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Datum

Datum

(OP)
Hi everyone,

We have these triggers that are dimensioned and toleranced as shown in the attached image. Recently we have switched the supplier and received parts that are not good. The main problem is that the center of the ears (slot) does not align with center of the tab (0.43 dimension) at the bottom. The part is not straight. The goal is to have centers of slot and tab to have controllable alignment.

I am thinking to add Datum A to slot (0.416 dimension) and location/position tolerance to tab (0.43 dimension) referencing Datum A.

Am I doing this right?

Thanks,
Vadim

RE: Datum

My opinion:
The thru holes (both of them) datum feature A (primary datum)
.416 width --datum feature B (secondary datum)

Position .43 in relation to A primary and B secondary (maybe even A at MMC and B at MMC)
Profile everything else to A(MMC) and B(MMC)including the curvature.
all radii must be basic.

Disclaimer: I don't know how this part actually functions, I imagine how this might function.



RE: Datum

I would keep dim. .415 primary

"For every expert there is an equal and opposite expert"
Arthur C. Clarke Profiles of the future

RE: Datum

No doubt the first drawing was sorely insufficient. My opinion is the datum reference frame should be as follows: make the .416 and the .43 widths two separate datum features (let's say A and B) then make A-B primary. The cross hole pattern should be secondary. Pick some other feature for the tertiary and dimension accordingly.

John Acosta, GDTP Senior Level
Manufacturing Engineering Tech
SSG, U.S. Army
Taji, Iraq OIF II

RE: Datum

Still go with my original answer. The bottom slot is driven not driving ( does not stop any degrees of freedom, it is clearance with the bottom frame)

RE: Datum

Hi, Vadim:

I agree with powerhound. I think it's better to make A-B as a primary. Reason is that neither A (.416 - slot) nor B (.43 - tab) is big enough to be a good primary datum.

Alex

RE: Datum

The primary control of orientation of the part is the cross-holes.

The width of the slot controls the location and would be secondary.

I'm basing this on the observation that the if the holes are misaligned by a small amount to the overall shape the overall shape will end up crooked in the handle. It looks like there is plenty of clearance in the width.

A complicating factor is that the allowable tolerance for the position of the tab depends on the clearance of the pin in the other part.

An alternative is to backdrive the holes. I would make a primary datum of the an RFS with targets at the width of the tab, the width of the slot and some other location along the trigger. The secondary datum is a target along the front edge of the slot and a point on the tab. The tertiary depends - either it is along the top of the slot or a shoulder of the tab.

Then set up a positional tolerance that is the difference in diameter of the pin hole taken from the mating virtual diameter of the clearance hole.

RE: Datum

Quote (3DDave)

The primary control of orientation of the part is the cross-holes.

Functionally you engage the slot first and insert pin thru cross-holes second.

Just like in any text book - flat surface primary, hole thru flat surface secondary.

"For every expert there is an equal and opposite expert"
Arthur C. Clarke Profiles of the future

RE: Datum

Probably is six of one half a dozen of the other.......

Holes (at RFS) primary and slot (at MMC secondary)
versus
Slot (MMC) primary and holes (RFS)

I agree with Dave that to reduce the variation the holes should be RFS.

I still consider that the A-B compound is not the way to go in this case, but I could be wrong. Again, my opinion.
(.430 slot does not stop any degrees of freedom and not suppose to rub against the frame therefore not arresting any DOF's)

RE: Datum

(OP)
Additional information to my original post. I got these images from QC. Hopefully this helps understanding the problem we are facing with these parts.


In this image the trigger is supported by pin that is sitting on both blocks.


When aligning the trigger from this side, the gap is larger than what it supposed to be.


When aligning the trigger from the other side, there is no gap. That's what causing the rubbing against the frame (one of the support legs in the casting), when assembled.

RE: Datum

This is a perfect case study to show how improperly defining a part can result in unusable parts that you still have to buy. I don't think anything about this part violates the print, does it?

"Using GD&T, raises the part cost!" Well how expensive is it to buy parts you can't even use?

John Acosta, GDTP Senior Level
Manufacturing Engineering Tech
SSG, U.S. Army
Taji, Iraq OIF II

RE: Datum

(OP)
Everyone,

Thank you for your responses. This is good and very helpful information.

Quote (3DDave)

The primary control of orientation of the part is the cross-holes.

I'm basing this on the observation that the if the holes are misaligned by a small amount to the overall shape the overall shape will end up crooked in the handle. It looks like there is plenty of clearance in the width.

Dave, after reading your response we took another look at the hole alignment and sure enough we found variance. We happen to have two samples in the QC office. One sample had a variance of 0.020" and the second was relatively aligned; variance of 0.001".

Another observation we made - center line of curved part is NOT parallel to the flat part (see image below). It is almost centered on top and it's quite off at the bottom as it can be seen in the image.


