Tips for turning to an accurate diameter

[mike 109444]

If you have a marker to hand, as you get to your final size run the marker along the surface as it's spinning to paint it. Allow to dry for a few seconds, then take either a spring or another cut and if you are getting an even cut you should see all the marker go. Think of the surface as a minutiae thread with "peeks" and "troughs" this is where your finish is graded. When you are a nats c00ck from size it maybe enough to polish off the "peeks" to get your finish size. Remember if you use emery then some will be left on the job and if the size is incorrect, running a tool over it will take the edge off the tool as the emery will dull it. Less of a problem when running coolant over work. Also is your set up balanced ? With the drive dog and face plate, could be unbalanced that doing in the final cuts

 

[Wallace]

Why do you need to get within 10 microns on a steering stem?

 

[Pete.]

Why do you need to get within 10 microns on a steering stem?

You don't. Couldn't if you wanted to and if you could it wouldn't stay like it.

 

[Wallace]

I was thinking the variations in shed temperature would preclude that sort of accuracy.

 

[waddycall]

I have two ktm steering stems, the original stem and the stem that was fitted to the new yokes I'm using. They are within 5 microns of each other at the various critical points. I'd like to replicate the fit of the original parts. I'm fitting later forks which need the newer yokes but a longer stem than either of the two I have.
Cheers, Pete

 

[Pete.]

Pete, with respect I think it's more important that you stick to scales of measurement that you can accurately work and measure work to. Even mentioning microns will leave you open to criticism or ridicule on most machinist forums. Having a digital micrometer that resolves to 0.005mm doesn't mean that it's accurate to 0.005mm and more importantly you couldn't turn to that accuracy on a lathe even if it was. Microns is never used to express a dimension in manual machining.

Scale down your measurements by a factor of ten and you're talking realistic numbers. 0.01, 0.02mm. Holding 0.01mm is a challenge for most people.

 

[Wallace]

Pete, are you using any lubricant on the tailstock fixed centre?

 

[waddycall]

Thanks for the comments chaps. I got to spend some time on it today. I concentrated on getting the first two diameters near the tailstock correct. I left the rest of the bar pretty much full diameter as I intend to get the critical diameters before spending time reducing the rest of it. The two diameters nearest the tailstock are for the top yoke and the top bearing which needs to be a sliding fit. The diameter for the yoke ended up 10 microns oversize and I left it there as the fit in the yoke is fine.
The next portion has been turned to 4 microns undersized compared to the two original stems. The fit of the bearing is a little looser but still ok.
I don't have a digital micrometer or vernier. I can't stand them. A non vernier micrometer measures to 10 microns and it's fairly easy to divide each division by two to get a resolution of around 5 microns.
The actual sizes are almost irrelevant as I am using standard ktm parts as gauges and comparing my new stem to the original with an indicating mic.
I'm lubricating the centre with grease.

 

[waddycall]

Sorry my photos always end up upside down for some reason!

 

[Wallace]

That's a very nice expensive looking micrometer.

 

[BrokenBiker]

i take it that micrometer measures to 0.001mm? We have one in the tool room in work that looks like that but ive never used it

 

[Pete.]

It's a comparator. Can be used as a micrometer by comparing it to a standard but it's function is to do exactly what Pete's doing with it - comparing two parts.

Talking in microns is still futile though. Breathe on the part and that device will detect the growth.

 

[MrFluffy]

Interesting, what grade of alloy are you making the new stem from? I have some white power forks here from a ktm and the stem is some funny titanium alloy when I got a friendly scrappy to hit it with the niton gun. I just used steel to make my replacement, I ate too many pies anyway for the weight loss from using Ti in a headstock to make any difference.

 

[Wallace]

The finish looks good, I take it that you are using the cutting feed? Love the way you've mounted the motor!

 

[waddycall]

The indicating mic was an eBay bargain at £20 a few years ago. I'm not sure where your coming from with the talking in microns comment. I would write it as 0.001mm but it's a pain on this ipad! If you re-read my original post you'll see I was hoping to get to 10 microns or 0.01mm of original size. Pete, you've told me I should be working in increments of 0.01mm so where's the issue?
I'm fully aware that I'm working to tight tolerances but I'll be happier replicating the fit of the original parts. So far I've achieved that on the diameters that I've finish turned.
The indicating mic has divisions of 1 micron on the gauge as well as standard micrometer functions.
I'm making the stem from 7075 t511 alloy (if I recall correctly!),

 

[waddycall]

The finish looks good, I take it that you are using the cutting feed? Love the way you've mounted the motor!

Thanks, yes I'm using the power feed. The motor was screwed to the wall a few years ago as a short term solution!

 

[brightspark]

[quote="waddycall, The motor was screwed to the wall a few years ago as a short term solution![/quote]well at least u wont get any vibration of it being mounted on wall as its not a resiliently mounted motor

 

[waddycall]

[quote="waddycall, The motor was screwed to the wall a few years ago as a short term solution!

well at least u wont get any vibration of it being mounted on wall as its not a resiliently mounted motor[/quote]
Spot on! When I bought the lathe it was mounted on an angle stand and the motor was hung on the back. I took it off the stand and screwed the motor to the wall to get rid of vibration causing finish problems. The bed on the Lathe is very week so I tend to do most work on it at around 100rpm which is a bit tedious.

 

[Pete.]

The indicating mic was an eBay bargain at £20 a few years ago. I'm not sure where your coming from with the talking in microns comment. I would write it as 0.001mm but it's a pain on this ipad! If you re-read my original post you'll see I was hoping to get to 10 microns or 0.01mm of original size. Pete, you've told me I should be working in increments of 0.01mm so where's the issue?
I'm fully aware that I'm working to tight tolerances but I'll be happier replicating the fit of the original parts. So far I've achieved that on the diameters that I've finish turned.
The indicating mic has divisions of 1 micron on the gauge as well as standard micrometer functions.
I'm making the stem from 7075 t511 alloy (if I recall correctly!),

Actually I meant 'expressing' in hundredths of a mm, if you can turn work to that tolerance (four tenths of a thou) then fair play to you. The thing is that the originals won't have been produced to such tight tolerances so replicating them to 'microns' isn't necessary nor practical, if the micrometer even repeats to that scale.
I didn't mean to cause you any offense, just point out that no-one uses microns as a term in regular machining.

 

[waddycall]

No offence taken. Just a little confused! Anyway, thanks for the advice chaps.