lapping process

[richard orrParticipant @richardorr31043]

[richard orrParticipant @richardorr31043]

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.

[colin hawesParticipant @colinhawes85982]

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

[richard orrParticipant @richardorr31043]

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.

[JasonBModerator @jasonb]

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

[colin hawesParticipant @colinhawes85982]

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

[ChrisHParticipant @chrish]

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

[richard orrParticipant @richardorr31043]

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

[JasonBModerator @jasonb]

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

[Cornish JackParticipant @cornishjack]

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

[MICHAEL WILLIAMSParticipant @michaelwilliams41215]

Bill ,

(1) Make an aluminium lap which just goes through the bore and with a working length which is much longer than cylinder .

Have lap running true in lathe and work cylinder back and forth over a long distance carefully by hand – turning end for end occassionally .

Use finest lapping paste . Ideally lap should be adjustable for wear and for sizing but with your sort of job this is not nescessary .

Take care that job does not seize – plenty of light oil will help .

(2) If it can be done set cylinder up to run dead true and use an Aluminuim lap held in tailstock . Make working section short in this case and run right through bore and almost out of the ends each stroke .

(3) Make a simple D bit style reamer from silver steel and work it through bore by hand .Size so that it just enters bore at the larger end .

Regards ,

MikeW

Edited By MICHAEL WILLIAMS on 20/09/2013 11:25:24

[MICHAEL WILLIAMSParticipant @michaelwilliams41215]

Richard ,

To lap such a big cylinder properly will require some proper equipment . There is no easy way of doing the job just by working a lap through randomly by hand .

Ideally work would be done properly on a lathe , milling machine boring machine or honing machine but in abscence of these here some suggestions .

(1) Similar to method for Bill above . Make a lap which will just enter the bore at tightest spot and with a working section much longer than your cylinder . Aluminium ideal but probably more practical to use wood . Has to be truly parallel and round .

Clamp the lap somehow and then with fine grinding paste work cylinder along and around lap by hand . Vary amount of turning and length of stroke randomly and also randomly turn cylinder end for end . Takes a while and lap may need to be renovated now and then but will eventually produce a smooth parallel bore .

(2) Use honing stones and make a device to hold the stones in a spider half way along a long round bar which will act as a centering guide . Arrange some sort of bush bearings in two bars clamped to cylinder ends and set long bar to run true in bore . Set stones to be just tight in worst part of bore . Arrange a drive and work honing stones back and forth through bore . Adjust if nescessary . Takes a while to set up but then works rapidly .

Could use Aluminium bar laps instead of stones .

It is also possible to actually bore cylinders this way – most of the large railway workshops had portable cylinder boring machines working on variations of the guide pole principle .

Regards ,

MikeW

[richard orrParticipant @richardorr31043]

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?

[Cornish JackParticipant @cornishjack]

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

[JasonBModerator @jasonb]

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

[richard orrParticipant @richardorr31043]

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?

[JasonBModerator @jasonb]

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

[richard orrParticipant @richardorr31043]

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?

[richard orrParticipant @richardorr31043]

Oops on that last post.

Question to Bill: what's delrin rod?

[JasonBModerator @jasonb]

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.

[Rik ShawParticipant @rikshaw]

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

[Ian S CParticipant @iansc]

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

[richard orrParticipant @richardorr31043]

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.

[JasonBModerator @jasonb]

Fixed head, No computers, which part are you refering to?

[Cornish JackParticipant @cornishjack]

Thank you Ian.

As written above, the piston is now as good a fit as I am ever going to get. The cylinder certainly looks like copper but I'm no expert! The use of aluminium for the piston is as the original Sieg kit.

Rgds

Bill

PS sort of runs, but not very well – more work needed

[Author]

Posts