
It's a Narex 12mm Mortice chisel

Bonus question: What's the best way to stick sandpaper to glass? I'm using contact adhesive but it doesn't hold for longer than a few minutes.
 
Get some engineers blue or possibly a sharpie pen and find out where the high spots are. Don’t move on to the next grade until they are all gone. Wet sandpaper won’t make this easy.

I stick my sandpaper to my table saw table and do it skewed at about 45 deg.
 
I would say the issue is the wet and dry paper, if you are using the "scary sharp" system it must be glued down flat. I would say the 120 grit is too coarse a grit and I can see its not flat, it's curled up on the edges, this is causing you the issues. Also when you have a coarse grit it rounds the edges of the chisel, if you stayed with a coarse grit you would not notice it, but as you work up the grits all the tolerances get finer and you start to see the problems.

I have tried wet and dry but now only use pre glued micro finishing films, designed for fine tolerance grinding.

https://woodworkersworkshop.co.uk/scary-sharpening/?sort=bestselling

My dvd/download explains these issues and more.

https://woodworkersworkshop.co.uk/peter-sefton-chisel-and-plane-sharpening-download/

Cheers

Peter
 
Doesn't need to be flat. Don't waste your time. Flattish will do. It's a mortice chisel not a scalpel.
Bonus question: What's the best way to stick sandpaper to glass? I'm using contact adhesive but it doesn't hold for longer than a few minute
Best to not stick it down at all.
Use thin paper backed (cheapest) wet n dry paper, very wet, with white spirit etc. It's designed for the job - there's a clue in the name!
Pour a pool of white spirit on to your glass, lay on the paper and pour more on top. After a few passes it will lie stuck down and very flat. Easy to remove and re-use, it also cuts much faster when wet. Dry is totally unsuitable for this job.
Having said that - I really wouldn't bother flattening a mortice chisel. Flattening can rescue a badly made plane sole but other than that is usually not necessary (within reason).
Modern sharpening is mostly just a crazy fashion, time wasting and really boring.
 
Last edited:
The cause is your abrasive isn’t lying flat and the curve up at the edges is causing what you see.

3m repositionable spray glue is decent- but you don’t need to obsess on the finish.

The question you haven’t answered is does the chisel cut a good clean mortice. The time spent practicing mortice cutting will likely yield a greater improvement curve than chasing microns on sharpening.

Sharpen for 5 minutes - work for 90 minutes - helps to think about the skills you need to spend time developing
 
Thanks for all the responses and suggestions. A few specific comments:

The cause is your abrasive isn’t lying flat and the curve up at the edges is causing what you see.
You're right, however, I don't understand how this would cause the issue. When I push the chisel down onto the curved edge of the sandpaper, it lies flat against the glass. The only extra upward force is from the curvature of the "not stuck down properly" section of sandpaper which I'm assuming to be negligible. What am I missing?

when you have a coarse grit it rounds the edges of the chisel, if you stayed with a coarse grit you would not notice it, but as you work up the grits all the tolerances get finer and you start to see the problems.
I had no idea that coarse grit rounds the edges. I'll try again starting with 240 grit instead of 120 grit.

Pour a pool of white spirit on to your glass, lay on the paper and pour more on top.
Thanks for the tip, I'll give this a go.

The question you haven’t answered is does the chisel cut a good clean mortice.
Not really because the edges aren't sharp. It requires a lot of whacking to get to depth and the top and bottom walls don't cut nicely.
 
....

Sharpen for 5 minutes - work for 90 minutes - ...
I tend to think work for 5 minutes sharpen for 30 seconds*. A little and often - and forget "flattening".
* PS or thereabouts, maybe 10 seconds - just a quick hone, turn and quick take off the burr. A bit like drawing with a pencil - you keep freshening the point.
 
Last edited:
..

Not really because the edges aren't sharp. It requires a lot of whacking to get to depth and the top and bottom walls don't cut nicely.
May be technique? First cuts are shallow but each cut pares off the vertical face of the cut before and goes deeper each time. Chisel always vertical, no levering. Leave the ends of the mortice until last, for a final paring cut
 
Last edited:
You're right, however, I don't understand how this would cause the issue. When I push the chisel down onto the curved edge of the sandpaper, it lies flat against the glass. The only extra upward force is from the curvature of the "not stuck down properly" section of sandpaper which I'm assuming to be negligible. What am I missing?
The curve only needs to exert a tiny bit of upward pressure to cause this. It is a very common problem. It is especially prevalent with coarse papers.

