Screw thread FIM callout 1

[craigmj]

Hello All,

I recently designed a custom screw with a 3mm X .5 thread. This thread is formed by cold rolling and is 1/4hard SS. Thread mates with a nickel plated brass insert that is ultrasonically welded into plastic.

Keep in mind I am new to this screw design stuff...

Turns out when I get the parts that the thread was rolled on at an angle (warbles when you screw it in). This causes some serious problems with force distribution on the threads of the insert and basically cases the inserts to strip out after only a few insertions.

My question is this. Wouldnt one think that if you call out a particular thread on a shaft that the angle at which you can cut that thread (or form it in my case) on the shaft would be limited to some range...inherently?

Anyhow, the vendor comes back to me saying that I never incuded a "FIM" number on the drawing (full indicator measurment) and that although the threads on the screw are at an angle we have no recourse to return them. The FIM measurement on the screws I recieved was 0.070...thats huge if you ask me!

Can anyone tell me how they deal with this issue on a drawing? Is FIM a standard thing to call out? Is what I am experiencing here normal or am I getting "screwed"?

Thanks!
Craig J

 

[ewh]

I would think that secifying 6g would cover you, the same as specifying UNC-2A (for example) on an english thread. They HAVE to go somewhere to determine what is required.

 

[KENAT]

I'd love to be wrong but certainly in any government type work both for UK & US I had to specify the thread standard either as part of the thread callout or in a standard note, same goes for things like welding etc.

Yes they do have to go 'somewhere' but whats from stopping that 'somewhere' being wrong. If you specify the standard then there's a lot less wriggle room.

 

[ewh]

Unfortunately, your thread callout was not complete, so the vendor may indeed have a leg to stand on. If the callout had been complete, I'd have to agree that it would be him in the wrong on this.
As far as specifying the thread standard (ASME B1.13M) on the drawing, in almost 30 years of creating mechanical drawings, I have NEVER seen that done. What is typical though is having a note somewhere on the drawing stating interpretation per ASME Y14.5, which includes references to standards for screw thread representation and gaging.

 

[MintJulep]

Since when does an ambiguous or incomplete call-out mean "Go right ahead and make a piece of crap"?

How about a vendor picking up the phone: "Hey customer, your drawing is not completely clear, what do you really want?"

 

[KENAT]

Mint Julep,

I agree, if a drawing is at all ambiguous they should contact you.

However it sounds like this vendor doesn't follow this rule.

It definitely sounds like primarily a vendor problem, but you've got to have your ducks in a row as it were.

 

[craigmj]

I understand now that my drawing was incomplete and I should in the future refrence a standard explicitly on the drawing.

BUT

I also agree with Mint J., that there is no excuse for threading at such an angle as to cause the part to be 0.070" out of round. Thats just bad workmanship in general!

Thank you all for the great discussion.

Craig

 

[aardvarkdw]

Ewh is correct, you don't need to reference every standard that may apply to every feature on your drawing. Reference ASME Y14.5M-1994 and that should cover most everything since Y14.5 references the other standards for you. There are exceptions but for threads and basic design geometry Y14.5 should be enough.

 

[KENAT]

If you're gonna go for the 'one reference covers everything' line then maybe Y 14.100 might be a better reference.

I'm just basing this on what our checker says though, I'm not actually basing it on a definitive statement in a standard. He's got lots of experience tho (40yrs +) much of it working on mil std packs.

That said he makes me call out the thread ref (it's in our standard notes).

 

[ewh]

UNC-2B = UNIFIED COARSE thread, size class 2B
The standard referenced is in the callout, no note needed.

 

[btrueblood]

Y14.100 does reference y14.5, which then references the screw thead standards...

But y14.100 also references such stuff as: "MIL-STD-882
Revision: D Chg: Date: 02/10/00
SYSTEM SAFETY PROGRAM REQUIREMENTS ".

I think ewh is correct that, for most of us, y14.5 is usually sufficient. If you are working with stuff for Mil/NASA application, then you are probably forced to use y14.100 anyway, and thus oughta call it out on your drawings.

 

[aardvarkdw]

For clairification I didn't mean that the Y14.5 note needed to be tacked onto the thread note, just that it needed to be in the standard drawing notes.

 

[KENAT]

At the risk of making myself unpopular (or more unpopular as the case may be) ASME Y14.6-2001 para 3.2 addresses the issue of whether to reference the thread standard.

In the US the name and number of the controlling thread standard is frequently omitted from the drawing. Reference is made instead to the designation symbols of the standard, such as series symbols and class symbols. To avoid misunderstanding, it is recommended that the controlling organization and thread standard be specified or otherwise referenced on the drawing.

It then gives examples of the thread spec as part of the callout.

So it’s only appears to be recommended to put the thread.

Also looking at our copy of 14.5M I don’t see the direct reference to ASME B1.1 or 1.10M or 1.13M.

In section 1.2 it does reference Y14.6 which in turn does reference these though.

I don’t have 14.100 to hand so can’t check.

 

[ewh]

Boo hiss KENAT!
Only kidding... as usual, it is good to know what the standards say. Now if they could only do away with that word "recommended".

 

[ringman]

I may be missing something, but it appears that the thing missing and causing the problem is the relationship between the threaded portion and the unthreaded portion of the part being discussed.

Without any control of the relationship, (FIM), as stated by the supplier, and without knowing the standards on the drawing for interpretation of dimensioning, it would seem that the drawing is lacking sufficient information.

 

[KENAT]

Ringman you may be right but that's not how I understood it.

I assumed the screw was threaded up to the 'head' and that it was a misformed thread causing the inserts to fail.

As such I would have thought having the thread properly designated should be enough.

Found Y14.100 (again it was sitting right on the shelf) and it doesn't directly reference the thread spec, like btrueblood says so I was wrong in suggesting it would be a better indirect ref.

 

[btrueblood]

Hmm, KENAT:

1. I didn't actually look up the spec., I surfed to IHS/Global website, and looked them both up, and clicked the link "specs referenced by this spec". They are pretty good about picking up secondary references as well (they should be, that would allow them to sell us more paper!).

2. Y14.100 references Y14.5, so it is a more encompassing spec., and just as valid a catch-all as Y14.5 for this problem. Just that it (Y14.100) also pulls in a lot of extra stuff that you may or may not want to comply with, or ask your vendors to comply with. Y14.100 is the ANSI continuation of the old DOD-STD-100 which I think was before that MIL-STD-100, and typically applied to any Mil/NASA/aero drawings by contract specification.

 

[ringman]

craigmj,

Does your drawing have a note which states in accordance with Y14.5 on it? If not, what is the standard that you invoke for your dimensioning interpretation?

Thanks
ringman

 

[craigmj]

I confessed a while back in this thread that I was lacking the specific standard callout...thats my fault and not one I'll make again.

Whats the old saying "bad experience is unfortunately the best way to gain good experience".

I did call out the thread tolerance class, but was not obviously enough. To my defence the parts were 0.050"-0.070" out of round, which on a 3mm x 0.5 screw is a crap load. You could actually watch the head of the screw wobble back and forth as you screwed it in. Ugly.

The vendor DID eventually take the screws back...

 

[KENAT]

I don't think any of us doubted it was primarily a vendor problem.

I was just trying to make sure you had your back covered if it got ugly.

Glad to hear it worked out.

Ken

 

[ringman]

No Drawing Standard being specified leads to 'BEEG PROBLEMS'. You just might have dodged a beeg bullet.