Lap Dance

[d-m]

Hi all
I have a lapping question I am starting my stirling fan project (Xmas present from the wife) for the power cylinder the plans recommend lapping in the finish.Surfing the web I found a lot of info on for gun barrels mostly better or worse type of post. Found a co that makes a expanding mandrel type lapping tool not real $$$ but I need other tools that those $$$ could buy. I machined the cylinder from ( what I call ledloy) 12L14 and the piston is graphite. So can some one show me a simple lapping tool, should I even bother with it. and run with "I would lap it with a split chop stick with emory cloth". use some fine stuff.
As tattoomike68 suggested I love the simple approach
Thanks Dave

 

[gilessim]

Hi, just turn a piece of close grained hardwood like maple to a close fit, hold that in the lathe and use metal polish ,like Brasso, if you have that there, if there are no tool marks that will work fine, emery cloth will tend to flare the ends of the cylinder if you're not careful!

Giles

 

[Bogstandard]

You can in fact use almost any material for the lap as long as it is softer than the part you are lapping. For steel, I use brass, for brass I use ali, for ali I would use a nylon rod.
You should make the lap a nice sliding fit into the bore, and ensure it is parallel along its length.
If starting off with a grinding paste, dab a bit of paste onto the lap, and roll it on a hard surface and use a piece of hardwood to embed the cutting agent into the lap. What you are doing is making a very accurate round file. For soft cylinder materials like brass or ali, on no account use one of the diamond lapping compounds, it embeds itself into the cylinder wall and will act like a file on your piston.
If possible make the lap a bit longer than the cylinder and always try to have the whole length of the cylinder on the lap at all times, this will tend to lessen the bell mouth effect.
Grinding paste will break down during use and go into smaller particles. You go from fine to finer compounds until you have a perfectly smooth bore. Normally you would then lap the piston to the bore, but in your case where you are using graphite, this can not be done, so the bore has to be smooth in its own right, then make the piston to fit.

John

 

[Brass_Machine]

...I am starting my stirling fan project (Xmas present from the wife) ...
Thanks Dave

Hey Dave,

Thats a great idea. My wife loves the sterling fans as well. Are you building from plans?

Hi, just turn a piece of close grained hardwood like maple to a close fit, hold that in the lathe and use metal polish ,like Brasso, if you have that there, if there are no tool marks that will work fine, emery cloth will tend to flare the ends of the cylinder if you're not careful!

Giles

Thanks Giles (and Bog) I was getting ready to ask almost the very same question about lapping.

Eric

 

[d-m]

John best advice so far, printed it off ,It's in a nutshell and I can easily follow. Can you recommend some compound?
Thanks


Eric
Yes building from plans published by Jerry Howell. Nice plans bit advanced for me never reading plans before.Luckily for me I have a good friend in San Diego who has more than likely read more plans than we would care to know. He has been vary helpful in this my first real project.
Here is the link to the one I am working on let me know if you are going to do one and we can compare notes.
I think spring is just around the corner and we all know what that means. Hunny do this hunny do that so this may take a back burner for a bit
http://www.jerry-howell.com/MiniFan.html
Dave

 

[Bogstandard]

Dave,
The way I do it works for me, someone else might have different methods.
For your leaded steel I would start with fine automotive grinding paste (used for grinding in valves), at small pot will last you the rest of your life.
The lapping action is, have the lap in the lathe chuck running at a very slow speed, and feed the cylinder on to it, get ready to let go quick, just in case it 'bites' and starts to turn with the lap. It doesn't take long to lap a part, usually only a few minutes. The hand action is (and no dirty thoughts here) while moving the cylinder up and down the lap, use a bit of wrist action to give a twist to the up and down action.
Then when the first charge has stopped cutting and gone to a sludge, have a look at the bore. It should look a dull finish all the way along with slight herringbone scratch marks from the lapping. If there are any machining marks still in there, they will have to be removed by further lapping with the grinding paste. Just recharge the lap and repeat. It will be the machining marks that will cause damage to your very soft piston. You just keep repeating until ALL machining marks are gone.
In your situation where you won't have a metal piston to lap fit, what I would do is turn up another lap to fit the just finished rough lapped bore. Then using something like metal polish or t-cut, pop a bit onto the lap and repeat the lapping process. You can go finer and finer with the polishes, it just depends what you want the finished bore to be like.
What you do is make the piston to fit the bore, for graphite you can polish it down with newspaper wrapped on a bit of flat wood to polish the outside of the piston and bring it down in minute steps, but make sure you keep everything square.
If done really well, the piston should slide really smoothly thru the bore, but when you put your finger on the end of the bore, the piston should stay where it is in the cylinder, held by the vacuum caused by the piston trying to drop. That is of course if you haven't already put a mounting hole thru the piston. In that case a bit of sticky tape over the hole will allow you to carry out the check.

I hope this has helped.

John

 

[gilessim]

I just wanted to add here that, although what John says is all true, there is "honing" and "lapping", honing is more of a "lets take some metal off" thing i.e. tool marks etc. whereas lapping is more of a polishing operation with finer paste or metal polish ,even toothpaste! , I've had problems with grinding paste in that it's normally made from carborundum (silicon carbide) which I think has a hardness on the Mohs scale of about 8.5 (diamonds are 10), some break down and some don't, I think that some are made from sand or some such grit, you should try it first or you could end up with a rougher bore than when you started!

Giles

 

[gilessim]

Sorry John , you got yours in while I was writing mine!!

 

[compound driver 2]

HI
Just honed the bore on my 2 inch tractor. Used a brake cylinder three point floating hone. Cost £16.00 from ebay and replaced the one I lost. The floating heads will reduce the chances of belling as long as you watch your over travel.
With out much effort an hour should see the job done.

Cheers kevin

 

[Bogstandard]

They are good Kevin, but anything under about 1" and it is a bit tight to get in.
I only just managed to do my poppet valve engine, and that was 1" bore, but it did a lovely job.

John

 

[d-m]

Thanks every body and I sure I have a handle on this now was going to give it a try yesterday. But the wife wanted to paint the laundry room lol. All she needed was a hand pulling the shelves and moving the washer dryer. Got the units out and found a vary rotten floor and some mold so you know were my weekend went. What a mess to say the least just got the new floor in and she is now painting :big:

John
Bogstandard
I want too thank you fro your help and I did have a chance this morning to read thought your post on the turbine.
I can only hope with some practice and time that I will gain the skill and knowledge to see work like that come from my shop
OUT STANDING