Interesting questions on datum feature simulators on linkedin 2

[greenimi]

Interesting questions on datum feature simulators on linkedin
Are datum feature simulator requirements (basic orientation relative to one another for all the
datum references in a feature control frame) still applicable to the threaded holes?
See link below

http://www.linkedin.com/groups/Datu...ack=.gde_1781397_member_227361160.gmr_1781397

I am interested to find out your opinion about this subject. I had an almost similar scenario 2-3 years ago when a gage design project has been scrutinized and over-analyzed for exactly the same reasons.

Thank you

 

[CheckerHater]

If I understood the linked content correctly, the threaded holes are NOT datum features.
Why datum feature requirements should apply to threaded holes?

 

[pmarc]

Agree with CH. This problem seems to have nothing to do with datum feature simulators.

Threaded holes are controlled features, not datum features. Could you clarify your concern?

 

[greenimi]

Yes, the threaded holes are not datum features. Threaded holes have a position wrt to A primary and B secondary.

And no, datum feature simulator requirements are not applicable to the threaded holes, but ARE applicable to the gages (gage pin and gage plate) because the gages are datum feature simulators, aren’t they?

And one more, yes (I agree) for (with) Pmarc, threaded holes are controlled features.

The concern is ON THE GAGES (!!)- where Ø.010 allowable tolerance SHOULD or is recommended to be considered or covered ? (between gage plate and gage pin OR between block to be verified and threaded portion of the gage pin)

See sketch

 

[CheckerHater]

That’s a nice picture you have there.

I would never use "undersized" thread gage. Just think of it.
First, how do you know the thread on the gage is perfectly located wrt thread in the part? So actual gap between gage and the part is not controllable.
Second, as long as threads are actually engaged, they can be tightened, thus self-centering against each other.

My vote is for using gap between cylindrical surfaces. Threads must be tightened (TRU-POZ® - style), or expanding.

 

[axym]

greenimi,

I would say that the 0.010 tolerance would be incorporated in the threaded portion of the 4 gage pins. The threaded portion would need to be sized so that it would always fit with a threaded hole that is at virtual condition - that is, MMC "size" minus the 0.010" Position tolerance. The gage pins would (in theory) have perfect orientation and location relative to the datum feature simulator plate and shaft. This is not because the gage pins are datum features, but because the tolerance zone or boundary for a feature controlled by Position must have basic orientation and location relative to the datums. I'm not sure if this is explicitly stated in Y14.5-2009, or if it's defined by example (I don't have the standard in front of me).

This gaging application is tricky, as the 4 gage pins would have to be threaded into the holes in the part while (or before) being pressed into the gage plate. We might be stumbling over terminology a bit as well - your DFS.pdf diagram includes the 4 gage pins as "datum feature simulators". Technically they are not - the correct term might just be "gage elements".

Evan Janeshewski

Axymetrix Quality Engineering Inc.

http://www.axymetrix.ca

 

[pmarc]

Question #1:
Why on greenimi's picture the gage plate does not have fully threaded holes and why the threaded portion of the gage pin is not longer, so that its minimum length is equal to maximum gage plate thickness plus maximum depth of the threaded holes in the workpart? The cylindrical surface of the gage pin would then be of no relevance - it could just serve as a handle. And the main concern would be to properly control projected position of axes of the threaded holes in the gage plate.

Question #2:
Wouldn't it be more reasonable to check projected positional tolerance of the threaded holes in the workpiece, and not position of the pitch diameters inside the holes? This workpiece mates with something and it is the position of extending portion of a fastener that is the key decider whether the two parts will mate or not. Isn't it?

Question #3:
Why not to use completely different approach (if projected tolerance zone is not subject of interests), that is to apply positional tolerance to the minor diameter of the threads in the workpiece ? Then two independent gage checks could be done:
1. One threaded gage pin unrelated to any datums could be used to see if all 4 threaded holes in the workpiece, as individual features, are OK.
2. Relatively simple gage with 4 fixed in size pins could be used to check if minor diameters of threaded holes in the workpiece are not violating their virtual conditions.

