Cutting nonstandard threads

Flashlight Modding and DIY Parts
1

Is there a quick and dirty shortcut chart out there that gives the basic size difference needed between major & minor for various thread pitches? Machinery's Handbook is great for established standards but it's just a little too much to sort through to find something like this. There's not much in there about how to do stuff the wrong way.

For one I just did, in 1" OD/.750" ID tube, I just arbitrarily picked .915" for the male thread OD, and guessed and estimated based on the 20TPI's thread depth, and came up with a number of .880" ID for the bore for female thread. A chart with that difference between OD and ID, for each thread pitch, sure would be handy when applying standard thread pitches to nonstandard diameters. I can't possibly be the first human to need a chart like this but I can't find the magic words to plug into the Google Machine.

2

I have an excel chart from someone on Hobby Machinist which has the formula. PM me your email and I'll send it to you.

From what I can see, the amount to deduct from the major diameter is 2 x cos30°/tpi x thread engagement (0.75 for alu, 0.65 for steel are good no.s). Metric is easy, just deduct pitch from major diameter eg. M4x1.0 = 4-1.0 = 3

3

Well that's just depressing, as it seems I'm the guy everyone else laughs at for not knowing trig.

4

I'm not sure we're talking about the same thing here. Sorry I'm not able to explain it better.

Say I cut a 20TPI thread on some arbitrary OD, say .915". The shape and depth and pitch of a 20TPI thread is the same whether it's 1/4-20 or 7/16-20 or 117/128-20, right? So what size do I bore the ID of the mating part to, before cutting another set of plain standard 20TPI internal threads, for it to fit my 117/128ths/.915" external threads? I figured there would be some established simple chart for that.

According to Machinery's Handbook, that difference between external thread part OD and internal thread part ID is the same for each pitch regardless of diameter. Or close enough to the same as to make no difference when the parts only have to fit each other, and the first piece is used as a gage for cutting the thread of the second part.

From "Table 2. Standard Series and Selected Combinations - Unified Screw Threads":

ext(max) int(min) difference
1/4"-20 0.2489" 0.196" 0.0529"
5/16"-20 0.3113" 0.258" 0.0533"
3/8"-20 0.3738" 0.321" 0.0528"
7/16"-20 0.4362" 0.383" 0.0532"
1/2"-20 0.4987" 0.446" 0.0527"

All have a difference of 0.052-0.053". Those examples are the extremes, max OD and minimum ID, and would give a too-sharp thread in aluminum, so the actual dimension for 20TPI in aluminum would probably be around 0.040-0.045"? That dimension, for various pitches, is what I was hoping to find. I'm not building a Space Shuttle here.

5

chuckling

I am NOT laughing at you, i am laughing at the whole situation.

To ME? “ooh …… T-R-I-G….” and “ooh…. G-E-O-M-E-T-R-Y…” were just little stepping stones on the way to the “real; actual heavy lifting math” like physics, statistics, calculus… and vastly stranger things.

i havent used much of it in decades, i would need a serious brushing up session, before i did any integral calculus or calculated the area under a curve algebraically from the raw equation(s).

but, geometry and trig? still come easy…

right off the bat? POOL knows what he is doing. ALTHOUGH, if you read his diatribe in his first big post? You will take note how he DELIGHTS in “making his boss look dumb” and likes to show off. (his words, not mine… he wrote it)

I would like to point out? While i saw blueprints, i did not see any “geometry” or “trig” being done. I really dont see any Trigonometry being performed here. I only see, application wise, fairly straightforward ALGEBRA. (algebra is the domain of 8th-9th graders, unless you were one of the slow kids)

if you follow his diagrams and examples? he’s giving away “plug in formulas”. The beauty of using “plug in formulas” is that its not necessary to understand the whole math class, you only need the formula and a few example problems, then you can calculate your own, for practical purposes.

=

ONE problem with math? Is that “a lot” of people like to make fun of those of us who did a lot of it in the pursuit of our higher education. Many people take some strange delight in showing off a 40 dollar an hour job, then proclaiming how they never graduated 8th grade, smiling and beaming proudly.

ANOTHER problem? Is that after being made fun of a lot in our life, many of us who DO MATH at the higher levels? Understandably like to smile and make it seem as hard as possible, when those people finally need some.

if ONE group of people would cease to make fun of people who are good at higher math, AND, the group of people that ARE good at higher math would cease “getting them back” by being smug about it, and “enjoying making people look dumb”?? MATH wouldnt have such a bad reputation.

its EMINENTLY possible to use some “plug in formulas” and follow some example problems, to then be able to calculate what you need to find out, without taking a whole class on it.

if someone cant go from “formula” to “calculator” by basic algebra… then it needs a further breakdown into a “cookbook” calculator recipe so hey can follow along.

