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Major Diameter Before Threading

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martik777

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#1
How much LESS than the major diameter, of the required thread, should work be turned to before threading:

With a die?

Single pointing?

For example a 3/8 NC bolt is always approx .367 not .375 and when one tries to cut the threads with a die, .375 is too large for the die. No charts I have seen specify this, so I usually just turn the shaft a few thou under.
 

T Bredehoft

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#2
I bought a 1/4-50 die some time ago, as delivered it cut .013 undersize. Is this what you're working with? I had to open up my home made die stock allow the die to enter it. It now cuts about.003 under, and the screws made screw easily into a hole tapped 1/4-50.

As to 3/8 bolt being undersize, those threads were very probably rolled, and who knows how accurately they were made. Measure the OD of you 3/8 tap. It's not undersize.
 

RJSakowski

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#3

Silverbullet

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#4
There are specs on threads that are undersized on purpose. Infact I forget the exact terms or lettering they use. Ck your tap chart it tells you percentage of thread with different drill sizes.
 

Bob Korves

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#5
You should pick up a copy of Machinery's Handbook. It does not need to be a new one, and used ones are not expensive. It is about 2500 pages, and it has charts that tell you minimum and maximum outside diameters for screws and bolts, among many, many other topics. Charts galore. I just used my handbook a half hour ago to see what size I should turn drill rod to for a 10-24 machine screw thread (class 2A threads, 0.1818" min., .1890" max.). Very useful book, and every machinist should have at least one. The newer versions cover metric and CNC stuff better and the old ones cover stuff pertaining to older machines and methods better. Either is fine, get one! I am using the 22nd edition, published 1985.
 

dave2176

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#6
For example a 3/8x16 tpi thread has a pitch of 1 ÷ 16 = 0.0625". Take the pitch 0.0625 × the magic number 0.859 = 0.0537 rounded up. Now take 0.0537 ÷ 8 = 0.0067. Take this number and subtract it from the bolt diameter. 0.375 - 0.007 = 0.368" diameter. The 0.007 is the peak (H8) in Bob's drawing but I find this works for any tpi i have tried. The full method is in the Machinist Handbook a I too recommend getting a copy for much more than this. Enco seems to have the best price on a new one. The book is a must have for cutting a thread using the 3 wire method and tons of other stuff. Hope this helps.
Dave
 

Uglydog

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#8
Thanks, I have that ebook, just a little overwhelming to look at sometimes.
martick777,
Definitely overwhelming. Yet, very doable.
I had to take one concept at a time, look up the terms and attempt to understand both the theory and application.
I'm still working on it! But, I am functional.
You can do this... I was fortunate to have both HM and some local mentors who patiently answered my questions.
What specific questions do you have? Let's work through this!
Daryl
MN
 
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Bob Korves

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#9
I am with Daryl. First, there is plenty of knowledgeable help here if and when you need it. Second, there is the satisfaction of knowing you can do it yourself. We want to help you get from the first to the second. There is an old proverb: Give a man a fish and you feed him for a day, teach a man to fish and he will be out with his buddies drinking beer all day! 8^)
 

joshua43214

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#10
A really nice companion to the Machinery's Handbook is the Machinist Calc Pro.
It has all the common thread sizes programmed in, you can input any custom size thread, and adjust thread percent. It will return all the info you need for drill sizing including close size drills if you are missing the correct drill, wire gauge numbers, etc. It is really handy if you are making an odd ball thread.
It also does instant metric to imperial conversion and a boatload of other things like hole patterns, trig, etc.
I all but stopped using my Machinery's Handbook for reference once I got it.


 

davidh

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#11
is there not just a simple rule of thumb similar to ( "X" % x nominal diameter ) that will usually work ? i thought i saw or read that somewhere. something like 93% ? ? sticks in my old mind.
 

Wreck™Wreck

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#14
is there not just a simple rule of thumb similar to ( "X" % x nominal diameter ) that will usually work ? i thought i saw or read that somewhere. something like 93% ? ? sticks in my old mind.
The UNS standard contain 3 thread classes 1A, 2A and 3A for external threads, 1B, 2B and 3B for internal threads, the higher the number the smaller the allowance. A 1A thread will always have a Max Major Diameter smaller then the nominal diameter.
All of this data may be found here http://www.efunda.com/designstandards/screws/unified.cfm?start=1&finish=63
I have a MH but keep it at work, if I need this data at home this is where I go. It only covers up to 4-16 threads however.

