measuring threads accurately

[stephen thomas]

.... a new thread micrometer from Mitutoyo or Brown & Sharpe is about $1000 if you include a range of anvils. Try to find used tools that weren't actually used...

There's a special shelf in my office & 'most every day i look up and give a little thought of thanks to PM member PeterM for the thread mics, standards, and anvil sets (metric, inch, and whitworth) he indulged me with.

I've cut plenty of threads in various forms but not qualified to discuss thread measurement, yet; so this "thread" and information is very much appreciated. Especially enjoying apestate's discourse.

I will say that before I was outfitted so luxuriously, the fast and dirty way to compare OD threads was over wires, any near-size wires (or same size drill bit shanks), so long as the thread being cut came out the same as the measurement over the same wires on the thread being compared.

smt

[Mydrrin]

Usually for common sizes internal I will order a gauge, it is pretty cheap if you order ahead and people bring it in. For odd sizes I usually make them or ask the customer for one. It usually takes an hour or two to make a good gage, so for the common sizes it is very reasonable to purchase. For external I use wires, you can also use thread mic's for "V" threads.

Like many here Gagemaker is great for checking with a "gun" style mic, but they are usually only for bigger threads like 2" (50mm) and bigger. Sometimes it is enough to make them from threat heights of Acme threads if you know and trust your inserts (always check thread form if you do it this way). Gagemaker also has great software for thread specs.

It is always a hard thing to decide what needs to be done. I usually charge a bit more if I need to make/buy gages, I have quite a few gages on my shelf gathering dust, so make sure you are paid for your time and efforts, unless they are for sure going to give you the repeat job and not just when they can't get it done on time.

Good luck.

[John_B]

I've found that I can buy a thread gage (OD set or ID) for less than I can make one and have any certainty that it is correct. The cost of the time in making a gage out of good material and inspecting it to ensure it is correct has always been greater than I get them for through my supplier. I get a decent discount off catalog prices from Bass Tool in Houston, who supplies me with Meyer, Vermont, or Pennoyer Dodge stuff.

I have made quite a few in an emergency, but I'm not keen on it! The aspect that bothers me is making a gage on the same machines that I will be making the parts on is suspect.

I typically buy the appropriate gage set, but I also have a OD thread mic set for external stuff - and insist that both are used. For internal threads I will use an indicator to check the thread crests for taper as well as using plug gages. This is in conjunction with always using the full form threading inserts.

[davycrocket]

Hi Turningboy
As you want to make an inside thread that's a perfect fit for the cusomer's part, why not get such a part from him to use as the gauge . ?

Davycrocket

[CarbideBob]

That's the exact reason I recommended sandvik. I have had to defend this opinion on here before, but their threading inserts are head and shoulders above everyone else's in terms of accuracy and function.

I agree that Sandvik sells top of the line tools.
However the prints from most of the top names are just about identical.
When making these one does need to realize that their checking methods vary.

Sandvik's inspection is tough as nails, no doubt about that. They are very hard to please.
You should realize that a fair amount of this work is outsourced.
The shop that ground your Sandvik full profile threaders may also be making them for Seco, Kennametal, Valenite, and others.

Single point toppers are relatively easy, lots of room to play.
It's nailing the seven tooth APIs that separate the men from the boys and pushes your measurement capabilities.
Bob

[CarbideBob]

Hi Turningboy
As you want to make an inside thread that's a perfect fit for the cusomer's part, why not get such a part from him to use as the gauge . ?

Davycrocket

Unless the part he gives you is low limit.
Now your thread works until the customer has some parts at high limit.
I thought we got rid of this custom made to fit stuff a long time ago.
Bob

[gregormarwick]

You should realize that a fair amount of this work is outsourced.
The shop that ground your Sandvik full profile threaders may also be making them for Seco, Kennametal, Valenite, and others.

I know that most of them stem from one place (vardex/carmex?), and I can't say for certain that you are wrong in the case of sandvik. I do think it's unlikely however, since they use their own proprietary style of insert that's entirely incompatible with everyone elses.

[litlerob]

I know that most of them stem from one place (vardex/carmex?), and I can't say for certain that you are wrong in the case of sandvik. I do think it's unlikely however, since they use their own proprietary style of insert that's entirely incompatible with everyone elses.

What does that have to do with outsourcing grinding operations?

Robert

[Gordon B. Clarke]

Hi Guys , What do all you recommend as a good way of accurately mesuring threads internal and external ? we currently use ring/plug gauges for some external/internal threads but this is proving expensive buying these for low volume runs .
We have for instance now a job to do an internal thread M56x1.5 Pitch and it has to be right , have any of you used thread wires and what are your thoughts ??
regards TB

There's some strange answers being given here

First off plug and ring gauges don't "measure" anything, they simply tell you if you are within the tolerances or not.

