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

 

[KENAT]

Craig,

What country are you in/standards are you calling up? I only have access to the US specs which for metrics are slightly different to the rest of the world in the way you designate threads.

That said the difference may not be relevant given that this is a special.

If you are doing it to the US specs let me know and I'll look it up.

Ken

 

[craigmj]

Hi Ken,

Im calling out an ANSI Metric 6g class thread.

Craig

 

[KENAT]

Can't find the spec right now. I'll check with my checker in the morning as he had it last I saw.

 

[ctopher]

As far as I know, your callout is good. I have used it for years without problems. I have never used the FIM callout. I'm in the USA.
We did business with a company in Denmark and they never had a problem with it either.

Chris
Systems Analyst, I.S.
SolidWorks 06 4.1/PDMWorks 06
AutoCAD 06
ctopher's home (updated 06-21-06)

 

[CoryPad]

This area is covered by ISO 4759-1 Tolerances for fasteners — Part 1: Bolts, screws, studs and nuts — Product
grades A, B and C .



Regards,

Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[KENAT]

OK, found the spec, it was sat on his desk out in the open, if only I’d opened my eyes.

The spec is now ASME not ANSI, shouldn’t make a difference in practice but if this comes down to a legal type thing I guess it might.

Looking at ASME B1.13M-2001, Metric Screw Threads: M profile, I see FIM mentioned at 5.7.

Circular runout is the full indicator movement (FIM) (see ASME Y14.5). Runout of the crest (minor diameter of internal thread or major diameter of external thread) relative to the pitch cylinder shall not be so great that the basic profile is transgressed.

Sounds like your screws may break this rule.

However, In section 7 ‘Designation of Screw Threads’ I don’t see anything about FIM numbers.

I think your callout should be:-

M3 x 0.5 6g

You’d also obviously need a general note saying something like “Metric screw threads are to ASME B1.13M-2001”

But you probably already new that much.

It would probably be best if you get a copy of the spec and look yourself as I don’t have time to look in detail.

What thread spec did you call up on the drawing and did your callout meet its requirements? If you didn’t explicitly call up the correct spec and designate the thread as it requires they may have wiggle room.

I would be very wary of relying on the ISO as you and presumably your manufacturer are in the US. The ASME spec designates ISO threads differently from the rest of the world, for starters you always have to put the pitch and tolerance class. We had that problem here, we used the international convention but called up the ASME and wondered why machine shops kept asking for the pitch, hence we now have a copy of the standard.

I've certainly never called up FIM though, either in the UK or here.

 

[craigmj]

Cory,

I dont have this spec. Online I can get it for $130, but first can you tell me what it talks about in regards to this topic. Meaning does it talk about inherent tolerances in to threading a screw on a shaft.

Am I in trouble here because I didnt put an FIM number on the drawing or is just calling out the thread enough to expect a good result (to some tol)?

These screws are so far out that Im thinking this FIM number thing is just an excuse...but I cant be sure.

Thanks for the info,
Craig

 

[KENAT]

Craig,

I guess you were writing while I was. Like I said, be very wary of using the ISO if you and supplier are in the US. Look at the ASME spec.

 

[craigmj]

Thank Kenat.

So many specs, so little time.

I wonder what they mean about "...basic profile is transgressed." Can you put a number to the "transgressed"? Seems like a subjective thing.

Craig

 

[KENAT]

I'll try and have a look at lunch but I think it would be covered by the 6g and whatever values that relates to.

 

[CoryPad]

Transgressing the basic profile means the part dimensions are outside the envelope allowed by the specification, so it is not subjective.

You don't have to worry about ANSI vs. ASME - they are the same. ANSI doesn't create any standards, they just approve ones from other organizations.

I wasn't paying close attention - if you are referencing ASME B1.13M, then you can use ASME B1.3M and B1.16M as your other references for tolerances, gaging, etc.

Regards,

Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[KENAT]

Cory,

From a practical point of view I agree on ASME v ANSI but if it comes down to a legal, 'you didn't make your parts right' 'well you didn't put the right specs' then it's worth making sure.

 

[KENAT]

http://www.boltscience.com/index.htm?

http://www.newmantools.com/tech/threadm.htm

Should help you.

It would be a lot of typing.

As Cory said it isn't subjective. The thread form and tolerances are specified in the spec. I didn't see the FIM number explicitly stated.

Talked to my Checker and we both think you've just got bad screws and maybe the manufacturer is trying to find some way out.

 

[MintJulep]

ASME B1.13M fully defines the thread form and tolerances. It also defines how to properly call-out a thread using the standard.

"FIM" is not mentioned in the discussion of call-outs.

Even if it were, it is an inspection method, not a manufacturing control.

Your vendor is full of crap.

 

[KENAT]

Very Succinctly put MintJulep.

Out of interest Craig what standard did you reference on the original drawing?

And yes I know I was wrong in my first post, M3x.5 isn't a special.

 

[craigmj]

Thanks much for all the good info.

Ok...Gulp. All I put on the drawing in regards to the thread callout was "3 x 0.5 6g". Meaning I didnt specifically call out the standard.

For the record I design plastic parts for a living and this was my first machine screw

I have a copy of the ASME standard on the way!

ty!
Craig

 

[KENAT]

Don't feel so bad, I've seen lots of drawings calling up not just threads but other things without specifying what standard they are to.

The bad news is that it probably means you don't have a leg to stand on legally but I could be wrong.

I would encourage you to always state the relevant standards on the drawing somewhere, usually in some standard notes. You don't necessarily have to actually have a copy of the standard, although it's a good idea.

 

[btrueblood]

Hmm. How did your vendor interpret the "6G" callout without referring to the standard? (Maybe they rolled them in a centrifuge at 6 G's? That might explain how they got them so f%^d up.) If you do much business with this vendor, or planned to do much business, I would have a brief conversation with the manager/owner/president, stating "make this right or forget any more of my business, and I will go out of my way to tell any and all colleagues of mine how I was treated".

 

[craigmj]

Thats a really good question! If 6g was on the drawing then how else are they suppose to interpert the tolerance on the thread?

This vendor has always been a pain to work with. Our procurment/materials deptartment must find some redeaming qualities about them...Of course given enough pain that could change.
Get ready for ssome pain.

Craig