How to converge to the desired diameter when fitting pieces?

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dml66
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How to converge to the desired diameter when fitting pieces?

Post by dml66 »

There has to be a trick to this which I don't know. When turning a piece, either inside or outside, I'll use a micrometer or gage pin to get me in the ballpark, then I try to sneak up on the final diameter. Every time so far, I overshoot :oops: . I think I'm patient, maybe I'm not. I've tried running a second pass without touching the tool post; a few more crumbs tumble, still no fit. Run the tool post in (or out) half a thou and now I have a loose fit :evil: .

What I'm looking for is the next fit after an interference fit, I just can't seem to get there yet. I have considered taper but, I've got that dialed out I think.
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rmac
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Re: How to converge to the desired diameter when fitting pieces?

Post by rmac »

In this ancient post, Harold_V explains why "sneaking up" is a bad idea, outlines a better procedure that works really well, and suggests removing the last few tenths of material by polishing rather than trying to machine it away.

-- Russell Mac
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tornitore45
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Re: How to converge to the desired diameter when fitting pieces?

Post by tornitore45 »

I learned that when I think (measure) 0.0005 over I am actually measuring the crests of the tool marks, a bit of emery cloth gets me on dimension.
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liveaboard
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Re: How to converge to the desired diameter when fitting pieces?

Post by liveaboard »

I always finish with emery.
As others have said, you need to be slightly oversize; I work in metric, for me .1 to .05mm oversize.
During polishing, take many measurements along the length of the piece.

Interference fits are still the most difficult work for me, even after making many over the last 10 years.
Threads are easy by comparison.
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Bill Shields
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Re: How to converge to the desired diameter when fitting pieces?

Post by Bill Shields »

Which is why I use loctite for many applications
Too many things going on to bother listing them.
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Re: How to converge to the desired diameter when fitting pieces?

Post by RSG »

dml66, you don't mention what material you are machining, for me it makes a difference.
Vision is not seeing things as they are, but as they will be.
dml66
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Re: How to converge to the desired diameter when fitting pieces?

Post by dml66 »

RSG wrote: Mon Jul 04, 2022 6:47 am dml66, you don't mention what material you are machining, for me it makes a difference.
RSG, in this last case I was machining a piece of 12L14 steel.
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GlennW
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Re: How to converge to the desired diameter when fitting pieces?

Post by GlennW »

Perhaps your tool grinding/sharpening technique needs reviewing.

What are you using for a tool bit?
Glenn

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dml66
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Re: How to converge to the desired diameter when fitting pieces?

Post by dml66 »

GlennW wrote: Mon Jul 04, 2022 9:00 am Perhaps your tool grinding/sharpening technique needs reviewing.

What are you using for a tool bit?
TCMT outside, CCMT inside.

I've yet to grind my own tools, it's on my list of things to learn. I've tried buying pre-sharpened HSS, the results have been awful, the workpiece looks like if has a fur coat.

I've installed a set of fine-feed change gears which gets the feed rate down from 0.0039" per spindle revolution to 0.0026 without compromising lathe safety. That change has improved the finish, still not a mirror, but better.
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GlennW
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Re: How to converge to the desired diameter when fitting pieces?

Post by GlennW »

There are also different grades specific to the material being turned, and chip breakers for the inserts that lend themselves to different applications such as Roughing, semi finishing, finishing, fine finishing, etc. . That can have an effect on the outcome of the finish on your parts.

Not all are created equal!

The "pre-ground" HSS tool bits may not be completely ready to use, as they may just have the rough configuration ground in them. Same with brazed carbide tools. They usually need sharpening prior to use.

I've never used the "Pre-ground" HSS, so I can't say for sure.
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rklopp
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Re: How to converge to the desired diameter when fitting pieces?

Post by rklopp »

Do you have a DRO? If not, you’d be surprised how much the cross-slide moves just from starting the lathe spindle. It even happens a couple “tenths” on my rigid Monarch 10EE. If no DRO you can put a DTI against the cross slide or toolpost to catch this unintended movement.


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dml66
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Re: How to converge to the desired diameter when fitting pieces?

Post by dml66 »

rklopp wrote: Mon Jul 04, 2022 5:06 pm Do you have a DRO? If not, you’d be surprised how much the cross-slide moves just from starting the lathe spindle. It even happens a couple “tenths” on my rigid Monarch 10EE. If no DRO you can put a DTI against the cross slide or toolpost to catch this unintended movement.
I've considered a DRO from Little Machine Shop, want to make sure it reads actual tool position and not just handwheel turns. I built my own DRO for my drill press using draw wire encoders, I know what you mean about just starting the motor causing flutter, and that's reading at 0.01 mm precision.
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