Locating Timing Pulley Tooth to Keyway 2

[donatim24]

I was looking at one of our drawings and we have a drawing for a timing pulley that requires the centerplane of a particular tooth to be aligned to the centerplane of a keyway within 0.5 degrees. Currently, the requirement is that the centerplane of the tooth is aligned to the keyway and not the surface of the tooth. I assume this is because the tooth form and size are controlled by the selected profile for the timing pulley.

The tooth profile is AT10. Therefore, there are no opposing planes that can be used to derive a centerplane. Is there an established way to determine the plane, or is the dimensioning approach flawed?

Best approximation of the requirement:

 

[3DDave]

You don't know how timing gears work. Clearly a problem.

If I say, go within 10° of North, does that mean 5°?

Do you know how angles and directions work?

Your alternative does not produce an angular tolerance zone.

I did.

 

[Burunduk]

"Within" is meant to note the total tolerance range. Since the allowable variation is symmetric about the reference plane "within 0.5°" means an inclination of 0.25° in either direction.

Your solution is ambiguous without providing the drawing with an obligating inspection plan generated by the design unit. When the requirement is un-interpretable without a specific inspection plan it's a sign of poor tolerancing practices.

 

[3DDave]

Within 10°of level means 5°? Within 10 minutes of noon means 5 minutes? Within 1 meter of the target doesn't mean a 1 meter radius?

Must be a local language thing that doesn't translate properly. That would explain your difficulty in interpreting it.

 

[Burunduk]

When a tolerance on a dimension has upper and lower limits, "within" considers both limits.
If one says that a shaft diameter is +/-0.2 mm that means that the size is "within 0.4 mm" total.
The same principle applies to angles.
If you understand it differently it is because of another layer of ambiguity, at best.

The distraction won't help you, the suggestion to keep a direct angular tolerance on a 0°/180° orientation is still a bad idea.

 

[3DDave]

As I said - you seem to have a language barrier.

In English we would say "Within a total symmetrical range of ..." if that's what is meant.

"Within", when referring to the general case of proximity, is one-sided. Within hearing distance, within reach, within shooting distance, within some distance to a target value, within a few dollars of the expected cost.

All are one sided.

 

[Burunduk]

You're still engaging in the distraction you started.

I don't have any language barrier. I know not everyone would be that wordy to say, "Within a total symmetrical range of..." - could just say 'within' with the same meaning.

Also,

Is that tolerance "within 0.05°"?

 

[3DDave]

"I know not everyone would be that wordy to say, "Within a total symmetrical range of..." - could just say 'within' with the same meaning."

They would not as the meaning is not the same.

That range is between 25.2° and 25.1°, inclusive. Values can be within that range and the range can be stated as 0.1°, but it is incorrect to say "within" to describe the half-extent of that range.

One could say the acceptable slope will be within 0.05° of 25.15°.

Are there more questions about what "within" means when applied to proximity?

 

[Burunduk]

Thank you for the attempt at an English lesson, which I don't need.
Your claims about the term "within" as an expression of one-sided proximity are OK in the context of general use, but not in the context of tolerances.

Your answer that "it is incorrect to say 'within' to describe the half-extent of that range" proves my point, thank you very much. You forgot to mention though, that the tolerance allows the angle to vary "within 0.1°" - which would be the correct description. And since limit dimensioning means the same thing as plus and minus tolerances, the same applies to a range expressed as plus and minus - the term "within" refers to the full range between the max and min limits.

Now that the distraction has run its course, you can get back to the topic by admitting that a directly toleranced angle is not a rigorous way to control orientation.

 

[3DDave]

Don't want is not the same as don't need.

"means the same" in this context makes no sense. Does 25.2°/25.1° mean the same as +25.2° and -25.1°?

The tolerance for the OP is the allowable deviation to either side from the nominal.

 

[Burunduk]

"Does 25.2°/25.1° mean the same as +25.2° and -25.1°?"

This question makes no sense. Why would it mean that?

The mean value is 25.15°.
So, 25.2°/25.1° or the equivalent 25.15°+/-0.05° can be described as "within 0.1°".

By the same token the OP's "within 0.5°" can refer to the full range of the tolerance; for 180°+/-0.25° it is "within 0.5°".

The important message which I already conveyed multiple times, is that it would be a horrible way to tolerance this.