Thread Gage Markings

Topics include, Machine Tools & Tooling, Precision Measuring, Materials and their Properties, Electrical discussions related to machine tools, setups, fixtures and jigs and other general discussion related to amateur machining.

Moderators: Harold_V, websterz, GlennW

User avatar
mineralman55
Posts: 10
Joined: Tue Feb 03, 2004 3:49 pm
Location: 30 miles north of 'Nawlins

Thread Gage Markings

Post by mineralman55 » Thu Jan 06, 2005 2:08 pm

Is there a short tutorial somewhere on the meaning of all the markings on one of those dovetailed thread gages used for helping shape lathe bits and set bit orientation?

Larry

User avatar
Victor_R
Posts: 681
Joined: Sun Jan 05, 2003 8:46 pm
Location: Taxland, Northern NJ

Re: Thread Gage Markings

Post by Victor_R » Thu Jan 06, 2005 4:52 pm

Larry,

Well, I'm thinking you're refering to a 60º center gauge (aka a fishtail gauge) used for shaping and orientating a threading tool. I have two of these. One is made in Germany and has only four thread pitch scales of 14, 20, 24 & 32. The other Lufkin gauge has the same four scales, and also has decimals that refer to the "double depth of sharp threads". But yours may be different.

This term (DDST) refers to the minimum root diameter one would expect to measure by subtracting it from the nominal major diameter. Since it is a sharp V form, you would need a sharp point micrometer or an optical comparator to measure it. It is a useless number in the real world especially since threads are truncated. I don't know why they chose these numbers.

Image
"I regard the brain as a computer which will stop working when it's components fail. There is no heaven or afterlife for broken down computers; that is a fairy story for people afraid of the dark." ~Stephen Hawking

User avatar
Victor_R
Posts: 681
Joined: Sun Jan 05, 2003 8:46 pm
Location: Taxland, Northern NJ

Re: Thread Gage Markings

Post by Victor_R » Thu Jan 06, 2005 7:58 pm

Ok, I found another one. This one's a Craftsman with American National double depth (now unified) which takes into account the truncation.

Image
"I regard the brain as a computer which will stop working when it's components fail. There is no heaven or afterlife for broken down computers; that is a fairy story for people afraid of the dark." ~Stephen Hawking

dgoddard

Re: Thread Gage Markings

Post by dgoddard » Fri Jan 07, 2005 2:00 am

I did the geometry and trig and can duplicate the numbers found on both gages.

The American National thread designation numbers assume a flat on the crest and in the groove each equal to 1/8 of a pitch. These numbers might be of some value if you were intending to cut a thread and then machine it off in some region.

The double depth of sharp thread numbers assume that the thread is cut so that there is no flat at the crest or the root of the thread. Any thread cut that deep will be pretty loose in the nut, a sloppy fit indeed.

What would be more useful would be knowing how far to feed the compound in order to cut a particular thread. If you need to cut a thread that does have the flat in the bottom of the groove (or even a "root radius" thread), you will have to sharpen the tool to have the correct flat on the tip for that particular pitch, or keep a tool around for each particular pitch.

It being inconvenient to have different tools for different pitches, or be grinding tools all the time, it is easier to have one that is "sharp" pointed and cut all the threads with that. As long as you can tolerate the reduction in strength of the screw and are threading fairly ductile metal that won't have a big problem with the stress raiser, that might work fairly well. ( One definition of a thread is: "A continuous helical stress concentration")

Theoretically if you are making a thread that is standard except that the bottom of the v is sharp the amount that you feed the compound should be 7/8 of the pitch. so that an 8 pitch thread should require you to feed the compound 7/8 x 1/8 = 7/64 = 0.109 in.

If you have put a proper flat or radius on your tool, you should feed the compound not more than 3/4 of a pitch if you have the sharpest permissible point and 5/8 of a pitch if you have the bluntest permissible tip. That would be 3/4 x 1/8 = 3/32 = 0.09375 for the sharp and 5/8 x 1/8 = 5/64 = 0.078125 in for the blunt. (lets not criticize my use of excessive decimals.... I did say "THEORETICAL".)

Those feeds on the compound assume that the entire cut is done by feeding the compound from the surface until the thread is cut. If the last part of the cut is done with the cross slide to improve the finish, obviously a littl less feed will have to be done with the compound.

Now, how about some of you with more experience tell us the practical way to use the compound and cross slide to get a good close fitting thread once you know the pitch to be cut.

User avatar
Victor_R
Posts: 681
Joined: Sun Jan 05, 2003 8:46 pm
Location: Taxland, Northern NJ

Re: Thread Gage Markings

Post by Victor_R » Fri Jan 07, 2005 12:03 pm

Now, how about some of you with more experience tell us the practical way to use the compound and cross slide to get a good close fitting thread once you know the pitch to be cut.


Ok, here it is. Use the proper thread wire with the proper constant for the desired pitch along with a micrometer to measure and know exactly where you are. Any other way is hit or miss.

