Rant: Customers don't follow their own tolerances

[guythatbrews]

If you are sure your parts meet all drawing requirements, either you are willing to put up with ignorance from these customers or you aren't.

I look at it this way. I agree to make parts to spec and on time. In return the customer agrees to pay for on-spec parts on time. What could be simpler, right? But some customers just don't get it.

If you decide they are worth the trouble just hope they don't pull the same crap on a great big job.

 

[boosted]

Are ya'll checking your minor diameters religiously?

Yes, but it was never really a problem until I started using form taps. It's pretty hard to mess up the minor on a cut tap.

Our system is same as yours, all GO-NOGO thread gauges have accompanying GO-NOGO pins for checking diameters.

 

[Ox]

Kind'a hard for a form tap to be loose fit - doo to the nature of the process.


-------------------

Think Snow Eh!
Ox

 

[VTM]

When I started doing this in the 90's I messed up a run of parts for minor diameter being out of spec. I was a rookie and admittedly didn't inspect that.

Now all my thread gage drawers have an accompanying no-go pin to make sure minor dia is not oversized for spot checking. We record actual minor diameter on First Article Inspection sheets.

I have a part-time guy that also works at another shop doing work for big-name semiconductor companies and he tells me they never check minor diameter. Kind of blows my mind.

Are ya'll checking your minor diameters religiously?

We are.

If we don't we will get rejected. Often times our parts must clear 3 different Inspection rooms all together. I put the Minor Ø call outs on every drawing that has an Internal thread, and Likewise I put the Major Ø call outs on every drawing that has an external thread. That way nobody has any excuses and my guys know to check It.

 

[Doug]

What worse was that they cut the part to see the cross section of the thread and saw that the threads were not pointy because I used a roll tap, which they have no idea what the process is. I explained the technical to them and showed them

It's very possible what they didn't like about the thread cross section was they weren't "pointy". Even threads in spec can look that way and it just doesn't look good.

Could it only be terminology when they say "too loose". Maybe what they really mean/want is pointy.

Having thread points not closing is something I don't like appearance-wise in form tapped holes. I even had a fine pitch thread where you had to be careful the fastener didn't screw into the unclosed points. When you look at commercial fasteners with formed threads usually the points are pointy (how do they do that, hot forming?).

Speaking of tolerance, etc, mostly those are a best guess for the designer. We built the demonstration model for a very complicated mechanism. print tolerances were generally loose, +/-.005". We kept everything to +/-.001" and it didn't work, too tight.

 

[ToolmakerTH]

Having thread points not closing is something I don't like appearance-wise in form tapped holes. I even had a fine pitch thread where you had to be careful the fastener didn't screw into the unclosed points. When you look at commercial fasteners with formed threads usually the points are pointy (how do they do that, hot forming?).

On our threaded holes, you couldn't see the unclosed portions on the thread crests. They all closed up at the crest but if you cut the threads to see cross sections, you could still see the tiny pockets below the crests. The top of the crests was shallow U-shape. We intentionally drilled the pre-tapped holes undersized so they would close up, however they would never form a uniform single radius crest like a cut tap unless we severely undersize the drill hole so that when the tap forms the threads the excess material gets rolled up and crushed under the threat roots on the tap. That shortens the tap life significantly.

 

[JS]

[]

 

[ttrager]

Just want to rant and get this off my chest because I am dealing with the same shit with multiple customers at this moment.

I have been gaining new works from both old and new customers this past year, so I have been doing alot of sample pieces for them before we go into production. This week, there are 2 cases of rejects from 2 different customers, both are repeat customers.

Case #1 - The sample parts were rejected due to threaded holes being "too loose". They checked with Go NoGo gauges just like I did and they passed the gauge inspection. However, they said they think there is too much slop even if the NoGo gauge can not go in. Well, isn't the point of Go NoGo gauges so you don't have to use your judgement?! What worse was that they cut the part to see the cross section of the thread and saw that the threads were not pointy because I used a roll tap, which they have no idea what the process is. I explained the technical to them and showed them videos of thread rolling. They still didn't care and proceeded to blame the thread rolling process for the "loose" thread. Now, how am I going to measure the threads if I have to go by their "feel"?

Case #2 - This is a very simple pin part. The diameter tolerance is not difficult to machine at all. The part was machined on a cnc lathe and passed all the inspection. Customer's QC gave these parts OK. But their engineer rejected the part. He wants the diameter to have 3 microns straightness and roundness even though there is no callout at all and the diameter tolerance is alot wider than that. I stood my ground and I told them to make revision to their drawing and I would quote it again. The engineer doesn't have authority to make a new design because it came from their oversea headquarter. He still insists on his tolerance though.



Enough ranting for now

How are you guys doing?