POOL did, by the way, give all three… formulas, example problem, and the final calculator cookbook recipe, where he shows what keys to press.

If POOL were a little more inclined to do so? he could probably render some more “fat” off of the whole process, and re-iterate the basic formulae and example problem more succinctly…

keep in mind, i dont see any trig or geometry knowledge required here… this is straightforward simple algebra… its just a “tedious” plug in formulae process… will take some TIME with paper and pencil.

6

My questions for Professor POOL, based on his homework reading assignment, lol…

i dont see “D, d, D1, d1, D2, or d2” explained anywhere in the original diagram. Most everything else seems to be calculated from P, IE, defined entirely BY the P

NOTE Profesor Pool DID SAY he will “finish this later”… he probably plans on boiling it down some, to make it practical for aluminum…

NOTE#2 as a lathe owner myself now? i need to know how to cut my own threads and do this myself… so dont feel bad, i will be following along bookmarking this thread too, LMAO…

7

You do realize that was a joke right? Machine shop managers tend to be “fry kings.” Anyways forget it.

8

I dont know what a “fry king” is either, and no, i didnt get tghe entire joke. Which is irrelevant…

my main thing is, if i cut a thick tube in two, and pretty up the cuts on my lathe… i want to turn down the OUTside of one, and the INSide of the other, then cut threads. I should be able to screw them together, the threads bottom out on the lip where i quit turning the inside, amd then if i have to i can make a finish pass on the entire thing screwed together tightly.

a practical example? would be whacking a good bit out of the middle of a maglight, then i want to thread the remaining shorter halfs back together.

i assume if i bookmark this thread, i can go over it 10 times then be able to do it, yes?

this is all how to “do it right” to specs, though… are you SURE there isnt a way to just turn the OD, turn the ID, cut threads on one, then cut threads of the OTHER… then turn one set down until the other “fits”??

guys that were not PROS in a machine shop, but that can never the less make some cool stuff? assured me that with a little experience one can just “wing it” slowly to get a cut piece of thick tube to thread the two pieces together… you basically just “make” the male threads, then cut the female threads slowly, using the male threads as your guage for when its done…

??? i hate to sound like the guy that said “I aint building the space shuttle” but… i just want a tube cut in two to thread together, i dont care if it doesnt meet “UN 4Z specifications”

9

the thread depth for any given pitch is always the same.

I’ve just remembered my rough rule of thumb for minor thread diameter of SAE threads, which is major diameter - 1/tpi for male threads and major diameter + 1/tpi for female threads. Note that major and minor here refer to the crest and trough of the threads, not the ID/OD of the material.

As for working out the major thread diameter of your female threads, I’d work off the minor thread diameter of the male threads + clearance (10-15 thou? ie, 20-25% of thread depth?) to give you the major diameter of the female threads, then the formula above to get the minor diameter of the female threads. Either way, I’d make the male threaded part first and use that as a gauge to test for fit when you’re doing the female threads. If it’s not a structural piece and you have plenty of threads engaged you don’t have to be super precise with thread engagement.

edit - forgot to say, when you’re threading make sure you bore or turn a relief at the end of the thread for the threading tool to go when you open the half nuts. If you cut it to the minor diameter of the thread, I usually see the threading tool start scribing a thread in the relief as I hit the right thread depth. It’s a good visual confirmation. I will but some thread wires though at some point…

10

Hm.

i “leaned” to cut threads myself. I say “learned” in quotes, because i picked a 60 degree “cuts both ways” tool with a somewhat rounded tip. I cut “some coarse male thread” looking design on the outside of a tube… my neighbor tried to thread something on it, after checking it with a thread pitch guage… and pronounced if i had “cut a wee bit more, a real female thread almost goes on it”

when you say the relief? i recognize that from making the outside practice threads as what i nicknamed my “landing zone”. I had to ignore the instructions that came with my lathe, as when i followed them “to the letter” it makes cutting more than one pass on the same index mark impossible.

my “landing zone” must be your “relief” i learned to make on my “looks like male threads” design, lol. I needed a “place” to stop cutting when i was thru the thread.

FWIW - “your way” sounds more like what was described to me. Apparently, this is the “non space shuttle approved” method… lol

11

yep, relief = landing zone. You can get away with it if you’re threading away from the chuck (sort of), a spindle proximity stop or one of those threading retraction tools, but most people don’t have those :slight_smile:

If you’re threading you really need to grind a proper 60deg bit or use an appropriate carbide threading insert. TCMT triangular inserts have the wrong geometry and won’t cut a good thread.

