# Why can one not use their thread gauges to cut thread?



## fixxer (Mar 7, 2018)

By that; I mean, why do all these videos I see show people using measuring wires or dedicated pointed anvils for finding the ID of the thread.   I'm talking about home-gamers when I see this and not talking about guys making aircraft parts for Boeing or JPL.  

Do my Mitutoyo thread gauges show me my threads are deep enough plus OD readings with normal layout tool mics not get the job done?  Do I need to invest in a dedicated screw-thread micrometer with different thread-form anvils?  It seems like overkill to a home gamer.  I haven't done much threading yet it just seems like 60 degree threads are habitually being over-complicated when I look at what's being touted.   Am I missing something?

I'd love to hear your thoughts.

Thanks folks.


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## mikey (Mar 7, 2018)

I had the same thought many years ago when I first started in this hobby. I tried it, too, and found that the resulting threads were extremely loose. Using the thread gauge caused me to cut way too much material off. You can try it if you like but it doesn't work well, at least in my hands.

To be honest, its actually faster and easier to cut threads to tolerances using a thread mike. They are cheap (Asian variety), direct reading, easy to use and the mating part fits every time. For me, this is a no-brainer.


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## Brain Coral (Mar 7, 2018)

"Do my Mitutoyo thread gauges show me my threads are deep enough plus OD readings with normal layout tool mics not get the job done?"

Are these thread gauges you talk about, what you would use to determine the thread pitch of an existing and unknown threaded bolt ? If so, they are not designed to be thread depth gauges. For most single point threading, turn to the proper OD, take some cuts, check it with an appropriate sized nut, when the nut fits, you are good to go.... simple...

If, however, you need to fit a bolt that you are making, into a machined and threaded hole in a machine, for example, this is where the "Machinery's Handbook", and a set of thread wires comes into play. You simply can't go wrong if you follow the charts.

Brian


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## RJSakowski (Mar 7, 2018)

Properly functioning threads have to have the correct pitch diameter.  A 6-32, an 8-32, and a 10-32 screw will all fit the thread gage the same way but they're clearly not interchangeable.  If you closely at an ideal thread, it has a small flat on the o.d.  The width of the flat has a has an effect on the pitch diameter even though the major diameter is correct which is why thread mikes are used.  The wires engage the flanks of the thread which are directly related to the pitch diameter so the wires plus conventional mike also gives you pitch diameter.

For the hobby machinist, an acceptable way to check threads is to use a mating nut.  It may not give you a textbook fit but it will work.  After cutting a few threads, you will get a feel for the desired fit.  It would be to your advantage to assemble an assortment of nuts and check the fit  on some mating screws to get a feel for the fit.  Set them aside as your own personal gage set.


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## markba633csi (Mar 7, 2018)

I use a Starrett #40 thread pitch gage both to check the form of the tool I've ground and the threads I'm cutting. Works great


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## benmychree (Mar 7, 2018)

RJSakowski said:


> Properly functioning threads have to have the correct pitch diameter.  A 6-32, an 8-32, and a 10-32 screw will all fit the thread gage the same way but they're clearly not interchangeable.  If you closely at an ideal thread, it has a small flat on the o.d.  The width of the flat has a has an effect on the pitch diameter even though the major diameter is correct which is why thread mikes are used.  The wires engage the flanks of the thread which are directly related to the pitch diameter so the wires plus conventional mike also gives you pitch diameter.
> 
> For the hobby machinist, an acceptable way to check threads is to use a mating nut.  It may not give you a textbook fit but it will work.  After cutting a few threads, you will get a feel for the desired fit.  It would be to your advantage to assemble an assortment of nuts and check the fit  on some mating screws to get a feel for the fit.  Set them aside as your own personal gage set.


Yes, I used nuts for a long time, but it is good to run a sharp tap through them before the first use.  Before e bay came along, I used thread wires, but now I use thread mikes, especially when I am making a thread plug gage.  Most of the time I simply use a nut.


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## benmychree (Mar 7, 2018)

markba633csi said:


> I use a Starrett #40 thread pitch gage both to check the form of the tool I've ground and the threads I'm cutting. Works great


Any more, I simply use the Aloris form ground threading tool, you just sharpen it on top and its good to go. for some special thread I use the "fish tail" gage to grind a tool and have both 60 deg and 55 deg tool ( for Whitworth threads) For common Acme threads, I use the Aloris form ground bit of the proper pitch; they make them in several (common) point widths, but the crest can be surface ground wider for odd pitches if necessary.  I also have a grinding jig made by the Clark Tool Co. to sharpen tool bits to acme form.


