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Why do you need a 3a GO gauge to measure a plated thread?

QM2300

Plastic
Joined
Aug 31, 2016
Why is the tolerance TIGHTER - but for only the GO gauge - when measuring a plated thread? I've been scratching my head over this one for a while. All the guides say "3A go gauge, 2a nogo" for plated parts, but don't say why! Thanks guys
 
Why is the tolerance TIGHTER - but for only the GO gauge - when measuring a plated thread? I've been scratching my head over this one for a while. All the guides say "3A go gauge, 2a nogo" for plated parts, but don't say why! Thanks guys

First noticed this thread now. I've never heard of the gauge type thread gauges you mention but I can give you some information on plated threads.

Plated can for example be galvanized, chromed etc. (plating that adds) or electro polishing (SS) that removes.
When cutting/machining a thread before it is plated you need to "compensate" for the thickness to be added or removed by making the thread smaller (or larger) before plating.

As if that isn't enough you've got to take into account the by plating a 60º thread then the effect plating will have on the thread will be 4 times the plating thickness. If the thread is an ACME 29º/metric Tr 30º then the difference will be 8 times! A thread, as opposed to a simply round item, has 4 flanks that are plated.

As to a possible reason for a tighter tolerance re the gauge/gauges then the only thing I can think about is that it/they should be used before plating as plating isn't always an exact science.

You don't mention what your thread is but with metric for example there are two standards (ISO 965-4 and 965-5) for hot dipped galvanized threads and their tolerances are given as az for external and either AX or AZ for internal.

If you want to know more you'll have to be more specific i.e. more info :) For example too, what "guides" are you referring to?
 
On the subject of plating has anyone ever noticed the brass "showing" on the thread of a brand new chrome plated brass tap?

If you see that then the machinist or the plater has got it wrong so the "extra" has had to be removed (reworked) to get the thread back within tolerance.
 
Very well maybe.... You give him too much info he has to digest it first before he can reply.:)
The first reply was almost three weeks after the original post.

He probably figured nobody was going to answer his question and stopped watching.

- Leigh
 
When I read this link then what I wonder is why anyone would specify a thread tolerance (the examples given are standard tolerances) for before plating without specifying the plating thickness or the tolerance required after plating.

http://greensladeandcompany.com/wp-...s/threads/Threads-10 Most Asked Questions.pdf

This is definitely a case where I see the metric system for calling thread tolerances as being advantageous over the inch system.

Page 3 of 5 shows what I mean - at least I think so.

https://www.fastenal.com/content/feds/pdf/Article - Metric System & Specifications.pdf
 
Maybe I'm missing something but nowhere do I see any reference so far stating what plating is meant. Not all platings (coatings) can be done equally accurately.

IMO the most (only?) relevant tolerance is for the finished thread. The tolerance (for pitch diameter) for the pre plated thread depends on the type of plating and the specified thickness. Elecro polishing even removes so the pitch diameter should be slightly larger to begin with.

ISO even has a special thread standard for galvanized metric threads.

Category:Metal plating - Wikipedia, the free encyclopedia
 
Maybe this is a topic for a new thread but this is how I would tackle specifying a thread to be coated. Not saying it's the only way but if anyone can come up with something better I'd certainly like to hear it.

1. Determine the coating layer thickness I want or need. To keep things simple let's say 0.0005".

2. Find the pitch diameter (PD) tolerance of the thread to be coated. As an example I'll use 1/2-13 UNC-2A which has a max PD of 0.4485 and a min of 0.4435.

3. As a coating of 0.0005 will affect the thread 4 fold then I must subtract 4x0.0005 (0.002) from the designated tolerance. This gives (before coating) a tolerance of a max PD of 0.4485 - 0.002 (0.4465) and a min of 0.4435 - 0.002 (0.4415).

4. The pre-coated thread must be inspected or measured (preferably measured) before being coated. The further it is from the max/min limits the less likely the chance of problems.

5. After coating the thread can either be inspected using thread ring gauges or the PD measured again.

N.B. ASME B1.1 (UNIFIED INCH SCREW THREADS) has 2 sections on designating and coating (or plating) 60º threads. Personally after reading them I'm not much wiser as to what I should do apart from when specifying I should add BEFORE COATING and AFTER COATING.

My conclusion? I think most will find that measuring PD is both easier and certainly cheaper that buying special thread ring gauges.
 
