GD&T Callout to Protect Wall Thickness in Valve Housing 1

[jimbod20]

Have a look at the attached hand sketch.

I have 10 round holes in a cylindrical valve housing and I want to protect wall thickness between the holes. The holes are .073-.077 inches in diameter.

I need to control location and orientation with respect to datum surface A. I want to control wall thickness between holes. Does the attached sketch adequately define.

Max allowed hole position tolerance will be .005 with .077 inch diameter hole.

 

[pmarc]

Yes, this can be the way to go.
I would, however, put B back into first compartment of composite positional FCF -- without it location of pattern relative to datum axis B is totally uncontrolled.

P.S.: Out of curiosity, which version of the standard are you using, 1994 or 2009, and which cylindrical feature(s) is/are exactly intended to serve as datum feature B?

 

[jimbod20]

I am using 1994.

Position to B controls orientation of holes to B diameter? I define the pattern as 'equally spaced'.

I do have another general question. Can I establish position of holes to B at MMC or RFS and position of holes to each other at LMC? Can I mix material modifier within composite FCF?

Thanks pmarc

Jim

 

[jimbod20]

Maybe I should explain a bit more.

Surface A is not a 'feature of size', therefore, material modifier is not applicable to surface A. Although not shown on my sketch I do hold surface A flat within .0005 and I define surface A perpendicular to diameter B within .001. I do maintain o-ring fits I want a diameter B. Sketch shows two o-ring grooves.

Diameter B is a feature of size, therefore, a material modifier would be applicable to B. Manufacturing will lean toward MMC, therefore, I would want to hold position of 10 .073-.077 inch diameter holes to B at MMC.

Jim

 

[pmarc]

1. Referencing to |B(M)|A| in upper segment of positional FCF means that all 10 axes of tolerance zones:
- cross datum axis B;
- are located at basic distance from datum plane A;
Their angular relationship will be controlled the "equally spaced" note.

2. You can control position of pattern to |B(M)|A| and mutual spacing of holes by position at LMC in one composite FCF - this is allowed.

3. So okay, you are using Y14.5M-1994. But how many cylindrical portions exactly will serve as datum feature B? 1 (as it is currently shown on the sketch), 2, 3 or 4?

 

[jimbod20]

4 cylindrical features define Datum feature B.

 

[pmarc]

In that case you have to find a way to tie all of them together and mark that all 4 consitute datum feature B. If this was '09 edition, there would be quite simple and ellegant ways to accomplish this (continuous feature modifier or zero position at MMC without datum feature references), however according to '94 this may not be that easy and straightforward.

You may want to read following thread that I started not so long ago:

http://www.eng-tips.com/viewthread.cfm?qid=350055

Although we were mainly talking there about group of features geometrically controlled from other datums (by position or runout tolerance), the very similar problem will occur in your case, where interrupted datum feature is used as primary datum feature.

 

[PaulJackson]

jimbod20,
you said:

Diameter B is a feature of size, therefore, a material modifier would be applicable to B. Manufacturing will lean toward MMC, therefore, I would want to hold position of 10 .073-.077 inch diameter holes to B at MMC.

WHY MMC! Is there any functional purpose to permit the ten tolerance zones (projecting radially, equally spaced, and perpendicular to the primary datum feature's axis) to translate at minimum or translate and rotate away from that axis variably according to their size? I cannot think of any functional reason considering given your design goal...

I want to control wall thickness between holes.

By coding the feature control frame with MMC modifiers in both the tolerance and datum feature you would enable the building of a "go gage". It would however be simply be 1/2 of an attribute gage strategy the other being a collection of "no go" local size measurements in each of the ten holes... not necessarily a functional gage strategy.

(Just curious)... If there are o-ring seals in the OD grooves of the "valve sleeve" I presume... then chances are those groove OD's as a pattern orient and locate the sleeve in its assembly? I would not use them however, as the feature to generate the functional primary to achieve the design goal nor would I use the OD lands (identified ) which are typically designed to be in clearance in the assembly... rather I would use the inner bore (typically the final finishing operation on the part) to accomplish the goal functionally.

