Mortising Large Timbers

Ideas about tools and techniques to efficiently cut lots of big mortises in 5x5 posts. April 9, 2008

I have a job coming up where a customer has 185 eastern cedar posts - 5x5. He wants me to make 3- 1" x 6" mortises through each of the posts for the rails. I have an old floor model Powermatic mortising machine, but I believe that the widest bit I can use on this is a 1/2" and I will have to flip the post and do both sides. Although I more than likely will have to do this anyway, what way would you recommend to do this? Would a router and mortising bit be quicker?

Forum Responses
(Cabinetmaking Forum)
From contributor G:
Do the mortises go all the way through? If so, you need a one inch drill bit on a press with a fence to bore six holes and a sharp 1" chisel to clean up the sides.

From contributor K:
3x185 is 555 mortises, so you need a production machine, and I would recommend a chain mortiser to produce the mortise from one side with a hardwood backer to the bottom to minimize break out below.

From the original questioner:
One of these chainsaw mortisers sounds good. Where do I see about one of these? 185 is just really half of the job. Later this spring he will want about this many more. Would this be way too much for a mortising machine?

From contributor J:
I agree, if it's profitable enough, you'll want a bigger mortising machine that can handle a 1" bit. Doing two passes will more than double the time it takes you to do this job. Often those bigger mortisers come up at auctions and used dealers at prices that would be attractive. Don't know where you'd find one new, or whether it would even be worth it for a new one.

From contributor O:
You might want to look at the power tools used by timber framers. I never liked chain mortisers for millwork. Noisy, dangerous, and if you keep the chain properly oiled, it gets all over your primo door stock. I googled it and found lots of timber frame stuff. You might even try EBay.

From contributor O:
I missed my own point there. If this contract is worth it, get a simple, fast, dedicated machine and whack those posts right out. Trying to find a machine that "kind of" does a good job but will serve some multi-purpose roll is not going to work. Hollow chisel mortising that much wood is an exercise in torture.

From contributor C:
Sub it to a CNC shop, and add profit.

From contributor K:
Go to and select chain slotters. These are different from mortisers in that they cut with the grain, not across the grain like a mortiser. I don't think a mortiser will cut smaller than a 1-1/2" wide mortise, but the slotters will cut as small as a 3/8" wide slot up to 3/4". You may need to make 2 passes to cut 1" wide slots. These machines work pretty fast. My mortiser will cut a 1 1/2" x 6" mortise in about 2 minutes in white oak timbers.