Bit Choices for a Slot Mortiser

Woodworkers discuss mortiser tooling. May 21, 2009

What type of cutter/bit should you use on a slot mortiser? Specifically the kind offered with an SCM jointer/planer. I recently saw what looked like a brad point being used, but this seemed to cause bumps in the side when the slot was cleared left to right after boring overlapping holes. Shouldn't there be a cutter that has a better side cutting action? I cannot imagine a router cutter because the speed is too slow.

Forum Responses
(Solid Wood Machining Forum)
From contributor J:
I use spiral upcut router bits. The biggest problem with the SCMI combination jointer/planer/mortiser units is that the cutterhead spins the wrong direction to use standard right-hand cutters. I wired a reversing switch into mine so that I can use standard bits. If you don't want to get into that, left hand end mills are available; they cut slower than bits intended for wood, but they do work.

From contributor S:
Isn't the speed of the cutterhead too slow for a bit like that? I say cutterhead because the chuck is on the same shaft as the jointer cutterhead.

From contributor J:
It's not the speediest thing in the world, but spiral router bits and end mills both work and cut cleanly. SCMI could probably sell you some cutters especially designed for the machine. So could Laguna Tools, since their combination machines are set up the same way. I use the spiral bits because they're cheaper and available in a much wider variety of sizes. A lot depends on the size of the mortises you're cutting; most of my work is rather small.

From contributor S:
Have a look at JSO. It is a German company that makes drilling tools. I use them on my CNC machine, but they could be used for mortising and drilling.

Door production on CNC machine, see lockset cutter Z2. Or drilling and routing tools for other machines - slot mortise bits are listed there as well. To order contact Guhdo USA.

From contributor K:
Onsrud makes bird's mouth style mortising bits that can be used in either rotational direction. You can get them through MiniMax as well as any tool supplier that deals with Onsrud. Most slot mortisers run at 3450 rpm, probably slower than your cutterhead shaft. Metal working end mills work well if oriented correctly to your shaft rotation.

From contributor R:
I have the same setup except I have Felder. I use bird mouth mortising bits. They work well at your machine speed and they don't care which way your machine rotates.