While your figures are correct (I assume) no practical machinest I know would ever go about it that way. It's all been done for you in the Government handbook H28. For me it would only be geometric mental -, if I could still do it. Of course, if one were a mathematician and got off on this sort of thing, that's another story.

And oh yea, IMHO someone who is too lazy to grind a proper threading tool probably shouldn't be at the lathe anyway.

Victor
"I regard the brain as a computer which will stop working when it's components fail. There is no heaven or afterlife for broken down computers; that is a fairy story for people afraid of the dark." ~Stephen Hawking

Harold_V
Posts: 17145
Joined: Fri Dec 20, 2002 11:02 pm
Location: Onalaska, WA USA

Re: Thread Gage Markings

Post by Harold_V » Sun Jan 09, 2005 4:13 am

Ok, here it is. Use the proper thread wire with the proper constant for the desired pitch along with a micrometer to measure and know exactly where you are. Any other way is hit or miss.

This is a subject that appears to refuse to die.

For some reason, the vast majority of the readers seem to think that measuring threads is something that only weird people do, that it's not really a part of learning to run machines properly.

Time for a wakeup call, folks.

Measuring threads by wires, thread mic's or triangles is the acceptable procedure for knowing where you are. There are no substitutes for measuring parts in a machine. It is not acceptable practice to cut threads by advancing the compound the prescribed distance alone. Measuring, or proper gauging should always be included. Would you trust turning a given diameter that way? Threads, especially small diameter threads, are very close tolerance. That type of operation simply isn't reliable enough.

Gages of all sorts will tell you if your part is acceptable, or not, but none of them will tell you where your thread is relative to the acceptable pitch diameter.

A relatively cheap set of thread wires is one of the best investments any home shop can buy. There's no excuse for anyone to not have a set, and to know how to use them. All the necessary tables for common, and many bastard or NS thread measurements are available in Machinery's Handbook.

Harold
Wise people talk because they have something to say. Fools talk because they have to say something.

User avatar
John_Stevenson
Posts: 531
Joined: Sun Dec 22, 2002 9:51 am
Location: Nottingham, England.

Re: Thread Gage Markings

Post by John_Stevenson » Sun Jan 09, 2005 5:53 am

All the necessary tables for common, and many bastard or NS thread measurements are available in Machinery's Handbook.

Harold

Only if you are fluent in Serbo Croat or served an apprenticeship at Bartnums Circus jumping thru hoops to understand them [img]/ubb/images/graemlins/laugh.gif"%20alt="[/img]

John S.

User avatar
AAA
Posts: 331
Joined: Sun Jan 05, 2003 4:35 am
Location: Hillsboro OR

Re: Thread Gage Markings

Post by AAA » Sun Jan 09, 2005 7:32 am

Harold,

this topic seems to be like a damn zombie!
We keep trying to kill it but it keeps on comming back! ha ha

Can you explain measuring threads by triangles? Is this the one where you use little "V" shaped blocks and a micrometer?

Mike

Harold_V
Posts: 17145
Joined: Fri Dec 20, 2002 11:02 pm
Location: Onalaska, WA USA

Re: Thread Gage Markings

Post by Harold_V » Sun Jan 09, 2005 1:35 pm

Is this the one where you use little "V" shaped blocks and a micrometer?

Mike

Yeah, that's the one. I tossed them in because I was trying to make a point. I don't endorse using them, though, no more than I endorse the use of thread mic's. In both cases they have a couple drawbacks. Thread mic's cost a lot and are a special application type tool that isn't a necessary expense for a guy working at home. Inexpensive wires sets take the place of that need, and cost very little.

Triangle have one drawback that I'm not well pleased with, which is that they measure the proper place on a thread only if the thread form is perfectly matched to the triangles. If the form is too narrow (less than 60°), they measure at the major diameter. If it's too broad, they measure at the root. Wires don't do that, although they do not necessarily measure at the pitch diameter, either, if the form is off. Very close, though. On coarse pitch threads it can make a substantial difference.

Truth be known, wires are the only really cheap way to go for a guy on the machine, thanks to the inexpensive wire sets available. As you and I know, buying the proper thread wires is hardly inexpensive, but the cheap sets put reasonably accurate measurement in a category much cheaper than any other method. Even if a guy doesn't own micrometers, he would be justified in buying them for other usage. I've seen 0"-3" import micrometer sets available for well under $100, which, while far from the level of quality necessary for missile work, would certainly serve the home shop type very well. Couple them with a set of wires that can be had for under $20, and the information readily available in Machinery's Bible and you end up with the ability to measure almost an endless combination of thread diameters and pitches that the average guy is likely to encounter.

By the way, I'm still very envious of the extensive set of proper wires you bought some time ago. [img]/ubb/images/graemlins/cool.gif"%20alt="[/img]

Harold
Wise people talk because they have something to say. Fools talk because they have to say something.

Harold_V
Posts: 17145
Joined: Fri Dec 20, 2002 11:02 pm
Location: Onalaska, WA USA

Re: Thread Gage Markings

Post by Harold_V » Sun Jan 09, 2005 1:44 pm

All the necessary tables for common, and many bastard or NS thread measurements are available in Machinery's Handbook.