So as not to sidetrack my post #14 then I've experienced more than once that a thread, after coating, is incorrect. To find where the fault lies I've had the coating removed. Except on one occasion then the fault was in that the pre-coated thread was outside the pre-coated tolerance.
 
Might be interesting to compare the ASME B1.1 specfications with the old B.S. 84, 1940 which covers whitworth form threads and B.S. 1095, 1942 which covers metric threads.

Judging by the condensed versions in my Newnes Engineer's Manual, (second version, second printing, dated 1964) these had a pretty sane and comprehensible way of going about things. Even the calculations are reasonably slide rule / book of table friendly. All summarised by the simple statement that for every 0.0001" of plating thickness to be applied the nominal dimensions of whitworth threads must be reduced by 0.00086" and BA threads by 0.00098" if both parts are to be plated. Half that if only one part is plated. A table going from 0.0001 to 0.0015 plating thickness too. Nowt about metric tho' so you'd have to do the calculations which don't look tricky. I guess something had to give when squeezing into two pages.

Clearly putting all the dimensional changes onto the externally threaded part is not going to fly if the plating is really thick or you are dealing with galvanised components but it does indicate that a simple approach can suffice. Caxtons equivalent book has the information for galvanised things giving dimensional modifications for both external and internally threadec components but it will be a bear to find.

Far as I can see specificationn suffer from feature creep after the first two or three iterations. Probably 'cos the real engineers have to get back to proper work so the academics and folk who do meetings take over.

Clive
 
Might be interesting to compare the ASME B1.1 specfications with the old B.S. 84, 1940 which covers whitworth form threads and B.S. 1095, 1942 which covers metric threads.

Judging by the condensed versions in my Newnes Engineer's Manual, (second version, second printing, dated 1964) these had a pretty sane and comprehensible way of going about things. Even the calculations are reasonably slide rule / book of table friendly. All summarised by the simple statement that for every 0.0001" of plating thickness to be applied the nominal dimensions of whitworth threads must be reduced by 0.00086" and BA threads by 0.00098" if both parts are to be plated. Half that if only one part is plated. A table going from 0.0001 to 0.0015 plating thickness too. Nowt about metric tho' so you'd have to do the calculations which don't look tricky. I guess something had to give when squeezing into two pages.

Clearly putting all the dimensional changes onto the externally threaded part is not going to fly if the plating is really thick or you are dealing with galvanised components but it does indicate that a simple approach can suffice. Caxtons equivalent book has the information for galvanised things giving dimensional modifications for both external and internally threadec components but it will be a bear to find.

Far as I can see specificationn suffer from feature creep after the first two or three iterations. Probably 'cos the real engineers have to get back to proper work so the academics and folk who do meetings take over.

Clive

Must admit you lost me somewhere in your post. I gave an example (suggestion) for external threads but exactly the same "calculation" would apply to internal threads.

I just happen to have a newer example of BS 84 than your 1940 version (mine is the 1956 and final version) and this is what it says about plated threads.

BS84.1956.jpg

What I did was to suggest (post #14) the easiest way I can think of to make a correct plated thread. Care to explain what you are suggesting?
 
First noticed this thread now. I've never heard of the gauge type thread gauges you mention but I can give you some information on plated threads.

Plated can for example be galvanized, chromed etc. (plating that adds) or electro polishing (SS) that removes.

electropolishing is most definitely not "plating", or even a "coating" process. it is a (reductive) surface treatment.

hot dip galvanizing, while a coating process, is also not usually defined as "plating", just as painting is not plating.

plating is usually electroplating or a chemical precipitation/ reduction such as "electroless Nickel".

the thickness of a plating deposition is highly variable, (less so for the electroless nickel), depending on the "throw" characteristics of the solution and the parts/anode orientation, agitation, temperature etc.
 
electropolishing is most definitely not "plating", or even a "coating" process. it is a (reductive) surface treatment.

hot dip galvanizing, while a coating process, is also not usually defined as "plating", just as painting is not plating.

plating is usually electroplating or a chemical precipitation/ reduction such as "electroless Nickel".

the thickness of a plating deposition is highly variable, (less so for the electroless nickel), depending on the "throw" characteristics of the solution and the parts/anode orientation, agitation, temperature etc.

I wrote "Plated can for example be galvanized, chromed etc. (plating that adds) or electro polishing (SS) that removes."

What you've written could be the subject for a new thread (no pun intended) but probably not in metrology.
 








 
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