Sliding Dovetail help needed

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BrianBDH

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Brian
I am thinking about a project that will need a sliding dovetail joint, so today I was fooling around on the router table and thought I would try one out of some scrap pine.

I used a 1/2" straight bit to cut a groove to start the slot so the dovetail bit didn't have to hog out all that wood.

Then I used the dovetail bit set at the same depth (maybe a hair deeper) to shape the sides of the groove.

Then, without moving the bit at all, so as to maintain the same depth, I made a series of passes on the other piece to "sneak up" on a good tight fit.

When the pieces finally slid together, the joint was, to say the least, total crap!

103_2850.JPG


I am hoping that somebody can tell me if there is a better procedure or I just have to try, try again until things come out better.

The depths seem to be off even though I never touched the bit set-up between the slot and the tail piece machining. I only adjusted the fence.

I tried to blow off the sawdust between passes, but there could have been some in the way at some point. Another question, should I have a small rabet at the bottom of my router table fence to give stray sawdust a place to go?

Any suggestions will be most welcome. This joint may not be a focal point of this next project, but it will not be hidden either.

Brian
 

Joe Scharle

New User
Joe
Brian, I've never had any luck with using a straight bit to take some of the load off of a dovetail bit. First time I read that 'tip', I tried it but didn't see any purpose other than to mess up my effort. Besides, I can buy a half dozen DT bits for the price of a good spiral, and DT bits a far easier to hone!

All fences benefit from a relief, and it needs to be shallow.

My method is to rout the slot part, then sneak up on the slider part. Try not putting too much pressure on the pieces to avoid flexing the router table insert and changing the depth of cut along the first 2 inches. And don't change the bit's height. If the 2 pieces mate but bind, another pass an hour later may free it up.

Cutting a longer sliding DT will require that the slot part be cut a little wider from the entry and narrowing to the last couple of inches of the slot. This is to compensate for the tendency to bind in the slot. A jig like this below can make the whole slot almost automatic. At any rate cuss freely and have fun!

 
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Charles Lent

Charley
Corporate Member
You cut the dovetail (not the dovetail slot) just a little too narrow, while trying to sneak up on the fit. If it was just a few thousandths wider it would sit lower in the mating dovetail slot. Notice that you have the same gap all the way across the joint. A wider tail would fix this.

Most people have trouble because their stock isn't perfectly flat. Any bow at all in the boards will cause problems. Cutting a sliding dovetail on a good dovetail jig (Leigh or PC type) is easier (but still not easy) because the board clamps flatten any slight warp in the stock while it's being cut. If you are using a router table you don't have a way to hold the board perfectly flat and cut it while holding it vertically against the fence.

Charley
 

eyekode

New User
Salem
I can't really tell from the picture but it looks like the striaght bit was a hair deeper. But I don't think that is the main issue. Are you sure the tail is perfectly square and the socket board is perfectly flat (without bow). It looks like something in the middle is holding the joint apart.

When I used a sliding dovetail on a stool I had a similar problem but because of the direction of the cup I had binding issues: http://ncwoodworker.net/forums/showthread.php?t=37424

Good luck!
Salem
 

BrianBDH

New User
Brian
Thanks for the help. Looking at the pieces a little closer, they are both bowed. And the fit is better in some areas than in others. So I can tell I need practice in maintaining equal pressure as the pieces go over the bit.

Also, as you take successive small cuts on the DT, it becomes harder to hold the piece horizontal. You are removing bearing surface with every cut. I wonder if it would be better to cut the DT first, one pass each side, good and square. Then sneak up to the proper fit on the slot. The piece with the slot is much easier to keep square as it doesn't get narrower. I'll try that next time.

But I can tell as you gus pointed out that the pieces need to start straight and square.

Thanks,

Brian
 

Charles Lent

Charley
Corporate Member
If you can use some clamping cauls across the board that will become the male portion of the dovetail just above your fence it may flatten the board enough for you to make a satisfactory cut. Leave them on until you have trimmed both sides to fit the female dovetail. When you remove them the board will take on it's original bow, but the width of the tail will be even. You will then need to force it back straight as you slide the two pieces together. It may be better to leave the cauls on until you have put the joint together and then remove them.

Charley
 

Joe Lyddon

New User
Joe Lyddon
I think your stock should be flat & square in all directions... without that, you will eventually have trouble.

You should not have to hold your stock down to remove a bow... IMHO.

Are you going to flatten the pieces to slide one piece into the other? Not a good idea... IMHO.
 

Joe Scharle

New User
Joe
Just got back and read what you said here:
Also, as you take successive small cuts on the DT, it becomes harder to hold the piece horizontal.
And that brings up the 'carrier' board jig. A lot of cuts are impossible without a carrier and a whole lot more are just plain asking for it without a carrier board. Here's one for a long lock miter...


So, I just came back from the shop where I made a sliding DT (in pine). I simply set the bit height to what I thought looked right and cut the slot somewhat centered and set that piece aside


I did measure the for shoulder cut and backed off about a 1/16". I do that so I will have a sneak up cut. One to CYA, but mainly to let me do a clean up pass. This next pic shows the second shoulder pass now using a carrier board. Some carriers like one required for window muntins must be an exact negative image, but most only require top and side support, like this one.
BTW, I've been timing both my making the sliding DT, and typing this and have just exceeded the 10 minutes it took to make! Just to point out that it's not labor intensive.
Next pic is the setup for the shoulder cuts. I did make 2 passes.


Next are 2 pics of the result.



You don't live too far away, so if you want come on over.
 

BrianBDH

New User
Brian
Joe - you are like the Yoda of woodworking!

That is what I would call going "above and beyond" to help somebody that you have never met face-to-face.

Besides the straight and flat pieces to start with, the carrier board is exactly what I need to hold the piece while making those final precision cuts.

Thanks so much. (May the Force be with you!!)

Brian
 

Joe Lyddon

New User
Joe Lyddon
Brian,

Sounded like you had a bowed piece... Bad... LOL

It's also a must to hold the wood down good over the cutter to make a uniform cut... given.

Hope you make it work. LOL
 
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