Pitch diameter is the diameter to measure and for a M56x1.5 (if not otherwise specified) the tolerance is 6H. In numbers the pitch diameter tolerance is min. 55.026 and max. 55.238. A tolerance of more than 0.2mm (0.008") should present no problem.

Take a look at http://www.f-m-s.dk/3,00%20pages2-15.pdfand for even more Screw thread types

As already suggested I'd use a full form thread tool to cut.

I'm sending you a PM and you can see even more

I sent you an email - not a PM.

[gregormarwick]

Unless the part he gives you is low limit.
Now your thread works until the customer has some parts at high limit.
I thought we got rid of this custom made to fit stuff a long time ago.
Bob

Exactly. I always shy away from fitting a thread to a mating part for exactly this reason.

[CarbideBob]

I know that most of them stem from one place (vardex/carmex?), and I can't say for certain that you are wrong in the case of sandvik. I do think it's unlikely however, since they use their own proprietary style of insert that's entirely incompatible with everyone elses.

All of the big makers press and sinter their own carbide grades.
They only job out finishing operations.
The originators of this tool were a nice size operation here in Detroit on 8 mile known at the time as the best in world at pressing intricate details.

As I said there can be no argument as to Sandvik's quality. They did not get to where there are by making and selling average quality stuff.

Bob

[gregormarwick]

All of the big makers press and sinter their own carbide grades.
They only job out finishing operations.
The originators of this tool were a nice size operation here in Detroit on 8 mile known at the time as the best in world at pressing intricate details.

As I said there can be no argument as to Sandvik's quality. They did not get to where there are by making and selling average quality stuff.

Bob

Fair enough, learn something new every day

I didn't mean to sound like I was questioning you, ultimately the only point I intended to make was that I have never had a faulty profile on a sandvik insert, but I have on another brand.

[Gordon B. Clarke]

My answer to this question is to use thread micrometers for OD threads, combined with thread rings whenever possible, thread plug gages for ID threads.

Measuring threads is no simple task. Look at some of the contraptions that have been invented to measure them, and their expense.

A thread is one half of a simple machine. 3/4"-16 UNF-2A, for example, has a pitch diameter tolerance range from .7049 to .7094", for a total tolerance of .0045". Not that bad, until you consider that this is a measurement from gage line to gage line, half way down a tapered groove which is helical, perpendicular to the axis of measurement. Basically that means the pitch diameter is only one of the important aspects of a thread, and it isn't a straight-forward measurement.

Above, I recommended the use of thread micrometers. In my use, a thread micrometer will measure a thread pitch diameter +/- .0005". I would not trust it to accept or reject threads measured within .001" of min or max. Therefore, the 3/4-16 thread pitch diameter mentioned has to be maintained +/- .0013" in order to run parts without a thread gage, and I would get very uncomfortable making more than a few dozen parts like this even on a good machine.

This attitude is a compromise between doing it right, and doing it with what you've got. Doing it right entails meeting you contract obligations as specified by the purchase order and the engineering drawing, which either specifies or infers a particular thread standard like ISO 68. In actual practice, when you've got thousands of dollars on the line, a $250 set of thread gages which are calibrated by a good accredited lab which uses thread master gages to check and set your thread gages, is a necessary and practical choice.

So far I've only been talking about external threads. Measuring internal threads by any other means than hard gaging (thread plugs) is a tough choice to make. It's no problem to own and maintain thread plugs for common hardware sizes such as M8x1.25, #6-32, 3/4"-16, etc. When such a thread as M56x1.5 is specified, you're talking around $600 for a pair of thread plugs, with a couple of weeks lead time. If the quantity of these parts is low, you're really up against a wall.

In practice, good craftsmanship would tend to suggest that the machinist makes his own thread plugs on a lathe, for this job. Even a single thread plug gage can be a good asset here, sized to the mean of the mating thread's pitch diameter tolerance.

A machinist would make this thread plug and then monitor the product's thread using the feel of senses that come from experience making good threads. The machinist is not going to have good sense in making good threads unless he or she has spent a great deal of time in the company of genuine, calibrated hard gages and thread mics and thread wires.

Another good approach, in gageless situations of making such a thread as M56x1.5, is to obtain a mating part from the customer. There's no shame in asking for this, especially if you can explain to the customer that such a thread costs $600 minimum for hard gages, and that not everyone is as scrupulous in producing high quality threads as you are, and perhaps the mating external thread is close to tolerances itself.