12

Yep, I'm the guy who failed 9th grade Algebra. Managed to fake my way through summer school. Geometry was never a problem, sailed through those with straight As because there was a 'thing' there that made the numbers and symbols make sense. The abstract stuff in Algebra never made sense. There was no physical object I could see in my head to relate to. And none of my brilliant teachers could ever explain what Algebra was for, except that it was a prerequisite for next year's classes. They would have helped me just as much by answering with 'Because I said so.' And I still wouldn't have been able to do it.

However, I'm not that dumb at everything. Internal threading to a shoulder with no relief groove? No problem.

Put the high/low lever in neutral, turn the chuck by hand (the leadscrew is driven by the spindle, not the motor). Make a sharpie mark on the chuck or workpiece so you know where in the rotation the tool will hit the shoulder and stop at the same spot each pass.

13

can’t see your pictures for some reason.

If you want to do manual threading, there are a bunch of examples on the net (deansphotographica is one, awesome site) where people have made hand cranks that slip into the other end of the spindle.

did any of the above tips on calculating thread diameters help?

14

Do they work with direct links? I don't see any 403s in the server logs. http://73.203.218.184/lathe/Img_1830.jpg http://73.203.218.184/lathe/Img_1829.jpg

That looks close, maybe a little too much thread depth (for AL, anyway). ID for the internal thread to fit my .915" external would be .865". 65% of that difference (0.050") is probably pretty close. I'll play with some different pitches and see what happens. Realistically, I can only see needing 20, 24 & 28 TPI for scratch-built parts for flashlight related stuff where I'm making both halves.

15

nope

16

I do OK in math, but one of my professors told me my spelling was approximately on the 4th grade level. Spell checkers hide it pretty well.
Many years later, I had a nightmare about having to go back to the 4th grade to learn how to spell. I was sitting in one of those old school desks that are connected with rails and each desk top is fastened to the back of the seat in front, with holes for ink bottles.

17

Nightmare! The convolutions in grey matter are as unique as fingerprints. I did some tutoring in high school and between that and my own experiences with teachers it became obvious to me that we each learn best from teachers whose methods fit the shape of our brain. Some don’t realize that and insist you are either slow or lazy when the truth lies elsewhere. I’m terribly disorganized, to the point that it feels like an operational dislexia.

18

i was a tutor in high school, and got paid by the university mathematics lab to do that at the university level…

i honestly have problems with “how” math is presented to people by most math teachers.

“math anxiety” is a real condition… a lot of people can do a lot better in math, and a number of factors co-incide to conspire to ruin it for most people.

1) it starts at the gradeschool level… kids making fun of other kids that do well in math. Its “sissyfied”, etc etc. Most of the kids seem to get this off of their parents. “Heck, I done never grad-i-dated da 8th grade? and i makes me 42 dollars an hour…. get yer nose out dat dere book, and you learns sumpin’, kiddo…. (spit for emphasis)”

2) many parents used to have some WEIRD idea that you could scare and punish a kid into “doing well” in school… IE, the kid brings home straight As, and they get 20 bucks… the next time? they bring the straight As home, and wonder how much money they will get? “well, see, now i know you can DO IT, if you dont get all As now? you get a beating…”

all THIS does in create anxiety.

3) most math teachers dont HELP ANY. Theres a very common teaching strategy, where the material is presented “this way” and thats the “correct way”… to THAT teacher. Another common strategy is for teachers to think they are “better” and “harder” by assigning hundreds of homework problems on every style of problem. Its a retarded approach.

4) showing students how to do ONE problem, then assigning a hundred identical problems for homework? isnt terribly helpful… the kids are memorizing one WAY to do one KIND of problem… and forgetting it as soon as they can.

5) I was hearing a “lot” about this “new math” stuff… i finally had parents show me what was going on in the classroom. I honestly cant fathom who created this “teaching method” its complete poop. This crap is being taught by the “gradeschool teachers”, and, my university had a large teaching department… by and large almost ALL the gradeschool student teachers? were helpless in the little math they had to demonstrate to pass the university gradeschool education program… and THESE halfwits are responsible for teaching the foundations of MATH to your children in america… its insane. Most of my tutoring in the university mathematics lab, was spoonfeeding these future gradeschool teachers… most of whom? started OUT wanting to be high school teahers, couldnt hack the basic math, and fell back into gradeschool education.

