Lapping a small bore

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Greg_Lewis
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Lapping a small bore

Post by Greg_Lewis »

I’m taking a break from the steamer and the unending home repair projects and am playing with a little PM Research wobbler engine. I’ve got the aluminum cylinder casting bored to within a couple of thousandths of the final 1/2-inch bore. I’d like to run some 400-grit paper into the bore to shine up the almost invisible tool marks from the boring bar. While I know there are commercial laps and hones that will do such things, I need to come up with something that I can make in a minimal amount of time and with minimal effort, as it will only get used for perhaps 10 minutes and then never again.

I tried slitting a 1/2-inch wood dowel but that presents issues of having the two sides of the slit being parallel, and the issue that the diameter of the dowel isn’t consistent along its length. I’d like to keep the bore straight, and not end up with a taper. I suppose I could try to even out the dowel diameter but that doesn’t solve the parallel problem. As I see it the two sides of the dowel would have to be such that the overall diameter could be reduced just a smidgen to get it into the bore, and then expanded back a smidgen so the paper would contact the bore. And all I see in this is some sort of fiddly method that I can’t visualize.

So what ideas do you have for this? THANKS!

The little aluminum cylinder. The tool marks from the boring bar are quite minimal but there just the same.
IMG_0415.JPG


I tried this bit of dowel. I could slit it all the way along but I but can't figure out how to get both sides parallel without fancy additions.
IMG_0416.JPG
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milwiron
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Re: Lapping a small bore

Post by milwiron »

I try to get within a tenth or two not thousanths before thinking about lapping but that's just lazy me.
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Greg_Lewis
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Re: Lapping a small bore

Post by Greg_Lewis »

milwiron wrote: Fri Sep 16, 2022 1:56 pm I try to get within a tenth or two not thousanths before thinking about lapping but that's just lazy me.

Agreed, but in this case the tool marks are probably .001+ deep, and the part doesn't need to be exactly .5 as the piston and rings will be made to fit.
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Our motto: "That looks about right."
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Fender
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Re: Lapping a small bore

Post by Fender »

The way I would do this would be to obtain a short piece of 1/2” OD copper tubing, and spin it in the bore with lapping compound. You might have to skim a few thousands off the OD of the tube to get it into the bore. Make up an expander with a long 8-32 socket head cap screw and a stack of rubber washers with steel washers and a thick nut on the outside. The washers need to be a fairly snug fit inside the copper tube. When you tighten the screw in stages, the rubber washers will gradually expand the copper tube, so go slowly on this.
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rmac
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Re: Lapping a small bore

Post by rmac »

Fender wrote: When you tighten the screw in stages, the rubber washers will gradually expand the copper tube
Really? Does the tube have to be slit lengthwise for this to work?

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Dick_Morris
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Re: Lapping a small bore

Post by Dick_Morris »

Could you push a hardened ball a thousandth or so bigger through the bore to burnish it? It would give a smooth surface and possibly harden it. Success may depend on the grade of aluminum.

My recollection is that you do some 3D printing. Something I've thought about is to print a hone with an appropriate cone shaped hole and plug to expand it. What I've read about home made hones is that they should be a softer material, like copper. The 3D print would certainly be relatively soft and valve grinding compound would imbed in the surface.

I cringe at the idea of "honing" with a dowel and sand paper.
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Dick_Morris
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Re: Lapping a small bore

Post by Dick_Morris »

Too late now, but another alternative would have been to bore undersized and use a reamer to take the bore to final size and finish. I've had a few reamers bought for specific jobs for many years, but a couple of years ago I finally broke down and bought a set. I've been surprised by how often I use them now that I have them.
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Greg_Lewis
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Re: Lapping a small bore

Post by Greg_Lewis »

Hm. Good thought, Dick. The bore is .005 under. I've got an adjustable hand reamer that could be fed with a center in the tailstock, but I'm nervous about that. A spiral flute chucking reamer would run me about $40 and I don't think I want to go that route for something I'll likely never use again and for a project that is really just a toy. But I'll check with a local friend who might have one to lend.
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Bill Shields
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Re: Lapping a small bore

Post by Bill Shields »

Can you put a piece of rid into a mill and slit it axially with a jewelers saw?

I have tapped the end of a bar with a tempered pipe thread, then slit the rod axially.

Inserting a socket head plug allows me to adjust the diameter....granted it results in a OD taper, but by the time you push it through the cylinder a few times with lapping compound, it is a moot point.

Note: Fenders trick with the copper tube and expanding plugs does work. It helps to anneal the tube....and be careful you do not over expand or you are going to turn the tube down to size.

It is just like using the expanding plug trick to expand tubes into a tube sheet.

I personally have not had a lot of luck with expanding reamers giving a really good finish.
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bushav
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Re: Lapping a small bore

Post by bushav »

Expanding reamers have never delivered on their promise for me. It’s a “guess o reamer”
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Greg_Lewis
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Re: Lapping a small bore

Post by Greg_Lewis »

bushav wrote: Fri Sep 16, 2022 10:06 pm Expanding reamers have never delivered on their promise for me. It’s a “guess o reamer”

That's why I don't want to chance it this time. I've used them where it really doesn't matter that much and the finish wasn't the reason I was reaming.
Greg Lewis, Prop.
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Bill Shields
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Re: Lapping a small bore

Post by Bill Shields »

You could always bore it slightly oversize and slide in a piece of thin wall brass tube with the desired ID.

I have never really liked aluminum in this application.

A bit of loctite and off to the races.. :mrgreen:
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