what am i doing wrong?

[Nick]

The opposite I think. Forstner bits clear chips poorly. No clue about the glazing. Sounds shiney to me. Shiney is usually good, no?

[stummel bum]

I would think shiny outside good, shiny (burned) inside bad. You want it smooth, but not shiny.

[KurtHuhn]

What's the disadvantage of a glazed wall?
And what's the difference between a forstner bit and a spiral bit as far as mortise drilling is concerned? Does the forstner bit clear chips more efficiently than a sprial bit?

Glazed wood is burned wood. It's also very unstable in moist environments since you've squashed the fibers rather than cut them. In a mortis it could expand to the point where the stem would be near impossible to remove.

Forstner bits are very poor chip-clearing bits. A standard twist drill bit is much better at clearing chips. When I use a 5/16" forstner bit for a mortis, I usually only advance the bit one rotation of the tailstock feed at a time, clearing the chips in between.

[m.c.]

Thanks. I did some experiment last night. Yes, clearing the chips is a must! Otherwise the bit squeaks badly in the hole. And being slow is the key, both RPM and advance! However, I still find some slight taper when measuring it with the calipers, about 0.2mm-0.3mm in difference between mortise rim and bottom. I will experiment more to figure out.