Inspecting straightness tolerance of .00005" on a shaft

R

rdonkers

#1
Hi everyone, I could use some help with finding a way to inspect a shaft for straightness.
I have a print (which I attached to this thread) that calls out a straightness tolerance of .00005 for 1.35 long section. Right now we are using a .00005 indicator between centers to check this but we need to come up with a gage that is more accurate than the callout. I was thinking maybe a laser of some sort but I haven't had any luck finding what I need. This is an aerospace part if that makes any difference. I don't want us to ship parts without a method that would be adequate to weed the truly good/bad results out.
Thanks everyone in advance-
 

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normzone

Trusted Information Resource
#2
Re: Inspecting straightness tolerance of .00005 on a shaft

Before this slides off the front page I'm going to give it a bump, and ask for some training.

I used to live in a precision measurement world, but those days are long behind me. The limits of rational hair splitting was about five ten-thousandths of an inch, or maybe one, back when I split hairs for a living.

What is it now, and how is it achieved ?

Thanks -
 
D

David DeLong

#3
Re: Inspecting straightness tolerance of .00005 on a shaft

Wow - still prints like this out there!

I don't believe that it should have stated straightness since the feature's length is so small. I believe it should have been roundness which makes more sense to me.

Since the straightness does not have a diametrical tolerance zone in front of the symbol, theoretically, it means straightness of the surface rather than axis. I would confirm whether it is axis or surface.

To check straightness (either of the axis or surface) you need a 2 point set up perpendicular to the axis preferably towards the end of the feature. See what I mean about the feature's length being too short to be practical. On a cmm, create a axis by contacting around one end and achieve a center and then then do the same on the other end. Theoretically, join the ends and now you have an axis. Contact around the center of the feature to create a center and then see how far it is off the created axis. Good luck.
 

Steve Prevette

Deming Disciple
Staff member
Super Moderator
#4
Re: Inspecting straightness tolerance of .00005 on a shaft

Don't know if this helps, but this reminds me of my submarine days - the main turbine had a "runout" indication as every startup was a bowed-rotor startup and the remote indicator would measure how much the end bearing "wobbled".

Could you get a very flat plate (glass? Metal?) and roll the item on it and detect how much it wobbled away from the plate with a laser?
 

Project Man

Involved In Discussions
#5
Re: Inspecting straightness tolerance of .00005 on a shaft

The easiest way to measure the straightness would be on a Comparator or Video Inspection equipment. An air gauge would also be a good consideration

P.S. The call out seems horrible and doubtful that it reflects what they need. I recommend asking them what they are trying to achieve.
 

optomist1

A Sea of Statistics
Trusted Information Resource
#6
Re: Inspecting straightness tolerance of .00005 on a shaft

I think David stated it best.....but a couple of cents, to follow on his comment, the drawing is very incomplete/inaccurate.....I may be nerding out a bit, but studying for GDT and fly specing all that I see....well said David ....
 

normzone

Trusted Information Resource
#7
Yeah, studying GDT is the way to go, and there's plenty of places that are using it. I'm sure it is more precise / practical than the old XYZ world that preceded it. My knowledge of it is sketchy at best, not much call for it in my current incarnation.

That said, while it's the way of the future, there's a lot of legacy stuff from days of yore that exists, and plenty of environments that have not made the step to GDT. And lots of prints with callouts that don't mean what either you or the guy who drew it think it does. Kind of a Princess Bride situation.

Some of the methods suggested above would not even get in the neighborhood of the level of discrimination that the print asks for.

And I have not seen an answer regarding what's possible, and with what tools. Just as well that I don't have to split hairs very often these days.
 
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