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Major Diameter Before Threading

Discussion in 'QUESTIONS & ANSWERS (Get Help Fast Here!)' started by martik777, Dec 12, 2015.

  1. Ken from ontario

    Ken from ontario Canada Active Member Active Member

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    I did look at the pictures and understood the different diameters, it sounds like I upset you with my question though, if that's the case, it wasn't my intention Bob.
    I was fine going with major diameter as the diameter that I should be concerned with but after reading MarkM's post about pitch diameter vs major diameter, I wasn't sure any more.
     
  2. MarkM

    MarkM Canada Active Member Active Member

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    Sorry I never meant to confuse you. When you use a pitch micrometer to check a thread it is the pitch diameter you are measuring.
     
  3. Ken from ontario

    Ken from ontario Canada Active Member Active Member

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    Thanks Mark for clarifying it , I'm just trying to gather a basic knowledge in this complex world of tool making and machining, needless to say , I have great respect for all the veteran machinists here but it is also important to me to learn and clearly understand what I'm trying to understand, that's the only way I know how, if a subject is not inline with what I thought to be correct or true, I ask for clarification, it should not be misconstrued or interpreted as a challenge , I don't have knowledge to challenge anyone here, rookies and veterans. I just want to verify that what I believe is right ,it is in fact right.
     
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  4. mikey

    mikey Active User H-M Supporter-Premium

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    Ken, it isn't that difficult. Look at Doogie's chart. Let's pick a 1/4-20 thread as an example and say you want to cut a class 3a thread. This is a close fit with no play.

    Screen Shot 08-28-17 at 06.00 PM.PNG

    You can see that the major diameter in the last two columns is actually a range of sizes, from 0.2500" to 0.2419". This is the diameter you would turn the work piece to, within this tolerance range. Now that you cut that OD, you start cutting your 20 tpi thread and as you get close to final size you begin to check the pitch diameter with a thread micrometer or with the three-wire technique.

    Doogie's chart does not specifically list the pitch diameter so I am attaching a chart that lists all the info you need to cut threads. Note in this chart that there is a pitch diameter range. When you measure your threads, you are trying to get your measurement of your pitch diameter within this range. Once you do, you're done and any class 3, 1/4-20 nut should fit.

    Hope this makes it clear.
     

    Attached Files:

  5. Bob Korves

    Bob Korves H-M Supporter - Premium Content H-M Supporter-Premium

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    OK, I did not even think of rolled threads being a possibility in this discussion, but now I do understand your point.
     
  6. Bob Korves

    Bob Korves H-M Supporter - Premium Content H-M Supporter-Premium

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    I was not and am not upset, Ken, just trying to help us all reach an understanding of how threading works. It can seem complicated until we get our heads around all the nomenclature, the names of the various dimensions of the thread, and how they all interact to make the thread we desire. Actually, a lot of Hobby Machinist threads are simply made to fit each other, fitted by testing as the thread progresses, and vetted by how the threads feel when test screwing them together. I often start with a major diameter that will work for the external thread in question, check the minor diameter of the opposing thread, cut until I am reasonably close to that number and then test each finishing cut until the threads mate to my personal satisfaction. Quite a lot(!) of my threads do not need to mate with another thread made thousands of miles away, they just need to fit each other, and that is a relatively simple thing to achieve, even for work that needs to have accurate fits between the mating parts. My thread wires will not wear out any time soon...
     
  7. MarkM

    MarkM Canada Active Member Active Member

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    Ken the way you are going about things will be with you forever and when you have to think on your feet with no aid to come up with a solutution is where it will pay off. Too often people go about it looking for the answer and not worrying about how or why the answer came about and no knowledge is gained and they have to ask the question again. Todays society in my opinion isn t producing those crafty souls because of all the aids out there like CAD. Real machinist in my opinion should be able to figure things out with a pencil, a piece of paper , a calculator and our Bible the Machinist handbook. I truly believe one with those skills will be super valuable as time goes on especially in the world of maintenance and repair machining.
    I should have kept it simple and just made the point that your pitch diameter is what gives your thread it s fit and class of thread and your major diameter isn t the deciding factor in it s fit instead of adding more for you to think about. Your on the right path.
     
