AdamAronson

Mortises for Leg Stretchers

35 posts in this topic

Ouch! Lotsa kickback and nasty mortises...

Here's the post I posted at Router Forums earlier today. I've edited the post for context. In case anyone is interested in following the discussion there I'll PM the URL. Not sure that "cross posting" here isnt punishable by death as it is across the forum world.

My setup for this particular operation was a PC 890 w/edge guide, 8931 (type 3) and a Whiteside 2" spiral upcut carbide bit 1/2" shank.

I am routing the stretcher mortises in ash and even with light passes (1/8") I am experiencing frequent kickback. Mostly this occurs at the end of the cut on either the first pass (routing the 1/2" W cut) or on the second pass finishing the last 1/16" inch). I rout the first channel to depth and then work on widening the mortise - again, 1/8" per cut. I am fresh to routing mortises and certainly have never routed any this deep. I tried various speeds between 10K and 23K to no effect.

Any tips from the more experienced here are greatly appreciated. Googl'ing around all I read are tips to take light passes which I thought I was doing.

Also, the mortises are intact but in some cases widened by the kickback. No way I'm starting over here... what is the suggested approach...? Wedges? Epoxy? I'm also hoping the draw bores will provide needed backup.

Marc, I see that you did the same operation, albeit with a router that costs 3x what mine does. ;-). I heard you mention light passes as well and have been doing that. I also noticed that my kickback was mostly happening when I reached the end of the cut so my suspicion is that the bit is grabbing the end grain at the end of the cut. I've been experimenting with plunging to reasonable depth at start and end first before routing across. Better...

Back to routing...

Thanks!

Adam

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Do I understand correctly, your router bit is 2 inches in diameter? If so then I think that may be your problem. That bit should run at a much slower speed than the 1/2" Marc used and I don't think your PC 890 has the torque for a bit that big.

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Adam, to prevent that catch you're referring to, I plunge full depth at the beginning and end of the mortise on or just shy of the line. Then I make the horizontal passes. That has cured the catching problem for me. I hope that helps.

Tom, I'm pretty sure the 2" is the depth of the router bit. ;)

Also Adam, there has never been a problem with cross posting here. It's been a common practice to refer to link to other discussion, even in other forums.

AdamAronson likes this

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Hi Vic,

I've been using the plunge start and end technique today with better results. Thanks!

And, thanks for the laugh! Tom,Vic is right... The bit is 2" in depth but I'd LOVE to see a hand held router (shaper?) that could handle a 2" width up cut spiral :-P

So, with that said... How do I make use of the wicked mortises I have with chunky evidence of kickback? Epoxy for the glue up for gap filling? Like Marc I am considering the fixed long stretchers (well, I sure am now). Will Titebond hold? Theres still a good amount of long grain walls in place (plus the draw bores). Would shims work?

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Adam, I'd have to see a pic to give an opinion on that. Remember Marc even made a "booboo" on one. With the drawbore method, if there is still good registration, I think you're OK with just the pegs. By the way. I didn't drawbore. I just partnered up some of my longer clamps and used epoxy. If I ever move, which I'm very much against, I can just take the tops and leg vise off.

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I'd say if you have about 75% contact, you're fine with just titebond. If it is severely chopped up, then you might try using epoxy just to be safe. But honestly, with the drawbores, I just don't know how much it would matter. Many folks leave out glue all together when using drawbores.

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Thanks for choking in, Marc. The mortises are tight-ish and hold well so I'm not that concerned about the hold - especially (as you point out) if I add the drawbores. As you I am also a "belt and suspenders" guy ... But I don't have any epoxy around. Should I pop over to the local Ace and grab some west marine?

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If you're drawboring, I think the epoxy is overkill for sure. I used epoxy because I wasn't drawboring and wanted to make sure overtime, mine couldn't rack. Your's wont rack if you do the drawboring correctly.

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Based on what you've said and without actually seeing it, I agree with Vic. I think the epoxy would be more than you probably need.

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I have had exactly the same problem with my current build of the Adirondack chair and the 1/2" x 1:1/8" deep mortises on the leg chairs.

So am I reading this properly and the forum here is saying that at each end of the mortise I plunge the full depth (1:1/8") first and then go back and forth at 1/8" passes and take the rest out ?

That is very different to how Marc suggests we do it in his instructional video (and I have had disaster with that approach).

I am using a 1/2" (diameter) x 1:1/2" deep upcut spiral router bit. What speed should I run that at ?

I have destroyed two legs so far with kickback, done want to create more trash.. Please help.

Cheers Chas

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I have had exactly the same problem with my current build of the Adirondack chair and the 1/2" x 1:1/8" deep mortises on the leg chairs.

So am I reading this properly and the forum here is saying that at each end of the mortise I plunge the full depth (1:1/8") first and then go back and forth at 1/8" passes and take the rest out ?

That is very different to how Marc suggests we do it in his instructional video (and I have had disaster with that approach).

I am using a 1/2" (diameter) x 1:1/2" deep upcut spiral router bit. What speed should I run that at ?

I have destroyed two legs so far with kickback, done want to create more trash.. Please help.

Cheers Chas

Chas, I've had good results plunging at both the start and end of the mortise. I've had problems when the bit engages the wall of it grabbing. It may be the quality of my routers. But, by plunging full depth on each prior to making the passes, the bit is fully encased and therefore isn't prone to what is essentially a minor kickback. I also use the highest speed with my mortising bits. Marc can possibly elaborate on why he isn't encountering the grabbing problem.

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It all depends on bit feed direction and which side of the bit encounters the material. I sometimes do Vic's method of defining the outer areas first. But depending on the wood, I find this causes the bit to overheat quickly. So you have to be kind of quick about it.

