Ramon "Tug" Wilson did a really good thread on lapping small, under 20mm diameter bore on another site, its probably non PC to mention it here, but it deals with model engine making. It basically demonstrated how to produce a suitable adjustable lap, how to use it, and the grit to use. Very, very interesting.

cheers

Bill

Edited By Bill Pudney on 10/08/2013 02:50:31

Edited By Bill Pudney on 10/08/2013 02:51:06

JasonB, I agree that lapping would in my oppinion be the way to go. Ian S C

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If I recall correctly:

Honing involves the use of hard abrasive stones, typically more than one, mounted on a mandrel … and finishes to a preset size.

Lapping involves a relatively soft "plug" loaded with abrasive paste … and the finished size is therefore perhaps less predictable.

Just to confuse matters: one of the classic Honing machines is made by Delapena.

MichaelG.

Reaming as against honing/lapping, the reamer brings a hole to a set size and depending on the quality of the reamer and the amount of metal removed gives a parallel hole with a good finish suitable for most engineering purposes.

Honing/lapping is used to bring a bore to a size so that a pre-made item (Piston) will fit with the correct clearance. Lapping is used to produce a very fine surface for rotary or longitudinal motion, again as in a model diesel motor.

Automotive honing has an automatic system (Sunrod) where the bore size is dialled in and away it goes until that size is reached. The spiralling motion of about 45 deg. suits the bedding in of the rings on the piston. In fact the bore when viewed is quite visibly marked. The bore, if badly worn can be bored to a set size and then is honed to tolerance. In all, 4 cyl. can take about 40 mins. with a bore size of 3" to 4".

For most modelling applications anything up to half inch is OK to ream, above that as the clearances increase then lapping can be used. Not always, as it can be difficult to hone or lap a large dia. hole of limited width so other methods are better.

making small spindles in a steel mount I allowed 1/100 mm for 10/15mm dia. This being a hand turned operating knob where any slackness would show up in the optical measuring system.

Clive

What was the problem you encountered with reaming? In what application?

Reaming is going to be a much more rapid metal removal method than honing. Honing is essentially a finishing technique.

Couple of things that make reaming go better include drill the hole undersize first, then bore the hole to within 5-10 thou of finish size. Boring gives a straight, concentric hole for the reamer to then follow. Machine reamer, or at least a fixed hand reamer, will more easily give a good result than an adjustable hand reamer. If I have to use an adjustable hand reamer, I will bore to within a couple of thou, then ream, then take the last half to one thou out with a brake cylinder hone. If you are reaming brass, a worn old reamer actually works best. A new sharp reamer will dig in. Running a small oilstone over the cutting edges of a new reamer will dull them enough to work well on brass/bronze.

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Ken,

Attempting to answer these points

• It's often better to ream more than a couple of thou
• Dulling the edge will effectively give negative rake [compare the clockmaker's five-sided cutting broach]
• I'm guessing that the first cylinder was better; either because the machined bore was slightly smaller than the second, or because you were pushing harder.

MichaelG.

I think I know where Ken is coming from as I had the same questions a couple of years ago. The question breaks down into getting a) an accurate bore of correct diameter, and b) a suitable surface finish. I would add a new variable in that striving for either the bore or finish can give an incorrect bore profile, especially a 'barrel' with a larger bore in the centre or a 'flare' with larger diameters at the outsides. I think this is the hardest error to correct for.

A reamer gives a good parallel bore of true diameter, up to perhaps 1" in non-professional hands, but the finish might be messy, as Ken says. I find that a flap of emery or wet and dry paper, held in a small wooden split stick, can produce excellent finishes, working from 200-1000 grit with plenty of paraffin or WD40 to wash away material. The job rotates in the lathe at a few hundred RPM and the stick is carefully moved in and out to avoid any creation of the barrel effect. Problem is it also enlarges the bore by 0.0005" or more.

Honing (using stones held on a frame) is obviously a superb technique in the professional machines described above as the bore is enlarged from its rough cut by a few thou and the finish of cross honing is perfect for cast iron rings in a cast iron bore, and parallelism is maintained. But, it is most suitable for bores of 2" diameter and upwards. I have seen sets of small hones at the ME Shows and wander what success people have had with them?

I did some web research and lapping (grinding paste on soft mandrels) is the described technique for getting accurate bores of suitable finish. I wanted to make some bronze bushes in which would run stainless pushers of diameter 0.8749 – 0.8750. The bushes were 1" long and the differential thermal coefficients of expansion (appx. 0.001" between stainless and bronze over 100 degC) meant careful measuring at a fixed temperature. I bored the bushes to 0.8735 – 0.8740 and then lapped them in the lathe using a home made mandrel with an aluminium body of nominal 0.8730 diameter and an end taper wedge to progressively enlarge it from the centre. The mandrel was designed to enlarge as a slight barrel, so as to counter the effect of it producing a 'flared' bore diameter increase in use. Diamond pastes, plus light oil, starting at 40 micron (400 grit) would remove a thou in 60 seconds. 14 and 5 micron then produced near polished surfaces. Absolutely thorough washing before measuring or changing pastes was essential.

Lapping this way produced bores that were so close in diameter to the stainless pushers that they were locked solid at 15 degC, just moved at 25 degC, and slid like glass at 35 degC. If it is an iron piston in an iron barrel one will not see this temperature effect.

I have no professional training but taught myself, so make a mandrel lap Ken and have a try! PM me for more if you wish as I have said far too much in this reply and will have bored too many.

on some of my hot air engines I have used a lap made of wood, and charged with grinding paste. The woden cylinder is split length wise, and a wood screw, or wooden wedgeis put in the open end of the split. The lap is held in the chuck of the lathe, and run at a moderately low speed, while the cylinder being lapped is moved back and forth by hand. This is one of the only places I wear a glove, there is a bit of torque, but the work gets quite warm, it's a job that takes time. Ian S C