View Full Version : Pin Sizes ?

08-30-2005, 10:35 PM
Hint. Sharpen up your math and learn to figure pin sizes for mating dovetails. In the end cutting to pin sizes saves a lot of time and gets better fits.

What does this mean?

08-30-2005, 10:37 PM

08-30-2005, 10:41 PM
Thanks !!

Paul Alciatore
08-31-2005, 01:49 AM
The easy way to do this is with a CAD program. Just draw two circles that are tangent to the bottom and sides and dimension the space between them. Most CAD programs will do the math with an accuracy that will be less than the diameter of an atom.

Paul A.

Your Old Dog
08-31-2005, 06:18 AM
I don't know how to do this so I have to ask. Been studying the drawing and have a question. Wouldn't the "final size" be the distance as you show it plus half the diameter of each circle X 2 ? (or the diameter of one rod/circle?)

Will this method work with anysize ball and any size cutter or do they have to be coordinated in some way?

If I can get this to work you have sure made things easy for me! Thanks a million for taking the time.

[This message has been edited by Your Old Dog (edited 08-31-2005).]

08-31-2005, 11:33 AM
I just threw the diagram up to show the concept of using pins for measurment. Review Machinery's Hand Book under measuring dovetail slides. The 23rd edition has it on page 686 (Should be close in other editions). The section explains what you need to do & has the required formulas. Pauls way is more fun though, everything I do starts off in cad so the all the info is there for me.
Have fun,

Your Old Dog
08-31-2005, 02:26 PM
sh_t http://bbs.homeshopmachinist.net//frown.gif ............................... http://bbs.homeshopmachinist.net//biggrin.gif I got to breathing quick there, thought it was gonna be easy!

[This message has been edited by Your Old Dog (edited 08-31-2005).]

08-31-2005, 05:05 PM
Hey, it's not bad. The sharp inside corner, a theoretical point, is calculated from the center of the roll pin. From the center of the pin to the inside corner is a tangent relationship - the rise over the run. You know the rise which is the radius of the roll pin. So, 1/2 diameter of roll pin divided by the tangent of 1/2 the inside angle of the dovetail (the rollpin against the two walls exactly bisects the angle) gives you a number. Final Size equals sharp corner dimension minus 2 times "calculated number" minus 2 times rollpin radius if you work it out visually like I do.

If you're mating to a male dovetail, remember that there will be a slight clearance between the flat faces at the top or bottom which also needs to be factored in but it is just added to the calculation for that one.

The last job I had the shop manager told me to cut the female first. Then, since I knew it was a 1-3/8 cutter, we could calculate and measure over the sharp corner of the male dovetail. Phooey. The 1-3/8 cutter does not mean 1.3750" and you can't measure the cutter because you don't know what tiny chamfer or radius is on the end of the teeth. Then you can't measure the male dovetail over the burr left by the cutter with any reliability and further you can't file it off and measure because you don't know what kind of flat you leave. I measured over rolls and didn't tell him. Finally had to quit in frustration over that kind of c**p.


Your Old Dog
08-31-2005, 05:18 PM
I have 24th ed of machinery handbook. On page 653 they have the formula as you mentioned but I cannot find one of the parameters "i". Using the diagram you drew the call "y" the final size.

it says, "To obtain demension y for measuring a female dovetail, add i to the cotangent of one-half the dovetail angle a, multiply by diameter D of the rod used, and subtract the result from dimension b. Expressing these rules as formulas:"

y=b-D(i+cot 1/2a).

THERE IS NO LETTER i SHOWN IN THE BOOKS EXAMPLE?? I'm still lost on this question.

If I can figure this out I'm good enough with Excel that I could write a small program so I can run some common sizes, post it on the wall and be done with it!

thanks for trying.

[This message has been edited by Your Old Dog (edited 08-31-2005).]

08-31-2005, 05:47 PM

That should be the number 1. It's probably a transcription mistake somewhere, or maybe they think "I" stands out better than "1". Whatever. In any case, it comes out to the same thing. The cotangent is 1 divided by tangent or the same thing as dividing half the angle by the tangent.

The formula could probably be derived algebraically, but I never quite trust just plugging numbers into formulas unless I understand them, and it's just easier for me to look at the right triangles in the problem and take it from there.

If stupidity got us into this mess, why can't it get us out? Will Rogers

Your Old Dog
08-31-2005, 06:11 PM
TGTool, thanks a lot. Now I'll get it figured out. It's most definitly a "i" in their equasions. I'm sure I'll be able to figure it out with this last piece of the puzzle.

thanks a lot.


08-31-2005, 06:47 PM
There's a free DOVETAIL program on my website. It will do all the math for

Regards, Marv

Home Shop Freeware - Tools for People Who Build Things

08-31-2005, 08:30 PM

It might be the letter i in the book but it is indeed supposed to be the number 1. My 26th edition, on page 683 (large print), shows the same formula as you posted but with the number one. In the book, the paragraph above the formula uses both the "letter" i and the number 1 when explaining HOW to use the formula. As Jan mentioned, it's just a typing mistake, nothing more.


[This message has been edited by Smokedaddy (edited 08-31-2005).]

Your Old Dog
09-01-2005, 06:21 AM
Mklotz, thanks for the heads up on the software. I loaded it, ran it but was not able to read the answer before it wiped off the screen recycled for another query. I'm running XP with Norton turned off.

SmokeDaddy, thanks for confirming that for me. I guess this particular "Bible" either has some errors in it or "i" may stand for "inverted" and my lack of math background didn't recognize it till jarred by you guys! At least I got to go to school before Affirmative Action became entrenched in the education system! I can still think for myself even if only on 3 cyclinders http://bbs.homeshopmachinist.net//biggrin.gif

09-01-2005, 11:16 AM

Windoze (spit) is closing the window automatically after the program finishes.
It's a problem with the POS OS, not the program. Read the last three
paragraphs in the Introduction section on my page to see how to overcome this
annoying 'feature'.

Regards, Marv

Home Shop Freeware - Tools for People Who Build Things

Your Old Dog
09-01-2005, 09:34 PM
Thanks Marv, it looks like it will fill my needs if I can make the answer stay up there long enough to read it. I will do as you suggest.

thanks again,

[This message has been edited by Your Old Dog (edited 09-01-2005).]

Your Old Dog
09-01-2005, 09:56 PM
Thanks Marv! Did as you said and the program works just fine. This solves a big headache for me and will save hundreds of pounds of valuable material http://bbs.homeshopmachinist.net//biggrin.gif

09-02-2005, 11:29 PM
You can use trig and do it.

09-03-2005, 12:39 AM
Thanks for the illustration. I think I asked about that awhile back and nobody answered. Thanks for re-posting the question.