Fine tuning the fit of a Dovetail

[Justin Jump]

Hello all,

So the last few nights I have been messing around with my Porter Cable dovetail jig, it's a 4210 with the half blind template.

If you take a quick look at the picture, you will see some small gaps.

I got the template dialed in where my pins and tails are nice and flush, but I have a loose fit.

Now, I can get those gaps to close by lowering (extending) the router bit and making a deeper cut, per the directions, but what it does is remove more material, leaving the area I have dimensioned really thin, which I don't like the look of, not to mention it's easily damaged.

Any suggestions for getting a tighter fit, but leaving some material there still?

This is 1/2", and I know that I could bump up to thicker material, but I would really like to stay at 1/2".

THX

[Alan Schwabacher]

While using the type of dovetail jig that cuts both parts of a half-blind dovetail at the same time, the pins and tails must be the same width. The width at half height must be the same as the gap, and it's determined by the template spacing. With a given template and bit, you must adjust cut depth until the width at half height matches, so you are stuck with a thin front. However, if you vary the width of the bit, you can vary this thickness with the same template. Since bits are sold by the width of the widest part, the way to change the width slightly at half height is to use one with a different angle.

[Floyd Mah]

I think I understand what you are trying to do. Before making your adjustment, the board with the tails, on the right, was proud of the board on the left. Then you lowered the router bit and got the tail board to nest deeper into the pins, resulting in the loose fit.

Anyway, I learned to cut dovetails by hand after struggling with a router template jig. It was from a small book by Ian Kirby. I think that in this book, he recommended sizing the material for drawer/box construction in such a way that the lengths were slightly long and then having the pins and tails slightly undersized. This resulted in a box which was slightly oversized, when measuring the cavity. Then one finished the box by planing the corners to get the pins and tails flush to the surface. If you had made the tails and pins too deep, then the cavity of the box would be too small and when you trimmed the pins and tails to be flush to the sides, your resultant box was too small.

What I am getting at is that your original bit depth probably gave you that box with the oversized cavity and a box that you could bring down to the desired dimensions by sanding or planing. Lowering the bit gave you a box that had gaps in the joint and that you could not remedy by trimming.

I used to have a friend who was constantly saying that "perfect is the enemy of good". In the case of router dovetail jigs, the joints are extremely sensitive to router bit depth and the care that you take in dimensioning the workpieces carefully.

[Joe Scharle]

To my eye, you are leaving plenty and could get even thinner by raising the bit height.
Remember - Heighten to Tighten.

You can see these use most of the end grain, but work just fine.



Oh, and here's some even closer. Done with a Harbor Freight! The 2 on the left were done using a Leigh #80.
The 1 on the far right were done with a standard 14 deg 1/2" bit.
You may get the result you're looking for by trying a 1/2" - 14 deg bit, instead of the 17/32" bit. You'll have to experiment with bit height in order to achieve balanced interference, because I doubt this is covered in the manual. BTW, I've used 1/2" drawer ends and sides when using false fronts but I cut through DTs when using false fronts because they're easier since I cut the tails with a bit and the pins by hand. But that's just me.