Dovetail myth regarding pin dimensions.

[Parlo]

I've seen several references regarding measuring over pins to establish mating dovetails.
The common statement is to use pins of the same diameter as the depth.
**This does work on 45 degree dovetails as the angular contact point with the pin is halfway for both male and female parts. This enables the dimension over the pins in the female slot to be the same as the dimension over the pins on the male part.
However this is not the case for other angle dovetails.
If cutting a common 60 degree dovetail mating pair, a pin diameter of 1/3rd of the depth should be used.
Hope this helps anyone considering using pins of the same diameter as the depth for any angle other than 45 degrees.

**Edit: I've checked the statements regarding the 45 degree dovetais and they are also false.
The ideal pin diameter for a common dimension for 45 deree dovetails is 41% of the depth.
Sorry, I should have checked before posting, it seemed plausible, I guess that it is why that size is often quoted.

 

[MyLilMule]

Instead of doing the math for stuff like this, I like to draw it up in CAD and just let the computer calculate the dimensions for me. The diameter of the pins is irrelevant and they don't have to be the same.

 

[3strucking]

If I can ever learn FreeCAD, I know it will make it easier for me. Math is not my strong suit.

 

[Parlo]

If I can ever learn FreeCAD, I know it will make it easier for me. Math is not my strong suit.

If you use the correct size pins then both dimensions will be the same. The only calculation is to either divide the depth by 3 for the pin diameter or make the depth 3x deeper than the pins you have. No need for cad with this info.

IMG_20221211_140940.jpg

drive.google.com

 

[ChazzC]

If you use the correct size pins then both dimensions will be the same. The only calculation is to either divide the depth by 3 for the pin diameter or make the depth 3x deeper than the pins you have. No need for cad with this info.

IMG_20221211_140940.jpg

drive.google.com

Yes, but . . . not all of us have two sets of Gage Pins, and it's a lot easier to measure over the pins if you can access them: in your expanses, measuring over the internal pins will be difficult. Just my 2¢.

 

[GeneT45]

I use what I have on hand. The math isn't hard (with a calculator) and CAD is good as well. Once you're OK with that, the only important thing about the pin size is that it be smaller than the dovetail is tall. This allows the convenience of using dowel pins, which most of us have in duplicate already.

GsT

 

[RJSakowski]

The original statement isn't true. Here is the geometry for a dovetail with pins.

 

[Parlo]

Yes, but . . . not all of us have two sets of Gage Pins, and it's a lot easier to measure over the pins if you can access them: in your expanses, measuring over the internal pins will be difficult. Just my 2¢.

I assumed that only one part would be measured at a time so just one pair would work.
The method I would choose for the female dovetail would be to wedge a pair of tapered parallels between the pins but measure over the protruding pins and not the tapered parallels.
For the male dovetail I would use a small clamp over the pins and measure over the pins.
If the two dimensions are the same, then the dovetails are the same, but only if the correct diameter pins are used.

 

[Parlo]

The original statement isn't true. Here is the geometry for a dovetail with pins.View attachment 429585

True, I trusted the info I read for 45 degrees as it seemed plausible and have amended the original post to read 41% of the depth for 45 degree dovetails.
Using these pin sizes the pins are directly above each other as my sketch shows for the 60 degree dovetail.

Your sketch for the 60 degree pin diameter is 2/3rds of the depth not 1/3rd as I posted.
The tangent point cannot be half the depth as the pin will be on the opposite side of the line for the mating part.
The pin sizes quoted will bring the pins directly above each other.

 

[Parlo]

I use what I have on hand. The math isn't hard (with a calculator) and CAD is good as well. Once you're OK with that, the only important thing about the pin size is that it be smaller than the dovetail is tall. This allows the convenience of using dowel pins, which most of us have in duplicate already.

GsT

This post was to highlight that it is natural to assume a common dimension over the pins for both female and male dovetails regardless of the pin size.
This is only true when the correct pin size is used.

If you are making your own custom dovetail, the depth can be calculated from your available pin sizes.
For example: A 60 degree dovetail using 3mm diameter pins will have a depth of 9mm to give a common dimension for the male and female parts.