It looks like the hole misalignment along with curved part not being parallel to the slot/flat section (0.416 dimension) is the cause of the problem.

RE: Datum

I agree with John. The parts are per the print.
Why you want to use GDT is that will raise the cost???
Isn't it better to have scrap?

RE: Datum

(OP)

Quote (powerhound)

This is a perfect case study to show how improperly defining a part can result in unusable parts that you still have to buy. I don't think anything about this part violates the print, does it?

"Using GD&T, raises the part cost!" Well how expensive is it to buy parts you can't even use?

Technically, yes, no violations. And this shows that the part needs to be more defined. Some things are implied to engineers/designers, but it seems that many times they are not to manufacturers. Is this true?

Some engineers can go to one extreme, while some production managers to the other. Engineers can spec out parts that will come out perfect, but too expensive. Parts didn't meet the print, but the production is trying to push - can we still use them? Being able to find the balance between engineering, production and purchasing is very critical to economical operation of the company.

Regards,

-Vadim
Design Engineer

RE: Datum

Dimensions can be implied. It is safe to imply that the .415 is centered on the curved handle. What can never be implied are tolerances. The reality is that the .415 will NEVER be perfectly centered in the handle so some degree of centering error has to be accounted for. The standard specifically says that all tolerances must be stated. In this case there is no tolerance stated for how off center the width can be so if it's .125 off center, there's nothing on the print saying it's unacceptable.

Keep in mind that "dimensions" and "tolerances" are two separate things and each must be addressed.

Dimensions can be implied, tolerances cannot.

John Acosta, GDTP Senior Level
Manufacturing Engineering Tech
SSG, U.S. Army
Taji, Iraq OIF II

RE: Datum

See ASME Y14.5M-1994 2.7.3 regarding showing parts alinged/centered/symmetrical etc. not implying any tolerance.


VD2108 - a properly prepared drawing where tolerance is properly addressed minimizes the issue of 'can I use it or not'. Tolerances should be driven by function. If you end up needing tolerances that aren't cost effectively met by available process capability then you need to look if you can redesign your part to be manufacturable.

Leaving tolerances off - as appears to be the case on your original drawing is just plain wrong and doesn't help production or engineering - but may allow an external supplier to force you to buy scrap.

Posting guidelines FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm? (probably not aimed specifically at you)
What is Engineering anyway: FAQ1088-1484: In layman terms, what is "engineering"?

RE: Datum

Also, you cannot imply the use of the title block tolerance for unstated tolerances if you are to comply with ASME standard. Many places think that it is "Industry Standard" to do such a thing but as of yet, I have never Had anyone show me the Industry Standard and point that paragraph out to me.

John Acosta, GDTP Senior Level
Manufacturing Engineering Tech
SSG, U.S. Army
Taji, Iraq OIF II

RE: Datum

PH - unless the slot is an interference fit, it doesn't control orientation. Since the primary datum (typically**) provides orientation control, the slot is not an appropriate choice for the primary. It would be different if the slot was an interference fit and the pin had a lot of clearance, but that's not been my experience with nozzle handles.

**I say typically as it's possible for a sphere to be primary and it controls no orientations, but it does control 3 location degrees of freedom.

RE: Datum

I agree with most of what you've said, Dave. My concern was that the cross hole was really short relative to the length and how far away the other width was. Considering the function, I didn't see any disadvantage to using the slots as primary since the datum would encompass more of the part, and the holes as secondary, with appropriate controls of course.

I don't know what you mean by primary datums typically providing orientation control. I've never thought of it that way. Can you elaborate on this a little? I see planes used as primary more than anything else. So to me, a plane is easily typical. Since my recommendation used a plane as primary, I'm at a loss as to what you mean.

John Acosta, GDTP Senior Level
Manufacturing Engineering Tech
SSG, U.S. Army
Taji, Iraq OIF II

RE: Datum

I am in Dave's camp in this particular issue. Anybody else is joining us? Or John's camp for that matter?
As I stated: one is driving and one is driven. In my opinion the upper width and the cross holes are driving and the bottom width/slot is driven into its position.

[color ][/color]

RE: Datum

Not knowing more about the assembly than presented in this forum, I would choose the cross holes as primary and the width as secondary. The cross holes locate the handle on a pin in the assembly and would restrict degrees of freedom on two planes. The holes IMO are driving the design requirements.

Drstrole
GDTP - Senior Level

RE: Datum

Drstrole,
That was my thinking too. I said even the same thing in my previous posts.
So one more in my camp.

