The Brownells sear fixture works well for its intended purpose, which is basically cutting a 90 degree primary surface, or trueing up an existing surface. The fixture is supplied with a .020" shim for spacing a cutting stone up off the surface of the fixture. An adjusting screw for positioning the sear is preset to hold the sear at the proper angle for stoning.
I wanted the ability to precisely set the sear primary face at an angle other than the standard 90 degrees. The sear can be positioned at any angle with the adjusting screw, except that on mine, the screw was frozen in position with a thread locking compound. Alternately, different shims could be used, along with some calculations. With a lot of heat, I was able to loosen the adjusting screw so I could remove it and clean off the compound. I could now adjust the sear within the fixture, but the setscrew is only accessible while the sear is rotated out of position. So it was trial and error to get the sear located properly. Another problem was that the sear was free to flop around within the fixture, as there was nothing to hold it against the adjustment screw (you have to hold the sear during use).
So I made some modifications.To provide solid working material for the new components, I flipped the fixture over to use the opposite side. I cleaned up the working surfaces to make them flat and square. A new hole was drilled and tapped for a socket head cap screw. The screw head diameter had to be turned down a bit, but still provides better bearing surface than the original adjusting screw, which was just a setscrew. The angle and location of the new adjusting screw allows access while the sear is in position. To maintain the sear in position against the adjusting screw, I added a tiny spring loaded bar to provide counter tension.
With the fixture working surface dimensions and sear pin center determined, the sear primary angle can be precisely set by a simple measurement between the sear rear edge and fixture. To make it even simpler, I made a brass gauge with two machined steps on it. One step locates the sear for a 90 degree primary. The other step locates the sear for an 87 degree* primary that I am experimenting with. The 87 degree primary face, if correctly centered over the sear axis, can approximate a true radius sear face to within .00012" (+/- .00006") over a primary face length of .020". This is as close to neutral as one can get with a flat primary face.
The sear is positioned so that the target primary surface will be parallel to the top surface of the fixture. For the standard 90 degree face, this can be simply done using the gauge. Then, to keep the cut parallel to the fixture top, a shim is selected that's equal to the sear protrusion above the fixture surface.
The 45 degree secondary surface was cut using a .125" shim. The Brownells instructions describe using a .020" shim, which is much too thin to provde the 45 degree secondary angle.
*actually 87.17 degrees, based on a .030" sear face.
These two CAD drawings show the original and modified fixture designs. All work was done on a LMS micro mill.
sear_fixture_original.jpg
sear_fixture_modified.jpg
These three photos show the separate components, and the sear in position for stoning.
sear_fixture_mod_01.jpg
sear_fixture_mod_02.jpg
sear_fixture_mod_03.jpg
-
Bookmarks