Bolt Circles - Relative Clocking

[MToomey]

Hi folks, I've done some digging around here and found a couple threads sort of relate to this, but I haven't found a solid answer.
If you know of one I missed, please point me there.

The basic question: How can I go about controlling the relative clocking of two circular patterns loosely while maintaining the radial position tightly?
The background: We are referencing Y14.5-2009. Check out the attached screenshot. Datum A is the OD of the wheel. The two inner bolt circles could overlap with each other if the relative clocking between them is not maintained.
The 1.188 holes are clearance for fastener heads to pass through and fasten other components together. (Inner race of wheel bearing to wheelbox. Outer race of wheel bearing is fastened to the .688 thru holes)
The tapped holes fasten a plate that transmits power from the motor.
The tapped holes have to match the hole pattern on the mating part, but they do not have any functional relationship to the clearance holes. We just have to maintain enough wall thickness to transmit the motor torque. They can realistically shift ±1.5° relative to the clearance holes and still be functional.

The best idea I've had to allow the most tolerance I can is to make the 1.188 hole pattern Datum B and use a two single-segment feature control frame to allow lots of tolerance with respect to datum B primary, A secondary, then refine the tolerance in the second segment with respect to datum A only, which should tighten up the radial position but leave the angular position alone. But does that leave the angular distance between holes within the tapped hole pattern loose also?
Is it enough to just put a note on the drawing that says the clocking of these two patterns shall be within 1.5°?

I appreciate any direction you can offer.
Best regards,
Matt Toomey

 

[3DDave]

You can use bidirectional tolerancing to control the radial location tolerance separately from the tangential location tolerance. In the tangential direction, use a composite tolerance to refine the hole-to-hole control in the lower segment.

 

[MToomey]

3DDave:
That is exactly what I should have been thinking about! (Or maybe I was thinking about without realizing it)
But without a clocking datum, I'm kind of dead in the water, no?
Does this lead to making one of the patterns a datum?

 

[Burunduk]

But without a clocking datum, I'm kind of dead in the water, no?
Does this lead to making one of the patterns a datum?

Since the two hole patterns you are concerned about are controlled in the same datum reference frame - [A] their relative clocking is controlled by a Simultaneous Requirement.
Same principle as in figure 7-47 of Y14.5-2018.

Only the third pattern is in a separate requirement, but there are no such issues with that because it will not overlap with anything.

 

[drawoh]

MToomey,

Look up composite feature control frames. You can apply an accurate positional tolerance for the pattern, locate it accurately to your centre feature, and sloppily to your clocking feature.

Does any of this matter? If your part is machined, there is no way they can take advantage of your sloppy tolerances to reduce cost. Is it worth your trouble to do drill jigs? A cardboard or plywood template will create an accurate pattern, but you centring error as well as clocking error. Burn brain cells for something important.

--
JHG

 

[Burunduk]

Look up composite feature control frames. You can apply an accurate positional tolerance for the pattern, locate it accurately to your centre feature, and sloppily to your clocking feature.

It's the other way around.
With composite tolerances, the upper segment locates the pattern to the datums, and it is the looser control. The lower segments, when they reference datums, are used to both tighten the tolerance for the mutual location and orientation between the features in the pattern, and the relationship to the referenced datums in rotational degrees of freedom - which includes clocking if the clocking datum is referenced in the second segment.

 

[axym]

MToomey,

This is an interesting application.

I agree with parts of all the responses, and disagree with other parts ;^).

Here is my understanding of the requirements for the two patterns:
-Each of the two patterns needs to be accurate "to itself". In other words, the relative angular location and relative radial location of the holes within each pattern need to be tightly toleranced. Relative angular location and relative radial location are equally important.
-The centering of each pattern relative to datum feature A needs to be accurate as well, to roughly the same degree as the within-pattern accuracy.
-The angular clocking of the two patterns relative to each other can be less accurate

I agree that two different position tolerances will be needed for each pattern. We need to keep the location constraint to datum feature A, so a composite FCF won't help us here. There isn't a convenient clocking datum feature, and I wouldn't recommend using one of the patterns. I would go further with the simultaneous and separate requirements. Here is what I would suggest for each of the patterns:

|POS|Dia .060(M)|A|
|POS|Dia .010(M)|A| SEP REQT

The tolerance values of .010 and .060 can be adjusted. The lower segment tolerance should be tight enough to achieve the within-pattern accuracy and centering accuracy that you need (it doesn't have to be the same tolerance for both patterns). The upper segment tolerance controls the relative clocking - it can be calculated to achieve the required minimum wall thickness.

Evan Janeshewski

Axymetrix Quality Engineering Inc.

http://www.axymetrix.ca

 

[3DDave]

Evan,

That's one way - it obscures the functional requirement, but it works on a CMM without the operator having to understand what it's for.

It should go on the GDTP test to see how many can calculate the minimum material between holes.

 

[MToomey]

Thanks for your input folks. As much as I try to allow all the tolerance I can, I think drawoh hit the nail on the head - It's a machine shop. They won't have any trouble hitting a .010 tolerance zone. Get the control you need and move on. My first thought was also Composite Control Frames, but I think they work the wrong way unless I have a clocking datum.
I'm still thinking about a note in English to give our inspector more leeway to accept a good part...

P.S.,
Evan: Are you implying that each segment of a MSS FCF can be a separate requirement? I had no idea! ... I wonder how many machine shops know about that

|POS|Dia .060(M)|A|
|POS|Dia .010(M)|A| SEP REQT

 

[axym]

MToomey,

Yes, I'm saying that each segment of a multiple single-segment feature control frame can be a separate requirement. To be honest, anything goes for multiple single-segments - there are no special rules like there are for composite FCF's.

If both patterns are going to be machined in the same setup and they can easily hit the .010 position tolerance on everything, then I agree that using a simpler-but-tighter tolerance is probably better. I would put my suggestion in the "nifty, but guaranteed to confuse" category ;^)

Evan Janeshewski

Axymetrix Quality Engineering Inc.

http://www.axymetrix.ca

 

[drawoh]

MToomey,

I started replying to axym's email, but it was not entirely intelligent or appropriate. Here is sort of was what I was going to propose.

[box][⌖][/box][box][⌀]0.8[/box][box]A[/box][box]B[/box][box]C[/box]
[box][⌖][/box][box][⌀]0.4[/box][box]A[/box][box]B[/box]
[box][⌖][/box][box][⌀]0.2[/box][box]A[/box]

Assume feature[ ]A is your primary datum, probably the mount face, feature[ ]B is your centring datum, and feature[ ]C is your clocking datum. If your machine shop easily hits [⌀]0.010", this whole exercise is a waste of time.

--
JHG

 

[3DDave]

From the initial post: "Datum A is the OD of the wheel"