Shaft key slot geometrical tolerances

BitMag · Apr 10, 2017

Hi,

I have this drawing of a shaft with key slot (see the attachment). The drawing has both symmetry and parallelism tolerances controlling the key slot orientation. I understand the point of symmetry tolerance in the given situation (or leat I think I do) but what I don`t understand is, why there is also the parallelism tolerance stated.

I also made a schematic of the interpretation of the given symmetry tolerance as I see it (see attachment 2).

As I understand, base A is the centerplane (green) of the shaft which is perpendicuar to the nominal (theoretically exact) key slot "floor". It is the centerplane and not axis, because the corresponding dimension does not have diameter symbol in front of the value. The symmetry tolerance zone is bordered by two planes (magenta) which are ideally parallel to the shaft centerplane and 0,08 mm apart (drawing is metric). The symmetry requirement states that the centerplane of the key slot (blue, derived from slot walls) has to locate inside the given tolerance zone.

So my question: What is the meaning of the parallelism tolerance? Symmetry already states that the key slot has to be parallel to the shaft axis (shaft centerplane)...

All opinions are welcome!

Silent_Man · Apr 10, 2017

I am no gd&t expert, but this is how i interpret that callout.

The parallel tolerance is tighter than symmetry. This controls the width of the keyway.

The symmetry tolerance does two things to the keyway; sets a tolerance for the keyway to be offcenter as well as tolerancing the width. However the control of the width in this case is superceeded by the tighter parallelism callout.

Sent from my SM-G930P using Tapatalk

BitMag · Apr 10, 2017

I`m not sure that I understand how can parallelism control the width of the slot.. Both tolerances are indicated as the extension of slot width dimension. This should mean that the object to be controlled is the centerplane of the slot (and not the with of the slot). The original drawing has a separate dimension tolerance for the width -> N9 (not showed on the hereby attachment).

Mtndew · Apr 10, 2017

BitMag said:
but what I don`t understand is, why there is also the parallelism tolerance stated.

Short answer: Because the engineer is a moron.

BitMag · Apr 10, 2017

Actually when I googled this problem, I found many drawings that had also symmetry and parallelism both stated (for a key slot). But some of them had looser tolerance for parallelism and tighter for symmetry, which is vice versa to my case..

MrSteve · Apr 10, 2017

Since the parallelism callout is 4X tighter than the symmetry callout the keyway must be much (4X) closer to perfectly parallel with the vertical plane thru the centerline of the shaft than it must be to being centered on the shaft.
That is, *each* keyway wall must lie within two vertical planes parallel to the centerline and 0.02 apart, also these two planes must lie within two vertical planes parallel to the CL 0.08 apart.

The parallelism does nothing for the width of the slot, only its walls orientation with respect to the shaft.

The engineer was (4X) more concerned that the slot be *parallel* than *centered* (i.e. symmetrical)

DMF_TomB · Apr 10, 2017

parallelism as a key in key way has to fit part that goes over it.
.
when you measure in machine if you indicate slot side thats parallelism and symmetry is how close to center when you set work coordinates.
.
endmills tend to deflect when cutting if you cut slot all at once often position is off, parallelism and size can vary. i cut keyway i always rough out and then take a finish cut. even on manual machine i would cut keyway 1/32 smaller and then finish mill. but some tool holders have some much runout you have to offset the 1/32 smaller end mill for each slot side when taking finish cuts

EPAIII · Apr 10, 2017

The symmetry spec. seems to be easy to understand. But, just what does that parallelism spec. refer to? It is shown on a sectional view and it is pointing to the vertical lines of the slot and the vertical dimension lines. So, does it only apply to those vertical lines in this view? Or does it also apply to lines along the sides of the slot that are parallel to the shaft axis?

For that matter, it is a linear spec. While parallel refers to an angular difference. So how is it measured? Total deviation between the two sides as measured in reference to their average spacing? Or measured to the nominal spacing? Or what? And does it mean that the slot must be parallel to the axis of the shaft to that tolerance also?

BitMag said:
Hi,

I have this drawing of a shaft with key slot (see the attachment). The drawing has both symmetry and parallelism tolerances controlling the key slot orientation. I understand the point of symmetry tolerance in the given situation (or leat I think I do) but what I don`t understand is, why there is also the parallelism tolerance stated.

View attachment 196018

I also made a schematic of the interpretation of the given symmetry tolerance as I see it (see attachment 2).

View attachment 196019

As I understand, base A is the centerplane (green) of the shaft which is perpendicuar to the nominal (theoretically exact) key slot "floor". It is the centerplane and not axis, because the corresponding dimension does not have diameter symbol in front of the value. The symmetry tolerance zone is bordered by two planes (magenta) which are ideally parallel to the shaft centerplane and 0,08 mm apart (drawing is metric). The symmetry requirement states that the centerplane of the key slot (blue, derived from slot walls) has to locate inside the given tolerance zone.

So my question: What is the meaning of the parallelism tolerance? Symmetry already states that the key slot has to be parallel to the shaft axis (shaft centerplane)...

All opinions are welcome!

BitMag · Apr 10, 2017

MrSteve said:
Since the parallelism callout is 4X tighter than the symmetry callout the keyway must be much (4X) closer to perfectly parallel with the vertical plane thru the centerline of the shaft than it must be to being centered on the shaft.
That is, *each* keyway wall must lie within two vertical planes parallel to the centerline and 0.02 apart, also these two planes must lie within two vertical planes parallel to the CL 0.08 apart.

The parallelism does nothing for the width of the slot, only its walls orientation with respect to the shaft.

The engineer was (4X) more concerned that the slot be *parallel* than *centered* (i.e. symmetrical)

Thank you!

A very good explanation, particularly the last statement.

BitMag · Apr 10, 2017

EPAIII said:
The symmetry spec. seems to be easy to understand. But, just what does that parallelism spec. refer to? It is shown on a sectional view and it is pointing to the vertical lines of the slot and the vertical dimension lines. So, does it only apply to those vertical lines in this view? Or does it also apply to lines along the sides of the slot that are parallel to the shaft axis?

For that matter, it is a linear spec. While parallel refers to an angular difference. So how is it measured? Total deviation between the two sides as measured in reference to their average spacing? Or measured to the nominal spacing? Or what? And does it mean that the slot must be parallel to the axis of the shaft to that tolerance also?

As MrSteve explained, the parallelism further refines the orientation of the slot centerplane (slot walls).
According to just the symmetry requirement, the slot centerplane can "swim around" in the stated symmetry tolerance zone (two planes 0,08 apart) and I think that the slot centerplane can also be at an angle to the axis centerplane, as long as it stay`s inside the tolerance zone.
But when we also take the parallelism requirement to account, then the slot centerplane can swim around in the 0,08 mm wide tolerance zone, but in whatever position in this 0,08 mm zone, the slot centerplane must lay also in the parallelism tolerance zone, which is 0,02 mm wide and ideally parallel to base.

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