For some reason this formula has been stuck in my head for the past 25 years, the Major Diameter of numbered machine screws is the number X .013 + .060", therefor a 10-24 is .130" + .060", or .190". Have forgotten more then I remember over the years but somehow this has become permanent, go figure.
 
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dave2176

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is there not just a simple rule of thumb similar to ( "X" % x nominal diameter ) that will usually work ? i thought i saw or read that somewhere. something like 93% ? ? sticks in my old mind.
The percentage varies significantly depending on tpi. That's why I just use pitch × 0.859 ÷ 8 and subtract the result from the major diameter.
Dave
 

martik777

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#16
I made a chart on the wall for all the numbered threads and generally go 3-7 thou undersize on the 1/4 to 3/8 threads depending on how hard the material is.

I don't understand why all the min/max dimensions for different classes overlap:
ie: for a 5/16 external class 1A is .3113 to .2982 2A is ..3113 .3026 and 3A is .3125 to .3038

Now if one turns the shaft to .305 it fits all 3 classes???? I see the pitch diameter are different but I cannot change those with a fixed die.
 
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Uglydog

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#17
martik777,
Have you ever noticed that some nut/bolt combinations are tight, loose, or sloppy?
That's the fit thing. As with all things there is a range of tolerance for both the internal and external threads. It is the combination of the two which makes for the fit.

There are three classes of external (1A, 2A, and 3A) and internal threads (1B, 2B, and 3B). The characteristics and typical application of each class are as follows:
Classes 1A and 1B: These classes are the loosest fit, in that, the largest amount play or clearance in assembly. These are used for work where ease of assembly and disassembly is most important, such as stove bolts and other general rough bolts and nuts.
Classes 2A and 2B: These classes are specified for the typical fit grade of commercial products, such as machine screws and fasteners, where interchangeability and reasonable quality and fit predictability is required. These classes are the most commonly used for commercial and aerospace applications.
Class 2AG: This application is specified to allow for rapid assembly, and to permit the application of a lubricant or for residual growth due to high-temperature expansion. In these applications, the G is specified to indicate, that when the thread is coated, the thread tolerance and size may not be exceeded by such coating.
Classes 3A and 3B: These classes are specified for high quality commercial, precision applications and products, such as machine screws, where a close or snug fit is required.
http://engineersedge.com/thread_strength/thread_classes.htm


As you have aptly identified there is likely a measurement which overlaps all the tolerances. However, the more metal you take off, the more difficult it becomes to hit the tolerance the project requires.

There is a pic on boltscience which shows me what happens when the thread tolerance is variable.
http://www.boltscience.com/pages/screw8.htm

Daryl
MN
 

RJSakowski

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#18
is there not just a simple rule of thumb similar to ( "X" % x nominal diameter ) that will usually work ? i thought i saw or read that somewhere. something like 93% ? ? sticks in my old mind.
If you noticed for metric threads, the tap drill size is the major diameter minus the thread pitch. Fairly simple for metric.

For SAE threads, the same formula will work. You just have to convert tpi to pitch. For example for a 1/2 -20 tpi thread, 1/20 = .050", .5" -.050" = .450". The recommended drill size is 29/64 is .453". For a 1/4-20, .250" -.050" = .200"; recommended size is #7 or .251". For 1/4-28, .250"-.036" = .214. Recommended is #3 or .213". This gives drill diameter for an approximate 75% thread.

Bob
 

DoogieB

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#19
Machinery's Handbook is great and all, but it would be nice to have a simple chart with the rod diameter you need to use with a die (or single point) for a loose or tight fit (2A or 3A).

If I'm tapping a hole, I usually use a chart like this:

http://www.littlemachineshop.com/Reference/tapdrill.php

With this chart, I can quickly grab the drill I need: tap/clearance and loose/tight. My copy is laminated and at a handy location in the shop.

