Delta Mortise machine

[rcplaneguy]

Do the sides have to be flat beyond the cutting edges?

[forrestb]

Do the sides have to be flat beyond the cutting edges?

No, but to get those edges sharp they have to be flat on the outside. They only need to be flat at the edge so you do not have to flat the entire side of the chisel just as you don't need to on a plane blade. Other chisels are a different story depending on how and for what you use them.

On a hollow chisel I find that the relatively short side at the end is not too difficult to flat. Check out this video (Oh! if were just this easy ):

http://www.finewoodworking.com/tool-gui ... -bits.aspx

Forrest

PS Full Disclosure: I have yet to cut a mortise on my Shopsmith!

[JPG]

Forrest,
Checked one side of the 1/2" chisel. It's not flat. That's enough for me. They'll all get flattened, now.
I'd have never thought to check them if not for your post. Thanks again, friend.

I be curious what the width(corner to corner) was prior to 'flattening'.

I'm guessing the bit cuts to the nominal dimension at the middle of the flats.

That implies there is nothing there for the chisel to cut in the middle of the 'flat'.

Not sure reducing the 'width' of the 'flats' to make the sides 'flat' is desirable. I'll go along with polishing.

[Gene Howe]

You might be (probably are) correct. Went and checked the 3 sides, and the flat area is about 3/4" up the chisel from the cutting end on all 3 sides. I guess a little narrower near the top might be a good thing, actually.
The mill marks are a bit disconcerting, though.
I'll still give it a couple swipes @3k.
When making mortises, I've never been overly concerned with their actual dimensions as I make the tenons to fit, regardless.
In my woodworking, very little is set in concrete.

I be curious what the width(corner to corner) was prior to 'flattening'.

I'm guessing the bit cuts to the nominal dimension at the middle of the flats.

That implies there is nothing there for the chisel to cut in the middle of the 'flat'.

Not sure reducing the 'width' of the 'flats' to make the sides 'flat' is desirable. I'll go along with polishing.

[forrestb]

Forrest,
Checked one side of the 1/2" chisel. It's not flat. That's enough for me. They'll all get flattened, now.
I'd have never thought to check them if not for your post. Thanks again, friend.

I be curious what the width(corner to corner) was prior to 'flattening'.

I'm guessing the bit cuts to the nominal dimension at the middle of the flats.

That implies there is nothing there for the chisel to cut in the middle of the 'flat'.

Not sure reducing the 'width' of the 'flats' to make the sides 'flat' is desirable. I'll go along with polishing.

JPG is correct that the bit should be cutting to, or very near to, the nominal dimension at the very edge of the chisel so chisel sharpness in that area is not critical to a clean cut. But just a small amount away from that, you have the chisel doing the work all the way up to the tips and that is where the 'grooves' were on my chisel in the pic above. So a good portion of the chisel needs to be sharp IMHO.

I don't know how to polish a non-flat surface without distorting the surrounding area But I am willing to learn!

And yes, you do not want to reduce the width of the cutting end to less than the bit diameter.

Also you want to be sure that the chisel sides up the shank from the cutting end are the same width (or less wide) as the cutting end. If it is thicker up to the cutting depth, you will be squeezing wood. Some would be acceptable, but not much. I think that flattening up to the depth of cut would be appropriate to minimize that. In addition, if the sides are flattened (and polished ) up to the cutting depth you should have less resistance during the cut. The Delta 17-910 (maybe just a bad chisel?) was definitely thicker up the shank to the depth of cut.

I sure would like to hear from a hollow mortise chisel designer weighing in on this subject 'cause I surely am an amateur!

Forrest

EDIT: If the upper shank is thinner than the edge obviously don't try to flatten it!

[JPG]

Took a close look at my chisels, and they are smaller above the cutting end. I assume that is to reduce the amount of drag. If so, concavity is a positive in that area.