I'm going to start off by saying that I don't claim to be a machinist, barely claim to be a gunsmith, and certainly have no formal education in either. That said, the idea of measuring threads by Pitch Diameter seems to be a relatively simple concept. So why are there so many people in this particular industry who routinely quote the Major O.D. as an indication of thread fit? I've worked in at least 3 other heavily mechanized industries, and never noticed the confusion until now.
Gunsmithing What's up with PD?
- Thread starter Ksracer
- Start date
For one thing the custom rifle business can be quite different than a commercial industry working with engineered drawing with a QC department. I was one of those that did just about everything by feel (action in hand) until I started doing AI's work in '09. Then I quickly got up to speed with several sets of thread mics. What I don't understand is the resistance by part time hobbyist smiths to even buy a cheap set of thread mics. It would make life much easier. I equate using thread mics to changing barrels. If you can change a light bulb/barrel you can measure threads.
Agreed. There's no reason not to have a $35 set of thread wires at the bare minimum.
But I've seen stuff like "I cut my tennons to 1.054 OD so I get a looser thread" stated. Other than root clearance, I guess I don't understand how that correlates to the depth the flanks are cut and the final "tightness" of the thread.
Measured pitch diameter doesn’t mean much if you are hand fitting threads. It does mean quite a bit if you are fitting a thread to a mating part you don’t have in hand. It also means quite a bit if you are trying to qualify a tool/part to a spec/standard.
If someone tells me to cut some part to a generic spec like "5/8-24" and they do not give me the mating part to fit check then I will use pitch mics/wires to make to the middle universal spec. If it doesn't fit, then I know that it's not my fault. The practice is probably not popular because it takes more time to mess with mics, (wires moreso) and inspection tooling is not free. Then there's the calculation of min/max PD on less common threaded parts (more time/more money). There is more software that make that easier today, but not many people are educated on how to or why to cut to a standard, especially if they have the mating part in hand.
That makes some sense, although I'd argue that I can get to size quicker by measuring and taking big cuts to get there, than I can by taking small cuts and trying to sneak up on it by test fitting after every pass.
If you consistently use the same tooling and you cut a specific thread many times you can get a feel for how much to cut before you even start fit-checking. "Sneaking up" isn't that time-intensive if you know you can cut 5 passes at .006" indicated on the compound to get you in the ballpark for X-TPI. Then you can either verify with pitch mics or see if your parts fits and go from there.
Gretan
I don’t think it’s any more complicated than the fact that the industry calls out the thread based upon it’s major diameter, and/or the person referencing it is being generic for conversation purposes. The confusion I see is usually coming from the customer side of things, because they don’t understand the relevance or they have never heard the term pitch diameter.
I just looked at EBay. There are always old B&S and Starrett fixed anvil mics for under $50. One guy is selling some for about $20. Just find the pitch range you need. Cheap imported 1" mic w/anvils $75 shipped. For about $150 you can have both 1" and 1-2"
Then you can get familiar with PD
I have a few sets of ring gages but I prefer thread mics. Sometimes I need to be close to the max PD for a good fit. Mics give me a number, ring gages give me an opinion. With mics I can also measure insert wear.
Then you can get familiar with PD
I have a few sets of ring gages but I prefer thread mics. Sometimes I need to be close to the max PD for a good fit. Mics give me a number, ring gages give me an opinion. With mics I can also measure insert wear.
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