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## mmcmdl (Mar 7, 2018)

Machining 101 .

When using wires , pitch mics or ring gages , you are checking the pitch diameter of the thread . There is no ID of any thread , the term does not exist ! There is a major , minor and pitch diameter . Ring gages , pitch mics and thread gages and wires should NOT be used to verify thread form  . Only an optical comparator will . Not that any of this matters as long as it's not for the military . Use a bolt or a nut to verify fit .


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## fixxer (Mar 7, 2018)

Thanks for the replies guys.  I put a thread depth mic in my online shopping cart weeks ago and then decided to hold off on it until I asked around.
I have the 29th and 30th ed. of Machinery's, SB How to Run a Lathe (which took me weeks to finish a little at a time).  Many references on hand in my shop but wasn't sure why I keep seeing people not use the thread gauges to check the threads they make.

I get using thread gauges following a scratch pass to check lineup but not why they cannot grind pitched gauges to indicate you have a deep enough thread belly, correct pitch and correct form angle.  It seems to me if they actually ground them to the correct pitch and depth that it would do this too.  Yes, the gauges are used to find thread pitch on existing equipment...  Except on almost any inside thread you can't bring to the bench.  Good luck with that.  I still have to figure that out.   I have a master set of strung nut and bolt checks that I use for this.  They go from nearly too tiny to distinguish up to 1-1/2" (or more I think) along with a metric set.  As far as fasteners go, that's how I spec what I need or sort what I have.  Very quick since I can just about ball-park most of them before hand on most.

Someone should make a truly pitched set that does this.  Or maybe a collection of nut and bolt cross sections that have been Q/A'd.  The wires seem very time consuming so if/when I go this route, I'll probably go with a micrometer type tool.  Maybe even digital for this one since I don't think I'll be working on anything for production.  I know I'll be making some acme (possibly buttress) threads in the future for leveling feet on a 1400lb (and counting) welding table I have gathered most all the materials for.  I might as well just get the layout tool that can support this.  I may only use it 4 times.

Yes.  Understand technically it is considered minor diameter and major diameter for threads.  And when you're cutting inside threads, it starts to get confusing to keep them separated.  I guess that's why it would be a good idea for someone like myself to work from notes instead of re-referencing the same tables throughout the cut.  I'll be shutting down the machine and rethinking every cut if I don't jot it down and roll.  Still though, thanks for the feedback.  It's easy to blow past these things and make them habitually incorrect.  Good thing I don't have to supervise this kind of work or I'd end up with a soup sandwich.

BTW, I should have bought the large print edition of Machinery's Hand Book.  Do yourself a favor and do this if you're on the fence.  I suspect I'm going to re-buy 30th in large print now.  It's almost as bad as trying to follow a tiny table inside McMaster or similar.  Maybe it's just me...


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## fixxer (Mar 7, 2018)

benmychree said:


> Any more, I simply use the Aloris form ground threading tool, you just sharpen it on top and its good to go. for some special thread I use the "fish tail" gage to grind a tool and have both 60 deg and 55 deg tool ( for Whitworth threads) For common Acme threads, I use the Aloris form ground bit of the proper pitch; they make them in several (common) point widths, but the crest can be surface ground wider for odd pitches if necessary.  I also have a grinding jig made by the Clark Tool Co. to sharpen tool bits to acme form.



I'm going to look for some grinding jigs.  I've come across a few online.  I'll take a look at these.  This will help me greatly.


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## mmcmdl (Mar 7, 2018)

Not to disquade ( not sure if this a true word ) ,  you from the purchase of a Machinist Handbook , but try this . If you have a tap supplier you use regularly , ask them to provide you with a thread form and limit book . I have them from Greenfield and OSG . It will give you ALL your limits on 1-4 class ID and OD threads , pitch diameter limits , tap classes ( GH1-5) etc . Best thing is they are free and much easier to read than the handbook .


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## mmcmdl (Mar 7, 2018)

Here is a home project you can easily make . I posted this years back . Maybe the 2nd one down . 60V and Acme thread grinding jig . 

https://www.hobby-machinist.com/thr...wn-as-g-or-government-jobs.43057/#post-369160


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## fixxer (Mar 8, 2018)

mmcmdl said:


> Here is a home project you can easily make . I posted this years back . Maybe the 2nd one down . 60V and Acme thread grinding jig .
> 
> https://www.hobby-machinist.com/thr...wn-as-g-or-government-jobs.43057/#post-369160


Thanks for that.

