Threading via Machinist Handbook (measuring)

[GunsOfNavarone]

I picked this metrology section for a reason. Though this is about threading, the questions are more about measuring than the process of threading.
As the ELS is up and running in my lathe, I really wanted to start to finish use the Machinist Handbook parameters and do lots of measuring from begining to end to come to the final product rather than grabbing a manufactured nut and do the old go/no go technique.
I took the outside diameter to 24.07mm as the hand book show a 24mm x 2 Major diameter to be 24.00 to 24.513
I did many passes with a 60 degree carbide threading tool to end up with a minor diameter of 22.108mm (M.H show max of 21.835 to 22.210)
Using the 3 wire technique, it shows I need to use the "45" wires. Now I hate using these, but this is about measuring so... I used a piece of foam to hold and came up with 24.408mm subtracting 1.7mm (card in 3 wire said this is the "constant") so I end up with 22.708mm. M.H says the pitch diameter should be between 22.701 and 22.925)
When I go to put the nut on the final product, I can only get on about 1 turn. I Dykem'd the threads and hit it with the nut again. It pretty much was removing ink from all places. This says to me everything is right...but just too large and I would need to take the major and minor down equally. Doing all the same measuring, I now had a pitch diameter of 22.563mm (the MINIMUM per M.H. is 22.701) but it now fit great....Maybe SLIGHTLY on the snug side, that is, there is zero wiggle from 2 turns in to all the way down. I will post the reference page just to show what I went off of (24x2). Now this is 6h tolerance class, not sure if that plays, but I only saw that or 4h. I would think 2a would have been more ideal but I would expect it to fit (?)
Any ideas based on measurements I used and the results? I wouldn't think the "best" home shop technique is trying a nut until it fits...? If your doing a 10mm thread, you pretty much turn that diameter and just thread what ever pitch until you nut goes on. BUT, if like me, you want to try doing it to "the Books" specs, well I didn't have spectacular initial results.

 

[extropic]

The book is "Machinery's Handbook", not "Machinist Handbook".

The table you posted is for the INTERNAL thread (nut). Since you say you checked your workpiece with a nut, it seems you're trying to cut an EXTERNAL thread. The good news is that, you can find the correct table and there is still material left to finish your workpiece.

TIP: The major diameter of a standard external machine screw thread is never allowed to be larger than the nominal (24mm in this case) major diameter.

 

[benmychree]

And, most of the time you need to file the OD smaller after threading is complete. so far as measuring is concerned, a thread mike is a good investment, they can be had on e bay fairly cheaply.

 

[GunsOfNavarone]

Sorry Extropic I get this a sensitive subject for you..
It says right there...internal threads...super. I don't need to save the work, I'm just wanting to use another method other than the testing a nut for fit. That now makes sense why the tolerance was allowed to be larger that the nut I was threading to work with!
As the usual, I can get to the end goal by having this conversation with folks here. Sometimes the answers are RIGHT THERE but you need a 2nd, 3rd, 4thset of eye on it. I'll try again when I come to the right table in the MACHINERY'S HANDBOOK (tm) and hope to get better results right off the bat. Sure does take longer with all the measuring...truthfully this is the first time it did it to specs...wrong specs, but still specs.
We'll see how the next one works out.

 

[GunsOfNavarone]

Ok, before I just in head first, I was again looking for a table for a 2a threading chart. That's when I hit the 4h/6h and just went back to the 6h (wrong internal thread chart) This chart is that one. Am I missing something obvious again?

 

[JRaut]

Those designations are the tolerance classes.

For SAE fasteners, the thread classes are as follows:
1A -- loose bolt*
2A -- normal bolt
3A -- tight-tolerance bolt

1B -- loose nut*
2B -- normal nut
3B -- tight-tolerance nut

*Thread classes 1A and 1B are super loose, and I don't think they're ever really used unless it's a special circumstance.


The thread classes for metric fasteners follow similar logic, but have different designations:
General purpose fit --- 6g bolt and 6H nut
Close fit -- 5g6g bolt and 6H nut

A quick googling brought up this PDF, which has a bunch of additional information

https://mae.ufl.edu/designlab/Online%20Resources_files/fastener_handout.pdf

 

[jaek]

Is your major diameter too big from the initial use of the internal thread chart?

The official thread form has flats on the crest and root, and fit happens at the pitch line If the threads are sharp at the crest and the PD is max allowable, the crest may interfere with the minor diameter of the female thread.

But yes, you have the right chart now. 22.562 PD is solidly within the tolerances.

 

[GunsOfNavarone]

@JRaut Yeah, I've been reading through the M.H. man it's dry and hard to stay focused but I did come the different classes but I only found the 4H/6H, not 5g/6g...I'll keep looking and I'll take a look at your PDF
@jaek yes, it was too big which makes sense as that would be slight loose fit as an internal thread. I keep the Maj/Minor the same ratios and just took them down evenly and it fits very snug but nice. I was just lost as to why all the measuring and end up winging it. My focus just is a bit muddled right now.
I have no pressing reason to do it by the book, but I'd like the learning experience. Press on!

 

[JRaut]

It's quite satisfying to cut a thread by the book, so to speak. Certainly worth pressing on.

One suggestion I have: if you're cutting an external thread (bolt), DO NOT PAY ATTENTION TO THE MINOR DIAMETER.

All you should have to care about is the major diameter and the pitch diameter. Assuming you've got your tool ground at 60* and your feed setting correct on your lathe, those are (just about) the only two variables that matter for the thread.

(Well, you can pay attention to minor diameter in a general sense, to see if you're getting close. But never rely on it as a factor in determining when you're done cutting.)

 

[mickri]

I have been trying to use the 3 wire method too. My 1915 Machinery's Handbook gives formulas for metric, imperial and whitworth threads. Simple to use because it gives you the micrometer measurement over the wires. The formula for metric threads and imperial threads is the same because they are both 60 degree threads. The formula is M = D- 1.5155P +( 3 W). M is the measurement over the wires. D is the outside diameter of the screw in inches ( major diameter). P is the pitch of the thread in inches. For metric threads you have to convert the metric diameter and pitch to inches. W is the diameter of the wires. Plugging your numbers in M = .9449 - 1.5155 * .0787 + (3 * .045). M= .9606 or 24.4000 mm. This is the measurement over the wires.