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deep mortise bit3/21
We are cutting 1.5" deep X .5" mortises in solid wood. (hickory cherry and maple) Can anyone share best router bit recommendation that has worked well for them? I am using a standard compression cutter that is designed for sheet stock and although it gives us a good finish we get some chatter and I feel a cutter with a shallower helix might be better? We are using a Thermwood CS45 router.
If the mortises need to be dead on .500 wide, get an Onsrud 60-907 heavy hogger and have it sharpened down to .485 or so. It gets you the cutting length you need, and the larger diameter will minimize chatter. It won't give you the best finish, but if it's a mortise for gluing it won't matter. If you need a better finish get a 60-957, which is a chipper / finisher. These are both upcut bits, for use only if your hold-down method can hang onto the parts. Downcuts are also available, but you'll generate a lot of heat if you're packing chips into your mortises. If you haven't tried a hogger / chipbreaker, they're awesome at devouring wood quietly.
Thanks Jared that helps. I am cutting both .474" and .551" mortises 1.5" deep. I have been using a .5" compression cutter that is sharpened down to .460" The mortises are for loose tenons so finish is not a concern. I will try the serrated tooth design. I hate to have a new cutter ground down but it looks like that is the only option to get the .47" mortise 1.5" deep.
Machinist supply places (MSC, etc.) have end mills of more types, sizes, etc. than you can imagine. There are 15/32" end mills with 2" loc., and .5" shank that might work for you. There are roughing (serrated) versions, too, if you want to go that way - I'd be a little concerned about compromised glue surface, however. I've used 1" dia. 6" loc mills to mortise small timbers on my router and have had good results - but as Jared notes, be sure your fixturing is very secure when using up-flute bits. Chatter should be avoidable with less aggressive cut levels (again, as long as the fixturing is good). I don't think a down-flute would be advisable - lots of chips and heat.
Definitely an upcut bit. I am not familiar with Thermwood, but are you cutting this in one pass or are you stepping it down? I always cut a mortise like this is two or even three passes. I might even stop the route to clean out the mortise.
Paul, I do cut the mortise in multiple passes and have gone to climb cut and this seems to reduce the chatter. I think the noise I am hearing may be because of the compression cutter not clearing the chips as it is the full depth pass where I hear it more. We have a vacuum table so I have a jig I built to hold the parts and it is vacuumed down to the table. It seems good and solid but it could allow some vibration I suppose.
We cut mortises that deep regularly. For 1/2" and wider we use a 1/2" 60-909 3 flute hogger. For the 3/8" and up mortises we use a 60-037 3/8" 3 flute hogger. The cut depth is limited to 1.125" but you can just cut the first 3/8" depth at -.005 stock to be left. This will create the clearance for the cutter shank without compromising the joint.
So you over cut by .005 for the first .375" depth? Or am I not understanding.
That is correct. It is a way to create the mortise with a stock bit that has a limited cutting length. In your case the the depth of cut for the mortise exceeds the cut length of most of the off the shelf cutters. The initial 3/8" deep cut is to create clearance for the cutter shank. You can dial it down to just a couple of thousandths if needed.
On another note, we generally use a helical toolpath when making mortises.
Thanks Gary now I just need to find how to do a helical toolpath with Aspire.