Sorry to all for posting this in the wrong sub.  I honestly looked for the correct subforum for a good while before throwing this in the general chat area.  It probably shoulda went into the metrology subforum.  

Thanks to all.
-Fixxer


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## RandyM (Mar 8, 2018)

fixxer said:


> Thanks for that.
> 
> Sorry to all for posting this in the wrong sub.  I honestly looked for the correct subforum for a good while before throwing this in the general chat area.  It probably shoulda went into the metrology subforum.
> 
> ...



I moved it to the General section. It is a machining subject.


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## ddickey (Mar 8, 2018)

You could go to a Kinkos and get this printed out.
http://www.nashua.edu/paradisem1/Machinery's Handbook 27th/27_Thread_09A.pdf


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## BtoVin83 (Mar 8, 2018)

If you have a sample of the thread you are to make it's real simple. Find a small drill that will fit the flanks of the thread. Measure the OD of the thread you are duplicating then measure the distance over the drill and OD. When getting close to finish measure the OD of your part and then measure the distance over the drill and OD. Compare the differences in OD and make appropriate adjustments to your over the drill distance.


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## mmcmdl (Mar 8, 2018)

BtoVin83 said:


> If you have a sample of the thread you are to make it's real simple. Find a small drill that will fit the flanks of the thread. Measure the OD of the thread you are duplicating then measure the distance over the drill and OD. When getting close to finish measure the OD of your part and then measure the distance over the drill and OD. Compare the differences in OD and make appropriate adjustments to your over the drill distance.




Simple yes . Accurate ? NO ! Why ?


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## BtoVin83 (Mar 8, 2018)

It is only as accurate as your sample and as close as you can make it. Do most projects require the utmost accuracy? I doubt it.


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## BtoVin83 (Mar 8, 2018)

We cut a lot of threads measuring on the fly using spring calipers, these threads were fairly coarse and large diameter, a loose fit did not hurt the application.


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## epanzella (Mar 9, 2018)

I problem I had when NOT measuring pitch diameter was making the threads too loose. If you don't have the proper flat on the thread crest it drags in the sample nut, giving the false indication that the thread is too shallow. If this is the case by the time you've deepened the thread enough for the sample nut goes on smoothly, you've cut too deep. Measuring pitch diameter eliminates any guesswork.


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## RJSakowski (Mar 9, 2018)

The problem with using an o.d. for measuring external threads or an l.d. for internal threads is that the unified thread standard has a flat on crest of an external thread and at the root of an internal thread.  Unless that flat was accurately re-cut with each pass, any measurement of o.d. or i.d. respectively will not give accurate results.

It would be possible to cut an external thread by starting with an oversize diameter and cutting until the o.d. was equal to the major diameter  + sqrt(3)/8 times the pitch, which is the diameter of an untruncated thread.  For a 1/4-20 thread, this would be .250 + .0108 = .2608.  The thread would be finished by turning the o.d. down to .250 which would create the proper flat.

A similar process could be used for internal threads.  A slightly smaller hole could be bored and the threading tool used to cut threads until the i.d. was equal to the minor diameter - 7*sqrt(3)/8.  For a 1/4-20 thread, this would be .250 -.0758=.1742.  The thread would be finished by boring to the minor diameter or larger, e.g. tap drill size.

This procedure would be for a basic thread.  Depending on the class of fit desired,an adjustment would be made to the diameters measured (slightly smaller for external or slightly larger for internal threading.
A copy of the standard thread geometry and an Excel spreadsheet giving values for the untruncated diameters for external and internal SAE threads are below.


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## BtoVin83 (Mar 9, 2018)

This might explain it better 
*One-wire method.*
In this method, one wire is placed between two threads at one side and on the other side the anvil of the measuring micrometer contacts the crests as shown in Fig. 13.14. First the micrometer reading is noted on a standard gauge whose dimension is nearly same as to be obtained by this method. Actual measurement over wire on one side and threads on other side = size of gauge ± difference in two micrometer readings.
This method is used for measuring effective diameter of counter pitch threads, and during manufacture of threads.
The difficulty with this method is that the micrometer axis may not remain exactly at right angles to the thread axis.
From the book *Measurement of Various Elements of Thread (Metrology)*


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## Holescreek (Mar 9, 2018)

The answer is: sometimes you can get by using a nut to check your finished size and sometimes you can't. Almost everyone starts out with a nut but soon learns that A) sometimes there is no "nut" or B)  what if the threads are on the other side of a part feature that can't be reached until the part is cut off?