RE: Datum

I'm not actually arguing my position over yours green. I agree that the cross holes are the functional feature. I stated that my concern was that the axis was extremely short relative to the distance between the widths. I thought a more stable primary datum would be the plane between the two widths. Doing it your way controls the location of the .43 width perp to the holes and centered on the .416 width, in that order. My suggestion controls the perp of the holes relative to the plane established by both widths simultaneously. Both work. I feel mine is easier to achieve and inspect with no negative impact on the function but either way, I think both are acceptable solutions.

BTW, I thought you said it was Dave's camp...bigsmile

John Acosta, GDTP Senior Level
Manufacturing Engineering Tech
SSG, U.S. Army
Taji, Iraq OIF II

RE: Datum

Re: " I thought you said it was Dave's camp.."

Yes , you are right here!

RE: Datum

I'm actually in the alternative camp which suggests locating the handle with a compound of the upper width, the lower width and some other anti-twist combination to orient the holes.

The problem with using the holes alone as the primary is that it's not possible to indicate the float allowed in the mating part without causing secondary problems.

If the holes are used as a datum RFS, then there's no float. If the holes are used as a datum MMC, then the hole diameter, which is used RFS by the pin, is not getting the benefit of the slop in the mating part.

OTOH if the width of the slot with an MMC callout (min size is the tightest fit)
+ the extended tab with an MMC callout (max width is tightest fit)
+ orientation targets (RFS)

then the pin holes can have the location and orientation tolerance allowed by the mating part.

If a user wants to take some tolerance away, then make all the datum references RFS.

This arrangement is the equivalent of putting the trigger into the handle and match-drilling the holes through the pivot hole in the handle.

Simple prismatic parts don't usually get this sort of treatment, but irregular parts like this can end up with more complicated datum schemes.

RE: Datum

Quote (3DDave)

I'm actually in the alternative camp which suggests locating the handle with a compound of the upper width, the lower width and some other anti-twist combination to orient the holes.

So this actually sounds exactly like my suggestion. I guess we're all in this camp together.

John Acosta, GDTP Senior Level
Manufacturing Engineering Tech
SSG, U.S. Army
Taji, Iraq OIF II

RE: Datum

PH - except you missed that the features you selected are unstable and need a third location to stabilize the part. And if your suggestion was familiar, it's because I made it first.

"I would make a primary datum of the an RFS with targets at the width of the tab, the width of the slot and some other location along the trigger."

RE: Datum

That's kind of weird because I don't see you posted at all before I made the suggestion. Maybe you didn't read the thread from the beginning. Also my suggestion completely constrains all degrees of freedom so I don't know where this idea of it being unconstrained is coming from.

John Acosta, GDTP Senior Level
Manufacturing Engineering Tech
SSG, U.S. Army
Taji, Iraq OIF II

RE: Datum

PH - to back up to a question you posed about orientation.

Let's say there's .125 nominal clearance with the slot width and .5 deep engagement.
Let's also say there's 0 nominal clearance with the pressed in pivot pin and .5 engagement.

Which is the more stable control?
Which affects the path of the trigger as it turns?

However - using the pin hole as a datum prevents the convenient transfer of clearance in the mating part as tolerance to the hole location.

If the handle is fixed in a frame of reference such that it clears the mounting boss, fits into the track, and is not twisted in the frame, then the cylinder of the pivot pin has limits on its location and orientation relative to both parts, limits which can be applied as orientation and location tolerances on the pivot pin holes.

RE: Datum

To be specific about what I've proposed, it's not just a single width that is used as the primary datum, it's both widths referenced as a compound primary datum feature. The secondary datum feature would be the coaxial cross holes, and the tertiary is just some feature to stop rotation around the axis of the cross holes. I agree that there will be clearance around both widths so referencing the A-B plane at MMB is certainly in order. Since the rest of the handle still needs to be controlled (the edges and curvature, perhaps with profile of a surface) I think it would be easier to control the outside profile parallel to a plane that extends from one end to the other, rather than perpendicular to a short axis at one end of the part. All that is left to do is apply the appropriate perpendicularity control to the cross holes relative to the compound primary datum.

Quote (3DDave)

Which is the more stable control?
They are both stable. I think we have different ideas of what stable really means. Allowing datum feature shift does not make for an unstable DRF.

Quote (3DDave)

Which affects the path of the trigger as it turns?
Definitely the pin. That has never been in question.

It's hard to ignore the fact that you're back to the "pin as primary" argument. This is your second flip flop in the same thread. You're just arguing to argue aren't you?

John Acosta, GDTP Senior Level
Manufacturing Engineering Tech
SSG, U.S. Army
Taji, Iraq OIF II

RE: Datum

Am I the only one who thinks that there is not enough information about how the trigger interacts with the mating parts (pin and frame) to constructively discuss about defining proper datum features, in proper order, and at proper material boundary conditions?

RE: Datum

PH - your method is over-constrained.

A pin constrains 4 degrees of freedom. a plane does 3. So that's 7 of a possible 6 required. Then you add a third point, so there are 8.

