Hi again Dee:
You wrote:
"I knew I was going to generate questions"
Sorry about that, but that's what I always do when I'm confronting a problem I don't immediately know the answer to...it helps me to frame the essence of the question so I can at least try to find a way forward in my head and answer in a way that will be useful.
Of course, for a project like this, there's really no substitute for trial and error, so take all of what I've said with due caution.
Having said that, my own analogous experience was lapping case hardened mild steel that had been cylindrical ground but wasn't as accurate as I wanted it.
It was for a column on a home built cutter grinder back in the late 1980's and I learned what I know of lapping at that time, mostly from reading of the experiences of others but also by taking chances on it, building the laps and finding out what worked for me.
I had three contact pads on the cylinder, and a big anti-rotation flat to make as parallel as I could so I could run the head on the column with as little clearance as possible.
So my task was not quite the same as your task is, because my goals were different. and I built different laps to accommodate my needs but I learned something useful from every fuckup and I did make a few before the project was done.
However I've lapped lots of cylinders and bores since then; the latest was posts for running a bronze bearing on that had to run with a couple of tenths clearance, and the starting stock was dowel pins which are neither round nor straight when you buy them from the store.
So I don't have exact experience, but I've solved similar problems and my little doctoral dissertation in post #2 was kind of a synopsis of what I've discovered about the process over time and trials.
So forgive my impertinence in asking loads of questions...it's just my way of framing the issue.
With regard to your specific questions in post #3 here's my take:
If your goal is better than 1 thou round and you are already very straight, the length of the lap will be less important so long as it's long enough that you can't twist it on the workpiece and make some areas smaller in diameter than others.
Since your lap to workpiece clearance is relatively small, you can accept a shorter lap; partly because your straightness is already very good, so you won't be lapping on a banana and so the lap doesn't need to straighten the geometry; it only needs to turn a cone into a cylinder; and a shorter lap can do that without much risk.
Even at one diameter long; your ability to twist the lap will be restricted by the clearance between lap and workpiece...you won't have the physical strength to fuck it up.
With regard to how many laps; well that's purely a function of what finish you need to achieve, because you need a new lap for every grit size and you can't just clean a coarse grit lap and put in finer grit.
One leftover coarser grain in the wrong spot and you'll scarf up that beautiful finish you're developing and all will look like a pig's breakfast in an instant.
For my big lapping job on the column I bought 150 grit, 320 grit and 800 grit Clover Compound (silicon carbide in grease for lapping in automotive valve seats).
I wasted my money on the 800 grit because I could lap for a bloody week with it and not change the part by a tenth over the surface area I was lapping (3" column 14" long).
I probably got within a tenth or two with the coarse grit and lapped with the 320 only out of a sense of masochism and a desire for "perfection" (I was very young at the time!).
The result was better than I could measure and it all ran so silky smooth when assembled, that I have to keep the head at temperature so it doesn't bind on the column.
With regard to the adjustment screw, it matters more depending on how far off the end of the workpiece you intend to run the lap.
The risk is barreling the workpiece, and the problem worsens as the lap is shortened.
I was hand lapping with no power at all, and I couldn't reverse the lap (asymmetric geometry) so the adjustment screws helped me a lot.
For your task they may or may not be worth it because you can just reverse the lap as you need to and you can also run off the end of the part a bit before you reverse the stroke.
But they're simple to put in so put in 3 screws; one at each end and one in the middle...use any or all of them as you see fit.
For such a big diameter lap you may also need to relieve the bore a bit, leaving only a few lands to actually contact the workpiece.
There's so much surface area potentially in contact you may not have the strength to hold the lap when you turn on the lathe, especially once you introduce grit into the interface.
Put handles or knurls on the lap but make sure they're still small enough that you can let go if the lap grabs and that you can't get pinched between the handles and the lathe bed or the carriage.
Take all the tools out of the toolpost and tailstock so you can't gore yourself on a pointy bit.
Wear a short sleeved shirt!
Lapping is normally pretty safe but you can hurt yourself pretty good if you are not on top of it and big diameters are quite a bit more dangerous than small ones when you're lapping in the lathe.
Last point; you want it all sloppy with lubricant...it's going to make a godawful mess and it's abrasive as hell so protect everything including the chuck jaw joints with masking tape before you splatter shit everywhere.
Cheers
Marcus
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