Need help figuring out a square thread pitch

[ARC-170]

I have an old Milwaukee vise from the 70's, I think. It was made in Japan. It needs a new screw. Material was free. However, I can't seem to figure out the thread pitch. It appears to be a 5/8-6, but there only seem to be a little more than 5 threads in an inch. It could be metric, since it measures 0.629" (16mm is 0.6299") in diameter. I could not find a thread chart for square, metric threads.

The threads themselves (let's call these the "male" part) are 0.108" wide and the grooves (the "female" part) are 0.116" wide. The thread pitch would then be 0.224" [5.69 mm]. I measured this at the ends where I thought there would be less wear. If this is a 5/8-6, then the pitch would be 0.166, right (1/6=0.166)?

I've included a picture. It's 7/8" between corresponding points on the thread (4" to 3-1/8" in the photo). I even checked a 1/4-20 and counted 21 threads, so I'm sure I'm missing something.

 

[mickri]

Have you checked Mr Pete's videos? I think that he has a video on cutting square threads. I'll also check my old machinery handbook to see what it has to say about square threads and report back.

 

[mickri]

The MH didn't have much to say about square threads other than they are difficult to machine. The numbers that you posted indicate a thread just under 5 tpi according to the chart in the MH. Sorry I couldn't be of much help.

 

[ARC-170]

Thanks, Chuck! (@mickri)

I think I figured it out. It's a 5/8-4.5. I counted 9 threads (male and female) in 2 inches and 18 in 4 inches. I also took a look at my lathe and noticed it can cut 4, 4.5, 5, and 5.75 threads per inch. It just didn't occur to me that there could be anything other than whole numbers!

I also found a chart that has thread depths and pitches for square threads and the numbers closely match my measurements.

 

[Alcap]

Would you mind posting a link to the square thread chart you found ?

 

[IamNotImportant]

 

[RJSakowski]

If determining thread pitch by counting the number of threads in an interval, use the largest intreval possible for best accuracy. One could assume some nice neat number for the pitch but that could be a mistake, especially when dealing with both inch and metric threads.

 

[ARC-170]

Here's the chart I used:

 

[MrWhoopee]

Does is bother anyone else that the OD is .629 (16mm)?

 

[ARC-170]

Does is bother anyone else that the OD is .629 (16mm)?

It bothers me, too, actually. The thread pitch on metric threads is measured between 2 corresponding points on the thread. I measure 2.62mm and 2.92mm for the "male" and "female" parts for a total of 5.54mm. I wonder if this could be a 16x5.5.

But, I couldn't find any reference for metric square threads. I found plenty of metric thread references, but nothing specifically addressing metric square threads.

The fact that the pitch was exactly 9 teeth in 2 inches and 18 in 4, along with the teeth dimensions being within wear tolerances of the dimensions in the table leads me to thinking this must be an inch thread. I used calipers to measure the diameter and they could be off a thousandth or two.