Making a Through Mortise

[Jeff]

I'd like some suggestions on how to make a through mortise using a plunge router without "blowing out" the back side. I have a homemade mortising jig that is pretty accurate, but I've only used it to make 3/8" blind mortises.
:dontknow:

I'll probably not attempt to square up the mortise because I've found it easier to just "round" off the tenon to accommodate the mortise.

Thanks in advance.

:help:

 

[gator]

I think that the simplest way would be to use a backer board.

George

 

[Jim Murphy]

I have a homemade mortising jig that is pretty accurate, but I've only used it to make 3/8" blind mortises.

If you can set up your jig with the proper stop blocks to ensure registration, couldn't you just plunge halfway, flip the stock, and keep going...

At least try that approach and report back. If it fails, re-think your jig positioning methodology.

 

[Alan in Little Washington]

Backer is fine but not always reliable if you want a clean, exposed mortise like on A&C furniture. In that case I usually just cut the mortises from the "front", visible side. I do it carefully so any blow out is minimal and covered by the tenon shoulders.

Also, a few more tips- either slightly taper the mortise or tenon so they only fits tight where the tenon it exits the front of the mortise, and so they can slide together most of the way easily.

 

[PeteM]

You can route the mortise about 2/3 -3/4 the thickness of the board then drill a hole and use a flush trim bit from the other side and clean up the corners with a chisel. A bit more work but no tear out.

pete

 

[fergy]

The other tip I've heard, but not personally done, is to set your depth so as to leave a super-thin amount of wood on the front side, then carve out from that side by hand.

 

[WoodWrangler]

Can't imagine a backer board securely seated wouldn't do the trick.

 

[DavidF]

Alan's tip is the way I have used, go in from the front with a backer board to minimise the tearout, which the tenon shoulders would cover. If one could remember at the time, you could leave the piece on the thick side, cut the mortise and then plane to final thickness. Similar to when through DTs show on the front of a chest drawers for the dust panels; you normally cut the sliding DT sockets and then rip to final width leaving a perfect surface.

 

[Joe Scharle]

Drill thru the upper and lower ends and rout out the waste from both sides.

 

[PeteM]

It's reassuring how we all agree on the best way to do this !!!!! :confused_ :gar-La;

 

[JimD]

I have done it from both sides and it will work if your setup is right. It is difficult to get exactly in the same place from both sides in my experience.

I normally use my dedicated mortiser and just set it up to almost come through. In practice, the remaining material is so thin I can just cut it out with a knife.

I have also left a bit more thickness before and just used a 1/4 inch flush trim bit to remove the remaining material with my Bosch Colt. This process should also work fine with a plunge router.

Jim

 

[dancam]

Backer board on the inside but put a piece of clear packing tape over the back side of the mortise before clamping the backer board. I've done it this way a few times and had little or no tear-out.

Dan C.

 

[DavidF]

Backer board on the inside but put a piece of clear packing tape over the back side of the mortise before clamping the backer board. I've doe it this way a few times and had little or no tear-out.

Dan C.

That's a good tip Dan!

 

[DavidF]

If you can set up your jig with the proper stop blocks to ensure registration, couldn't you just plunge halfway, flip the stock, and keep going...

At least try that approach and report back. If it fails, re-think your jig positioning methodology.

I've found that it's not generally a good idea to try and go from both sides even with setup blocks, it just never seems to work perfectly. Then you have to fettle the mortise and end up widening or spoiling one opening DAMHIKT

 

[Alan in Little Washington]

Can't imagine a backer board securely seated wouldn't do the trick.

Depends on who is doing it! :BangHead::BangHead::BangHead:

Just like the time I ended up with a few more sets of through tenons because the stop on my mortiser slipped and I blew all the way through one leg while making a blind mortise. :embaresse I just made some lemonade, and only I know they weren't supposed to be through tenons!

 

[Ozzie-x]

Backer board on the inside but put a piece of clear packing tape over the back side of the mortise before clamping the backer board. I've doe it this way a few times and had little or no tear-out.
Dan C.

Good tip Dan. Taking your tip a step further, since I use a lot of walnut, I use a lot of masking tape that helps with splintering at cuts AND (most important benefit) helps me to be able to actually see my layout marks, I just can't see these fine marks well enough (any more) on the walnut wood itself. I also keep a bottle of super glue on the bench to immediately glue back any pesky splinters that happen to pop off, I always seem to get some of these especially when I'm dry fitting.

Back to the through M&T's- I've done lots of these over the years, and I just cut from both sides of the workpiece. If the stock is square and the mortiser is set-up properly, and you use good technique, it should work fine. A lot of folks never do a proper set-up on their new mortisers, and suprisingly, very few new mortisers are true/correct from the factory, you have to set them up for accuracy, see Tom Hintz's site for a tutorial. Sharp bits are also important. Do a careful layout of the mortises on both sides of the workpiece and use the same edge as a point of reference to the fence, and just cut them from both sides of the workpiece. Any time you can avoid another tool or process to complete an operation, you lessen the goof-up risk. Following is an example of one of the through M&T's in one of the legs of a pair of walnut nightstands I made. The project had 12 total through M&T joints showing on the front reverse tapered legs, and they turned out fine cutting from both sides of the legs.

 

[merrill77]

No one else has mentioned this...which probably means it isn't a very good idea. But it worked for my on my one-and-only attempt - through tenons on a bench.

I didn't have any sort of mortising jig and my horizontal router table setup wasn't going to work for a mortise in the middle of a large panel like the bench. So I drilled two large holes and cut out most of the waste with my jigsaw. I then used a homemade jig and router bit to clean up the edges. Since there was no plunging down involved, there was no chance of blowout.

YMMV, IANAMW, etc.
Chris

 

[Jeff]

Thanks for the input folks. I'm gonna have a go at it later today. With all of the tips that have been offered I hope to find one (or a combination) that'll work well in my hands (if I'm not my own worst enemy).

I can appreciate the idea of blowing through a blind mortise when the stop guide slips. I've had that happen with my plunge router, but not a fatal error in my case. I like to consider these little mishaps as the individual craftsman's signature which few others would recognize.

I'll keep ya'll posted. :icon_cheers

 

[dancam]

"I like to consider these little mishaps as the individual craftsman's signature which few others would recognize."

I bet there are a whole bunch of these individual craftsman's signatures out there...I know there are quite a few coming out of my shop.

Dan C.

PS...Jeff post a pic(s) of your through mortises

 

[Jeff]

Well your tips and advice has worked nicely for cutting 3/8" through mortises without back-side blowout. Here's what worked for me:

Use a backer board and put painter's tape on the backside of the piece as well. After plunge routing the mortise simply peel off the tape and voila! Very clean. :icon_cheers

The taped side would be the outside face of the workpiece so any imperfections on the inside face will be covered by the tenon shoulders.

I have some pics to post within this thread, but I haven't figured out how to import them from my Mac to this site. HELP please. :dontknow:

If I wanted to wedge the tenons how would I get the very small wedges needed? Can you buy them somewhere?

Again, thanks for the tips and advice.