So I'm thinking that perhaps this is one of those things that deserves a little more attention and detail. I had no idea folks were having issues with the technique. I'll see if I can drum up a supplementary video.

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It all depends on bit feed direction and which side of the bit encounters the material. I sometimes do Vic's method of defining the outer areas first. But depending on the wood, I find this causes the bit to overheat quickly. So you have to be kind of quick about it.

So I'm thinking that perhaps this is one of those things that deserves a little more attention and detail. I had no idea folks were having issues with the technique. I'll see if I can drum up a supplementary video.

Awesome! Thanks Marc! It does heat the bit up. I forgot to mention that. I'd love to see if there is a better method I can be using.

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Here are some ideas from a non-guildie who isn't sure exactly what you're talking about.

  1. Someplace I read that it's better to think of the router as a trimming tool rather than a tool for removing lots of material. I find that to be good advice.
  2. If I understand correctly, you're trying to cut a mortise that's the same width as the router bit diameter. When you do that, it's kind of like one side of the bit is doing a climb cut and the other side isn't. I've always found that to be a little dicey.

So how would it work to take a page from the Neanderthal book and start by removing most of the material with a drill. Then swith to Normite mode and trim up the sides of the mortise with a router (instead of doing it with a chisel like the Neanderthal would). It seems to me like there would be two benefits to this technique:

  1. With most of the material already gone before the router gets involved, it has an easier job, the cutting forces are all reduced, and the magnitude of any kickback will be lower.
  2. (Probably more important) With most of the material already gone before the router gets involved, you can trim the walls of the mortise with a smaller diameter bit. That way, the bit will only be cutting on one side, and I think that probably eliminates the potential for kickback almost completely. Of course this means you need a template or something to guide the router instead of just using an edge guide, but from what you guys are saying, it would be worth the extra effort to set up the template.

-- Russ

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  1. (Probably more important) With most of the material already gone before the router gets involved, you can trim the walls of the mortise with a smaller diameter bit. That way, the bit will only be cutting on one side, and I think that probably eliminates the potential for kickback almost completely. Of course this means you need a template or something to guide the router instead of just using an edge guide, but from what you guys are saying, it would be worth the extra effort to set up the template.

Seems to me you could use an edge guide - you'd have to set it for the inside edge, and then reset it for the outside edge.

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Seems to me you could use an edge guide - you'd have to set it for the inside edge, and then reset it for the outside edge.

The trouble there is that for the outside edge, you'd have to be double dang sure to hold the edge guide firmly against the workpiece throughout the cut so that the bit didn't stray into unwanted territory. (For the inside edge, the edge guide itself would prevent disaster; if the edge guide came away from the workpiece momentarily, the bit would just move harmlessly into the mortise cavity.)

So I guess if you weren't confident about your ability to hold the edge guide firmly against the workpiece, another approach would be to run the edge guide along one side of the workpiece for one side of the mortise, then flip everything around to use the edge guide along the other side of the workpiece for the other side of the mortise.

I think I'd still make a template. :)

-- Russ

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Thanks for the response Mac!

If I understand correctly, you're trying to cut a mortise that's the same width as the router bit diameter. When you do that, it's kind of like one side of the bit is doing a climb cut and the other side isn't. I've always found that to be a little dicey.

It seems to me that any time you're trying to rout a mortise the first pass is going to involve the cutter climb cutting on the opposite edge. Right?

A

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Yes indeed. But the effect is reduced by taking light passes at multiple depths as you work your way down. The deeper you go, the more likely you are to feel the effects of the climb cut.

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Perhaps it would be safer to set up straight-edge guides on both sides of the router when doing deep mortices?

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Perhaps it would be safer to set up straight-edge guides on both sides of the router when doing deep mortices?

Yeah. Almost like a template! (Okay, no more smart-ass remarks for me this weekend. I promise.)

-- Russ

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It seems to me that any time you're trying to rout a mortise the first pass is going to involve the cutter climb cutting on the opposite edge. Right?

Right, except that as long as the bit is as wide as the mortise, the climb cutting occurs on all the passes, not just the first.

-- Russ

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Right, except that as long as the bit is as wide as the mortise, the climb cutting occurs on all the passes, not just the first.

-- Russ

And this is what I noticed. However, the start and end of the mortise was where things went nasty. What greatly reduced the possibility of the kickback was the full depth plunge at the beginning and end then working slowly between them. Vic also suggested this.

A

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Right, except that as long as the bit is as wide as the mortise, the climb cutting occurs on all the passes, not just the first.

-- Russ

In this particular instance, the mortise is actually wider than the bit.

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And this is what I noticed. However, the start and end of the mortise was where things went nasty. What greatly reduced the possibility of the kickback was the full depth plunge at the beginning and end then working slowly between them. Vic also suggested this.

A

From my experience, when pushing from left to right, there really isn't much of a tendency for kickback at the far end of the cut. At least not any more of a tendency than when making the standard pass from left to right (which I find to be minimal). But if you go back to the start of the cut and try to finesse that starting point, you could very well run into a heap of trouble because you are climb-cutting. So I would only work that starting area with a plunge and not a pull, if that makes sense.

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It all depends on bit feed direction and which side of the bit encounters the material. I sometimes do Vic's method of defining the outer areas first. But depending on the wood, I find this causes the bit to overheat quickly. So you have to be kind of quick about it.

So I'm thinking that perhaps this is one of those things that deserves a little more attention and detail. I had no idea folks were having issues with the technique. I'll see if I can drum up a supplementary video.

Yes that would be great Marc, a good tutorial on Router techniques and there associated pros/cons would be great. Also suggestions on when to use tables vs using fence's. But what will help me most is explaining all about kickback and best to avoid it.

I'm going to try direct plunge approach on each end this weekend and will let you al know if I stuff up another pair of Adirondack legs or not :)

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