Hi Tony,
My choice of lubrication is simple: white mineral oil (like the stuff you can buy at the pharmacy). I chose this because I knew it had no additives that would "stain" the brass, and had a low enough viscosity to keep viscous friction low (in other words, to not gum 'er up).
During run-in, I stopped the engine every few minutes, and squirted a healthy dose of the oil into the steam inlet. The goal was to flush out all of the junk. The oil came out dirty for a good half hour.
Now that she's run in, I'm still using mineral oil, but every few lube cycles I'll use some way oil I fortified with ZDDP (an anti-wear additive). After that, she'll run slow for a spell as the way oil is much more viscous. But, the ZDDP is then allowed to do its thing.
However, the secret to getting her to run at such a low pressure isn't so much the choice of lube. Having built several small block Chevy race engines (full-size
) over the years, my mindset was on proper clearances. So, rather than take the approach of building each part to print as an individual, hoping all the tolerance stackups we're in the right directions, and allowing run-in to take care of friction, I "blueprinted" the engine during assembly and measured/adjusted lash and clearances with the goal of allowing a 0.001 clearance between sliding surfaces (except for the slide valve which wants to be tighter to prevent leakage). The crosshead needed the most tweaking. This allows whatever lube I use to operate hydrodynamically. If I used the run-in to fit method, the best I would theoretically ever get is a line to line fit - which wouldn't allow space for an oil film.
I've already had several people ask, after rotating the engine by hand, whether I hid ball bearings in there!
The other secret is getting the valve timing exactly right. I found the best way to do this is to disconnect the connecting rod so the piston can move from end to end under air power without rotating the crank, then adjusting the valve timing so a piston move occurs at or slightly after the crank throw passes TDC or BDC. This ensures the piston never tries to buck the system. It seems the PMR was designed to not have much, if any, cut-off (which is likely best for optimum power), so valve timing is a bit picky at low speeds.
The sldie valve can also be adjusted in offset as well. This is much less sensitive, but still with the connecting rod disconnected, I adjust this so I get more or less equal force/speed on both fore and aft piston moves.
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Getting back to the center/edge finder... I use the center finder in a slightly different way which yields much better results... Rather than run the mill and adjust so she stops wobbling, I do it without the mill running. Now, I can use my fingernail to detect where there is a step between the tip and body sections and adjust X or Y accordingly. The fingernail technique is remarkably sensitive and not only tells me there is a step, but whether the step faces down or up so as to give knowledge of which direction I need to tweak. This gets me within 0.001 as verified by a DTI. I really hate raising the head to get the coax indicator in there, or craning my head all around to see the DTI when it's facing the wrong way... If it need to be prefect, I'll always use the DTI. The coax indicator has a few tenths of runout in itself.
All of my lazy shortcuts come as a result of not having bought a knee mill (my mill is a RF45 clone mill-drill).
If I understood you correctly, you mentioned the use of a DTI in lieu of an edge finder. Can this be done? If so, how?
I've seen the machinists here at work use DTIs in the mill to do a lot of clever things. Recently I was taught how to set the mill spindle directly over the top center of round stock with a DTI as would be required when cross drilling, although I'd be hard pressed to explain the method in words for the benefit of the forum. That technique might make an interesting video if I get a chance. In any case, I'd love to learn more clever uses for the DTI myself... Any takers?
John
