Figuring out dovetails

[bigsteel15]

Maybe I'm beating myself up over this before even trying it, but best to put my thoughts down or I forget.

I recently bought the PC 4212 jig, but I think my question will apply to any DT jig.

Example: PC jig has 17/32" x 7 deg DT bit, combined with 13/32" straight for through DT.
If I wish, I should be able to change the diameter of the bit used, as long as it is 7 deg. So if I go DOWN to the standard 1/2" DT bit, it makes sense to me to go UP to a 7/16" straight becasue the tails need to be smaller.

Am I thinking this out correctly? My assumption is that the increase in one bit need to be offset by the same decrease in the other bit.
I'm trying to know that I can use a more standard bit on this jig.
Also looking into making my own templates for it of any design I want at work with our CNC mill.

[Tom Hintz]

I enjoy experimenting with woodworking procedures as much as anyone but when it comes to dovetail jigs and all the geometry behind them, I stick with the bits specified by the manufacturer. The difference between a good fitting and terrible fitting dovetail is so slight that messing around with bit sizes is just asking for trouble.

[RayintheUK]

If I wish, I should be able to change the diameter of the bit used, as long as it is 7 deg. So if I go DOWN to the standard 1/2" DT bit, it makes sense to me to go UP to a 7/16" straight becasue the tails need to be smaller.

Am I thinking this out correctly? My assumption is that the increase in one bit need to be offset by the same decrease in the other bit.
I'm trying to know that I can use a more standard bit on this jig.
Also looking into making my own templates for it of any design I want at work with our CNC mill.

I'd agree with you, Brian, that - in theory - a decrease of 1/32" on the dovetail bit woud seem to be compensated by an increase of 1/32" in the size of the corresponding straight bit. Surely, the determining factor is the relationship of the bit size to the collar (guidebush) used.

However, the two bits supplied differ by 1/8", whereas your proposed change only separates by 1/16" and there will be a reason for the size of the separation. Having said that, I've no experience of the PC jig at all, just trying to think it through aloud.

Also, when making your own templates, don't forget that specialised jigs will only take specific sizes of board for the joints to work correctly at each end, due to how the outer pins work out. You might want to take this into account when at the design stage.

BTW, keep on beating yourself up in print - that's how we all learn!

Ray.

[LarryG]

IINM, the dovetail joint a given jig produces is a combination of

1. the template's finger spacing
2. the guide bushing's diameter
3. the bit's diameter
4. the bit's angle

I'm thinking that if you change any one of those except the last, things are going to get hosed. IOW, if you changed bits, you'd also have to change guide bushings, and possibly the finger spacing. At least I'm fairly sure that's the case for half-blinds. For through DTs it might be different.