Lapping compound

[SmoggyTurnip]

I am trying to get together the materials to build a stirling engine. I have stainless steel for the power cylinder and graphite for the piston. Any ideas for lapping compound? I don't know much about it. Remember I live in Canada.

 

[Brian Rupnow]

And why would living in Canada be an excuse for not knowing about it? ??? Jeez, come on---Those good old boys down in USA think we live in igloos and trade furs for a living as it is!!! ;D ;D
(Okay, enough smart a$$ing---I don't know either). I do know that for aluminum and brass, toothpaste is a good lapping compound. For something as hard as stainless and as soft as graphite, toothpaste might work fine there also, except I think any material removed would all come off the piston.---Brian

 

[Mike N]

And why would living in Canada be an excuse for not knowing about it? ??? Jeez, come on---Those good old boys down in USA think we live in igloos and trade furs for a living as it is!!! ;D ;D
(Okay, enough smart a$$ing---I don't know either). I do know that for aluminum and brass, toothpaste is a good lapping compound. For something as hard as stainless and as soft as graphite, toothpaste might work fine there also, except I think any material removed would all come off the piston.---Brian

Typing paper works great to lap Graphite. Comet or Ajax mixed with light oil makes good lapping compound. Or you could just buy the real stuff from McMaster Carr.

 

[joeby]

Mike N has the right idea, I wouldn't use lapping compound with graphite, it will most likely turn the graphite into a black, messy paste, and could cause problems from grit embedding itself into the softer graphite.

I actually just use the back (cloth) side of emery cloth, it's slow going; but controllable.

Kevin

 

[malcolmt]

Hi Brian
I may be misunderstanding you..... But i get the impression that you are thinking that you would use the piston in the barrel with a lapping compound between the two.

My understanding is that you would lap the bore with say a piece of nylon or some such material charged with paste to lap the cylinder then lap the piston with something as has been suggested "the back of Emery paper" to make the piston fit the bore.

If i have misunderstood what you where saying please forgive me.

Kind regards

Malcolm

 

[Brian Rupnow]

Malcomt--You raise a good point. My understanding, in terms of an aluminum piston in a brass cylinder is this. You finish the cylinder bore first, preferably by drilling and then reaming it to size. Then you turn the piston untill it is a very light interferance fit into the cylinder. You then coat the outside of the piston with toothpaste (which has a superfine abrasive in it to get your teeth really sparkling white). Then you attach the connecting rod to the piston and manually slide it into and out of the barrel untill the interferance fit becomes a "sliding fit". This should be a staight linear "in and out" motion, with little rotation involved, and the piston should be rotationally aligned with the cylinder in the same position it will have when things are "finish assembled". That is my understanding of how one "laps" a piston/cylinder.

 

[Kermit]

I think you DO want the rotation. As I have come to understand the process it would involve wearing away the metal by rotation AND the in and out motion you mention Brian. This would increase the "roundness" as well as enlarge the bore leaving all the scratches going at 45 degree angles to the ultimate work driven friction contact. The added benefit of the angled scratches is to increase the flow of lubricant along the entire length of the cylinder during the stroke of the piston.

Kermit

 

[Brian Rupnow]

Kermit. ---You may be correct. I know that when honing a cylinder on an internal combustion engine (automotive) you try for a diagonal cross crosshatch pattern on the cylinder walls to hold some lubrication and to allow the compression rings to "seat" better. However, on a small air or steam powered engine with a couple of radial grooves around the piston to carry the oil film, and no "piston rings" it may not be as necessary. Personally, I have not had a great deal of success lapping cylinders with the method I described above. This may be out and out blasphemy, but I have had more success machining things for a "snug" sliding fit, then assembling everything, giving it all a good coating of 30 weight oil, chucking the variable speed electric drill onto the output shaft, and let the sucker run for a couple of hours on the floor to "break everything in".

 

[joeby]

Kermit,

Might you be confusing lapping with honing? Typically, a cylinder hone will leave the crosshatching.

Lapping a cylinder would normally be done with a lap turned from a softer material than the cylinder, and with rotation. Most times, I would rather use an expanding lap to take up wear and allow you to adjust the lap oversize and recut it to true it up. They can be purchased (barrel lap) or easily made. Lapping is not good for removing a lot of material; but more suited for a finishing operation to improve surface finish and bring to size. You can make an external lap to do pistons also.

I've never lapped a piston in it's bore. I don't like the possibility of the lapping compound becoming embedded in the piston (or cylinder), and I think you can get a closer fit by lapping the cylinder and polishing the piston to fit (the lapping compound has to fit between the two, so you have built clearance in there already).

Just my thoughts on the subject.

Kevin

 

[Mainer]

I'm with joby. I lap the cylinder with an inside (barrel) lap and the piston with an outside (ring) lap. If you try to do them together, what comes off what is totally random.

In the case of graphite, with my Miser I took Jerry Howell's suggestion and used a piece of paper wrapped around the graphite piston to take it down to a nice fit in the previously-lapped cylinder.