For single pointing a thread on the lathe or using a die, I haven't been able to find a chart that's as concise and easy to read as the link above. The link from Wreck is close, but the information isn't very concise nor is it in a printer-friendly chart.
 
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Tozguy

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#20
Thanks Doogie, it is a well thought out chart. Gonna laminate them and keep handy.
 

Ken from ontario

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#24
I've created a chart for the Major Diameter sizes for external threading. The information was pulled from the efunda.com site, heavy cropped and edited to be in a similar format to the handy charts from LMS.

http://doug.freeshell.org/files/machinery/Unified_External_Screw_Threads.pdf
I know this is an old thread but I just wanted to thank you DoogieB for posting the simplest chart ever.
I spent a good hour trying to find a chart for rod diameter prior to threading, just a simple chart for the most common sizes but it wasn't easy, I finally found it here on the good old HM. it's already printed and tapeed on the wall in my shop.
Thank you again DoogieB .
 

MarkM

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#26
What is more important is the pitch diameter. It is the point of contact on your threads. It is an imaginary diameter on the thread where the v of your thread has an imaginary line that goes across your thread where the v has an equal distance that matches the sides to an imaginary line on the top of the v. Now draw a line on the top and bottom of the thread where the v s are equal to the sides and top of the v. There is a tolerance for your class of thread. To simplify it take a letter v now draw a line across the top of the v. It should be the equal distance as both sides are. When rolling a thread vs. Cutting a thread you would turn your stock down to the pitch diameter vs. The major diameter.
A simple formula for cutting internal threads is to subtract your pitch from your major diameter to get your drill size. 5/8 -8 thread. .625-.125. Drill is half inch.
 
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Bob Korves

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#27
When rolling a thread vs. Cutting a thread you would turn your stock down to the pitch diameter vs. The major diameter.
Huh? If you turn your stock down to the pitch diameter it will be way too small and you will only have half a thread when you are finished... You indeed turn your stock down to a point between the minimum and maximum values of the major diameter for the class of thread you are cutting, and then cut the thread. Am I missing something?
 

Ken from ontario

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#28
Bob, what you said made a lot of sense to me , but I'm still not sure which diameter I should machine a rod before external threading. I found this "thread builder" on the net and this is how they describe diameters:
Diameters:
Major diameter: the maximum and minimum sizes of the outside diameter of a screw
Pitch diameter: the Maximum and minimum sizes where the 1/2 Pitch intersects within the "V" shape of the screw thread. (this is the most important dimension of a thread)
Minor diameter: The maximum and minimum sizes of the smallest diameter of the screw threads
Over wires distance: these are the maximum and minimum distances if using 3 wires of the same size to measure the dimension close to the pitch diameter of an external thread.
Wire Diameters: Diameter of the wires used to measure the dimension close to the pitch diameter the external screw thread.

http://theoreticalmachinist.com/Threads_UnifiedImperial.aspx
 

Bob Korves

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#29
Did you look at pictures to understand the different diameters? If so, you will see that the pitch diameter is a imaginary point half way along the flank of the thread, and it indeed defines the thread size/location. Read the first diameter on your list:
Major diameter: the maximum and minimum sizes of the outside diameter of a screw
That is the outside diameter of the stock, ready to cut the thread.
 

MarkM

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#30
Huh? If you turn your stock down to the pitch diameter it will be way too small and you will only have half a thread when you are finished... You indeed turn your stock down to a point between the minimum and maximum values of the major diameter for the class of thread you are cutting, and then cut the thread. Am I missing something?
When I refer to turning your stock down to the pitch diameter it is when you are rolling a thread not cutting a thread. It displaces the metal similar to knurling. It is a much stronger thread. Some times this type of thread is requested or for production when you may be doing multiple operations at the same time like on a screw machine where your drilling from the turret and at the same time a cross slide has a thread roller that comes in from the side. Full speed when thread rolling. The roller comes in and applies pressure and displaces the metal to form your thread.
When cutting a thread it is still your pitch diameter that classes the thread. Obvioulsy within reason but a few thou below or ten though won t make much difference as far as strength but it will make it easier to cut as long as you can still cut to your pitch diameter.
Take threaded rod for example. Major diameter is usually a bit below it s named thread major for sake of easing production.
 
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