RUNOUT ON A THREADED PIN. HOW DO YOU REFERENCE PITCH DIA (SEE DRAWING)

[johnchrc]

I have threaded shear pins with 1/4-28 thread to capture and hold part. I have a .001 runout with cylindrical pin CL Datum A and the pin thread pitch dia. However, I have added note indicating that checking the pin thread Major OD against .158 cylinder CL would achieve the same results and be more practical to inspect. I feel confident the pitch diameter is appropriate but how do you check this?

Calling out .001 runout to major OD probably yields same functional result as PD being held to.001 since the thread would be machined or tapped together. This assumption may be invalid

I attached a sketch to illustrate and would appreciate any comments that would help me to understand the reasoning and what kind of leeway is allowed to decribe intent.

Thanks and Best Regards,

CJ

- CJ

 

[powerhound]

A better solution is to call the PD datum A and formulate an equivalent runout spec on the shaft. Never assume you know how a component will be made. You may think they'll single point something like this on a lathe but they may actually just run a die over it or use a geometric head. The point of this being that the major diameter is not equivalent to the pitch diameter. As far as I'm concerned this is not a checkable callout

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[CheckerHater]

I would agree with powerhound on that.
There are several applications where runout would be preferred method, but this is probably not one of them.
It looks more like a position to me.
If it is really important for your part to operate, you can possibly specify both controls, or even have some sort of statistical requirement as in “check position of the PD so many times; check runout of the MD so many times”.

 

[johnchrc]

If you were inspecting, how would you determine your datum axis from your pitch dia? Is it not much easier to find a datum axis on a smooth dia?

I have not been trained in GD&T and as soon as I think I understand the proper way to inspect a part, I'm wrong. If you make the PD the datum, how do you set up to check the cylinder?

- CJ

 

[johnchrc]

More info... I have attached a pdf showing how the pins are installed in the assembly.

- CJ

 

[powerhound]

johnchrc,

There is nothing easy about inspecting a callout like this. The runout callout to the pitch diameter is not checkable since runout requires surfaces that either revolve around or are perpendicular to a datum axis. The operational word is "surfaces." A pitch diameter does not have a surface. The functional part of an internal or external thread is its pitch diameter so checking the major diameter as an alternative is not the same thing. Changing the pitch diameter to a datum feature means you can use an internally threaded datum simulator. Now you can check the runout of the non-threaded part to the axis of the pitch diameter. You haven't stated which standard you were working to. I'm assuming Y14.5M-1994. This is the standard by which my answers are formulated. If you are working to ISO, then I don't know if what I've stated lines up with it.

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[johnchrc]

Position seems to be a very useful control. However, on this simple cylinder I don't see how postion tolernace is required. The second sketch will provide more insight into the fit of part. I'm not implying you are not correct. If an explanation can be made without too much trouble, it would sure help. Thanks.

- CJ

 

[johnchrc]

Powerhound,

I'm following you now... Internally threaded datum simulator. I can understand this and agree this sounds like an excellent method for holding the thread datum. I'm not sure what this part would be and how involved making one is. As the designer, If a call out PD as datum, I assume the machine shop must now make a simulator in order to check. However, I don't want them to make a $5,000 simulator to make 100 screws thus making my parts excessively costly.

What can be used as a datum simulator for screw threads?

Back to Major OD. I agree it is not the same thing as pitch dia. and does not control the alignment of the thread. However, since these features are made on same setup, they should be on same axis. If a datum simulator isn't practical, could this be reliably used (Major OD) even though it isn't the controlling feature?

Regards,
CJ

- CJ

 

[CheckerHater]

Imagine the screw being checked in the gage shown (very roughly) on the picture.
This gage represents tolerancing scheme shown on the right.
If using gage like that will be satisfactory to tell good screws from bad screws, then position is your thing.

 

[johnchrc]

OK Checkerhater. I understand your point with position.