In the attached photo I had both situations on the stainless steel part.  It had to be parted off of the bar on the left side as it sits in the photo, and there was no "nut".   No one would want to part it off the bar without knowing if the threads were correct before hand, right?  I couldn't run the part in the lathe the opposite way because I had to cut internal threads concentric to the outer threads.

The answer to the problem was to use pitch micrometers. Cutting the threads to the correct pitch diameter assured that the finished OD threads were the correct size.  In the past I'd have just used thread wires (3 wire method) but I upgraded to pitch mics several years ago.  By cutting your threads to the correct PD you know that your parts will always fit interchangeably with other correctly made parts.


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## magicniner (Mar 10, 2018)

Holescreek said:


> B)  what if the threads are on the other side of a part feature that can't be reached until the part is cut off?



It is not unknown in engineering to have to hold a threaded section on a part in a lathe and the correct size of collet will hold a correctly formed thread concentric for further operations, another option is to replicate the mating part in a sleeve, it's all fairly easy stuff to achieve, you just have to stop thinking about what can't be done and envisage ways in which it can, 
Regards, 
Nick


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## crazypj (Mar 10, 2018)

You seem to be using the term 'thread gauge' when you really mean PITCH GAUGE. They are completely different tools. A thread gauge will actually 'gauge' the fit of a thread,(as noted, an optical comparator/shadow graph is more accurate but very few people have them sitting around) A pitch gauge just verifies the pitch is correct which seems to be what your doing.
BTW, don't worry about it too much, 70% thread engagement in nut is normal in general engineering and is about 90% strength of 'full engagement. Majority of fasteners are over-sized for application wit very few exceptions. (only one comes to mind at present, axle adjusted bolts on many Honda motorcycles) They are often 7mm as 6mm isn't 'strong' enough and 8mm is  extra material/cost- Japan has to import just about everything before they can re-sell it. Individually it isn't much but several million will be a substantial saving


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## RJSakowski (Mar 10, 2018)

I some analysis the one wire method as well as some measurements and it works well.  Using only one wire and making a measurement with and without the wire will accomplish the same thing as using three wires.  The measurement with the wire minus the measurement without the wire gives you the amount that the wire is above the thread crest. With the three wire method, the diameter measured will be the crest to crest measurement plus twice that amount.  If d2 is the measurement with the wire and d1 is without, the measurement over the wires, as defined in the Pee Dee thread chart. "M" = (d2-d1) x 2 + d1 =2d2-d1. The pitch diameter can then be determined from the Pee Dee thread chart or from the equation E = M-3W+.86603/n where E is the pitch diameter, M is defined as above, W is the wire diameter, and n is the thread pitch equal 1/TPI.

The obvious upside is you don't need three hands to make the measurement as with the three wire method.  The downside is that there are now two measurements instead of one and any measurement error is doubled on the d2 measurement, tripling your possible error.


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## BtoVin83 (Mar 10, 2018)

Holescreek said:


> B) what if the threads are on the other side of a part



You mean like this?


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## BtoVin83 (Mar 10, 2018)

Cut the head off and finished


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## Holescreek (Mar 10, 2018)

magicniner said:


> It is not unknown in engineering to have to hold a threaded section on a part in a lathe and the correct size of collet will hold a correctly formed thread concentric for further operations, another option is to replicate the mating part in a sleeve, it's all fairly easy stuff to achieve, you just have to stop thinking about what can't be done and envisage ways in which it can,
> Regards,
> Nick



In my experience engineers spend all sorts of time dreaming up all sorts of ways to throw money at a simple solution.  Down on the floor where the work gets done is where it gets sorted out.  Thread wires are a little more practical for the home machinist.


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## BtoVin83 (Mar 10, 2018)

That is the point I was tying to make as a hobby, resources can be better applied on things you need. A thread mic can be a big outlay for something that may not be used often even thread wires can be pricey. You may not always have a small drill or rod laying around to gauge threads but most everybody has access to some kind of wire, with it  and a little imagination you can get the job done. I would rather buy something I'm going to use on a regular basis.


