My problem is that I have been asked to teach a basic course in Geometric Tolerancing and the thought of standing in front of a group of people makes my face break out.   I'm thinking of adopting a whole new persona.     Since I'm losing my hair, I'd like to shave my head like Kojak or Michael Jordan.     But my skin is pasty white and my head is so lumpy that I'm afraid it might look like a semi-collapsed Casaba melon.     Also, and I'm ashamed to speak of it, my feet stink.    Not that I plan on taking off my shoes during a lecture, but the smell becomes more intense when I'm nervous. So, when I'm standing up there drenched in a flop sweat, dark rings forming under my arms, with my head shaved and  lopsided, my face covered in hives and pimples and my feet smelling like aged cheese, I wonder if anyone will be listening to the lecture or just wondering what planet I am from. Do you think cowboy boots with spurs and a ten-gallon hat would help? I could dress all in black and wear wrap around sunglasses indoors. People might think I'm cool.

Signed,
Searching for a New Me

Dear Searching:

We don't handle personal grooming tips in this forum, but thanks for writing anyway. I suddenly don't feel so bad about myself.

Dear ASTE:

The people around me are always doing something.  I look out of my office cubicle all day long and watch them.  They scurry about, talking to each other, answering the phone, glaring at their computer screens, typing in stuff and then running off for hours, only to return again and start over. What the heck are they doing?

Signed,
Baffled in Buffalo

Dear Baffled:

They're probably working. Why don't you try it?  It will give you something else to do.

Dear ASTE:

About a year ago, I started a new job. In this place, I'm around toxic waste all the time.  My skin has recently taken on a greenish tint and my hair and teeth are falling out. After my weight dropped from 180 to under a hundred pounds, my wife said she couldn't stand the stench on my clothes anymore and she ran off with the paper boy. A welder I work with says the company has been illegally dumping poison in the ground for the last 50 years here and that I shouldn't drink the water. What scares me is that I've worked with welders before and I know that they will drink about anything, moonshine, kerosene.  I even knew one that used to drink skin bracer.  I'm beginning to think I made a huge mistake by coming here.

But, this is a government run underground facility.  Some even say it's a front for the CIA. The government wouldn't do anything that wasn't in the best interest of the people. Would it?

Signed,
Skinny and Disgusting

Dear Skinny:

You need to start watching the X-Files. And consider it a documentary.
 

Dear ASTE:

I am concerned about some of the datum references being used at my company. For example, engineers have recently started using a couple of holes as datum features. I'm not talking about the common usage of one hole (call it D) as a secondary datum feature and the other hole (call it E) as a tertiary datum feature. No they will either use them together as D-E (or D at MMC dash E at MMC), or they will use both holes as a pattern datum (calling both holes datum pattern F, for example). My first question is; Is this legal and how are these
two concepts different?

My second question is about how they reference these datum features. As Isaid, sometimes they will reference them at MMC (maximum material condition)and at other times at RFS (regardless of feature size). Is this legal?

Signed,
Ted

DearTed:

The concept of using two holes as datum features such as D-E is exactly like using them as a pattern datum consisting of two holes. For example, if I called a two hole pattern (or an 899 hole pattern) datum pattern F, it is perfectly legal. The question is what does it mean and how do I measure it.

In the Y14.5 standard, it allows patterns of features to be used to construct datums to be referenced at MMC. In other words, F at MMC. This relates to your question, in that if D and E are both referenced at MMC, it is exactly like calling out a pattern datum at MMC. It means, in general that the virtual condition boundaries of the features within the pattern datum generate a central axis for the pattern datum (something we sometimes call a centroid of revolution). It is from this axis that things will be measured. Now, in your situation, D-E, if both D and E were referenced at MMC (D at MMC dash E at MMC) and both D and E were in the same plane on the drawing, all you would have to do would be to construct a plane (perpendicular to the primary datum plane) through D and E's virtual condition boundary centers (axes) and also a plane perpendicular to this plane that is halfway between the two centers. This would be the central axis of the datum D at MMC dash E at MMC, and subsequent features could be dimensioned from this axis.  Without the MMC concept, it is a more gray area, but if they insist on it, I think we can extend the principle of coaxial holes to say we expand into D and E (regardless of feature size). This will construct an axis that is halfway between the two. The problem I see is the exact definition of what is being specified.  

