Straightness per Unit Length, within Bound Section of Shaft

[TopPocket]

Hi,

I have a shaft (Ø10 mm, 210 mm long) I want to be straight. It passes through a 92 mm long bore. The worst case stack up shows the different in diameters to be 0.013 mm.

For simplicity I assumed constant curvature of any bending so I could apply this as a simple straightness (to axis) to the whole length. So 210 / 92 = 2.28, 2.28 x 0.013 = 0.03 mm.

The manufacturers are telling me this is tricky and if I can give them more. So I figured MMC (MMR actually because I'm an ISO guy) would help.

Problem is I've now scaled the straightness the MMC should also be scaled by the same amount, I assume there isn't a way to do this?

So I would have to use the strict definition of my straightness value, which is in the image below : [ - | Ø 0.013 / 92 (M) ]

I believe this is correct.

Technically the tolerance would only apply to the first 140mm section of the shaft.
How do I include a boundary condition of straightness (axis) over a length of a shaft?

This might all give them more in theory, but inspecting it is a completely different matter. MMC to straightness of an axis, hmmm maybe, now make that straightness over a sliding length, yikes. My CMM operator is not going to be happy.

Is it even worth the hassle...

 

[pmarc]

The straightness callout that you propose will work as long as the definition of the 92 mm long bore assures that the MMVC of the bore is not smaller than dia. 10.000.

If you want to apply the straightness tolerance to a limited length of the shaft, there is a few options to consider:
1. Define points A and B on the shaft and add A<-->B notation to the straightness callout.
2. Draw a thick chain line close to the shaft contour in a longitudinal view to indicate limited length and its value and then connect the straightness tolerance frame with that line using a leader line. In that case, you will have to add (A) modifier after the tolerance value to make the callout apply to the axis/median line.

Is it worth the hassle? I would say depending on how hard you must fight for each extra tolerance micron.

 

[TopPocket]

Is this correct?

I can't get the arrows of the K and L letters vertical but I guess that doesn't matter. The position of K is defined elsewhere.

 

[pmarc]

Yes, that's OK.

Just for the record. Since this is ISO, you may want to consider application of a more generous overall form tolerance to avoid incomplete definition of the remainder of the shaft.

 

[TopPocket]

Thank you pmarc.

Okay, yeah I can see having that additional overall straightness spec may be useful. Would it need to be a completely separate frame to avoid confusion with the bounded one?

 

[greenimi]

Since this is ISO, you may want to consider application of a more generous overall form tolerance to avoid incomplete definition of the remainder of the shaft.

Why this is applicable to ISO only? (more generous overall tolerance)
I would say it is applicable for ASME too?
Or would you say because of rule#1 default in ASME, my above statement is not quite right?

 

[pmarc]

TopPocket,
If you are thinking of applying overall median line/axis straightness, then I would stack the two frames together and place the between notation in line (after or before) the frame defining per unit length straightness.

greenimi,
Yes, the same applies in ASME.

 

[greenimi]

Yes, the same applies in ASME.

pmarc,

A quick question for my own education and knowledge:

For the OP's embedded picture, single FCF (not for your proposed solution with two frames stacked together) do you think the shown frame is an exception of rule#1 or not? So, I am asking if rule#1 is applicable or the straightness per unit basis modified at MMC is considered as a derived median line straightness (and consequently exception of rule#1 in ASME)? (I know it is a separate discussion from what OP’s requested -ISO system)

 

[pmarc]

greenimi,
I would say that if this was ASME drawing, the DML straightness callout that we see on the first drawing attacched by the OP would override Rule #1 just as if this was a regular DML straightness callout.

Unless the straightness tolerance applied to a limited length only. In such case, for the remaining length, assuming the size callout was still applicable to it, Rule #1 would stay in charge.