- CJ

 

[powerhound]

Remember when I said never to assume how a part will be made? Why are you so sure of this? Granted, it makes sense and if the parts are made on a CNC lathe, then it will almost certainly be as you assume; however, the manufacturer could make this in two separate operations. I'll tell you a cautionary story.

I used to work at a place that designed equipment for the oil field. There was a large part that had an ACME thread that had to be located within .002" to a pair of o-ring glands that were cut just above thread. This part weighed about 250 lbs. I said we needed to specify a positional callout on the thread and the glands to ensure that they came out coaxial but was overruled by the engineer who was sure that it would all be done in the same set-up. The parts came in, the threads were up to .015" out of position to the gland and the parts failed. When the engineer was told that because of the size of the piece the threads were cut on a different machine than the gland, he was aghast. He was also told that had they known the threads has to be positioned so closely to the gland, they would have processed the part differently. We wound up paying around $15,000 for parts that went straight to the scrap bin. What's worse is that the engineer never learned the lesson and put all the blame on the machine shop that made the parts, never mind the fact that the potential for disaster was brought to his attention beforehand.

A datum simulator for this part should not be anywhere near the amount you mentioned. If it only needs to check 100 parts then I couldn't imagine it costing more than $200, if that. I guess another question that could be asked is why this callout is so tight. What is the diameter of the hole that the smooth part goes into? As is it, it better not be less than .002 larger with a 0 at MMC positional callout. Even that isn't really sufficient since the pin part could run out to the thread more than the thread could run out to the pin. If the clearance is more, then your runout tolerance could potentially be more, it just depends on how the hole is currently called out.

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[powerhound]

CH has a good suggestion. I guess he posted it while I was typing my novel.

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[CheckerHater]

Isn't picture worth a thousand words?

 

[johnchrc]

Powerhound,

I work in the oil industry and most engineers carry an air of arrogance because they have a degree in Engineering (which generally has nothing to do with a person's mechanical ability) and partly due to the R&D environment at places like SLB where you are expected to know more than the machinist, toolhand, etc.. even though 99% of us have no practical shop experience, field experience or inspection experience. We only know how to draw up parts and calculate a few things. I have tried to be open minded and learn. In fact, perhaps I defer too much because of my own inadequacies. I have seen many like the engineer in your example learn the hard way, continually working up hill because of their know-it-all attitude. It makes me sick and I love watching those with nothing to learn fall on their face.

GD&T is the way to dimension parts but I have done without it for years. Suddenly I have to apply to drawings where, in the oilfield, concentricty between shaft and seal surfaces was the most used callout. Other special tolerances had to be described with notes. I keep asking my supervisor to pay for a GD&T class but he keeps stalling. He wants us to go to a designer that worked in Aerospace and has experience to save money on training.

Back to Pin. I would be glad to share my calculation regarding the fit of the pin. It involves 3 parts. See attached drawing for assembly.


- CJ

 

[johnchrc]

wrong pic... This shows pin we have been discussing in Assembly.

CH - pics are worth lots of words IF you send the right damn pic

- CJ

 

[powerhound]

What is the size tolerance and positional tolerance on the .166 hole?

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[johnchrc]

PH,

Pin: .158/.157 with .001 positional tolerance between thread and cylinder,
Outer Part (Fishing Neck) that screws thread into to has .003 positional tolerance RFS and .160/.163 hole ssize.
Shear Ring has .163/.166 hole size with .000 Max material positional tolerance.

.158 pin + .001 positional + .003 positional on Fishing Neck = .162 < .163 shear ring hole. Shear Ring Hole can only move if diameter get bigger so fit should be assured. Is that how you see it?

- CJ

 

[powerhound]

If the datum structure is set up correctly then I think you're okay with these calculations. You do have an extra .001 that you could add to the pin piece if you design the fit line to line. I would add that you're really engaging on the thread so that's what is locating the shaft, not the other way around. This would seem to confirm the idea that your thread is better served as datum feature A. Also, because the length of the thread is much shorter than the length of the shaft, the runout of one to the other is not equal. I really think position is a better callout since you can make use of the benefits of MMC.

Powerhound, GDTP S-0731
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