Lapping a cylinder with dowel + paste

Good morning all,

It's very shortly going to be time to lap the cylinder of my little steam engine - and rather than buying a honing tool I thought I'd use the old trick of a hardwood dowel and lapping paste to finish the cylinder bore.

So now in my hand I have a dowel of about 11/16" and some fine and coarse grit lapping paste (silicone carbide + lithium grease) and I'm suddenly at a bit of a loss as to what to do.

I believe I need to split and wedge the dowel so the cylinder is a tight fit over it (and thus reasonably likely to maintain the concentricity and parallelism of the bore) then apply the paste to it, put it in my lathe chuck, cover the ways with some towel and then move the cylinder back and forth over it until a good finish is obtained. Is that about the sum total?

Any tips on splitting it and what to wedge it with? I've only got basic woodworking tools!

Many thanks,

I'd split it with a fine-bladed saw, and use the same tool to cut the wedge. That part of the operation does not need great accuracy.

More to the point ensure you keep the lap moving, and as axially as you can.

Recently I carried out the opposite, externally lapping the end diameters of a shaft, with an aluminium lap turned to a close sliding fit on the shaft and in a die-holder. I cut the split in the lap with just a hacksaw, with a touch of a file along the cut edges to remove the burrs..

First off turn the dowel so that you have a length that is the same as the cylinder you want to hone, reduce the diameter after the turned diameter to allow you to pass the dowel right through the cylinder. Now you can slot the dowel and as the diameter behind the lapping part is smaller you will get the springing action you need for pressure on the internal wall of the cylinder.

the coarse and fine lapping grit will be embedded in the dowel so maybe make two dowels ffor the differnt grits.

Good advice from Nigel Graham, I've used wood dowel for lapping small bores by making the lap a fairly tight fit and not bothered to split it, and it's and it's always worked out ok.

When I did my Lion cylinders I used a plastic pipe split along it's length and packed with some foam, I did a video of it.

I've also used a lead lap, casting lead around a tapered bar, turning to diameter and the tapping it further on the tapered bar to expand it, it worked pretty well but I now prefer to use aluminium, plastic or wood.

Nigel

I would advise against any cloth, close to a rotating shaft. Use paper/tissue instead for safety reasons.

I would rough the dowel until it is a slightly larger dia than the bore and then take it out of the chuck and saw the slit into it, make the dowel longer than the cylinder that way it will always be in contact as you slide it up and down, sam elength means you end up lapping more of one end than the other.

Put slotted lapo back into lathe and then turn to a close fit in the cylinder. Charge with paste and lap, you may not need to expand it as you get finer a sit may well swell as it absorbs the oil carrier. Run about 250rpm and gently slide cylinder up and down the lap. I tend to put a board over the lathe bed as there is no way you will get tangled up in that. No gloves either.

I would also say throw the fine/coarse valve grinding paste that it sounds like you have into the back of a cupboard as it is far too rough for cylinder work. get some silicon carbide powders down to at least 600g, 1000g better for the final finish and these are mixed with a light oil to make them into a paste. Work your way down through the grits until you can't see tool marks and the bore is a dull grey.

I am a little surprised that one would chuck the lap & hold the cylinder. Surely, as the cylinder , often being of irregular shape would be better in a 4 jaw chuck. Then if something jammed it is just a round lap that slips in the hand & not an irregular shaped lump of metal. Worse still if one then dropped it.

Of course it should never jam, but bad luck( put that down to inexperience) happens

I only ask out of interest, having never tried.

Edited By Sam Longley 1 on 25/06/2021 16:42:49

Good advice Sam. I'd rather hold the lap than an irregular shaped component.

Surely, using a wedge would just expand one end of a parallel slot and result in single point annular contact? Not sure how easy it would be, but I would have thought a tapered slot to suit the wedge, or identical back to back wedges would be a better idea.

Depends on how you cut the lap, you can do a length wise slot almost all the way through and half way axial cut then drive a long wedge into the side rather than a short one in from the end. I I mostly make metal laps and have not used a wooden one for quite some time.

That's one reason why you hold the cylinder - you can swap it end for end regularly so you get a more even lap rather than a bell mouthed end.

Holding cylinder also means you don't get any lapping past coming out the back and running all over the chuck.

I actually find it easier holding an odd shape than a round one once your hands are all oily, infact for finned round cylinders I tend to stretch a bit of inner tube over them to save your hands. Same for honing I tend to hold the cylinder and run the hone in the pillar drill most of the time then muck just drips out the end and straight down where it won't muck anything up.

Ah so I think my first problem is that my dowel is 1/16" SMALLER than my cylinder - and all the advice so far has suggested turning the dowel DOWN to the relevant size - do I need to get a larger dowel? (at this point I'm eying up one of my brooms...). As it stands I have two of those 11/16" dowels of about 8" each and the cylinder is only about 1-1/2" long.

With regard to chucking the lap vs the cylinder - given the harsh warnings about using emery paper or cloth near the ways of a lathe, I would have thought that lapping compound INSIDE a chuck would be much worse?

To be 100% clear, am I slitting the END of the dowel, about half way, and inserting a narrow wedge into the end? Or am I splitting it in the MIDDLE of the dowel (with both ends somehow held together?)

My method of lapping my Lion cylinders seems to have attracted some criticism, which is fine, I'm always happy to learn from the experience of others.

While lapping my cylinders I didn't experience any significant torque applied to my cylinder, should I have let go of the cylinder there would not have been any possibility of the lap jamming or gripping the cylinder and have it flying around, the weight of the cylinder alone would just leave it hanging on the lap. In any case I was being very careful through out the process and didn't get any lapping compound in the chuck.

There are commercial honing machines where the part is held by hand, I would expect honing will be much more aggressive than lapping, at least in my case anyway, with much more torque applied to the part.

Regarding the cloth, well it was felt actually, that's a fair enough comment. The felt was packed inside the plastic tube to expend it with no real risk of it flapping around and getting caught in rotating machinery, but all the same with hindsight not recommended.

Cheers

Nigel

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Jason,

what speed do you run the three legged hone? Min speed on my pillar drill is 500 rpm

I tend to use the 2nd speed on the old Narok which is 730rpm.