Thread gages

[litlerob]

So as some of you know I am a machinist that pretends to be QC when our guy is out. Our guy is not out at the moment, but has brought a new quality standard to the business. From now on when we check thread depth we need to measure the over all length of the thread gage, zero our caliper (or do a little math with dial or vernier calipers) and measure the remainder of the gage once bottomed out in the hole. Okay fine, company policy. Here's the kicker, he doesn't want the tapered tips ground off, they are to be included in the final hole depth. INCLUDING the no go end of the gage. Does ANYONE do this, EVER?

Robert

 

[ewlsey]

There are a few methods, most are approximate at best. We tend to treat all thread depths as minimum dimensions. Even the best CNC machinery floats a bit on the tapping depth. IMO +/- .030" is not unusual.

The method I use is to measure from then end of the gauge (the tip of the no-go side) to the first thread on the go side. All gauges I have seen have a higbey (SP?) thread on the go side. This part relies on some eyeballing. I record that number. Then I screw it into the thread until it bottoms. I measure from the end to the top of the part. The difference in the measurements is your thread depth.

The other method is to screw the gauge in until it bottoms. Then back it out and count the number of turns. The total number of turns times the thread pitch gives the depth. This method does not account for the lead in chamfer. It does not work for ports or other things with large chamfers.

The best method is to have the gauge made with a notch for the thread depth. However, that is limited to just one measurement.

I'm still trying to train our lead man that .005" shallow on a thread depth is not a reason to panic. At least he is checking...

 

[citizen_snips]

Always do it the way your QC guy wants it done. There may be a good reason for it, or it might just be that he doesn't want the guages to be other than factory - none modified, all the same.
The guages themselves need to be guaged or calibrated from time to time, perhaps this is on his mind. They may be unacceptable if modified. Or, a customer requested it, and his boss told him to do it that way.

ALWAYS do it the way your QC guy says!!!!

 

[citizen_snips]

Yeah, you're right about discussing any disagreements. But I don't mean "I was just obeying orders". I mean, QC needs to maintain consistency, so if there are three different guys measuring with a modified or different guage all over the shop, what has he got?

So that is why I advise to measure the way QC wants it done - of course, if your QC guy is an idiot and it really is wrong, then it's time to talk to da boss.

 

[litlerob]

A funny update, this issue became a real issue. The parts we were building were for an Aero/Defense contract. I pushed the issue to the point of reading the government document describing the method of thread depth verification. It does in fact state that the OAL of thread gage should be measured and used to verify the depth (their standard). So we shipped these parts----and they were rejected because of the thread depth. Our quality manager sent photos of himself measuring the thread depth, and their response was that he was doing it incorrectly because the conical part of the gage is not actually part of the gage. We re-made the parts

 

[The real Leigh]

... then why isn't there a standard made giving guidelines or rules for how it should be measured?

Standards exist because something is important. As you said, tighter standards cost more, so each situation must be evaluated independently.

For example, NASA has at least four standards that I've encountered:
1) manned flight safety of flight;
2) manned flight non-safety of flight;
3) unmanned flight safety of flight; and
4) unmanned flight non-safety of flight.

Any of these may apply to any particular part, and multiple standards may apply to the same part (a bolt, for example) depending on its usage.

Any attempt to combine all of the available standards would result in a tome so voluminous that the printing cost would bankrupt most countries,
and the storage space required for a single copy would exceed the floor area of the palace at Versailles.

I worked at an aerospace company some years ago that had a complete set of military specifications on CDs. The set occupied 24 file boxes IIRC.


- Leigh

 

[litlerob]

Interesting thought Gordon, the fact is; there was a toleranced drill depth, it was .300" the tap depth was .250" both with a tolerance of ±.020" (just under one pitch). It was a 4-40 2B thread. We ended up deviating from the drill depth-given the material (304 SS) and chip control. I agree with Leigh about the standards needing to be--------uhhhh standardized, but this particular situation was unique just because one person was following the standard to the best of his knowledge but the customer was defining or re-defining the tools we used, and how to use them.

I was satisfied through vindication as this is an on going argument between myself and QC.

Robert

 

[citizen_snips]

Litlerob - wow!!!!

Awesome for you to look into the specs, and what a surprise ending. Go, dude!!!!

 

[Putch]

Why not just bottom tap ALL holes? what's another .05" of thread depth really gonna cost in labor time or tooling? I always bottom tap or tap thru when allowed

 

[The real Leigh]

The customer is always right. When's the last time you told a customer he wasn't right?

No, the customer is not always right.

But the customer is paying the bill... only if his QC department accepts the parts.

- Leigh

 

[litlerob]

My mistake. I should have put "The customer is always right" in quotes.

My point was that it's rarely easy to tell a customer that (now I'm being diplomatic) they might not be right. I've found that the larger the company the harder it is to get through to someone that can do something. It's always someone else that's the "sinner".

Gordon

That could not be more true, multiple engineers, multiple drafts persons, multiple sales persons-----our end, multiple machinists, multiple Quality Assurance people, and one smart ass.

Litlerob - wow!!!!

Awesome for you to look into the specs, and what a surprise ending. Go, dude!!!!

Hell yeah

Why not just bottom tap ALL holes? what's another .05" of thread depth really gonna cost in labor time or tooling? I always bottom tap or tap thru when allowed

Doesn't work that way in Aerospace, the deal is if it's to print they buy it, if not---we do, and they start dinging (recording) your company for failures, regardless of whether you get them more good ones in time or not. Boeing has a 3% rejection acceptance, when you go over that they drop you.

Robert

 

[machinistrrt]

Things sure must be different now. When I was working in the aircraft component outfit, tapped holes were all 3B fit, tap drill depth was the same as your situation, but thread depth was either given as through, or two place decimal depth, minimum full thread. And yes, we all resorted to counting turns on the go plug as well as argue with the inspector whether or not there was an allowance for the chamfer.

 

[racen857]

When threading to a required depth I have sometimes made "bolts" with a shoulder, and made the other end flat. Then cut the length to either the minium or the max lenght making a "go - nogo gage". Always worked good for the shops and takes the guess work out of it. One of the shops I worked in where we did this alot was making alot of parts for electronics. lots of +-.02 on thread depths.

 

[The real Leigh]

Full thread depth is trickier.

{Counting turns of a gage is WAY too imprecise IM[-H]O. }

This is my suggestion (assuming the hole is in a planar surface to which the test system can be set perpendicular):

You need a thread gage accurately sized to minimum thread height with NO chamfer (i.e. square thread end) with both gage ends flat and parallel,
and a dial indicator of suitable range and accuracy, with a support for same.

Set up the DI and gage offset from the threaded hole. Set the gage perpendicular to the surface with the DI stem on its end, and zero the DI.

Move the system to center on the thread. Run the gage into the hole until it stops with very gentle pressure (you don't want to cut new threads).

The DI reading should be the depth of the full threads (see discussion below regarding gage form).

If the workpiece doesn't meet the criteria stated above, the method can still work but might be much more tedious.

There's one minor point regarding thread dimension and gage configuration:
How do you define "thread depth"? Is it to the peak of the male thread, or to the peak of the female thread?
This will determine whether the flat end of the gage intersects the maximum or minimum thread depth.

If the gage is designed to be flat at the full depth of the thread (i.e. all full threads on the gage, as I would recommend),
you would need to subtract half the thread pitch from the resulting reading if the customer spec uses the other definition.

- Leigh