Didn't find the steel bit I was after, but the mystery brass I have that turns horribly should be a quicker test. The rough start was about as good as I could get turning this piece, it also tapers to the chuck down 0.05mm. My first try and to get a feel for the lapping tool was just paper, Autosol polish and oil. I wanted to see the contact pattern of the lap before deciding where to go next, the result was surprisingly even bar a couple of hollow sections on the lap. I guessed this pattern might wear down as some heat flattened the lap high spots, I'm not sure that's what i got, but it did improve. The lap was washed between paper changes to make sure no slurry was left behind, just warm water and washing up liquid.
Next up was a hybrid of 50/50 Emery paper and plain paper, the Emery was 360 grit and really did lap the bar. I took out the taper and had it less than 0.015mm across the bar in a short time. I didn't fancy hunting down some finer grit paper, so jumped to final polish with Brasso. The finish is dull but to be fair the surface needed much more work getting the grit down. However, you can see by the dull polish that it has smoothed the surface quite a bit and the contact paper is more even. Once finished the lap was again washed and checked for wear, none present, the lapping tool did open up with the paper and Emery and is now a loose fit on the Delrin shaft it was tested on. You have good feel with these laps and can identify high spots by the increase in drag of the lap. The spring in the lap has been useful, I think if you needed more pressure an O ring would be adequate round the lap. I was able to squeeze one side to apply pressure and adjust a high spot near the shoulder. I am impressed, for a tool you can print in just a couple of hours and tailor to pretty much any size, not wasting aluminium as you only use the material you need to print the lap. As this one is PLA you won't feel so bad if it ends up in the bin at some point.
The CAD model I drew for this incorporated 0.5mm radius corners at the slots, so the Emery paper or beer can lap wouldn't have a bulge at the corner. My worry over the slurry was not a concern as the slot allows a place for the slurry to accumulate. Yes Mark, give it a go be interested to see the result you get on harder metal.