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## benmychree (Mar 10, 2018)

BtoVin83 said:


> That is the point I was tying to make as a hobby, resources can be better applied on things you need. A thread mic can be a big outlay for something that may not be used often even thread wires can be pricey. You may not always have a small drill or rod laying around to gauge threads but most everybody has access to some kind of wire, with it  and a little imagination you can get the job done. I would rather buy something I'm going to use on a regular basis.


I remember many years ago, a old machinist friend brought me a threading job to and had measured the part to be duplicated with wires; as you mentioned, everyone has access to some kind of wire; in this instance the three wires were baling wire!  RIP  Bill Hesse.


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## mcostello (Mar 11, 2018)

Yes but when You get tired of messing around holding wires...............


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## Firestopper (Mar 11, 2018)

I'm a simpleton when it comes to cutting threads. I use a nut or bolt as a gauge. It's worked for years. Another thing I do often is use dies for final cuts (if I have the correct size) for smaller threads. I also have a decent assortment of taps. For gunsmithing, I use the counterpart as a gauge for the best fit.

Paco


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## benmychree (Mar 11, 2018)

Most of the time I use a nut to gage the fit of a thread, nearly never do I use a die to fit a thread, except possibly in the case of a very small size; where I want a particularly nice fit, I use thread mikes, all of which I bought fairly cheap on EB.


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## magicniner (Mar 11, 2018)

Holescreek said:


> Thread wires are a little more practical for the home machinist.



Well for those for whom thread wires work better than adequate work holding solutions I say go at it, but I was just pointing out that there are other practical options and that that which was alluded to as being impossible is not ;-)


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## benmychree (Mar 11, 2018)

One important thing is to spread a rag under the area being measured with wires to catch them when they are fumbled, (nearly inevitable) to keep them out of the chip pan and getting lost.


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## RJSakowski (Mar 11, 2018)

If using a wire or three wires to measure the pitch diameter, be aware that any error in the actual diameter is multiplied be three in the pitch diameter calculation.  For accurate measurements, it is important that the wire be round to within a tenth or two of a thousandth.  To that end, baling wire would probably be a poor choice, at least without pre-qualification.  

Secondly, the wire diameter should be between 60% and 90% of the thread pitch.  If it's too  small the wire won't stand above the crest of the thread and if it's too large, the wire won't be making contact within the flanks of the thread.


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## RJSakowski (Mar 11, 2018)

I gave some thought to using nuts as a gage.  In particular, I was concerned that an interference at the top of the threaded piece might give a false indication.  In reviewing the situation, if the rod is turned to the major diameter prior to threading, there will be no interference at the crest as an internal thread is cut deeper.  There could be interference at the root of the threaded piece but this would only occur if the cutting tool had a larger radius than for the thread being cut.

If using nuts as a thread gage,   I would turn the rod to the major diameter and my threading tool would have a radius of 14% or less than the thread pitch.  If those conditions are met, the nut will contact the thread properly. 

As the threading progresses the test nut would go from not starting to threading with difficulty to threading easily to being noticeably loose.  The subjective part is to determine how loose you wish the thread to be.  Comparison to another thread will aid in that.

As a final comment, go/no go thread plug and ring gages are commonly used to determine progress and/or suitability when threading.


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## magicniner (Mar 11, 2018)

RJSakowski said:


> As a final comment, go/no go thread plug and ring gages are commonly used to determine progress and/or suitability when threading.



And they're often cheaper than you'd expect


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## benmychree (Mar 11, 2018)

RJSakowski said:


> If using a wire or three wires to measure the pitch diameter, be aware that any error in the actual diameter is multiplied be three in the pitch diameter calculation.  For accurate measurements, it is important that the wire be round to within a tenth or two of a thousandth.  To that end, baling wire would probably be a poor choice, at least without pre-qualification.
> 
> Secondly, the wire diameter should be between 60% and 90% of the thread pitch.  If it's too  small the wire won't stand above the crest of the thread and if it's too large, the wire won't be making contact within the flanks of the thread.


As to the diameter of thread wires, that is why they are sold in sets, with a specific pitch for each different set of wires; the set that I use (occasionally) was made at the Mare Island Navy Yard during WW-2 with a purpose made pouch with compartments for each wire set with the pitches marked with an fountain pen.


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## RJSakowski (Mar 14, 2018)

The formula used in the three wire method is E = M-3W +.88803/n while the one used in the single wire method is E =2M2-M1 -3W +.86603/n.

