# Thread: turning 36" +- .005 long bar

1. ## turning 36" +- .005 long bar

hello guys , I have about 3 years of experience doing set ups in a cnc lathe and i have to turn both ends of a bar .25 diameter and 36" +-.005" on the length, never turned such a small tolerance for a long bar so i dont know whats the proper way to get that tolerance any ideas or suggestions are good to me, thank you very much for your help.

2. Originally Posted by nascad
hello guys , I have about 3 years of experience doing set ups in a cnc lathe and i have to turn both ends of a bar .25 diameter and 36" +-.005" on the length, never turned such a small tolerance for a long bar so i dont know whats the proper way to get that tolerance any ideas or suggestions are good to me, thank you very much for your help.
I don't know about turning it but that 37" micrometer must take 2 people to lift. LOL

The hardest problem that I see is measuring it.
Also at 36" long I bet thermal expansion starts to come into play. A 25 deg F temp difference will probably move the bar out of spec.

Not That I have any experiance but I would machine both ends clean, measure, then take another cut and sneak up on the dimension. If the bar is long enough and you measuring device will reach, you coud have it stick out the back of the head and measure it in the lathe between cuts.

3. Originally Posted by TOTALLYRC
...Also at 36" long I bet thermal expansion starts to come into play. A 25 deg F temp difference will probably move the bar out of spec.....
The coefficient of thermal expansion for steel is 0.00000645in/in/deg F so I think he is safe there.

Measuring it will be a challenge even with the 80lb, 36-37" micrometer

4. Originally Posted by Geof
The coefficient of thermal expansion for steel is 0.00000645in/in/deg F so I think he is safe there.

Measuring it will be a challenge even with the 80lb, 36-37" micrometer
0.00000645in/in/deg F * 36" *25deg F = 0.005805" which if he cut it at one end of the tolerance it would move the bar out of spec. If my math is correct and I counted all of the zeros properly.

Not that his work shop is at 100 deg F and his inspection area is at 68 deg F. or any thing like that, he didn't sound like a sadist. LOL.

5. Oh dear, I am the one who has to wear the hair shirt, rub ashes in my hair and flagellate myself; I screwed up the zeroes.

Too much wine after dinner I guess.

6. Originally Posted by Geof
Oh dear, I am the one who has to wear the hair shirt, rub ashes in my hair and flagellate myself; I screwed up the zeroes.

Too much wine after dinner I guess.
It could be not enough wine after dinner, you never know.