Constraining rotation of primary datum axis with a secondary orthogonal datum axis

[ewans]

Morning all,
Hoping for some input/feedback on whether or not an orthogonal datum axis feature can be used to constrain the rotation of a primary datum axis. I can't find an example of this scenario in Y14.5-2009 or in any other online resources. The closest reference I could find was section 4.16.8 in Y14.5-2009, but I would have an orthogonal hole in lieu of the slot.

I have attached a PDF drawing of this scenario, and was seeking to confirm if the established A,B datum reference frame is valid.

Additionally, any input/feedback on the application of projected tolerance zone, and the use of a minor thread diameter to establish datum feature C, would be most appreciated.

Many thanks.

Regards,
Ewan

 

[drawoh]

ewans,

Wow!

Geometrically, I think your datums A and B completely constrain the part. Your datum[ ]C seems to be concentric to datum[ ]C.

Practically, building a fixture for this, particularly datum[ ]B, will be a challenge. The tool that picks up datum[ ]B must account for errors in positioning the hole.

How about calling up your datum as |tpos|Ø0.2|A|B-C|, where datums B and C work together to define an axis that locates up and down, and constrains rotation?

--
JHG

 

[ewans]

Hi drawoh,
Thanks for your reply... appreciated.
Agreed... a hard fixture to establish Datum B at RFS (or RMB as I gather it's now called) would be challenging. Fortunately, the supplier has a CMM

I had considered using |A|B-C| as the datum reference frame, but every example I found of using a 2 coaxial features to establish a single datum axis, had the axis as the primary datum, not secondary. Also, it seemed to me that inspecting/establishing the B-C axis as a secondary datum would be even trickier than using B alone, which itself would be tricky.

Cheers,
Ewan

 

[drawoh]

ewans,

I understand theoretically how Datum[ ]B can control rotation. I don't see practically how it would do it. It is too short to define an axis. Datums B and C define the axis you require. This does make the tooling interesting.

--
JHG

 

[ewans]

Hi drawoh,
My sense is (and I am quite possibly incorrect) that the datum feature B does not require much length to constrain the rotation of datum axis A. Would the datum feature simulator for datum B not simply be required to expand until it makes contact with datum feature B as the part is rotated about A?

Cheers,
Ewan

 

[drawoh]

ewans,

Your datum[ ]B feature is 6mm long. How accurate is your CMM? Divide that accuracy by 6, and you have your datum angle precision.

Using two features to define a datum probably is not functional for a tertiary datum, which usually (always?) is only a single point. Usually, a secondary datum is a line, which is definable by two features.

--
JHG

 

[ewans]

Morning drawoh,
Thanks for your reply... appreciated.

Ewan

 

[ModulusCT]

Interesting post.

drawoh - Could you explain (or point me in the right direction) "datum angle precision"? Also, in a practical sense, what do you mean by 'divide your CMM accuracy by 6'. I'm assuming your post was sort of off the cuff, but I'm interested in your meaning and the possible implications this could have on fixturing.

Thanks,

Mod

Please, please, please, use the correct terms!They're Datum Features NOT DATUMS!!! AAAAAHAHAHAHHAHAHAHHAAAAA" -- Don Day

 

[drawoh]

ModulusCT,

You are using a 6mm long cylinder as a datum feature. Let's say that your CMM is accurate to within 0.1mm.

Datum angle error = CMM Error / Length = 0.1mm/6mm = 17[×]10^-3radians.

This works out to about a degree. I ignored the possibility that the CMM error works in opposite directions at each end of the cylinder. I also ignored the possibility of you averaging multiple test points.

This, probably, is why they are discouraging the user of the concentricity specification on ASME Y14.5.

--
JHG

 

[pylfrm]

There is no translation modifier for datum feature B, so the axis of the datum feature simulator is fixed at 65.5 from the axis already established from A. B doesn't have to define an axis by itself, but in concert with A. The length of the datum feature would much less important than the distance from the axis of A, 32.5 to 38.5 in this case.

- pylfrm

 

[drawoh]

pylfrm,

If datum[ ]B is used to control location, its length is irrelevant. If you do it this way, you need to control rotation. It would be nice to have a side with a straight edge. The OP's original concept sort of does this, but the tooling could be fun. I am not familiar with CMMs. Maybe this is easy.

--
JHG

 

[pylfrm]

drawoh,

What if instead of a cylindrical secondary datum feature B you had a segment of a sphere of the same size and in the same place? Wouldn't this be enough to constrain the two degrees of freedom left over from the primary A despite having zero length? You'd just construct an line perpendicular to and intersecting with the axis established by A, out to the point established by the sphere.

If so, how/why would the length come in to play for the cylinder? There's not a lot of difference between a (very) short cylinder and a (very) short segment around the equator of a sphere.

Back to the original drawing though, datum B is used as primary for one tolerance, so the short length could be a problem there regardless. I'd be concerned about the short length of datum A for the same reason.

- pylfrm

 

[ewans]

Hi pylfrm
Thanks for your input to this discussion... appreciated.
I did have concerns about the short length of datum feature A relative to the overall size of the part. It is, however, the most functionally significant feature on the part.

Cheers,
Ewan

 

[3DDave]

ewans,

if the bearing shoulder is not perfectly perpendicular to the shoulder will pressing the bearing into place result in a gap due to the perpendicularity variation?

 

[ewans]

Hi 3DDave,
Are you referring to the shoulder on the datum feature A bearing journal, or on the datum feature B bearing bore?
For the datum A journal, if the shoulder is not perpendicular to the axis it may result in a gap, but more likely it will result in a very unhappy cross-roller ring. I suspect the clamping ring would force the inner bearing race to conform to the shoulder surface.
I have attempted to control the shoulder attitude and position with the surface profile gtol (it's not very clear on the PDF)

Cheers,
Ewan

 

[3DDave]

I'd guess the shoulder is the primary source of orientation and location (3 DOF) with the shaft supplying 2 DOF. Since it's round, there's no need to control the remaining DOF. Using those as primary and secondary allows setting up the rest of the part in a straight-forward way. Like the current B datum features would just be positioned to the primary shoulder and secondary shaft.

The trick for this sort of situation is that no one seems to do an FEA on the allowable varied shapes to see what the real effects of that variability is. Other than that, selecting tolerance values is just guessing. It may be good guessing, but guessing nonetheless.