To 'work,' some of the controls need to be ignored.

Width, first constraint:
Z u v

Pin, secondary
X Y u v <- ignore u and v, the angles the pin is the primary control for, or ignore the initial u and v, which requires instability in the primary constraint)

Point/line, tertiary
w.

So the pin, the feature that controls the installed orientation of the part, when used as a secondary, is ignored as the primary orientation control that is used to prevent the tab at the other end of the trigger from dragging along the side of the frame.

I have two equally useful positions - either the pin interface is primary, as it is the feature that controls the most degrees of freedom or it isn't used as a datum at all and is derived from the functional requirements for the positioning of the trigger.

In the fabrication of the trigger, and the particular problem here, there is apparently a failure to appropriately locate the holes following the forming operation. This suggests it is best to create the holes after all other features are formed, which prevents using the holes as a datum reference.

RE: Datum

Dave,
It is interesting what you are saying about degrees of freedom, because if I were to follow this logic in case of figure 4-2 in Y14.5-2009 for example, it looks like 9 DOFs would be constrained. I must be missing something, right?

RE: Datum

You've completely lost me now, Dave. Never mind, I give up.

John Acosta, GDTP Senior Level
Manufacturing Engineering Tech
SSG, U.S. Army
Taji, Iraq OIF II

RE: Datum

pmarc - the apparent difference is a side effect of the standard using precise prismatics for many of its examples.

Same method:
1st datum: Z, u, v
2nd datum: X, v, w; ignore the redundant v because, unlike the pin, there's nothing indicating the part will be oriented with the side.
3rd datum: Y, u, w; ignore both u and w because, unlike the pin, there's nothing indicating the part will be forced into alignment with the smallest side.

9 degrees - 3 ignored = 6.

Note the lack of explanation in the standard for what happens to the redundant controls in secondary and tertiary, et all constraints.

Now imagine that the real part being constrained had all surfaces convex and that the second surface was known to be the primary orientation source, but the large surface was selected for being conveniently large. Then some of the primary constraints have to be ignored for the secondary ones to be meaningful.

RE: Datum

Dave,
If I understand your last two posts (reply to powerhound and to me) correctly, you seem to be sure that the holes play the biggest role in orienting the trigger in the assembly. I am not saying you are wrong, I would just like to understand why you think this is the case. Because, as far as I see, apart from knowing that the width of the opening in the trigger is .416 +.015/-.000, we have not been provided with any meaningful information about dimensions and tolerances in the interface area? Do we know the size and tolerance of the holes in the trigger? What is the size and tolerance of the pin that goes through those holes? What is the size and tolerance of the width of the tab that mates with the .416 opening?

Is this standpoint simply based on your experience with nozzle handles?

RE: Datum

Pmarc, I didn't reply to your inquiry about being the only one ..., because it's an obvious observation that doesn't affect how various requirements might be analyzed. I did reply to your remark about Figure 4-2, but see no hint of its consideration in your latest inquiry.

Based on the apparent surprise the OP had at how misaligned the holes in the failing part were, following my suggestion about their affect, I'd say the hole alignment was of previously unrecognized importance.

Is your experience that high duty-cycle pivots are very sloppy and depend on secondary guides?

RE: Datum

(OP)

Quote (pmarc)

Do we know the size and tolerance of the holes in the trigger?

0.203 +0.005/-0.000

Quote (pmarc)

What is the size and tolerance of the pin that goes through those holes?

0.194 +.005/-0.000

Quote (pmarc)

What is the size and tolerance of the width of the tab that mates with the .416 opening?

0.38 +/-0.015

The size of the slot that 0.43 handle feature fits into is 0.50. This is sand casted handle.


-Vadim
Design Engineer

RE: Datum

Thank you, Vadim.

Knowing these values, I agree that the holes in the handle should be selected as primary datum features. Their coaxiality could be controlled by datumless position callout or by using CONTINUOUS FEATURE concept (if you follow ASME Y14.5-2009). The .416 opening should be secondary datum feature, controlled by perpendicularity callout relative to A. The .430 tab should then be controlled by position callout to A|B. I would personally define both A and B at MMB (MMC, using Y14.5M-1994 terminology).

Dave,
Actually, I did consider your reply to my inquiry about fig. 4-2 in my last post, although not directly. Last part of your reply was about customization of orientational degrees of freedom, which lead me to the conclusion that you really did not consider any other feature beside holes as orienting feature.

This is also where the over-constraining issue came from in your reply to powerhound, I believe.

RE: Datum

pmarc,

Re:" the holes in the handle should be selected as primary datum features.................The .416 opening should be secondary datum feature"

I said exactly that a week ago or so..... That is called beginner's luck ....Thank you for agreeing with me.
If I have you on "my boat" I am not afraid of anything:)


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