You could provide a generous but sufficient tolerance limiting the material loss between the holes considering their "least material condition" and reference the inner valve bore as the primary datum feature at RFS. Also if the axial position of the pattern is less critical you could reference the position only to the axis [A] and then provide generous limit dimension to constrain the pattern to the assembly feature that stops translation along the axis. I would weigh the controversy of putting a LMC modifier on the tolerance for the holes (in terms of my down-stream customer not understanding the additional latitude that it gives manufacturing for conformance based on hole size) and capitulate with RFS if there was significant resistance... but I would not succumb to MMC on the holes unless forced... because then I would have redo all of my calculations and protect for thin walls between holes due to manufacturing's leaning toward non-functional "attribute gaging" (typically referred to as "FUNCTIONAL GAGING" whether it is or not).

 

[jimbod20]

Paul/Pmarc,

Appreciate feedback.

My question regarding mix of MMC and LMC in composite FCF was motivated more so by rules rather than application in this specific example. I do understand my explanation of LMC/MMC mix in my specific example is at best poor as outlined by paul.

I have also had a chance to study the concept of functional gaging applied to composite position tolerance applied on an MMC basis.

My takeaways;
I need to control wall thickness between the 10 holes. I'm comfortable with number of holes, hole size and position on LMC basis.

Both datums B and C establish a datum axis. I do need to establish holes cross datum axis. I do believe datum C datum feature/surface (2x diameter) is more explicit than Datum B feature being 4 locations. I'm not sure if this is rigorously correct as pmarc noted. I use Y14.5M-1994. Datum C is more functional (so to speak) than datum B.

I use generous pattern locating tolerance of .010 RFS to C and A. Datum C is primary. I maintain feature relating position tolerance of .005 LMC.

Thanks

 

[pmarc]

LMC concept does not have to be automatically used when control of what is commonly called "wall thickness" is required. Each positional tolerance, regardless of material condition applied (RFS, LMC, MMC) creates two boundaries - inner and outer boundary of a toleranced feature. In other words:

1. Current lower segment of composite positional FCF with tolerance value modified by (L) creates two boundaries for each of the holes within the pattern:
- outer boundary, which is virtual condition of the hole = .077 (hole LMC size) +.005 (pos. tol. at LMC) = .082. This outer boundary "is responsible" for "wall thickness" between the holes.
- inner boundary, which is worst-case resultant condition of the hole = .073 (hole MMC size) -.005 (pos. tol. at LMC)- .004 (bonus tol.) = .064

2. However, the lower segment of composite positional FCF could contain a tolerance value modified by (M), and it would also create two boundaries for each of the holes within the pattern:
- outer boundary, which would be worst-case resultant condition of the hole = .077 (hole LMC size) + [pos. tol at MMC] + [bonus tol.]. Now, to keep the same .082 as in case #1, [pos. tol at MMC] + [bonus. tol] would have to be equal .005, correct? This in turn would be possible only if the value of positional tolerance specified in lower segment of positional FCF was .001 followed by (M).
- inner boundary, which would be virtual condition of the hole = .073 (hole MMC size) -.001 (pos. tol. at MMC) = .072

3. The lower segment of composite positional FCF could contain a tolerance value at RFS as well, and it would also create two boundaries for each of the holes within the pattern:
- outer boundary of the hole = .077 (hole LMC size) + [pos. tol at RFS]. To keep the same .082 as in case #1, [pos. tol at RFS] would have to be equal .005.
- inner boundary of the hole = .073 (hole MMC size) -.005 (pos. tol. at MMC) = .068

The disadvantages of such "conversion" can be easily noticed:
A. Sizes of the inner boundaries are different in each case.
B. Total available positional tolerances as defined by lower segment of composite FCF are different in cases #2 and #3 comparing to case #1:
- case #1 = .009 = .005 (specifed on the drawing) + .004 of bonus;
- case #2 = .005 = .001 (specifed on the drawing) + .004 of bonus;
- case #3 = .005 (specified on the drawing)

In return in case #2 you gain a possibility of verifying the composite positional callout by using 2 hard gages.

At the end of the day it is up to you to decide which scenario fits the most to your application.