Help my decroded brain with some math

[DPittman]

I'm wanting to cut some 60 degree dovetail grooves in an air rifle barrel to accept a front site. I'm having trouble with figuring out the math in order to cut them correctly. Maybe my childish drawing might help to convey what I'm trying to achieve.

I think I have to use the cosign of 30 degrees to determine the travel of the cutter inwards to achieve the proper distance of the top of the groove which is indicated in the picture as 8.65mm.

If I move the cutter down 3mm from the top of the barrel down to achieve the correct distance for the bottom of the groove, how do I determine the inward movement to achieve the 8.65mm distance between the two grooves?

 

[thestelster]

I'm wanting to cut some 60 degree dovetail grooves in an air rifle barrel to accept a front site. I'm having trouble with figuring out the math in order to cut them correctly. Maybe my childish drawing might help to convey what I'm trying to achieve.

I think I have to use the cosign of 30 degrees to determine the travel of the cutter inwards to achieve the proper distance of the top of the groove which is indicated in the picture as 8.65mm.

If I move the cutter down 3mm from the top of the barrel down to achieve the correct distance for the bottom of the groove, how do I determine the inward movement to achieve the 8.65mm distance between the two grooves?
View attachment 40370

I think we need to know the diameter of the barrel.

 

[DPittman]

I think we need to know the diameter of the barrel.

.750" or 19.05mm

 

[Susquatch]

Stel is getting you the math, but even once you have it, I would recommend using wires to measure your reference dovetail and then machine to that wire distance on the new one. It's the best way to avoid letting chamfers and roots ruin your day

 

[DPittman]

Stel is getting you the math, but even once you have it, I would recommend using wires to measure your reference dovetail and then machine to that wire distance on the new one. It's the best way to avoid letting chamfers and roots ruin your day

Ah never thought of that, I'll give it a try.

 

[DPittman]

Ya I'm probably over my head here but I'll try. I pulled out my Machinery's handbook to try to understand on how to accurately measure a dovetail. I don't even remember what the hell a "cotangent" is, let alone how to use it!?!

So using .040" Gauge thread wires I get a measurement of x being .400"

I'm not sure how to measure the height "h", but to the best of my ability I determined it to be .079"

The dovetail cutter I am using is a 60 degree cutter so is one half of dovetail angle "alpha" then 15?

And how do I get dimensions "a" ?

Clearly I am missing much.

I'm trying to duplicate a dovetail groove as on this barrel.


I'm so mixed up now I'm not even sure if this is a male or a female dovetail! WTF? (and I don't mean "well that's fantastic").

This is all I want to accomplish.

 

[YYCHM]

I hear ya Bro..... I found cutting dove tails on QCTP tool holders quite challenging. 8 out of 10 ended up too wide and needed to be shimmed and re-cut.

Do you have a sample dove tail that's correct?

 

[DPittman]

Do you have a sample dove tail that's correct?

Just the one on the barrel in the picture. The one I need to cut is actually on a sleeve that fits over the barrel end and is of a different diameter of the one pictured.

 

[Susquatch]

Don't try to use math on the thread wires. Just duplicate the reading.

In other words, measure the distance across wires on your master, then cut the new ones, then measure over wires, then subtract and then cut half of that much more on each side.

This assumes your height measurement is good too.

Might be a good idea to practice on a piece of 1/2" bar.

 

[DPittman]

Don't try to use math on the thread wires. Just duplicate the reading

Good point.

Might be a good idea to practice on a piece of 1/2" bar.

Ya tried on a piece of same diameter. Depth of grooves was excessive and they got too big/small (the front dovetail sight was a loose fit over the grooves)

 

[Susquatch]

Ya tried on a piece of same diameter. Depth of grooves was excessive and they got too big/small (the front dovetail sight was a loose fit over the grooves)

Yup. More practice needed......

 

[thestelster]

Somebody has to double check my math please.

 

[YYCHM]

Use a regular end mill mill a flat down to the 3mm depth. I.E. center the end mill on the bbl then step over 8.65mm/2 - EM dia/2 and mill the flat on both sides. Then come in from the side with your 60 deg dovetail cutter taking care not to narrow the 8.65 dimension.

 

[thestelster]

The dovetail cutter I am using is a 60 degree cutter so is one half of dovetail angle "alpha" then 15?

No. The 60° dovetail cutters are in fact 60°.

 

[thestelster]

what the hell a "cotangent"

 

[thestelster]

And you know, whenever in doubt, just draw it out x-times larger in all dimensions. As a sanity check.

 

[PeterT]

If I honor the 3mm, the distance across the corners is 9.53mm

If I honor the 8.65 across the points, it works out to 4.32mm vertical

The only reliable way is measure using pins because corners are burred or chamfered or whatever. If you tell me the wire diameter & outer distance combination (example 3mm & 13.06mm) that becomes the constraint & I can tell you the rest of the corresponding dimensions which result.

 

[thestelster]

And, just as aside, most north american rifle dovetails are 3/8".

 

[DPittman]

Well flying by the seat of my pants mostly, my second attempt on a blank produced acceptable results. Now I'm going to try to comprehend all the math that you fellas have provided. I hoping to duplicate my last results on the actual piece but would feel better if I could make the process a little more scientific/mathematical this time.

 

[DPittman]

And, just as aside, most north american rifle dovetails are 3/8".

Yes. This is on a German made air rifle. On other air rifles 3/8" is sometimes used or 11mm is also common. This is for a front hooded sight and the dovetail groove is not the same dimensions as the one on the receiver for a scope.