Sounds like you covered the situation with an agreement up front on what gages would be used. Periodically I've seen issues like this where there was a difference of opinion between two parties, that, in the end, could be traced back to the Work Order / Quote NOT stating how specifications would be qualified for acceptance.

In your case it sounds like that did happen.

Good luck, hope this works out so you aren't taking a loss in labor/cost.

 

[Fal Grunt]

Are ya'll checking your minor diameters religiously?

Man... I can't remember the last time I checked a minor diameter (major all the time). There was a weird bastard interrupted acme I did a few years ago.... and a extra fine bastard that I had to make go/nogo gauges for.... I can't think of another time! I guess I need to think about that going forward?

So glad I worked in a plant that the parts I made were only used in our plant. I knew where every part I made was used and what tolerance I needed to make them fit. Would hate dealing with outside customer BS.

Wow, were you lucky. Internal customers were as bad or worse than external in my past experience.

 

[Ox]

Checking minors how?

Other than a shadow?

That's what rings are for.


-----------------

Think Snow Eh!
Ox

 

[EmGo]

Yes, but it was never really a problem until I started using form taps. It's pretty hard to mess up the minor on a cut tap.

Have you considered skipping the form tap thing for this particular customer ? Are form taps saving you so much money that it's worth going through this hassle ? In fact, at the end of the day are they saving money or costing money ?

 

[metal-ica]

Checking minors how?

Other than a shadow?

That's what rings are for.


-----------------

Think Snow Eh!
Ox

I was referring to tapped holes.

I almost exclusively use form taps because of the material I'm cutting. As Boosted mentioned, it's harder to f-up when using cut taps.

 

[Ox]

Well I agree b/c a wee bit of change on a form tap drilled hole makes SO much difference on the other end!

I used to run a lot of form taps as well 15-25 yrs ago, but not many now.
I still like them for high volume jobs for no chips, but ...

A formed internal thread on min minor will strip way easier than one with a cut tap.
You need to git to a smaller minor before the formed ID thread is as strong as a cut thread.

Everyone's concept of stronger formed threads points back to external threads that are rolled up to full crests, THEN, and only then - you have a stronger thread.


--------------------

Think Snow Eh!
Ox

 

[EPAIII]

That's an interesting thought on form taps. And this is perhaps the first time I have seen it expressed. I have not used form taps, yet.

Is this due to formed threads not having a fully formed crest? If so, what is the amount of the crest that is typically missing? I assume that the crest on a formed thread would be a radius, not a flat.

And could that be changed by using a smaller tap drill?

Well I agree b/c a wee bit of change on a form tap drilled hole makes SO much difference on the other end!

I used to run a lot of form taps as well 15-25 yrs ago, but not many now.
I still like them for high volume jobs for no chips, but ...

A formed internal thread on min minor will strip way easier than one with a cut tap.
You need to git to a smaller minor before the formed ID thread is as strong as a cut thread.

Everyone's concept of stronger formed threads points back to external threads that are rolled up to full crests, THEN, and only then - you have a stronger thread.


--------------------

Think Snow Eh!
Ox

 

[Garwood]

That's an interesting thought on form taps. And this is perhaps the first time I have seen it expressed. I have not used form taps, yet.

Is this due to formed threads not having a fully formed crest? If so, what is the amount of the crest that is typically missing? I assume that the crest on a formed thread would be a radius, not a flat.

And could that be changed by using a smaller tap drill?

The hole has to be dead nuts on size to form tap nice threads. The crests will look jagged and if the hole is oversize too much, like 5 thou or so the threads look like double start threads. I stopped using form taps. I don't like futzing with drills that much for the volume I do. I cut tap on size threads. I thread mill oversize threads for plating and type3.

 

[Ox]

That's an interesting thought on form taps. And this is perhaps the first time I have seen it expressed. I have not used form taps, yet.

Is this due to formed threads not having a fully formed crest? If so, what is the amount of the crest that is typically missing? I assume that the crest on a formed thread would be a radius, not a flat.

And could that be changed by using a smaller tap drill?

I may have a bit more experience with this than many - simply b/c I have set up many thread roll heads on multi-spindles. I Have rolled many Dryseal pipe (NPTF) threads back-in-the-day, and to make that 6-step profile gauge fit, it's got be be dead nuts! When dooing that, you will be looking at your form a lot with a magnifier while dialing in your timing. You can also see it when setting up your Fette head, but since timing is not an issue on a Fette, you typically are not studying your thread form much. Mostly just measuring PD. But you will see the problem easily on form tapped holes with too large of a start hole as well.

