Wednesday, 27 June 2018

Fettling the Crossbow

I've made an improved version of the scope' elevation mount with longer side cheeks from steel plate to improve sideways rigidity. I've added lots of small improvements all to remove slop or improve alignment.
When adjusting the wheel to raise the scope the feel was good but due to gravity and the steep angle of the triple start screw thread it was a bit loose when lowering and could even creep downwards with vibration. To add some friction I filed a slot into the side of the hole in which the brass nut rotates and glued in a tiny strip of leather, that did the job.

Spring pressure held the upper part of the mount down against the top of the screw, but this meant it could move, rattle or bounce upwards so I pinned it to the top of the screw, the hole in the top of the screw shaft had to be elongated to allow for the upper part rotating through an arc, filing to elongate an 2.5mm hole was a right pain and I had to grind down a needle file to get in there. Patience and cutting fluid helped.
The Aluminium wheel has had a groove turned in it and a brass plate added to engage in the groove, this is to stop the wheel lifting. The temporary fixing screws were replaced with nice socket head ones with a built in flange. These are screwed straight into the wood, this may seem surprising, but using an M6 screw gives a 1mm pitch on the threads (the pitch drops to 0.8mm on M5) and these cut in very nicely with a decent tap. I did a pull out test first threading a 1/4"scrap of Ash and putting an M6 bolt through it, I couldn't pull it out using a claw hammer.

The trigger pull was improved with some very careful work with a diamond file and a padding piece screwed into the nut between the two fingers that hold the string and the block which resets the safety, this meant the excess travel between re-cocking the trigger and re-setting the safety was removed.
I then noticed something pretty fundamental, the bolts didn't seem to sit right on the track ! The tip of the bolt was touching the prod mounting! What?? I'd sorted this all out already ages ago... but of course taking the prod on and off numerous times and making other adjustments had allowed things to move.
I opened out the shoot-through hole in the prod assembly, but it still looked lopsided. I measured from the nocks to the centre of the trigger mechanism nut and it showed the prod was slightly out of true. by abut 1/4". A couple of thin tin plate shims on one side of the prod mounting fixed that and the bolts sat cleanly on the track.
Reassembling it all for the umpteenth time and re-sighting the scope, it is now shooting much better.
Next step is to sight it up at 20, 30, 40, 50 yards.

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