I am putting two halves of a round tabletop together by way of a Full Blind Tapered Sliding Dovetail... I just need to know what is going to make the strongest or most durable joint between a 7 degree and a 20 degree bit. I want it to be able to be taken apart so there won't be any glue used. I was looking at cutting a 1" thick piece of wood (or 4/4) with a 1/2" dovetail bit. The weakest/narrowest point on each side would be 1/4".
I think you might be asking the wrong question. Depending on the orientation of your work, the angle of your tail will have little to no impact on strength.
You could sit down and calculate the optimal angle, based on the material, but it's impractically difficult, and it will only give you an approximation. What we were taught at school (also the claim made in The Complete Dovetail by Ian J. Kirby), is that softwood dovetails are traditionally angled at 1/6, hardwood dovetails at 1/8. While this will affect joint strength, it is mostly an aesthetic consideration, and anything between 1/3 and 1/9 should be strong enough.
The considerations regarding dovetail strength are as follows: Imagine a dovetail joint like this:
_______________ \ / ___ _\_/_ _____ | | | | | |
The angle of the tail is supposed to prevent the tail-piece from sliding out of the grove, a force pulls the pieces apart (as if someone pulled the tail-piece downwards in the illustration). The angle of the dovetail affects the joint-strength in this scenario:
- Too narrow an angle means the joint might be pulled apart, as the wood can be compressed until the sides of the tail becomes parallel.
- Too wide an angle might allow breakout on the tail. Imagine the wide end of the tail becoming wider; the added material isn't supported by the rest of the tail, and might break off.
Hence the wider angle for softwoods, that are more prone to compression than breakout.
This makes sense only if your joint is stressed by a force parallel to the length of the tail piece! If the force is perpendicular to the length of the tail piece, tail-thickness will likely be all that matters.
A side note on angles: In joinery, it is common to measure angles (other than square and 45 degrees), not in degrees, but as the ratio between the side and hypotenuse of a right triangle. Thus 9.5 degrees is 1/6, and 7.1 degrees is 1/8.. A 1/6-slope tail should be measured out like this:
___1__ __1___ \ | | / \ | | / \ | 6 | / \ | | / \ | | / \| |/
Note: The illustration is of course not to scale, as the "6" sides are not 6 times as long as the "1" sides.