We could certainly approximate the theory by just finding the center of each hole and constructing a plane through the center of both and then a perpendicular plane half way between the two.  That is pretty simple and pretty correct. But the exact theory is (in my opinion) that both holes would have an expanding gage pin located at the basic dimension apart and the gage pins would expand at rates that would allow both to contact the hole surfaces simultaneously to create their D-E axis regardless of feature size. 

That is just what we ask of the coaxial diameter datum feature situation that uses a compound or simultaneous datum (something like twin chucking devices grabbing both features at the same time to create the common axis between the two).  The only difference in representing datum features referenced at MMC and datum features at RFS is supposed to be that when referenced at RFS the gage pins or fixturing pins (if used) expand (instead of being represented at one fixed size-virtual condition or maximum material condition, as appropriate).  

The argument you may get is at what rate they expand. Since this is not well covered in the Y14.5 standard, some will argue that the gage/fixturing pins expand at different rates so as to engage the holes simultaneously (this is the one I think is correct). But others will argue that the gage/fixturing pins expand at the same rate engaging the smallest, most geometrically imperfect hole first, then using the other hole to give only angular orientation control (stopping only rotation).  I believe this definition to be inconsistent with the most common use of compound or simultaneous datum features referenced at RFS, which is with the coaxial diameters as datum features situation.  In any case, what they want to do, I believe is legal, just more legal, well-defined and therefore, more understandable if referenced at MMC (D at MMC dash E at MMC or datum pattern F at MMC). 

I cover all this in my textbook on Geometric Dimensioning and Tolerancing in Section 10.2 on pages 212-215.

I hope this helps.
Jim Meadows

Dear Jim:

Recently there has been a lot of controversy over the appropriate design and tolerancing of Receiver Gages.  I know you can't cover the whole thing here. But, since you are the chairman of the B89 (as yet unpublished) standard on Fixtures and Gages for Geometric Tolerances, can you give me just a few guidelines.

Dear Writer:

Here are a few from my text book on Measurement of Geometric Tolerances in Manufacturing (pages 249-250).

There are three gaging policies practiced throughout the world. They are:

1. Accept most good parts, reject all bad parts (reject some borderline good parts).  This policy is the ANSI preferred policy. 

2. Accept all good parts, reject most bad parts (accept some borderline bad parts). Not an ANSI preferred policy. 

3. Accept most good parts, reject most bad parts (reject some borderline good parts and accept some borderline bad parts). Not an ANSI preferred policy.
 

Gage Guidelines:

- Receiver gages=GO gages and Functional Gages

- GO gages check MMC only (perfect form at MMC envelopes)

- Functional Gages inspect geometric tolerances and virtual condition boundaries (MMC
  concept)

- Functional gages are made to the virtual condition (MMC concept) of the features they
   inspect

- Recommended gage tolerancing policy is that no bad parts will be accepted; therefore, a
   small percentage of borderline technically good (in tolerance) parts must be rejected

- If so, gage tolerances only add material to the gaging feature. This means that gage pins
  are toleranced to begin at MMC or virtual condition and then be only larger than this
  boundary they are inspecting and gage holes are toleranced to be begin at MMC or
  virtual condition and then be only smaller than the boundary they are inspecting. In other
  words, gage pins get only plus tolerance (no minus) and gage holes get only minus
  tolerance (no plus)

- ANSI standards recommend 5% of the part tolerance be used as gage tolerance, with an
  optional 5% for wear allowance.    This is a total recommended 5-10% of the part
  tolerance to be used as gage tolerance.

- Part tolerance for GO gages is equal to the difference between the inspected feature's
  MMC and LMC.

- Part tolerance for Functional gages is equal to the difference between the inspected
  feature's LMC and Virtual Condition (MMC concept).

- To calculate the virtual condition (MMC concept) for all internal features of size (holes),
  use the following formula: MMC of the hole minus the geometric tolerance applicable at
  MMC

- To calculate the virtual condition (MMC concept) of all external features of size
  (shafts), use the following formula: MMC of the shaft plus the geometric tolerance
   applicable at MMC

Good luck.
Jim Meadows

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