lapping process

Am wondering about an effective and simple way to lap my large cast iron cylinder. Am thinking about using a polishing wheel and compound hooked up to a hand drill.

I would not recommend using lapping compound on cast iron. You are likely to do more harm than good because the lapping compound will impregnate cast iron and itself form a lap which will rapidly wear away anything running on it. Colin

O.K. That sounds reasonable. The question is how then would a c.i. cylinder be polished? I have thought about making a graphite impregnated leather piston ring and initially breaking the engine in with it, using it to burnish the cylinder walls.

Whats the diameter of the cylinder. What type of ring packing will it eventually have, what use steam, IC slow or fast reving????

Options are to make a metal or even hardwood lap and load that with lapping paste or powder/oil mix. Silicon carbide or "timesaver" should not impregnate but diamond paste will

Just using something like a narrow piston ring will risk making teh bore crooked.

Edited By JasonB on 19/09/2013 19:49:58

The best but not cheapest way is to procure a cylinder honing tool. i've used a split wooden piston wrapped with fine emery cloth so as to apply a light springy pressure. Colin

When lapping in ships Diesel engine fuel valves many years ago, the lapping blocks for lapping smooth and true the mating faces of the fuel valves which were of some special steel were made of CI. The grinding paste was held in the softer CI which wore away the harder steel. Just reinforcing what Colin was saying..........

Chris

To answer Jasonb: Cylinder dia. is12 inches (twelve). Steam at 40 pds. 80 to 120 r.p.m.

What rings/packing will you use? I would have thought at that size honing would do particularly if you are using iron rings. If you can't get the cylinder to a specialist then search out a Sunnen portable hone.

Whats the finish on the bore at the moment, freshly machined, old and pitted/worn?

Edited By JasonB on 20/09/2013 07:44:07

G'day all.

OT but different materials - I am in the process of making a replacement piston for my Sieg hot air engine. The previous one was leaking. By some mischance, I have made an aluminium piston which is a super fit in the (copper) power cylinder ... at one end only! It enters the cylinder for about a quarter of its length. Question - how does one get an EXACT fit full length? I have tried hand-lapping (I suppose that's what one would call it!) with Brasso, but it seizes up afte a couple of turns - using a balsa wood dowel of approximate size, soaked in Brasso. Using the new piston itself causes the same thing.

Any clues, please, chaps and chapesses?

Rgds

Bill

The packing that I would prefferably use is some sort of graphite empregnated woven square yarn.. The idea is to make a piston with a removeable top. Packing could be replaced from time to time without having to remove piston from cylinder. I happen to have some woven 3/8ths. square packing that I bought 10 yrs. ago that is so tough that I could not cut it with a knife. Can't remember what it is called , but it may be tough enough to to burnish cylinder by simply running it in.

Cylinder was bored out some years ago and has been stored with a thick coat of grease.Perhaps lapping may ultimately be unnesessary.

On a slightly different, but related note, I believe the leather packing rings in some of the earlier steam feed pumps were set with a loose fit so that steam pressure could get in back of the ring in order to expand and snug them up against the cylinder. Am wondering weather that is the practice also with yarn packing rings in larger bore applications?

Thank you, Michael.

Used your principles but different material - Delrin rod, turned slightly undersize , split and inserted fine wet and dry in split to wrap around rod, then lapped with light oil. Followed with Brasso and finally used piston with toothpaste to polish.

Somewhat gob-smacked to find that piston moves freely over full length of cylinder and (even more so) that with piston in place, filling space above it with penetrating oil, NOTHING drained through!!! I think that will probably do!

Rgds

Bill

That's going to be a nice size engine, I did a 1/6th grasshopper earlier this year from scratch which would be about 12" bore at full size so about the same size as yours.

First thing I would do is check what the existing machined finish is like, if its very rough you will be there for a month of Sundays trying to lap it smooth. You may need to remachine or if its reasonable could get away with honing either by taking it to a specialist of using a portable honing machine as I mentioned earlier, this is what the full size traction engine boys use to refurb the cylinders without removing them from the boiler.

If the honed finish is still thought to be too rough (which is doubt) then when the engine is erected you could use an iron ring in the packing grove and run teh engine with an external power source to bed things in. Or make an expanding wooden lap and make use of the cylinder cover to help guide it .

J

Jadoson, good advice. Perhaps there is a rentable hone of that size , If not , making an iron ring sounds like the second best choice. Do you have a pic. or two of that grasshopper on line?

The whole build is covered on MEM Forum and if you want to cut out teh chat between posts its currently being done on MEWS

J

Jadoson, good advice. Perhaps there is a rentable hone of that size , If not , making an iron ring sounds like the second best choice. Do you have a pic. or two of that grasshopper on line?

Oops on that last post.

Question to Bill: what's delrin rod?

Its just a plastic that is easily machined and reasonably hard though at your sort of diameter may be hard to find let alone afford a piece say 300dia x 300 long

Available from here and many others

Thats why I think wood would be the most cost effective at that size for a homemade lap maybe even glue up several discs of MDF.

Many years ago I used to watch a particular fitter hone the 8" diameter cylinders that took a cast iron piston which carried a punch for punching ally milk bottle caps through a die. He used a portable honer and Ken - as was his name - was recognized as the bloke who above all the other fitters was the only one that could achieve the desired fit. The punch and die drawings called for a clearance of .0002" so the fit needed to be perfect. I used to grind the OD's of the piston to within a nats but Ken provided the magic by finishing them of.

I think that what I am trying to say is that no matter what degree of accuracy you want to achieve when honing, its all down to practise and experience.

Rik

Bill, for your little hot air engine, a lap made from hard wood will do the job. Make a wooden cylinder to fit the bore, cut a slit length wise for a couple of inches, in the open end of the slot put a wood screw, or a wooden wedge, this is to expand the lap. Fit the lap in the lathe chuck, apply the grinding compound, and a bit of thin oil, run the lathe slowly, hold the cylinder to be lapped in your hand, this is the one place I do wear a glove, because the work can get quite warm. Take your time. The dry piston should fall down the bore under its own weight, and if you block one end of the bore the piston should stop, or move very slowly when you have the right clearance. Are you sure the cylinder is copper? I would have thought it would be brass, or better still bronze. Aluminium is not the best friction wise, or expansion wise either. Sorry bit OT. Ian S C

Took a look at your grasshopper build. Beautiful work. Cannot figure out how the multiple angles are acheived unless your milling machine has a swiveling head that is hooked up to a computer.