New thread micrometer uncovers a problem with theoretical machinist data?? maybe

woodchucker said:
that doesn't help me. The 2 threads I needed to make were not standard. I need to make a 12-20 for that Union plane right now. The books show nothing for that thread. So ....
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So, determine the work od and cut a 20 tpi pitch. Whats the problem?
 
woodchucker said:
maybe you would like this
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I've watched Breaking Taps for a while. Tech ingredients and Action Lab are good too.

-- so you don't own a copy of the book? I'm now going to obsess on threading until I find a reference I can digest. I've always used MH as a chart only, but would liek to get a reference the *explains* the whole subject...
 
mikey said:
All kinds of calculators and stuff on line. I have apps on my phone and never use them, either. I use the attached chart to tell me the size I need to turn the work to for each class I need and the thread pitch range I need to hit when threading. Everything I need for any class of fit I commonly see in the hobby shop is here, at a glance. You can't get more simple than this chart. It is accurate, complete and quick to use.

So, you pick the tolerance class you need, like class 2a (normal fit external thread like any hardware store bolt), and you turn the work piece to the size range called out for that thread. For example, if you chose to turn a class 2a 1/4-20 thread then you would turn the work piece to within the max and min major diameter (0.2489 - 0.2408" OD). As long as your work piece OD is within this range, the thread fit will be adequate for a class 2 fit. Then you chamfer that work piece, set up your lathe to cut the thread pitch you need (1/4-20 in our case) and cut your threads. There are formulas to estimate total depth of cut; look them up. As you get within 0.010" of max estimated depth begin checking your thread pitch. You can use a mating nut or work piece or you can use your thread micrometer. The chart will tell you that you need to cut until your thread pitch falls within a range and for our 1/4-20 thread, that range is 0.2164 - 0.2127"; cut until your direct reading thread mic gets you inside this range and you're done.

Always clean the thread before taking a reading. Use a wire brush to knock off debris and burrs off of crowns and blow off debris before using your thread mic. High end thread mic anvils are hardened and ground; not so sure about imports so be careful with them.

I can't make it any simpler than this. Hope it helps.
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It's important to remember that if you are not using a full form insert then the o/d is no reference nor is is the dimension over the wires. If the crest radius is too sharp it will still affect the fit, even if the measurement over the wires is correct. If the crests are not adequately formed or controlled then either the parts wont fit or; will seem ok but will be engaging on the crests only.
 
mikey said:
So, determine the work od and cut a 20 tpi pitch. Whats the problem?
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well, making sure I have cut to the right depth. That's what I picked up the thread micrometer for. That's why I am asking. I don't want to leave it to chance. The problem .. just being accurate, and consistent.
 
Sorry to say you gots two choices. 1. Figure out what standard your screw was made to. Hint IT IS NOT Unified Thread, may not be American Standard, might be Sellers.
Find a copy of that standard and look up the info needed to cut your thread.
or
2. Recognize that you have chosen a graduate level thread cutting job for your first effort, dig into the theory of threads and figure it out and then git er done.
 
Female threads are made larger than the nominal, male threads are smaller than the nominal. It's just the size of tolerance they are made to that determines the class of fit. I remember it as maximum and minimum metal condition. Basically, if your male thread is under the nominal size it will fit any class of nut and if your female thread is larger than nominal then it will fit any class of male thread
 
Asm109 said:
Sorry to say you gots two choices. 1. Figure out what standard your screw was made to. Hint IT IS NOT Unified Thread, may not be American Standard, might be Sellers.
Find a copy of that standard and look up the info needed to cut your thread.
or
2. Recognize that you have chosen a graduate level thread cutting job for your first effort, dig into the theory of threads and figure it out and then git er done.
Click to expand...
First effort? I've been cutting threads.for years. Just not a lot of oddballs. And certainly when I have the mating part.

Sent from my SM-G900V using Tapatalk
 
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