But essentially, when you are forming, you are pushing down the root, and displacing the material to fill in the form on your tap into the crests. But seldom doo you ever get to 100% on an internal thread. I used to run a 3/8-16 hole in 6061 in an oil slinger that way, and I loved the look of those threads, but that would likely not be attainable in steel, nor in coolant. (never tried, so not sure)

So, your formed thread is a line of W's.
Looking at that W, envision that the bottom of the W is the bore of your hole, and the W represents only one full thread pitch.
The outside edges of that W are where the fwd and aft thread on your tap is forming.
The V (upside down for this arguement) in the middle is the zone that is getting squeezed and pushed up into the form of the tap.

Now the W shown here is an over-dramitization as it is all that I have handy, and I say this b/c the V in this W goes all the way to the root diameter, when in real life we are starting at a little under pitch D.

But the catch is that until you get to full crests (almost never on an internal thread) you are essentially running a double thread. Picture your 3/8-16, with 5/16-16 running inside of it. If you have a large start hole, it is not uncommon for the customer to actually have troubles getting their fastener started - as it wants to try to start in that in-between thread 1/2 the time. This is the noticeable issue that folks are aware of.

But what most either don't understand, or prefer to disregard simply b/c they want to run this style tap, is that even when you have your 70% thread that so many are OK with, the 25% of the crest that has been formed up past the 45% start hole, is unsupported. Go back and look at that W aggin.

So, lets say that if you look at the cross section of your thread, and at the diameter of the root of your unformed crest* (see daffynation below) of your W shows 1/3 formed V, then 1/3 unfinished crest, and then finishing with another 1/3 formed V. We started at 45% full thread, and it has been formed up to - what? If you are measuring your Minor D now, and you show 70%, the root of that open area between is going to be more like 50-55% prolly. So, your formed thread has a good solid 50% thread, and another 20% that has very little strength since there is not much backing it up.

So, you drop to a smaller drill size, and now you are up to 75% on your Minor D. Now the middle gap of your thread is pushing up quickly as you have more material pushing up into an ever smaller void, and maybe now your gap is at 65%?

Now you drop to another letter size and your Minor D is now up to 80-85%. Now the space that your V occupies is getting much smaller. You have much less V sticking up above your gap, and your bolt doesn't even want to think about cross threading anymore. I would say that by this point your 80-85% formed thread is likely as strong as your cut tapped 70-75% thread, b/c the gap between your threads on that cut thread is flat, and fully supported at your measured Minor D.

There will ALWAYS be a W profile until you get to 100%.

* Meaning, look at your W thread. I am talking about the depth of what is left of that upside down V in the middle of your W.

You can prolly find some pics online that would show this better than my text.


----------------------

Think Snow Eh!
Ox

 

[Fal Grunt]

Great post Ox.

 

[MMC Design]

A couple of questions for the group.
1) When you meet the print, but they reject the part, do you try and force them to pay you or do you walk away?

2) How do you handle a situation in which you meet the print and have the inspection data from inspection equipment that is in calibration to prove it, but their internal inspection has a different result that shows the part fails and they reject the part.

Mike

 

[Fal Grunt]

I'm not a production guy, so I am sure my experience will be different than others. I have been blessed, as yet, to not have had this problem.

1. If the part meets the print, they would absolutely need to pay for the parts.

2. If there is a discrepancy, then I would want them to show me how they are inspecting the part. Sometimes knowing how the part is being inspected is as important as the tolerances. Is the discrepancy small? Is this 10 parts or 10,000? Are they amiable or assholes? Can you afford to walk away?

 

[Ox]

A couple of questions for the group.
1) When you meet the print, but they reject the part, do you try and force them to pay you or do you walk away?

2) How do you handle a situation in which you meet the print and have the inspection data from inspection equipment that is in calibration to prove it, but their internal inspection has a different result that shows the part fails and they reject the part.

Mike

Only time that I can recall this really being an issue for me, was prolly about 20 yrs ago. A customer wanted some fittings post haste. But I didn't have gauges for that thread, and it was a thread that I didn't see needing much, and this job didn't justify buying them, and so they gave me a set to use. (rings - for what was Shirley a rolled thread)

I delivered a partial in a cpl days, and by the time I got home, I had notice that they had called and said that the threads were junk. I grabbed my gauges and went back right away.

The parts fit the set pf gauges that they gave me like a glove, but the ones that they were using out on their floor read something COMPLETELY different. Both sets had been certified for ISO within the last year.

They sent the other set back out onto the floor and kept going in their shop, and accepted my parts and sent me home to run the balance. No red flags in any way from what I could tell. I was just beside myself!
I don't recall what the thread was, but it was a little odd, like maybe NPS? I am pretty sure that it was a straight thread, as the size really mattered. Not like a taper that would work regardless.

I sure am glad that it was their gauges that didn't match!
It may have been a bigger fight if they were mine?

IF I had any respect for ISO and it's kin, it would have been lost that day!
And I learned that day that I don't want to send my gauges out for cert either!


-------------------------

Think Snow Eh!
Ox