Safely making French dovetails

[wizardgmb]

My wife purchased some raised planting bed kits from Home Depot a couple of years ago and wants more. Rather than pay exorbitantly for the kits, I decided to build my own. The kit consists of 3" square by 11" high pressure treated corner posts with French dovetails on four surfaces and 1x3x4' pressure treated rails with dovetail ends that are stacked between the posts. I'd prefer to use 2x stock  and skip the dovetailed corner posts but that isn't an option.

I have a P/C 6920 router with a 1/2" shank 5/8" wide, 14 degree, carbide dovetail bit.  It's attached to a basic tabletop router table with a taller fence that I built to replace the short factory pieces. I also built a sled that slides in the miter square track and holds the the 1x3 stock upright to make the end cuts. After considerable tinkering with the sled settings, I can make suitable dovetails that match the original factory posts.

The posts are another story... They were ripped from 4x4 pressure treated stock that has been in my barn for several years. I have the fence set so the bit cuts a dovetail down the center of the post face with a plastic feather board holding the post against the fence. Feed direction is from right to left, opposite of hand routing with the 6920. Of the first three posts I made, two ran wild between 1/3 & 1/2 way through and created misshapen dovetails. I was able to run them through again to create a dovetail sufficient for our needs (definitely not cabinet joinery). Hoping for better control of the feed and cleaner dovetails, I took the remaining corner blocks to the table saw and cut a 1/4" dado in the dovetail path to get rid of some of the material. The slotted posts worked well until I decided to change the bit that was getting dull after many slats and posts. When I fed the slotted post into the new bit, it ran wild, jammed and locked the router shaft. After shutting off the power and removing the post, I ran the same dovetail again without a problem but the second side ran wild. I bought a new 4x4 that I plan to cut additional posts from, maybe it will be easier to cut than the old hardened stock I've been using.

The problems cutting these dovetails have been scary but didn't result in any injuries. Saving $25 a planter box isn't worth it if I lose a finger or worse while dovetailing corner posts. I'm looking for suggestions for a safer method to cut these dovetails or adjustment to my current method to stay safe. Maybe it's as simple as switching to fresh lumber.

[sjk]

I had to search to figure out what a "French dovetail" was - turns out it's a sliding dovetail.  This article says it came from sales people. (http://www.popularwoodworking.com/article/french-dovetails-or-sliding-dovetails)

Anyway...

How wide of a dovetail are you trying to create? (just the width of the router bit or wider?)

How fast are you feeding the wood?

Do you have a featherboard keeping downward pressure too?

Have you tried removing even more of the wood than that 1/4" dado? 

 

[wtnhighlander]

"pressure treated" - now THERE'S your problem! I've never seen even a simple lap joint that executed cleanly in pressure treated lumber. That stuff is terrible to work with.

[wdwerker]

That might work with cypress, cedar or redwood but regular treated pine isn't going to cooperate well. Sliding dovetails and moisture won't behave well under changing conditions so don't make the fit tight at all.

There is one kind of treated that might work KDAT (kiln dried after treatment) They start with a better grade of wood which helps too. Lumberyards that cater to higher end builders might have it or can bring it in. It's not cheap !

[rodger.]

PT lumber is notorious for being soaked when its purchased.  This will wreak havoc on your joinery.

I hope you are protecting your lungs -  PT is nasty.

[Coop]

OP, you didn't have a chance coming out of the chute?! I agree with what others have said. But good luck and welcome to the forum.

[wdwerker]

I found a picture of those raised bed kits. That is definitely a much higher grade of treated pine.

[wizardgmb]

sjk -

"How wide of a dovetail are you trying to create? (just the width of the router bit or wider?)"

The dovetail is the width of the bit 5/8" at 15/32" deep and 3/8" wide at the surface.

"How fast are you feeding the wood?"

On a good clean cut, it takes me 20-30 seconds to cut the 12 inch dovetail. That includes a 2-3 second pause every 2-3" to allow chips to clear.

"Do you have a featherboard keeping downward pressure too?"

I only have a single featherboard and it's set up horizontally. A second vertical one may be the answer but I'm not sure where to find one in the vast technological wasteland of Central Virginia. I'm already behind schedule, the beds should have been planted 2 weeks ago and I can't wait for one to be shipped to me.

"Have you tried removing even more of the wood than that 1/4" dado?"

I have a couple of posts that were slightly larger than the rest and they ended up with a 3/8" dado. They went through easily before I changed the bit. The posts cut from the new 4x4 will be a little more consistent and I'll cut a 3/8" dado in them.

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wthighlander - "I've never seen even a simple lap joint that executed cleanly in pressure treated lumber. That stuff is terrible to work with."

wdwerker - "Sliding dovetails and moisture won't behave well under changing conditions so don't make the fit tight at all."

The good news is this isn't cabinetry! The dovetails need only engage once and a cap on the post covers up the joints if they are a little loose, so I'm leaving a 1/32" clearance.

