Making/measuring threads... A comedy of errors.

E

ErichKeane

Making scrap at ludicrous speed.
H-M Lifetime Diamond Member
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So this is a multi-part screw up. I'm trying to make an internal boring bar for my shaper. I cut the profile down pretty quickly/easily, with only 1 small 'issue': I copied a dimension off the normal tool holder, and later discovered that IT is about 60 thou smaller than it needs to be. SO, I removed a little more material from the back flange that I could have kept. However, the real trouble started trying to cut the 1-20 UNEF threads:

This is the story of my first attempt.

First, I have a set of thread mics, but could never get them to work consistently. I would 'zero' them out, then when I went to measure it was an impossibly small and inconsistent reading! So, they sit in the back of a drawer.

SO, I decided to try to use my new-to-me-but-old-to-the-world thread wires! These are probably 40+ years old and never used. I put some cardboard down underneath the chuck, used some grease, and went to measure. I adjusted the micrometer and started tightening it to start reading and.... bumped the wires. Two of course somehow skipped my cardboard and dove right into chip tray. I managed to find 1 of the 2 digging through my (admittedly quite full) tray, but the other one is lost to time.

Not having a thread wire to measure with anymore, I decided to pull out my thread mics. I realized that the 'tail' piece (non moving jaw) would still move even with the lock on! I swore at the mics for a few minutes before I had an idea: I swapped the pointy and the 'v' jaw around so that the 'v' jaw was in the moving bit, and wouldn't you know it, it worked! The tail piece now stays still. I did a little jig of success, then measured my part:

.975. I check my thread dimensions and see that the minor diameter is something like .9405-.9269, I have a ton of room to go! So, I do 2 .005 passes (so 20 thou off diameter), and pull out my thread mics. The astute amongst you will already know what I did wrong. I measure, and its right at .950. Its at this moment _I_ realize what I did wrong above: Thread Mics measure PITCH DIAMETER, not THREAD DIAMETER. More swearing...

The pitch diameter range I was aiming for was .9675-.9641 (for 3a), or .9662-.9616 (for 2a). So I'm now more than 10 thou too small with a 20 TPI thread. Even the 1a thread requires at least .9593, so I'm about 10 thou too small there.

Looks like I scrapped the part!

However, I just (while typing this!) realized I ALSO screwed up in another way: I hadn't made the nut yet. I likely want to cut those threads FIRST so that I can just cut the threads to fit. So how many mess ups? I've lost count...
 
You won't be the only guy in the forum that has a male and female thread that will only work with it's mate. Ask me how I know :)
This is how we learn.
I enjoyed your adventure!!
 
Janderso said:
You won't be the only guy in the forum that has a male and female thread that will only work with it's mate. Ask me how I know :)
This is how we learn.
I enjoyed your adventure!!
Click to expand...

Yep :) Hence why I'm a hobby machinist and not a real one! I keep trying to do 'real machinist things' and end up messing up left and right! Its just my life I guess :)
 
SOOOooooo
Since you have to make the nut yet, You can just make the nut to fit the thread and all is good, nothing scrapped.
 
Flyinfool said:
SOOOooooo
Since you have to make the nut yet, You can just make the nut to fit the thread and all is good, nothing scrapped.
Click to expand...
:) I was actually hoping to make 2 of these with the nuts fitting both, and without wasting a bunch of material I don't know how to make the nut 'to fit'. The protrusion would require a through-hole, so I'd have to cut enough off to hold in the chuck with a hole all the way through.

I think I'm going to do my best just making the nuts to spec (cut the ID correctly, perhaps on the tighter side!) and cutting until the blue almost disappears. If after I part it off it fits, I'll keep the part I made, otherwise it can go in the scrap bin.
 
FWIW, you only need one wire to measure thread pitch diameter. No fumbling around with three wires or dropping wires into the swarf tray. Also, you can use pin gages or the shanks of drill bits rather than having a PeeDee set. Just mike the pins first to get an accurate dimension and use the formula in Machinery's Handbook.

I did an error analysis of using a single wire vs. the conventional three wires and the probable errors are comparable.

First measure the major diameter of the thread. It doesn't have to be geometrically correct but it should be uniform across the thread. Then insert one wire and measure across the thread and the wire. The PeeDee diameter is equal to 2 x the second measurement minus the first measurement.

The PeeDee diameter is the diameter that you would measure with three pitch wires (labeled M in the Pee Dee chart and Machinery's Handbook). You still have to go back to the table that is furnished with the pitch wires or to Machinery's Handbook to determine the actual pitch diameter based on the thread pitch, the value of M and the diameter of the wire.
 
Neat! I didn't know that! I've now got my thread mics working, so thats likely my weapon of choice, but its great to know for >1" threads!
 
Janderso said:
You won't be the only guy in the forum that has a male and female thread that will only work with it's mate. Ask me how I know :)
Click to expand...

And you won't be the only guy in the forum with a thread that won't fit any other thread in existence!
-brino
 
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