Today I finished machining up the front end of the belt tensioning shaft, and drilled the holes in the adjuster nut to take the handle, and the grub screw to secure the nut to the shaft.
First I cut the hex. To my surprise, I got within 2 thou of a tolerable fit first time out of the blocks... so after a little skim, it slipped on perfectly, with only a tiny bit of wiggle.
Next job was to cut the thread. To aid me, I'd purchased a new split die (I have a 1/2" BSW die, but it's not a split one, and it doesn't have a lead-in taper), but then I hit a snag.... by making the centre post that goes in the nut a little larger than Boxford's original... meant it interfered with the thread slightly! And being hexagonal, I couldn't get the die centred properly.
So, plan "B" was invoked, and I single-point cut it all the way. The serrations on the hex are where the toolbit just touched the corners of the hex. I'd say it looks neat, but TBH it just shouts "amateur!!!". A few thou smaller would have made the problem simply go away... Anyway, we are where we are.
Once the threads were done and the adjuster collar was able to be screwed on/off, it's back over to the mill for some drilling. Ironically, you have to screw it on over the hex! It's a bit wobbly to start, but works surprisingly well considering how shallow those cuts are! I hemmed and hawed about the grub screw, but eventually went with an M6 thread, as I have about a bazillion cap-head screws, one of which I could cut down. Fitting the shaft and nut together, with the nut clamped precariously in the vice, I carefully drilled a 5mm hole into both nut and shaft (to assure perfect alignment. Purely by luck, the depth of the hole in the shaft was
exactly 1/4"). Then I withdrew the shaft and tapped the M6 thread into the nut. Whilst I was there, I also drilled the 8mm (! yes, really, it's pin on 8mm) hole for the handle.
The cap-head screw has 1/4" length of thread turned off, to fit snugly in the 5mm hole in the shaft; then the cap was milled away to leave a grub screw. The original Boxford grub screw used an Allen key, but feeling lazy and in need of a coffee, I cut a slot (freehand with a 6" grinder, I'll have you know!) for a screwdriver. Unlike the original, this is just to hold the two parts together, there will be very little force exerted on the screw itself.
So... the pics show the component parts; then the collar screwed all the way in (out?) to allow access to the grub screw; an finally an overview of what it should look like in service.
The last remaining jobs on the shaft are to trim it to length (optional, quite frankly) and deburr the far end, drill and tap a hole for the "cam" which rides in the thingybob underneath the motor plate, and does the actual pushing in/out. I'll turn the "cam" bit out of phosphor bronze I think, since I accidentally ordered 2 pieces, so I've got more fozzy bronze than I'll ever need now!
There's one more thing to do to the adjustment collar; the big chunk that's missing from the relieved section (see 2nd pic) needs filling with weld, then turning back to diameter. The original Boxford setup had a grub screw protruding into that relief, which the collar then bore against when being adjusted. In theory, it shouldn't see a massive load (because you'd not turn the adjuster with the belts tensioned), but as we can see, it's vulnerable to being hit. Think I'll turn up a piece which is a nice sliding fit in there, drilled and tapped for a grub screw (or maybe just pinned, like my shaft). It wont be as strong as just using the grub screw, but it won't offend my OCD either. Somehow, just having the end of a grub screw sticking into that recess doesn't feel
right.
More tomorrow, hopefully.