If you can make a measurement within +/- .0001 of the true measurement and you assume a worst case erroro stacking the three wire method would give a maximum error of +/- .0004" while the single wire method would give a maximum error of +/- .0006".  For most practical purposes they are equivalent.  I have a set of PeeDee wires and FWIW, i miked them and found that they can vary from the nominal size by +/- .0001.

Weighing the slight decrease in potential accuracy against trying to balance three wires while making an accurate mike measurement, I believe that I will use the single wire method going forward.

I bought my set of wires from Enco just before they left the scene and they were quite reasonable.  However, if a person didn't want to spring for a set of thread wires, I would suggest getting a set of 010" - .060" pin gages.  They run about the same price as the thread wires but they have other uses in the machine shop


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## P. Waller (Mar 18, 2018)

Thread gauges are used for thread cutting every day worldwide, these are the round kind so they cost a good deal more.

Look here https://www.threadcheck.com/unified-inch-thread-gages/


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## Bill Gruby (Mar 18, 2018)

To my knowledge the gauges you link to will not cut threads. They are for inspection of threads already cut. They are go and no go gauges. There is no taper in those threads hence they will only follow what is already there.

 "Billy G"


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## magicniner (Mar 18, 2018)

Bill Gruby said:


> To my knowledge the gauges you link to will not cut threads. They are for inspection of threads already cut. They are go and no go gauges. There is no taper in those threads hence they will only follow what is already there.
> 
> "Billy G"



I suspect he meant "for measurement during the process of thread cutting" ;-)


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## GK1918 (Mar 29, 2018)

the sole purpose of a thread gage is to verifiy threads per inch  'gear box or change gear setting'  guess I'm a gettin
to old amazingly all the young ones have to be concered with wires thread mics math>>>>dumpster by by!
The best wires are your ears and eyes.  All these threading posts I read  =  (oh I gotta be perfect first
time so I need math wires & mic)  People you are over killing your brainm unless your are a cnc'er. which I am not.
thousands of threads under my belt and I feel hear and see the perfect form THEN try the nut.  Spring cuts ya best 
friend.    So the nut a little tight?  try valve grinding coumpound 'under power back an forth'


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## P. Waller (Mar 29, 2018)

Bill Gruby said:


> To my knowledge the gauges you link to will not cut threads.
> 
> "Billy G"


No, they only assure that that part is within pitch diameter limits.


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## gonzo (Mar 29, 2018)

I can't imagine my 76 year old fingers holding those wires while I simultaneously manipulating a measuring device with my other hand all while bending over my lathe. I just use nuts to check fit. After all. I don't do work for NASA.


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## P. Waller (Mar 29, 2018)

gonzo said:


> I can't imagine my 76 year old fingers holding those wires while I simultaneously manipulating a measuring device with my other hand all while bending over my lathe. I just use nuts to check fit. After all. I don't do work for NASA.


Very few  tasks are more annoying then measuring small threads over wires, Large threads are much easier until one reaches leads that are wider then a common micrometer anvil where a gauge block is required.


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## magicniner (Mar 31, 2018)

Plasticine, Blu Tack, etc. there are many options which avoid "fumbling wires" ;-)


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## epanzella (Apr 1, 2018)

I absolutely HATE using 3 wires but I use them because I don't have a thread mike and I believe the 3 wire method is the only accurate alternative to a mike. I know some guys use the one wire method but as it uses the thread crests on one side it will only be accurate if the flats on your new thread are exactly the same as the thread you'r trying to replicate. The one wire method removes 50% of the error that would be introduced by the no wire method. (Just measuring the OD)


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## RJSakowski (Apr 1, 2018)

epanzella said:


> I absolutely HATE using 3 wires but I use them because I don't have a thread mike and I believe the 3 wire method is the only accurate alternative to a mike. I know some guys use the one wire method but as it uses the thread crests on one side it will only be accurate if the flats on your new thread are exactly the same as the thread you'r trying to replicate. The one wire method removes 50% of the error that would be introduced by the no wire method. (Just measuring the OD)


Not so.  Two measurements are made, one with the wire (M2) and on without(M1).  The measurement without the wire determines the location of the surface opposite the wire and effectively cancels out any effect caused by crest geometry. 

The formula used in the three wire method is E = M-3W +.86603/n while the one used in the single wire method is E =2M2-M1 -3W +.86603/n. There is an increase in potential error in the calculated pitch diameter from about .0004" to .0006", assuming all measurements are +/-.0001".  The biggest factor in determining error comes from the measurement of the wire diameter.


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