Understanding Body Taper on a Reamer Print

[surgeon260]

I need some help understanding how to determine body taper from a reamer print. Consider the following fictional dimensions from a fictional reamer print:

-A (Length from bolt face to body/shoulder junction): 2.000"
-B (Chamber diameter at bolt face): 0.500"
-C (Chamber diameter at body/shoulder junction): 0.490"

Given these dimensions, my intuition is to calculate the body taper as (B-C) / A = 0.005", or 5 thou of taper per inch.

Here is where I get confused. There is a table in the bottom right of the reamer print that summarizes taper in terms of "TPI in 20 inches." Let's say in our fictional reamer print the table says:

Body TPI = 0.050"

I don't understand this number. If I multiply my calculation of body taper per inch using the actual dimensions from the chamber by 20, I do not get 0.050. Rather, I would get 0.100. The Body TPI from the table seems to disagree with my calculated Body TPI by a factor of two.

What am I missing? Where is this factor of two coming from?

Another way of describing this observation is that the Body TPI (taper per inch) value in the case taper summary table is always 10x greater than the Body TPI (taper per inch) calculated using the (B-C)/A method described above. I would expect the difference to be 20x since it is labelled "TPI in 20 inches." This observation has been true for every reamer print I have seen.

Clearly there is some sort of convention here that I don't understand.

 

[WildRose]

Body taper as I understand it is the difference between the body diameter just above junction of the case head at the widest point just above the rim and diameter at the body/shoulder junction.


In the case of belted magnums the difference between the case diameter at the junction with the belt and at the body/shoulder junction.

 

[surgeon260]

Understood, but why do you get two different Body TPI values when calculating the Body TPI from the chamber dimensions and reading the Body TPI from the taper summary table?

 

[Edd]

For that 20" number to be accurate the calculation would have to be made using the centerline of the reamer. I suspect there is something about the way reamers are manufactured that requires a spec from the centerline.

 

[Gord0]

The body TPI of .05" on 20". Take .05" and divide by 20. It has .0025" of body taper on each side of the centerline, so .005" if you include both sides

 

[can1010]

your numbers come out to be 0.05555554 which will be pretty light and might have high pressure extraction issues.
that was figured out assuming 0.500 was at the 0.200 line above bolt face. some drawings will use a lower number and that will change the calculation
some guys will tell you to stay at .080 or higher to avoid possible extraction issues

 

[Edd]

I think he just made up those numbers to use as an example. But a 6.5 Creedmoor isn't far from that number.

 

[Rich Coyle]

The Weatherby cartridges have about .010" taper from right above the belt to the shoulder as far as diameters go. I guess that would be .005" per side wouldn't it?

 

[can1010]

using the 300 weatherby
0.5137 above belt
0.496 shoulder
body tpi 0.083
the body tpi on reamer prints is already one side

 

[rickjoty]

Gord and Edd stole my thunder but you have to look at it like the difference between the diameter and the radius. Calculate from the center line (radius) of the reamer slope to get accurate taper because (obviously) you are dealing a cylinder/cone. It reminds me of calculating neck squeeze. The diameter of the bullet plus 2x the thickness of the brass.

 

[MudRunner2005]

Just checked, SAAMI reamer print shows .300 Wby Mag has .020" taper.

Same for most all Weatherby cartridges.

 

[can1010]

Just checked, SAAMI reamer print shows .300 Wby Mag has .020" taper.

Same for most all Weatherby cartridges.

the numbers I used were from quick design. the SAAMI print is slightly different than the CIP print that is where the difference is, also the cartridge print is also different that is showing .020 but who knows I have seen so many different numbers on prints

 

[Edd]

The SAAMI spec for the chamber TPI on a 300 Weatherby is .00863". For the smaller cartridges it is .0100".