Thursday, November 20, 2008

Slavia ZVP, Dovetail Math

I really need to make that front sight. The first thing I need to do is to measure the dovetail slot.

A caliper and a set of PeeDee thread wires.

I chose a wire size that was just below the top of the slot.

With wires in either side of the dovetail I measured the width, I got .217"

But when I verified this measurement with a gage block stack it would not fit. Calipers aren't that accurate.

So I tried .216" and it was spot on.

A side view.

Here's a drawing.
The value we want is the width of the bottom of the slot.
We know the diameter of the pins, and the thickness of the gage block stack.
I assumed the dovetail was 60 degrees. It seemed to be.
So, knowing those and looking up the formula for measuring dovetails in Machinery's Handbook:
y=b–D(1 + cot 1⁄2α)

b= base length
Since we need to know b, it becomes:
b=y+D(1 + cot 1⁄2α)

"cot" is a fancy way of saying 1/tan

tan 30 = 0.57735026918962576450914878050196
cot 30 = 1.7320508075688772935274463415059

So that's how I got a value of .3881"
Honestly, I just wanted an excuse to do a little math tonight.

1 comment:

Anonymous said...


You're slipping, buddy. I'd have gone to AT LEAST thirty-five decimal places for those calculations.