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  8. Ken from ontario

    Ken from ontario Canada Active Member Active Member

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    I'm just happy I got my question answered and thank you all again for chiming in and contributing to the discussion.
    Mikey, thanks for posting the pdf file, very much appreciated.:cheerful:
     
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  9. mikey

    mikey Active User H-M Supporter-Premium

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    Ken, let me give you some food for thought. Bob and Mark are right; in a hobby shop where one-off parts are the norm, we can make the threads fit however we want them to. As long as they go together and hold well, the fits are not that critical. However, there are times when fits are important, such as threading a part you're making to fit a thread that is already made by someone else. That's when you need to be able to cut a thread to tolerance.

    There are also times when you need to make an internal thread that is a close fit. As you know, you cannot measure a female thread directly with a thread mic or wires. The cheapest, easiest way to get the thread right is to make a go/no-go plug gauge. The no-go end will have male threads that are just outside the tolerance range for your thread, while the go end will have threads near the middle of the tolerance range for the fit you need. While this isn't something you will do often, there will come a time when you want to do it and it will be worth the time it takes to make such a gauge. Being able to cut external threads to close tolerances will enable you do just that.

    Cutting threads to the tolerances in the chart is no more difficult than not cutting them to tolerances so why not just cut your threads accurately? Its good workmanship to do so and I think that's something we can all strive for.

    You do need to buy some thread wires or a thread micrometer to work this way, though. Until I found my Tesa thread mics, I used a borrowed Fowler thread mic and found it to be pretty good. Shars makes a similar tool (probably identical) for under $100.00. Worth it in my book, and they make it simple and fast to cut threads you can be proud of.
     
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  10. Ken from ontario

    Ken from ontario Canada Active Member Active Member

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    A thread micrometer,:cry:, Mike you give great advice on tool making or machining but I tell you the advice you give on tools is always hard on the wallet:D I know you are a "buy once,cry once"- kind of a man but I must admit I have had zero regrets listening to your advice and recommendations.
    I'll work on a thread mic next.:encourage:
    Actually I need two don't I, one metric and one imperial.
     
    Last edited: Aug 29, 2017
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  11. mikey

    mikey Active User H-M Supporter-Premium

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    The cheap way to go is to buy a three wire set. They are accurate. I just don't like them because I always drop one or two of them. Yes, yes, I know how to use wire/putty/etc, etc. I just don't like them, and then I have to do a calculation to get the info I need. A thread mic is direct reading and takes a few seconds to figure out where I am.

    You do not need a Tesa thread mic. A Shars-quality set will do fine and no, you don't need an Imperial and Metric set. The anvils with the Imperial set will also measure metric, although you'll need to convert inches to mm. You can also buy these in Metric if you prefer to work in those units.

    Screen Shot 08-29-17 at 12.08 PM.PNG

    You can get by with the 0-1" mic for most work. If you go bigger then buy a bigger one. This is the equivalent of the Fowler/Shars tool: https://www.amazon.ca/Accusize-0-00...=1504044906&sr=8-1&keywords=thread+micrometer

    Sorry about the wallet hit, Ken. We, as members of the HM community, are duty-bound to get other guys to buy stuff! :)
     
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  12. Larry Hoy

    Larry Hoy United States Iron Registered Member

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    all the calculations is making my head hurt. Just find a bolt that is the size you want mic it and cut your thread.
     
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  13. 4gsr

    4gsr Global Moderator Staff Member H-M Supporter-Premium

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    I've designed lots of threads over the years, they all are cut to specific tolerances. Most can only be cut on CNC machines.
    In my shop for any 60 degree vee thread, I leave the OD a couple of thousandths big. Doing this allows for deburring of the thread flanks with file and a little polishing with emery to make the threas look pretty. Once done, the OD is generally right on size.



    Sent from my SAMSUNG-SM-G920A using Tapatalk
     
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  14. Ken from ontario

    Ken from ontario Canada Active Member Active Member

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    Great tip Ken, thanks.
     
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  15. MarkM

    MarkM Canada Active Member Active Member

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    Just interested but 4gsr could explain what you mean by most can only be done on the cnc. Other than a manual machine not having the right pitch in the thread box why couldn t a manual machine cut a thread a cnc could?
     
  16. 4gsr

    4gsr Global Moderator Staff Member H-M Supporter-Premium

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    Imagine a thread with a variable pitch or lead that changes from start to end. A thread with a variable depth to it. A thread with a taper that changes from end to end. Just a few examples of threads that can't be cut manually. And most CNC machines can't cut them either!
     
  17. MarkM

    MarkM Canada Active Member Active Member

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    Interesting. I ve made variable screws on screw machines but a wood to metal thread and tapered threads on a manual lathe with the taper attachment. To change actual pitch. Not possible on a manual machine. Learn something everyday. Thanks for posting!
     
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