 

[greenimi]

Looks like CH and Evan gave conflicting suggestions for this issue (where Ø.010 should be or is recommended to be found on the GAGE ELEMENTS) and pmarc stirred us in some other direction (projected tolerance zone)
CH and Evan and all,
What advantage or disadvantage you see for these two way of designing the gages? Why one is better than the other from your perspective?
Per Y14.5-2009/page 31: Each tolerance of orientation or position and datum reference specified for a screw thread applies to the axis of the thread derived from the pitch cylinder.

Evan,
Are you suggesting to have the PD (on ½-13 UNC thread) on the threaded gage element ground from its MMC (.4500) to .4400 (.010 positional) ? I just would like you to confirm I did understand you correctly since you said “The gage pins would (in theory) have perfect orientation and location relative to the datum feature simulator plate and shaft”

And yes, I agree with you: gage plate and gage shaft are datum feature simulators, but the gage pins are “just gage elements”.

Pmarc,
I will start with:

Q#2: Yes you are right here, but it is what it is. I don’t know how the drawing on original post (on linkedin) looks like, but on drawings I have to deal with, I don’t have the projected tolearance zone for the threaded holes. (yes, you know the old “aggrement”—if we have projected tolerance zone, nobody will understand it - so stick with the devil you know).
I picked up this subject (from linkedin) because it’s a subject of a great interest for me. I did create the picture with the original dimensions just to make it consistent and have somehow a link between those threads.

Q#3: I have to agree here with you (from a gaging perspective), but from the product engineering perspective the argument is: the minor diameter is a non-functional diameter, the mating part is not centered on the minor, but on PD and is no control (manufacturing cannot control) between PD and the minor. How far PD axis is from minor diameter axis? Don’t know and to sacrifice some of the tolerances (from .010) just for an easy (non-expensive) inspection method did not appear a very good option back them (2-3 years ago) for the projects I was involved in.

Q#1: I did not pay too much attention (lengthwise) when I created the above picture, so you could be right about “the threaded portion of the gage pin is not longer, so that its minimum length is equal to maximum gage plate thickness plus maximum depth of the threaded holes in the workpart.
My concern with this approach is that you have 4 gage pins and you will never be able to get the gage plate sitting flat on the work piece to simulate the primary datum feature A (because of the how the threads works - to thread the gage pins into the workpiece while they are threaded into the gage plate and have the gage plate sitting “perfectly” (acting as a primary datum) would not be the easiest thing to do. Just my opinion.

 

[CheckerHater]

Let set a virtual experiment:
A) Create two pieces with male and female threads.
B) First we will have them perfectly match each other.
C) Then we shrink male thread to create radial gap around it.
D) But did we really create radial gap? Let start turning male piece around. The radial gap will shrink.
E) And then finally surfaces of male and female threads will touch each other and gap will turn to zero.
This is what they mean, when they say threads are able to self-center.
In real-life assemblies threads are always tightened, so surfaces of male and female threads are always touching each other.
This is the reason why many sources suggest to always specify threads RFS, because bonus tolerance is virtually non-existent.
This is why using undersized threaded pins to check location of threaded holes is questionable to say the least.
Not only “The gage pins would (in theory) have perfect orientation and location relative to the datum feature simulator plate and shaft”; the gage pins will also have to be perfectly oriented to actual threads made in the part, so crests and roots are aligned to achieve maximum gap. With all due respect this fixture concept seems to be completely unrealistic.

 

[pmarc]

greenimi,
As for my 3 questions, I agree with your answers, that:
Q1: The approach I described would not be good idea, exactly because of the reasons you mentioned.
Q2: The projected tolerance zone is often not used because it is poorly understood.
Q3: This approach would not reflect real functionality of the assembly.

However, in my opinion, as long as projected tolerance zone is not defined, your gage will not be verifying true functionality of the assembly. In other words, the gage will check something, but it will not give you full answer whether two (or more) parts can really be assembled properly with the use of fasteners. If the projected tolerance zone was used, the approach to gage build shown in fig. B20 of Y14.43-2011 could be used.

 

[fsincox]

It is nice to see the issue of projected tolerance zone even brought up. It is another example of the distance between were the committee is and where we in the "working world" are.
Frank