As my first attempt at this ended in failure (I widened out the holes too much), I decided to have another go at it today.
I used the same setup as before, using my digital caliper jig to get the holes drilled accurately.
I originally planned to do the base of the tracker bar using John Smith's honeycomb method, but advice from Melvyn Wright ( http://www.melright.com/busker/ ) pointed me at using an all-wood method, described in a post by Walt Lysack on Melvyn's website. After studying Lysack's post I decided that I'd do a slight variation on his method.
I cut a length of hardwood about 7/8" x 1" to use as the top and center, and a piece 1/4" x 1 3/4" to use for the tubing connection base.
I drilled the center holes about 1/2" deep into the block, then sliced off the tracker bar top.
I then took the middle section (with its half-deep centerline holes) and glued it to the undrilled base piece.
I then drilled all of the offset holes through the middle piece/base assembly, again using my caliper jig.
This is what I wound up with:
And here's how the pieces looked once I finished with the Dremel tool spiral blade and some sandpaper:
I started work on the trackerbar today. I first used my steel rule and marked off the position of the holes on a piece of paper. Just to be safe, I double checked it against the Melvyn Wright test roll holes to make sure everything lined up. I then made a photocopy of the template (just in case) and I cut it out and glued it to the top of a 1" x 3/8" length of the mystery hardwood that I've got so much of.
I made a jig for drilling on the drill press and figured out a way to attach my digital calipers. I first drilled all of the holes with a 1/16" bit to serve as a pilot hole, and carefully double checked my measurements with the caliper jig as I moved the piece along. Rather than "zeroing out" the calipers after each hole, I went with cumulative multiples of 6mm as I went down the row (9mm for the last 3 holes).
(On a test piece of wood that I had tried earlier, I zeroed out the calipers after every hole, and the minor variances added up along the way and threw the measurements way off.)
After I had all of the holes drilled to their proper size (5/32"/3.96mm for the first 17 and a "C"/6.14mm drill bit for the big three), I stood the piece on its side and cut it in half.
This left me with two identical trackerbar pieces, which I figured I might need if I really screwed the first one up somehow. As it turns out, the half that was originally on the bottom during my drilling, was pretty fouled up on the bottom side due to splintering from the drill bit. I could still use it in an emergency, but it would require a lot of sanding to make it sound.
Next, I set to work with my needle files and squared of the leading edge of the holes (after double and triple checking John Smith's instructional video to make sure I had them pointing the right way.)
Then I double and triple checked the video again and marked off the back of the piece so I wouldn't lose track of which holes widened in which direction.
To gouge out the back of the holes, I used a rotary cutting bit on a dremel tool and worked my way along very slowly and carefully.
Here's what it looked like after all of the machining and filing.
I found that a rolled up scrap of 220 sandpaper made a good file for getting the little fuzzy bits out of the holes.
Thinking that I should probably give my steel crankshaft parts a coat of shellac to keep them from rusting, I thought I'd try an experiment with a mixture of shellac and a few drops of yellow food coloring.
I brushed two coats on a scrap of aluminum rod, one coat on the top of a razor blade, and two coats on part of a chrome-plated shaft, a ball bearing housing (avoiding getting shellac inside), a piece of steel from a ratchet, and part of a slotted screwdriver shaft. I'm not sure how well the colorant will resist fading - especially prolonged sunlight exposure, but for the interior organ parts I may give it a go.
Now that I've got the reservoir/bellows assembly fairly complete, I figured I'd better return to my work on the crankshaft, since my first attempt with it failed so miserably.
I originally attempted to make the crank arms out of aluminum, but those turned out to be too soft, and the holes for the shaft widened out enough to make the shaft wobbly and useless. So I repeated the machining process, this time from a solid piece of 1/2" x 1/4" steel bar. This time, to ensure a tight fit, I drilled the shaft holes with a "D" sized drill bit (0.246"), which is slightly undersized for the 1/4" shaft. The shaft wouldn't quite fit into the holes, so I drilled them a little on both sides with a 1/4" bit, leaving about 2 or 3 mm. in the center at the smaller "D" size bore. I then polished the steel up so it would look as shiny as the chrome plated shaft steel, using 220 then 1000 then 1500 grit wet sanding, and finishing them to a near mirror finish with some white rouge polishing compound and a felt wheel on the dremel tool.
I then hammered the shaft sections into the crank arms and got a super tight and square fit for all the pieces. Even though there was no lateral or axial movement, I decided to play it safe and drill completely through the arms and shafts and tap in some 3/32" spring pins. I put a drop of loctite in each of the spring pin holes before inserting the pins. I would have used loctite on the shafts ends, but they were in so tight that the loctite couldn't find a gap to penetrate.
Here are most of the part laids out. I haven't attached the single arm/short shaft/rod end to the end yet, as I'll likely wait until I have it installed through the wooden panels. I'll also wait until I've got the whole pressure box assembly built, so I can tell how much to trim off the end of the shaft where the crank would traditionally go, since I don't want that part of the shaft protruding from the case.
