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
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
What drawing standard if any do you work to, ASME Y14.5M-1994?
What is Engineering anyway: FAQ1088-1484: In layman terms, what is "engineering"?
RE: Datum
I uploaded and attached the file again. Let's see if it works this time.
Yes, that is correct.
RE: Datum
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
"For every expert there is an equal and opposite expert"
Arthur C. Clarke Profiles of the future
RE: Datum
Thank you for your responses.
Attached is an image of the trigger and its mating part. 0.43 fits into 0.5 slot/passage.
RE: Datum
John Acosta, GDTP Senior Level
Manufacturing Engineering Tech
SSG, U.S. Army
Taji, Iraq OIF II
RE: Datum
RE: Datum
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 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
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
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
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
"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
Thank you for your responses. This is good and very helpful information.
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
Why you want to use GDT is that will raise the cost???
Isn't it better to have scrap?
RE: Datum
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
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
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.
What is Engineering anyway: FAQ1088-1484: In layman terms, what is "engineering"?
RE: Datum
John Acosta, GDTP Senior Level
Manufacturing Engineering Tech
SSG, U.S. Army
Taji, Iraq OIF II
RE: Datum
**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 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
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.
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RE: Datum
Drstrole
GDTP - Senior Level
RE: Datum
That was my thinking too. I said even the same thing in my previous posts.
So one more in my camp.
RE: Datum
BTW, I thought you said it was Dave's camp...
John Acosta, GDTP Senior Level
Manufacturing Engineering Tech
SSG, U.S. Army
Taji, Iraq OIF II
RE: Datum
Yes , you are right here!
RE: Datum
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
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
"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
John Acosta, GDTP Senior Level
Manufacturing Engineering Tech
SSG, U.S. Army
Taji, Iraq OIF II
RE: Datum
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
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.
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
RE: Datum
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
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
John Acosta, GDTP Senior Level
Manufacturing Engineering Tech
SSG, U.S. Army
Taji, Iraq OIF II
RE: Datum
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
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
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
0.203 +0.005/-0.000
0.194 +.005/-0.000
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
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
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:)