After getting the conrods done, I'm itching to get enough of the moving parts done to see some motion, so next are the crossheads.
While I had the Emco set up for turning, I churned out a couple of piston rods from precision-ground 303 stainless, leaving the piston end unfinished (for length adjustment later), and putting a bit of thread and a landing on the other end to fit to the crosshead.
The plans show the crosshead and piston rod as a single forging, but I didn't fancy turning both from solid, and without grinding ability, I didn't have the confidence to get a good even diameter on a rod the length of the piston rod. So I'm doing a fabrication. I haven't yet decided whether to silver-solder or use Loctite to hold the parts together.
So here's some material for the crossheads and the slide plates (which bolt onto the underside of the crosshead). The crossheads are 1144 stressproof, and the plates are O-1 gauge plate.
The first operation on the crossheads was to drill the corners of the bearing opening:
after which I opened them up with an end mill:
leaving a little spare for later filing. After that, each crosshead had three 5BA threaded holes made: two to hole the slide plate, and one for the bearing wedge:
Initially I was going to do the piston rod recess in the 4-jaw chuck, but then thought I'd get better alignment by doing it in the mill. First I drilled through for tapping size,
then went part-way through with a larger drill. Then I could tap:
and finally bore to fit the piston rod end with a tiny boring bar:
This joint is critical to get the piston rod lined up with the crosshead, so it was worth some care to get a good fit. I did have to do quite a bit of fiddling to get the pistron rods to screw all the way in.
The final operation while the vise was set up was to mill away some of the material to reduce the amount of interrupted turning later:
Then it was time to set up each crosshead in the 4-jaw, using the piston rod to center:
which allows the radius to be turned:
I was getting a bit of chatter there, but it should clean up with a file.
Both done:
This is a mockup that shows the arrangement of the screw that pushes down on the bearing wedge:
What I don't see on the plans is anything that locates the bearings in place; are they just held in by the wedge?
Now the bearing openings need to be cleaned up, and the only way I have to make square holes is by filing! To get the corners nice and square, I made up a couple of L-shaped guides from gauge-plate, and hardened them. Then, by clamping them on either side of the part to be filed, I can be sure that my filing is square.
Setting up these guides was a bit fiddly, but a toolmakers vise helped. The strategy here was to find a drill bit of the right thickness to line up the bottom edge to be filed, snug up the vise, then tap the part to align the perpendicular edge. Then tighten the vise, and apply a toolmakers clamp to hold things together for transfer to the bench vise, for filing:
A triangular file helps get into the corners. Four setups per part, and the bearing openings are reasonably square and sharp-cornered
Final steps are to mill the slight taper on the back face of the crosshead, and to take it to thickness, using the screwed in piston rod to make sure the sides are parallel with the direction of travel. A bit of draw-filing removes the milling marks, and we're here:
What I didn't show was the hour spent struggling to get the slide plates moving smoothly in the slides, which involved lots of filing, rubbing on stones, some TimeSaver, and milling off the odd thou here and there. My fingers still hurt from the sharp edges
All was rewarded, however, with a test-fit with the cylinder covers on the cylinder block. Everything seems to fit
Next I think I'll do two sets of bearings: those for the conrod, and the crosshead ones.
Simon