You've seen this in your experience, because you are a bad-ass at measuring threads. And that is the moral of the story. Measuring threads isn't as straight-forward as you might think, and hitting tolerances requires a bit of study and a wealth of experience.

You're not in touch with what's available
It's not that what you write is incorrect but you'd be more at home in a technical school teaching theory that working on real threaded components.
We're dealing here with measuring a simple straight forward fine metric thread and it's far from rocket science.

Measuring threads is both simple and inexpensive. Try clicking on my name (Gordon etc.) and looking at my website. External threads for 1/4" have a reasonably large pitch diameter tolerance (around 0.004") and a 2" thread is around 0.01". The pitch diameter tolerance on internal threads is usually about 30% more than for external.

The pitch diameter tolerance for the thread in question is 0.008" so what's the problem?

[turningboy1979]

Thanks to All for the info Guys !
I called the Customer and requested he sent the Male Part which he had no problem with (just shows that sometimes its best to ask rather than struggle by when he just had a male part sat on his desk ready to send)
@ Gordon , Thanks your stuff looks good and i will be in touch shortly as i am interested in buying.
I actually used a full profile sandvik insert and tried the method suggested to turn to the mean minor diameter and when i tried in the sample it was perfect so that was a good call , but i was glad to have the sample as when running quite a few you never know if the insert starts losing its edge and push off occurs , you just have that piece of mind of trying the male part in before removal from chuck .
Thanks All TB

[Gordon B. Clarke]

I'm just going to add this as the "apestate" way of thinking might scare people into thinking that making correct threads is complicated.

The closest I can come to thinking about how to make a "perfect" thread is by measuring pitch diameter and using a GO thread plug or ring gauge.

This lets you know where the thread is with regard to the tolerance and the gauge lets you know that the thread profile is OK.

In a pinch you can also use a nut or bolt to check thread profile as long as you measure pitch diameter. You can even make your own gauge if you measure pitch diameter and the cost of a real gauge seems a bit over the top.

Again, it surprises most just how big thread pitch diameter tolerances are.

I made this table long ago but it gives a good idea of just how big "big" is

0.10mm = 0.004"

http://www.f-m-s.dk/2.03.pdf

And to those wanting to know more

http://www.f-m-s.dk/1.04.pdf

To get from mm to inches just divide by 25.4

[aerodark]

I just talked to a guy that did threading overload work for Air Research/Garrett/Allied Signal in Phoenix in the 1960's through mid 90s. He recommended this company's product line for internal thread checking:

Internal Gages, Internal Gage Frames, Internal Gage Fingers, Internal Gage Stands: MTG: Carson City, Nevada

[Gordon B. Clarke]

I just talked to a guy that did threading overload work for Air Research/Garrett/Allied Signal in Phoenix in the 1960's through mid 90s. He recommended this company's product line for internal thread checking:

Internal Gages, Internal Gage Frames, Internal Gage Fingers, Internal Gage Stands: MTG: Carson City, Nevada

There isn't anything wrong with that except perhaps the price I think many tend to forget that many measurement gauges need setting references and they certainly add to the cost. The inserts I make for internal thread measurement requires a setting master too. Of course I have the advantage that just one of my setting masters covers all pitches from 0.5 to 8mm / 48 to 3 TPI for all threads with a flank angle between 50 and 80º. To some that's spamming yet to others useful information

Using a "nut" or "bolt" to make a good external or internal thread can be very dangerous as 1) you don't know where it is with regard to tolerance and 2) the part being made is almost always a much tighter fit than need be. A nut and bolt should be easy to screw together, not a "tight" fit. A tolerance is there to be used.

Cut the thread pitch diameter as close as possible to the middle of the pitch diameter tolerance and probably the only thing you need to be on the look out for is a worn or broken threading tool.

Gordon

[aerodark]

Oh, I guarantee this stuff isn't cheap.

[Dave K]

et a high quality (recommend sandvik) full profile insert. Bore the hole out to about 0.1mm under the min. minor diameter of the internal thread, then start threading. Carefully increase the thread size until you hit the mean minor diameter.

The only problem I see with this is, if the tip of your insert wears down, that minor will still be measuring good, while the major could be undersized.
I wouldn't trust this unless you're diligent enough to keep a close eye on your tool tip quite often.

[Gordon B. Clarke]

The only problem I see with this is, if the tip of your insert wears down, that minor will still be measuring good, while the major could be undersized.
I wouldn't trust this unless you're diligent enough to keep a close eye on your tool tip quite often.

Remember too that the min. (bore) diameter tolerance of an internal thread is twice that of the pitch diameter tolerance. If you go with that method measure something first as you'll need to know where you are.