6) most math concepts? tend to have two (or more) different approaches to teaching it… most teachers use “whatever one they like” and either dont know the other(s) or simply skip it. MOST of my students i tutored? when i showed them “the other way” would pick it up quickly and easily. I DONT KNOW why more math teachers dont use this approach.

7) MOST otherwise normal to good students? actually do FINE with using math… the problem comes for test time… they get anxiety, they get scared, they freeze up. When i get them in the mathematics lab? they do the work FINE, they freeze up when you put therm ON THE SPOT and on a TIME LIMIT.

8) Getting a good grade in a higher math class? Is obviously needed to keep going onto more of it… but… USING math, in a relaxed and practical way? is more USEFUL…

so? lets begin to LIST all of the problems? there are M-A-N-Y… and we wonder why people being good at higher math is so rare…

getting a good GRADE in math, and… being able to USE IT to solve a real problem? are 2 complete different things.

I was a more or less “B student” at university in my heavy higher math courses… i would constantly run into smug “memorize everything, straight A students” who would laugh and spit out equations i didnt memorize… but, they couldnt solve problems in a real world creative fashion… but they are WONDERFUL at memorizing equations, and MEMORIZING how to do practice problems…

give them a complicated “no clear way to do it” problem with math? they flounder just like the students that cant do math…

its always or usually possible? to USE a higher math than you are capable of… simply by finding the right equations, and maybe getting someone to boil it down to a plug-in cookbook….

PRACTICAL results matter more than any grade you ever get.

===

math anxiety? people get basically “scared” and get butterflies, similar to what a fighter gets before a “match”… most times, when you take the TIMED grading aspect out of it?? people can sit and think, and remain calm… and slowly figure out a way out of the real life math problem…. where TIME doesnt matter. I dont care if it tales me a WEEK to restructure an equation so i can just use USE IT in a practical way after that…

for a country that wishes more people were comfortable with higher math? society and the education system pretty much go about it in such a way as to completely ruin it and try to get the worst results overall. Even professionals that are doing well in it? are just going thru the motions, to get through it… which serves no practical purpose in the end.

==

i taught some people the basics of FIGHTING too before? its about the same thing… all the same problems with the WAY everyone goes about teaching it…

19

Here’s a method to try:

Lets say we are using 1” pipe with .75” ID and we want to cut a 20 TPI thread.

If we wanted the threads to be centered between the thickness of that pipe, we’d use a pitch diameter of .875”.

Add .65 times the pitch to the pitch diameter and we’ve got the OD of the external thread: .65*.05+.875= .9075”

To cut the thread, set the compound rest at 29.5°.

The infeed of your compound rest at 29.5°, if you used a tool with an almost perfectly sharp tip, should be .875 times the pitch. (this value is unrelated to the pitch diameter of .875”)
.875*.05=.04375” (Be sure and cut the thread in several passes. Set the cross slide dial and compound rest dial both to 0 with your threading tool touching the material. After each pass, exit the thread with the cross slide rather than the compound. Adjust the compound rest infeed and return the cross slide to 0 for the next pass.)

If there is a flat on the tip of your threading tool, the infeed will need to be less by an amount of .86603 times the width of the flat.

So, the external thread is cut.

For the internal thread minor diameter (the bore) multiply the pitch by .86603, then multiply the result by .625, then multiply this by 2.
So: .86603**.05**.625*2=.0541”

Subtract the result from the OD of the external thread:
.9075-.0541=.8534”

Bore the ID to this. This is the minimum minor diameter of the internal thread, and it would be fine, but aim for a + tolerance (slightly larger bore) rather than a - tolerance.

As with the external thread, the infeed of your compound rest at 29.5°, if you used a tool with an almost perfectly sharp tip, would be .04375” (.875*P) but only if the ID bore were exactly .8534”. If it is bored slightly larger, the infeed will be less by .86603 times 1/2 the difference between the actual bore size and of our calculated bore ID. So if it were bored to .860”: .860-.8534 = .0066” / 2 = .0033” *.86603 = .0029”. The infeed would be less by .0029”. (And again, if there is a flat on the tip of your threading tool, the infeed will need to be less by an amount of .86603 times the width of the flat.)

Your external (male) thread should be cut first and used for fitment of the internal thread.

20

Well hot damn, I finally understand how the pitch diameter relates to this particular thing. I mean I knew what it was, the book definition, but understanding what it is is different. If that makes any sense. Maybe a little insight into how my brain works, or how it doesn't.

I'm using a homemade 1/4" HSS for external threads because that's easy. Grinding one for internal threads is hard so I'm cheating, using an 11 IR insert (upside down and on the backside of the part, so I don't have to change the compound angle).