Calibrating a Hollow Chisel Mortiser

Careful adjustments to the spacing between the chisels and the bit can prevent overheating, smoking, and chisel breakage. January 3, 2012

I bought a used Delta hollow chisel mortiser on an auction. I just installed a new bit and chisel. When I drill a hole in soft maple the bit starts to smoke and gets hot. I've never used a HCM before and wondered if that is right?

Forum Responses
(Furniture Making Forum)
From contributor N:
Try dropping the auger bit a little farther down. The lips should be at least 1/16" below the bottom of the chisel.

From contributor L:
Getting the delta HCM to work well can be a real pain. As suggested the bit has to set just right. At a minimum it high quality bits that have been smoothed on inside and outside and well sharpened. Spraying it frequently with a blade and bit lubricant helps a lot. Also, plunging at the right speed allows the chips to carry away a fair amount of the heat.

Beware: In my case with the Delta the head and bit was not aligned parallel to the ways, therefore when it plunged the bit wanted to move sideways and would bind the wood if it was clamped too hard.

From contributor F:
Personally I think 1/16" is a bit much. Bottom line you don't want the bit scrubbing the chisel as the heat buildup will cause the chisel to quickly turn blue and then crack, then you get to buy a new one.

I set the gap by placing about .008" shim between the base of the chisel and the mortiser before securing it into place. Then I use a block of wood to push the bit tight against the chisel and secure it in the chuck. Then loosen the chisel and remove the shim/s, push the chisel up against the mortise, re-secure, and you will have a precise gap between the end of the chisel and the flukes of the bit. I also keep a block of paraffin handy which I touch to the end of the chisel periodically which helps keep things running smoothly. Proper tuning and sharpening and high quality tools also help as others have pointed out.

From contributor N:
Somewhere, too, I found the following info, generated from testing:
"A gap the thickness of a nickel—roughly 0.072” thick—results in operating temperatures roughly 40 or 50 degrees higher than if you increase that gap to 0.117” (the gap provided by the built-in spacer, on the Powermatic PM701 mortiser, used in their testing"). So I now use a 0.125" gauge block to set the gap. The importance of sharpening and polishing the chisels and bits can't be overstated!