What is happening is that the paper bends up at the side of the chisel. This removes a tiny bit of the side, more with each pass, and the curve gradually develops. It will get worse the more you work at it.

For initial flattening you can use carbide grit straight in to a sheet of glass with some water to make a slurry. This eliminates the problem.
 
....

For initial flattening you can use carbide grit straight in to a sheet of glass with some water to make a slurry. This eliminates the problem.
or paper-backed wet n dry used very wet. It's designed for the job. Also it doesn't destroy your flat plate. I use the out feed on my combi.
 
We know your views about how flat the back of a chisel/plane iron needs to be
The question was why cant I flatten the back not do I need to
Well I answered that too! Basically you are overdoing it big time but it's probably the coarse stuck down paper that causes the most problems
I'll repeat: don't stick your paper down, instead use thin paper-backed wet n dry dropped into a pool of white spirit on your flat base, then wet it on top too, very wet not just a spray. Keep it running wet. It'll lie dead flat and stick well enough for your purposes. It might need pressing down a bit if it started curled up. Best stored between boards to keep it flat
You don't need all those grits and diamond plates either. Just one grade of paper, say 120, is all you need.
Then hone in the normal way on a fine stone, at which point you start losing flatness little by little as you take off the burr, but it doesn't matter.
Modern sharpening is crazy. These are not scalpels for brain surgery, or samurai swords!
I did wonder about your mortice chisel technique too, but that's another topic!
 
...
You don't need all those grits and diamond plates either. Just one grade of paper, say 120, is all you need.
PS or start with 80 for speed, but not beyond 120.
 
There's a couple of reasons why flattening on wet and dry can be troublesome. As said previously it only takes a small amount of curling to make things out of flat, especially as sandpaper is compressible. Another factor that contributes is that silicon carbide breaks down quickly, you tend to wear out the middle of the abrasive much quicker than the edges.
Sandpaper on glass has it's uses but I think it's pretty naff for general sharpening.

If you are flattening by pulling the chisel backwards, also make sure to start off overhanging the edge, or else you will get a belly from front to back.
 
There's a couple of reasons why flattening on wet and dry can be troublesome. As said previously it only takes a small amount of curling to make things out of flat, especially as sandpaper is compressible.
Which is why thin paper-backed wet n dry is best (used very wet).

Sandpaper on glass has it's uses but I think it's pretty naff for general sharpening.
Agree. Has its uses though - 80 grit is good for flattening plane soles (used as described!)
 
For what it's worth my observations are:- Once the chisel has rounded slightly you won't be able to hold it flat and it will roll. The narrower a chisel is the more likely it will roll, the thicker the chisel the harder it will be to maintain consistent central pressure again causing roll. Wet and dry (in my opinion has issues with the surface tension of the liquid in the middle causing a higher point in the middle causing roll. Float glass bends under pressure you need at least 12mm thick or a flat strong support underneath. The other observations about glue down etc are also valid. So by the time you add it all up it's not a very satisfactory way of flattening despite all the support for it. I'm not a fan you can probably tell.
 
.... Wet and dry (in my opinion has issues with the surface tension of the liquid in the middle causing a higher point in the middle causing roll.
News to me. Never happened so far. Can't see how it would happen anyway. In fact as you use it it gets flattened down tight and the white spirit squeezed out. Are we talking of the same stuff i.e. thin paper backed, not cloth?
Float glass bends under pressure you need at least 12mm thick or a flat strong support underneath.
agree. I use the out-feed on my combi
The other observations about glue down etc are also valid. So by the time you add it all up it's not a very satisfactory way of flattening despite all the support for it. I'm not a fan you can probably tell.
Glue adds thickness and holds irregularities.
Wet mounted the thin paper settles as flat as it can go. And is easy to lift off, dry, and store between boards for re-use.
n.b. I use whole sheets as it comes (A4?) and don't cut anything. A No7 plane sole and I use 2 whole sheets edge to edge.
Sorry to boring and going on about this repeatedly, but it does work really well and I'm continually amazed by the struggles people go through doing these very simple routine things, as per our OP.
 
Last edited:
It’s an interesting idea @Jacob. I will give it a go. In the past I’ve used sticky sandpaper stuck to my table saw table, or spray glue on an old cast iron surface plate that I never really managed to get off. Sticky sandpaper is expensive and remarkably hard to find (when I looked).

The downside I can see, is you don’t want any sparks.
 

Latest posts

Back
Top