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Pug - "PT lumber is notorious for being soaked when its purchased.  This will wreak havoc on your joinery. "

In what way(s) do you expect it to wreak havoc?

[Tpt life]

Oh man, take the advice. I have never bought treated that did not warp as it dried. That is very likely the wrong joint for the material. 

[wizardgmb]

C Shaffer - "Oh man, take the advice. I have never bought treated that did not warp as it dried. That is very likely the wrong joint for the material. "

Changing stock is not an option; I already have $120 worth of wood cut to length for the project. We can't afford to write off the present investment plus spend 3X as much for new stock. I'll switch to 2X stock without posts before I do that.

As I said 2X before, these joints don't have to be cabinetry quality. I actually built two P.T. beds last spring as a trial run, without post routing, problems, that have held together just fine,

[wtnhighlander]

Wizard, could you post a photo of what you are doing? Maybe we just don't properly understand the question.

[bleedinblue]

Since it's not cabinetry, why not whip out a pocket hole jig and finish this whole thing in a half hour?

[Tpt life]

2 hours ago, wizardgmb said:

C Shaffer - "Oh man, take the advice. I have never bought treated that did not warp as it dried. That is very likely the wrong joint for the material. "

Changing stock is not an option; I already have $120 worth of wood cut to length for the project. We can't afford to write off the present investment plus spend 3X as much for new stock. I'll switch to 2X stock without posts before I do that.

As I said 2X before, these joints don't have to be cabinetry quality. I actually built two P.T. beds last spring as a trial run, without post routing, problems, that have held together just fine,

 

2 hours ago, bleedinblue said:

Since it's not cabinetry, why not whip out a pocket hole jig and finish this whole thing in a half hour?

That's what I said. Wrong wood for the joint. I would adjust the joinery to match the wood. Just me and my two cents, but advice was sought. Don't shoot the messengers. 

[Chestnut]

If I'm getting this right your trying to cut a groove or slot along end grain with a router bit. How long are the boards that your trying to stand on end and slide along your table? I honestly think you might just be approaching this wrong. Do you have a band saw? If so trace the cut needed and take out almost all the material there first and use the router bit to basically clean things up if needed. If you get good with the band saw you could get your sliding dovetails to work strait from that.

[Pwk5017]

37 minutes ago, Chestnut said:

If I'm getting this right your trying to cut a groove or slot along end grain with a router bit. How long are the boards that your trying to stand on end and slide along your table? I honestly think you might just be approaching this wrong. Do you have a band saw? If so trace the cut needed and take out almost all the material there first and use the router bit to basically clean things up if needed. If you get good with the band saw you could get your sliding dovetails to work strait from that.

See, I think hes routing a long dovetailed dado/groove. It sounds like the bit is catching and going a little squirrely on him. Since the OP is already at the table saw, I would use the 1/4" dado stack to hog out the waste, and then just stay at the saw. Bevel your blade to a suitable dovetail angle(softwood is 1:8 ratio or 1:5? I always confuse that) run the board through to get one side of the dovetail, and then flip it to get the other side angled. With both hands on push blocks and maybe a feather board, I cant see how the table saw gives you issues on this one. IMO, the router table certainly has more potential for unpredictable things to happen. 

[sjk]

I think he's doing a stopped sliding dovetail along the length of a piece of pressure treated 4x4 that's been ripped to 3x3.

@wizardgmb, does the dovetailed groove run all the way through both ends, or is one end still solid wood?

 

On ‎4‎/‎27‎/‎2016 at 5:32 AM, wizardgmb said:

I only have a single featherboard and it's set up horizontally.

You can slap together a shop made one in a few minutes with your table saw. (feel free to message me if you're not sure how to make one).  It's worth doing - it will help stabilize the work piece and make the operation safer.

Definitely remove as much of the waste as you can with your dado stack or a straight router bit, so you lessen the work the dovetail bit has to do.  That will let the wood react however it is going to when you're using a straight bit, rather than a more trapped dovetail bit.

[Bombarde16]

The OP said that these posts had been sitting around for some time, so I'm guessing they're not as wet as a PT 4x4 right off the bunks at Lowe's. I think the router is the wrong tool altogether. How about using a table saw? Set a suitable bevel, run the pieces twice per face and then hog out the middle however is easiest. Perfectly centered dovetail slot done lickety split.

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[Tom King]

For our raised garden beds, I just made some stakes out of treated 2x4's, drove them in the ground, and screwed 2x12's to them.  It took maybe 15 minutes to build each one. If I'm remembering correctly, they're about 4 x 28 feet. 

 We have two herds of deer on our land, so we put up a deer fence, instead of feeding them vegetables this year.  I had this plot leveled several years ago when I had a Cat loader and dumptruck here.  There's room for expansion on the other side for at least 6 more beds.

View from the road is mostly blocked by large Hybrid Azaleas 30 some years old, a really large native Holly, and a couple of Leyland Cypress, but it still gets full sun most of the day.