Plunge router mortise woes

[Myles Moran]

I used to cut all my mortises with a plunge router and an edge guide. It works well enough, with the only downside being marking all of the mortises out and then having to very carefully sneak up on the marks while routing each one.

So when I started a set of tables that have a whopping 160 mortises between the two of them (I really should have counted them out before finalizing the design). Given the amount of time this would take, I decided to finally break down and build a jig to make the router mortises.

After browsing the internet, I decided to build a jig that uses a bushing to guide the router. The bushing rides in some 1/2 Baltic Birch, and the rest of the jig is built out from 3/4 Baltic birch. The first runs went great, cutting all of the end grain mortises. I set up a fence to quickly clamp the pieces in position, and knocked out 80 mortises in like 2 hours. Never before would I have been that fast and accurate. Today I went to cut the side grain mortises. I had basically the exact opposite results. If I made multiple passes, the mortise ended up too wide with steps on the side. If I plunged the router home and abused my 1/4" upcut carbide bit, I was able to get a properly sized mortise, but it was very fuzzy.

Not exactly happy with the results, gave the jig a look over to see what was going on. The bushing was nice and tight in the jig.
With my finger between the bit (router unplugged) and the jig where I clamped the wood, I couldn't get the router to move enough to feel that level of bit movement that I was seeing. If I pushed hard enough on the router the whole jig flexed, but as far as I could tell it was rock solid relative between the bit and the clamped wood.

Has anyone made a similar style jig and had similar results? Everything I've seen online seems to be with excellent results, not messed up mortises that make you want to chuck the project across the shop. I'm not sure what is wrong or what I could do to fix it.

As far as fixing the table, I'm thinking I can go back to the old style setup I have successfully used with the edge guide and upsize the mortises from 1/4" to 3/8. At least for the aprons. I also have a lot of 3/4x1.5" pieces that made up a shoe rack at the bottom of the tables. They have 3/4" x 1/4" mortises currently, and I'm not sure if there would be enough room to upsize those to 3/8" thick without the mortises between them taking out too much meat.

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Edit- the only thing I can think of is that I set the jig up so the router plunge rods run parallel to the mortise, as opposed to the edge guide that has them perpendicular. I'm not sure if they could have enough play in their movement to do this, but I can see that the perpendicular arrangement would be more stable against this sort of racking.

[Robert Hazelwood]

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Edit- the only thing I can think of is that I set the jig up so the router plunge rods run parallel to the mortise, as opposed to the edge guide that has them perpendicular. I'm not sure if they could have enough play in their movement to do this, but I can see that the perpendicular arrangement would be more stable against this sort of racking.


Most plunge routers have a noticeable amount of play when the plunge lock is released, but tighten up when locked. So I try to plunge straight down, release the plunge lever to take out play, then move the router along the mortise length, back to where I started, and withdraw. Then adjust depth stop to next setting and repeat. I always plunge in the same spot to minimize the overall effect of the plunge mechanism play.

The other thing you need to look for is something that is not flat, in your jig, router base, or work piece. If things can rock side to side even a little you can get a pretty big discrepancy at the tip of the router bit.

[glenn bradley]

Since you initially had success I would inspect the jig. Has the hole worn? Is the surface that the router rides on still flat? If this all checks out unplug the router, remove the bit, remove the collet and clean everything. Reassemble and make a test run. Better? This is a quick and simple way to eliminate any irregularities that may have developed over the many mortise operations you did so far.

[Myles Moran]

Okay, so I took a closer look at everything, and took another (test) run at cutting a mortise.

As far as I could tell, the router baseplate was flat. The top piece of 1/2" Baltic birch (where the router sits) was flat within 1/64" (contact would be at the edges, so the router wouldn't be necessarily rocking). The bushing still runs nice and tight in the guide slot. What I did notice is that the top has enough flex (probably because of the bushing slot) that if I were to push the router down on the unsupported portion of the top there is a chance it could tip. I thought I was being careful to hold the router on the supported side, but it's possible I was not, and that caused the top to flex and let the router tip.

The other thing I noticed was that the plunge mechanism definitely seemed to have some play. With it locked I was able to flex it enough to visibly see the non-locking column move. Not sure how normal that is, but I'd have to guess it's not as much of a factor in that 1/2" plywood flexing.