As far as a source for lapping compound: Brownell's www.brownells.com at one time sold a multi-grit kit of something like 6 or 8 assorted grits in 2-ounce cans, which I've found to be more than sufficient. I don't know if they still carry the assortment, but it would be worth checking.

 

[Kermit]

Joeby I think I'm confusing Lapping with Honing...

Gotta keep an eye on me


Thanks guys

 

[dwentz]

Honing is done with a set of stones in most cases.
Lapping is done by a charged carrier that holds the lapping compound. I use copper laps as I have a set of them and they work real well. I will try to get a photo to show everyone. I have home made ones and a bunch of commercial ones that I found at an auction.
Lapping is also not done in most cases with the piston in the bore. Make a blank to use in place of the piston, and lap using that. Clean up real well, then hand fit the piston. I have also not had much luck lapping aluminum. It works, but the aluminum seems to always keep hold of a little bit of the lapping compound.

As mentioned, ToothPast and Bon_Ami make good lapping compounds. The auto parts store sells valve lapping compound but it is too course for most of the work we do. You can buy it commercially as Clover Lapping compound, but it is spendy and you have to buy a large amount.

A good source for find lapping compound is the lapidary shops. They sell it in multiple grades by the pound in a dry form What you are looking for is Silicon Carbide, and Aluminum Oxide. It comes in all kids of sizes, and I like 1000 and 1500 A pound should run you about $5.00 Which is a life time supply. I mix it with a little machine oil and it works winders. They also sell a aluminum oxide polish that works really well as a final lap and as a polish.

Dale

 

[joeby]

One point to keep in mind when lapping is that the lap needs to be softer material than the part being lapped. The grit will be embedded in the softer of the two, and you don't want that to be your finished part.

The lapping compounds made commercially come with different types of abrasive materials. Some break down rather easily, I think carborundum is one; but others can stay behind to ruin your day. I don't like any of the diamond paste for this reason. It's difficult to say for sure if you've gotten it all cleaned out, and if not, it will keep cutting.

I have made some small diameter laps (1/8" and under) from brass. It's a simple job to make these up, then smear a little lapping compound on a piece of steel or glass. Take another piece of like material, put your lap on the first plate, set the other piece on top and roll the lap between with some downward pressure. Now your lap is charged and if you are careful to not force it, it will cut very well.

Kevin

 

[Kludge]

I use toothpaste and Bon-Ami, dependent on what's being lapped.

Bon-Ami is made of feldspar and breaks down as it's used, thus earning the self-proclaimed reputation of "hasn't scratched yet." Ajax et al don't work as well (in my opinion and ONLY in my opinion) since they tend to be more abrasive ... though that may be a good thing. I don't know. In any event, I also use it with oil instead of water which isn't a universally accepted method. This works for me; I can't and won't speak for others.

BEst regards,

Kludge

 

[dwentz]

Here are the photos of the laps that I picked up at an auction.

 

[dwentz]

Here are some photos of the laps that I purchased at an auction. I think I gave $3.00 for the lot.

Dale

 

[BobWarfield]

Nice selection of laps!

I have a vague recollection Mcgyver has made and used similar laps, but I don't know if he still frequents this board.

FWIW, another lapping compound I see used a lot if Mother's Mag and Aluminum polish. See here, for example:

http://www.cnczone.com/forums/showthread.php?t=53421&highlight=lapping+ways&page=13

Cheers,

BW

 

[Paula]

One point to keep in mind when lapping is that the lap needs to be softer material than the part being lapped. The grit will be embedded in the softer of the two...

Not necessarily, Kevin. Many commercial laps are made of softer metals, but it's not a requirement for successful lapping. If the lap is not soft enough for the grit to become embedded, then there is simply no embedding. For example, I often use a steel lap on a cast iron cylinder with excellent results. The grit cuts until it breaks down, and doesn't become embedded in either the lap or the cylinder.

In fact you could even lap a brass cylinder with a steel lap, as long as you used an abrasive hard enough to cut the brass, but soft enough to not embed itself (e.g., garnet).

Paula

 

[Bluechip]

Hi Folks

Paula ... 100% right, I have used steel laps together with this stuff made by some of you nice USA folks

I recommend it .. non embedding, cuts fast, ...

http://www.newmantools.com/lapping/time.htm

Had mine for years, forgot the UK lot who were the sole importers at the time, several grades IIRC, better to say what you want to use it on maybe .
One of the few products from any source that I've bought that actually did what it says on the tin ;D

Dave

 

[Brian Rupnow]

Jeez----After seeing all the posts on this subject, I don't know how I could have got it so wrong!!! I'm pretty sure that the advice I gave on putting toothpaste on the piston and working it in and out of the cylinder by hand was given to me by someone on this forum, way back a year ago when I started making my first steam engine. So---No friggin wonder I never had much success with it. Ah well, colour me stupid. : :