Wednesday, October 10, 2012

Diopter Riser Base for the Tech Force TF79 / AR2078A

Removed the stock diopter sight from the Tech Force TF79 target rifle to swap it with the higher quality Russian IZH diopter.  Unfortunately, the IZH sat too low on the gun and the elevation was maxed at 10 meters and still needed a few more clicks higher.  The TF sight attaches to a dovetailed riser block bolted to the top of the breech.  It wasn't too difficult to look at the elevation scale marks on the Russian sight and see that it was about 0.175" above center of the adjustment range.  If the new riser base is about 0.185" taller than the stock base, that will put the sight well into the middle of the adjustment scale and give a bit of additional adjustment for greater distances.  I looked at the metal stock on hand and found a piece of 0.500" thick 6061 aluminum plate--except it was it was twice the width necessary for the sight base.

While I'd made a slitting saw arbor for the Taig lathe a couple years ago, it won't fit on the milling machine.  This was a good excuse to make a tool for the mill that utilizes the same blades.  Some 0.750" drill stock and an hour or so later...

And I had a saw arbor.

With a couple custom-sized washers, it accommodates all the various sizes of saw blades on hand.

OK, here's the base on the TF79.

Removed the two M4 screws.

It's about 18mm wide.   Did some layout on the aluminum leaving a bit of extra width to play with.

Quite a bit taller.

With the workpiece on parallels, it took a few passes to make the cut.

Note that this is not a proper endmill for aluminum.  The aluminum really wants to weld to the cutting edges ruining the finish.  But keeping it lubed and brushing it off after each pass left a beautiful finish.

Cleaned up the ends. 

Marked center line.

The gas tube on the TF79 was roughly 19mm.  Close enough to use a 0.750" ball mill to cut the profile.

Not quite centered as you can see by the edge thickness on the left compared to the right.  Not shown:  Milled the right side down until it matched.

Measured the thickness of the stock base and added 0.185"

Milled away the excess.

The original base has 60 degree dovetails with the top of the base measuring 11.5mm.

By only cutting on one side and flipping the workpiece in the vise for each successive cut, the dovetail remains in the center of the work.  Advance the cutter, make the cut, flip the workpiece, make the cut. 

I stopped when the top of the dovetail measured 11.5mm.

The mounting holes are exactly 80mm apart on the gun.  Spotting drill to locate the hole,

drilled for clearance,

and countersunk.

Test fit with new, longer, mounting screws.  Each countersink is finished when the head is completely flush with the surface.

Deburred the edges and it's done.  OK, maybe not done.  I'll put a satin finish on it later so it matches the other aluminum parts on the gun.

Not too bad.  Missed my self-imposed height goal by 0.001".  The base mates perfectly to both the gun and the sight.  Dialed the sight to the middle of the range and in 5 shots it was good to go.

Thanks for reading.  Check back for more soon.