# pseudo lapping technique



## PeterT (May 10, 2020)

I've had some challenges making smallish cylindrical shaft surfaces for my radial engine, kind of learning as I go. Compared to the valves which are turned down from SS stock, the rocker axles seem dead simple. They are made from O1 tool steel as they need to be hardened. Tool arrives looking nice & shiny but its actually about plus minus 0.0005 oversize. The issue I have found is its actually elliptical in shape, not circular like a gauge pin. You can verify this my taking measurements at right angles on the same portion of shaft. Correcting both the diameter (within tenths) and shape simultaneously sounds like a job for lapping. I have made a few tools like this squeeze clamp style with replaceable brass or aluminum cartridges. It works but lapping is messy business. And you cant get a lap on & off unless the the shaft is the same diameter. So for example turning in a lathe where you have the enlarged end knobby in place related to tail stock support.

So I gave this method a try fully thinking it will be yet another way of how NOT to do it. But I'm actually very impressed. In about 15 minutes I've made myself some nice sections of 'true' 5mm shaft stock that are within a tenth of target dimension & desired finish. And its a very controlled process. I got the idea by (carefully) mic-ing various wet/dry papers in the 600, 1000, 1200# range. I was impressed with how consistent they are across the sheet & across other sheets of the same vendor. For example the 600# I have measure 0.0075". Amazingly for some reason my 1000# thickness was quite close to 600#, within tenths, otherwise I would make a separate lap for each grit. So I worked this dimension in as an allowance, both annular (surrounding the stock) and between each lap face. Hopefully the sketch makes sense.

So I started by milling a block of aluminum, cut in half so it gave 2 symmetric pieces. Clamped them together face to face in the vise & drilled the oversize hole (0.0075 annular = 0.015" diametric). It would have been much better to ream the hole for a better finish but I didn't have the right size. So I chose the next closest numbered drill I had. There will be some buffering with the paper itself I guess. Then separate the 2 lap halves, position in vise & simultaneously mill off the face allowance from both. make a small transition radius on the hole corners The idea was to place 2 strips of abrasive paper clam-shelling the stock. If everything is dimension-ed reasonably close, by the time you fully squeezing the assembly together, it should be getting close to target dimension. You could spin the stock in the lathe, or as I did, just grip it in an electric drill. The benefit is the 'tool' never changes shape or wears out by lapping action. Fresh abrasive is a postage stamp area of wet/dry paper.

Well it worked out way better than I expected. You can actually feel a bit of vibration initially, which are the eccentric bumps of the shape. I used 600#, then 1000# mic-ing every so often. A longer section of shaft (3-4") means you are probably lapping more consistently across any one area. I figure 5 seconds of lap-sanding was about 0.0001" so it was actually quite easy to sneak up on the target dimension & finish simultaneously. I tried 2 strips of paper & also a single wrap just to get a sense of how much of the paper was in contact & cutting.

This is not a good way way of removing too much stock. Its kind of like lapping but without all the mess. Digital photo's tend to exaggerate machining marks so hopefully you can see the contrast of the finished rod & the aluminum tool. Drill holes are nasty looking surfaces under magnification once you split them open.


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## PeterT (May 10, 2020)

I posted the above on a model engine forum & another individual mentioned this YouTube video. The first part shows a similar dry lapping technique.

Anyways a relatively easy trick if you have some oddball dimension shaft to make or condition.


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## Marc Moreau (May 10, 2020)

Look so  simple  but this engine is very expensive . Those engine are jewell and very good. Nice engine.


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## RobinHood (May 10, 2020)

Fabulous work as always Peter. Your attention to detail is out of this world.  Makes my stuff look like you know what...



PeterT said:


> The issue I have found is its actually elliptical in shape, not circular like a gauge pin. You can verify this my taking measurements at right angles on the same portion of shaft.



I wonder if that is related to the interaction between tool pressure, the modulus of elasticity and the aspect ratio of the part.  Maybe even tail stock pressure, if used as a support. Here are my thoughts:
a) tool engages part, and pushes it some distance away because part is elastic
b) pressure increases until a chip is starting to form, now the part “rebounds” toward tool (maybe even “dragged“ into the tool by the geometry?)
c) because of the bigger chip, the pressure increases and overcomes the part’s elasticity and pushes it away, resulting in a smaller chip and lower tool pressure
d) the part “rebounds” again, and the cycle continues

this is happening rhythmically, depending on RPM, DOC, and feed rate. A higher aspect ratio would amplify the elliptic path of the part. So would TS pressure as it would make the part appear to be more “plastic”.

we actually use this property to tune arrows: the TS is our string supplying the “push force”. We increase (or decrease) the point weight to decrease (or increase) the “spine” (stiffness => apparent  elasticity) of the arrow. All arrows flex when shot. It is of importance how fast they “recover” from the flex that is key to good performance and accuracy.

Getting back to machining: I wonder if a feed through bushing support would help you get more round parts? I saw a video quite some time ago where a fellow did exactly that. He was turning really thin, long shafts very accurately using that method. Just had to keep the bushing lubricated.


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## PeterT (May 10, 2020)

Thanks Rudy. I think all your comments apply. I agree the in-feed cutting forces become more significant relative to skinny parts. And some materials like stainless don't like to be shaved & dusted. they just work harden & you end up rubbing. 

The bushing/collar support has merit for sure. I suspect the stock OD & collar ID should be pretty close fit & finish. It would be good method for constant diameter stock like drill rod. It might not be as practical on turned/reduced parts (if I understand you). This valve work-in-progress is an example. I had to do the turning & finishing in one setup. Live center support is necessary to keep the shaft as straight as possible. I left it on right through to block sanding and (split) lapping. Even so I could feel different pressure on the live center engagement in the center drill hole. I bet stress relief, changing temperature through the cut... etc.

I've have seen some neat, miniature travelling steady ideas for turning or (worse yet) threading real long, slender rods. Some use 2 bearing races that form a vee to the stock. Another was a brass notch, birds beak principle. I guess you'd have to increment the steady contact every pass so its making contact with the material ahead of the cut, but that doesn't look too difficult. But with that extra support you have the mass of anchored toolpost supporting the cut, not just the skinny part itself.

My electric drill method only works because I am cutting the shaft stock into little segments, I have no way of controlling straightness. Probably why on centerless grinders its a completely different principle. No end support at all, its a contact wheel pushing against the abrasive wheel. Tom Lipton did a video spinning a shaft  kind of cantilevered out from a spin fixture using his surface grinder. Cant recall why kind of accuracy he achieved but seemed like a good way on short (especially hardened) shafts.


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