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Realistic straightness callout on turned threaded bar.

Realistic straightness callout on turned threaded bar.

(OP)
I am looking to get a piece of bar turned with two threads of different pitches (differential screw), there is to be an M8 and M10 thread and the piece is to be 90mm long.

Am I right in thinking run out cannot be specified as it threaded?

I want it to be very straight without specifying something unreasonable.

Am I right in thinking that 0.1 means 0.1mm per metre deviation of the axis?

Is straightness callout of 0.05 too tight????


Cheers

RE: Realistic straightness callout on turned threaded bar.

"0.1" only means 0.1mm per meter if you specify it as such.

Achieving low straightness deviations on threaded parts is not, in and of itself, very hard- but measuring them accurately can be.

Is this a production part to be made in high volume, or a one-off?

The other question is, what happens to you assembly if this part is very slightly over that (very tight) .05 tolerance?

There is way more deviation than that built into the thread- so what do you gain by spending a bunch of money to make this part super duper straight?

RE: Realistic straightness callout on turned threaded bar.

When you say "straight" do you really mean the Pitch Diameters of the threads needs to be functionally coaxial to each other?

If yes, I would suggest using the dimensioning schema in Fig 4-24, page 68 of Y14.5-2009, where no datum is used to control the location between two coaxial features

Certified Sr. GD&T Professional

RE: Realistic straightness callout on turned threaded bar.

(OP)
Thanks for the replies.

It is to be a one off (or low vol. at least) and is part of a precision assembly.

It will be used to wind a dowel in and out, hopefully the clearance in the threads will be greater than any variation in straightness and concentricity etc. however I feel I want to specify something to ensure the part is made with precision.

As the part reasonably short at 90mm it should be possible to make it fairly accurately .

RE: Realistic straightness callout on turned threaded bar.

Emo01

Remember: Tolerancing defines acceptance criteria. How will the mfg or QA know if the part is "good" or "bad" if limits are not available. Trusting machinist and machine tools to make the part as good as or better than required is dangerous and may result in a pile of rework, or worse, scrap.

Certified Sr. GD&T Professional

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