Harold

Only if you are fluent in Serbo Croat or served an apprenticeship at Bartnums Circus jumping thru hoops to understand them [img]/ubb/images/graemlins/laugh.gif"%20alt="[/img]

John S.

Chuckle!

Trust me, John, it gets easier each time you try to make sense of it.

Can you imagine the confusion this guy had when, as a young lad of only 18, he was assigned to chasing threads and had to understand pitch diameters, proper thread forms and classes of threads, when the only thing that was important to him was getting his check on Friday?

Talk about a rude awakening.

I was fortunate to have one inspector take me under his wing and keep steering me in the right direction. He was kind enough to pass on to me a large manual that detailed each and every feature, along with proper formulas for calculating proper pitches for various lengths of engagement. Threads can be one huge PITA, but are almost always very manageable, even to the uninitiated.

You'll be surprised how quickly this all starts to make sense once you give it a go.

Harold
Wise people talk because they have something to say. Fools talk because they have to say something.

User avatar
AAA
Posts: 331
Joined: Sun Jan 05, 2003 4:35 am
Location: Hillsboro OR

Re: Thread Gage Markings

Post by AAA » Mon Jan 10, 2005 2:26 am

As you know, up until quite recently I'd never used thread wires, they're not commonly used here in Australia, and they not in the current machining apprenticeship courses anymore.
When I went looking for some, I had a hell of a time finding them - the standard response was - either "What?" or "why don't you just use a thread mic?"
At the time I was making a hydraulic fitting for an aircraft. The specs called for the thread to cut and checked with thread wires only - with certs to be provided showing specs to ANSI B1.2. As you've mentioned earlier the reason for this being that the thread wires ensure correct thread form, which was obviously important for this part.

The thread wire set I ended up getting was a second hand set. Mine are Van Keuren ones from the '50's. The guy gave me a real sweet deal on them and was amazed that anybody would want them.
Unlike Pee Dee wires it has a different thread wire set for each pitch, and there a both metric and inch pattern ones.

There's 64 vials each with 3 wires. There's 30 metric ones going from .2-10 pitch and 34 inch ones going from 4-80 TPI. That pretty well covers any thread I'm ever likely to cut!
There's also some extra vials with other ones that are marked for ACME threads as well that have been added to the set.

Just out of interest, what's a set like mine go for over there price wise?

Mike

Harold_V
Posts: 17145
Joined: Fri Dec 20, 2002 11:02 pm
Location: Onalaska, WA USA

Re: Thread Gage Markings

Post by Harold_V » Mon Jan 10, 2005 4:49 am

Just out of interest, what's a set like mine go for over there price wise?
Mike

I certainly have no clue, but if you are ever in a charitable mood and would like to make an old retired machinist happy, feel free to ship them to me. I'd certainly give them a good home! [img]/ubb/images/graemlins/cool.gif"%20alt="[/img]

If I was to venture a guess, I can't imagine you could buy them (new) for any less than $50 per set of three these days. Could be wrong. One of the problems with an outdated set would be the lack of certification of the enclosed wires. Here, there is a demand for reinspection and certification by a certified secondary standards lab, which, no doubt, is a good part of the cost involved in the wires.

The last time I bought wires was back in the 70's, and they were well over $20 US per set at that time. Due to closing my shop in '83, and entering a totally unrelated business field, I lost touch with almost everything related to machining. By then, I was well burned out and welcomed the change. It is for that reason that I am so woefully uneducated in the ways of modern machining.

Interestingly, Van Keuren was the maker of the wires I used when I worked at Sperry, and as you described, they came in a wooden box, with each wire size in a vial. The wires I purchased later were made by Deltronic, but in both cases they were certified and traceable to the U.S. Bureau of Standards. The Deltronic wires each include a plastic sleeve with the wire size and constant (each to .XXXXX") printed for ready reference. I don't recall the Van Keuren wires having the plastic sleeve, but the constant was printed on the label within the vial, on what I recall to be orange paper.

I could hardly believe my eyes as I read that wires appeared to be out of favor in Australia. I think it's safe to say that thread mic's are close enough for the vast majority of work, but it's also safe to say that the one and only way to measure threads reliably is by the three wire method, which is reinforced by the requirement you found when machining the hydraulic fitting. For me, that was routine. The world from where I came revolved around such requirements, so I more or less assumed that industry, in general, also did. Should have known better, however, after working in one shop where there were no gages of any kind, and every part that came off the machine was useable.

To be very honest, I miss the environment of the aero-space industry. It was a tough place to be, but you came away with skills that are difficult to master in many other industries. The rigid requirements forced workers to become highly skilled, or they didn't last. It was shocking for me to leave the industry and work where specs were not enforced. Later, when I worked in a job shop, and then when I started my own business, the majority of the work involved was defense related, so once again I was back in my preferred environment.

Harold
Wise people talk because they have something to say. Fools talk because they have to say something.

Post Reply