Advanced GD&T Question Composite True Position LMC

[afotion]

This is the FCF is question. I would love to hear how this FCF is interpreted by multiple people. I am almost certain I understand it but I would like to be sure.


For info:
J is the surface these holes are drilled into.
G & H are larger diameter holes also drilled into this surface.
Nominal Diameter is .250
Measurements are in mm

Thanks Everyone! I am also new to engineers edge.

[Belanger]

Welcome aboard!


It simply means that each of the 128 holes shall be positioned within Ø0.05, with respect to the J, G, H datum reference frame, if made at 0.263. But as a hole's size is made smaller, it will be allowed to deviate more off center (bonus tolerance).

Then the lower part of the FCF says that each of the 128 holes shall be positioned within Ø0.025 if made to a size of 0.263, but the only datum here is J. So this controls the perpendicularity of each hole, but it also locates the holes to one another -- don't forget this aspect.
If a hole is made to a size smaller than 0.263, then a bonus tolerance is allowed here, just as in the upper part of the FCF.

I suspect you pose the question because it's LMC, rather than the more common MMC use of composite tolerancing. Using LMC here is legal, but I can't say whether it's wise; that totally depends on the function. I will say that this doesn't allow you to use functional gaging, which is allowed when the MMC modifier is shown.

[Kelly_Bramble]

I suspect you pose the question because it's LMC, rather than the more common MMC use of composite tolerancing. Using LMC here is legal, but I can't say whether it's wise; that totally depends on the function.

I'm having the same thoughts..

So afotion, what is the function or application of these hole features?

[afotion]

Welcome aboard!


It simply means that each of the 128 holes shall be positioned within Ø0.05, with respect to the J, G, H datum reference frame, if made at 0.263. But as a hole's size is made smaller, it will be allowed to deviate more off center (bonus tolerance).

Then the lower part of the FCF says that each of the 128 holes shall be positioned within Ø0.025 if made to a size of 0.263, but the only datum here is J. So this controls the perpendicularity of each hole, but it also locates the holes to one another -- don't forget this aspect.
If a hole is made to a size smaller than 0.263, then a bonus tolerance is allowed here, just as in the upper part of the FCF.

I suspect you pose the question because it's LMC, rather than the more common MMC use of composite tolerancing. Using LMC here is legal, but I can't say whether it's wise; that totally depends on the function. I will say that this doesn't allow you to use functional gaging, which is allowed when the MMC modifier is shown.

Thank you so much for the help everyone!

The function is simple for these holes. Ink is simply jetted through these holes and basically supposed to make lines parallel to each other and within a certain distance apart at a consistent width. It is for a print head.

I was also very curious as to how the Bonus Tolerance for this FCF is calculated.

[Kelly_Bramble]

I was also very curious as to how the Bonus Tolerance for this FCF is calculated.

For the upper horizontal frame:

When the holes are as-manufactured at LMC dia. 0.263, the allowable position tolerance is dia. 0.050. As the as-manufactured hole size decreases an increase in the allowable position equal to that amount of size divergence is summed to the starting position tolerance specification. So, drill the hole at dia. 0.262 - position tolerance allowed is 0.051. drill the hole at dia. 0.261 - position tolerance allowed is 0.052 and so on until MMC size.


For the lower horizontal frame:

When the holes are as-manufactured at LMC dia. 0.263, the allowable position tolerance is dia. 0.025. As the as-manufactured hole size decreases an increase in the allowable position equal to that amount of size divergence is summed to the starting position tolerance specification. So, drill the hole at dia. 0.262 - position tolerance allowed is 0.026. drill the hole at dia. 0.261 - position tolerance allowed is 0.027 and so on until MMC size.

[afotion]

For the upper horizontal frame:

When the holes are as-manufactured at LMC dia. 0.263, the allowable position tolerance is dia. 0.050. As the as-manufactured hole size decreases an increase in the allowable position equal to that amount of size divergence is summed to the starting position tolerance specification. So, drill the hole at dia. 0.262 - position tolerance allowed is 0.051. drill the hole at dia. 0.261 - position tolerance allowed is 0.052 and so on until MMC size.


For the lower horizontal frame:

When the holes are as-manufactured at LMC dia. 0.263, the allowable position tolerance is dia. 0.025. As the as-manufactured hole size decreases an increase in the allowable position equal to that amount of size divergence is summed to the starting position tolerance specification. So, drill the hole at dia. 0.262 - position tolerance allowed is 0.026. drill the hole at dia. 0.261 - position tolerance allowed is 0.027 and so on until MMC size.

Kelly,

Thank You so much!

Perfect, easy to understand, and well written explanation!