As far as the test mortise, I had the same issue with the side stepping. This one I ran with the router turned 90° from before. The ends where I plunged were cleaner and there was less runout during the plunge, but the movement with the plunge mechanism locked in place definitely had the same issue. To me this means my theory about the router is only a factor when I'm actively plunging, but the jig is causing the issue with the mortises.

[Jim Dwight]

They are not cheap but your project sounds like a potential justification for a Domino. I have the XL and it is by far my favorite way to make mortises (I have used a plunge router and still own a hollow chisel mortiser but I do not use it any more).

[Myles Moran]

They are not cheap but your project sounds like a potential justification for a Domino. I have the XL and it is by far my favorite way to make mortises (I have used a plunge router and still own a hollow chisel mortiser but I do not use it any more).

I would definitely love to have one, unfortunately my bandsaw and router table are in far more desperate need of upgrade. My router has been good enough at least until I built this jig. I think I'm going to have to put the domino on my shopping list after the bandsaw and router table but before I get a lathe. In the mean time I think my jig just needs to be completely rethought.

[John TenEyck]

Build a horizontal router mortiser and you will never have this problem again. I think you posted earlier about this project and I provided a link to one you can build from free plans for less than $100.

With regards to your problem, end grain and side grain behave differently. Even with a perfect setup 1/4" bits will flex more than 3/8" ones, etc. Also, you want to use the shortest bit you can that will reach the mortise depth you need. With a 1/4" bit there's no real benefit of making the mortise deeper than 3/4", so a bit with 1" cutting length is a good choice.

John

[johnny means]

A picture of the jig might help.

[Myles Moran]

A picture of the jig might help.

You're probably right haha, in my rage at the messed up mortises I didn't think to take a photo of the jig too.

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Pretty common design I've seen online. Basically a 3/4" plywood box with a 3/4" plywood front cut for the matchfit clamps - those I can say are awesome and I'm glad I didn't go with toggle clamps. Maybe my slot is too long, thinking about it more I can't ever see myself cutting a mortise wider than 4" so I probably should have used that dimension to drive the width instead of looking at photos and saying "that looks about right".

I'm thinking that if my problem truly is related to the 1/2" sliding top that the bushing rides in that I basically need to replace the sub top so the router edge guide can ride on it and it has stops for either the router or the edge guide.

[Charles Lent]

A couple of suggestions -

1/4" bits will flex under side loads, so you need to minimize the forces that you apply when routing slots. Plunging straight in many times to the full depth first to clear out the bulk of the wood, and then going back with side ways motions at increasing depths to clean out the mortise will significantly reduce the side load seen by the bit.

Putting an Arrow mark on the top of the base of your router, and then always pointing this arrow in the same direction will help remove off center guide bushing errors from the mortise, since they will always be in the same direction. You still need to do the centering calibration, but any error left will just shift the mortise location slightly and not change it's width.

If you have enough wood left to go up in bit size to 3/8" you should experience less bit flexing, but I would still do the straight in full depth plunges to clean out the wood before moving side to side to clean out the remaining wood, and it would be best to do this at several increasing depths as well.

Even when using an Leigh FMT Pro jig the manual recommends plunging to clear out the bulk of the wood before moving at depth to clear out the remaining wood. I think you will get much better results, even with the 1/4" bit, if you follow my suggestions.


Charley

[Tom Bender]

When cutting the side grain mortices the wood will come out in chunks because the bit is facing end grain. It doesn't cut so much as it grabs the end grain and yanks off a chunk. Assuming you are cutting a 1/4" mortice with a 1/4" bit in one pass you are climb cutting on one side. This makes one side smooth except that it can yank the bit forward. This all adds to a jumpy cut.

A couple things that can help;

If you are using a bit with a 1/4" shank, go to 1/2" if you can. This in addition to miles's suggestion.

Make two passes, the first with a smaller diameter bit, 3/16". The cleanup pass with the 1/4" bit should be much smoother.

[Rich Engelhardt]

Did you use a centering cone to make sure your base /bushing is centered?
If not, that's the first place I'd start.

[Edwin Santos]

What size bushing are you using?

I ask because a possible problem could be inadequate clearance for the waste being generated by the bit.

[mreza Salav]

This is the jig I built many years ago and used on several projects (before buying Domino). It works perfectly and produces flush joints everytime. no bushing or clamp needed.

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