Threading

[rwskinner]

I thought I understood External Thread cutting calculations but evidently I'm doing something wrong. From what I've read, if using a Sharp 60* V tool, which I am. the thread depth would be (P x 0.866), but when I follow this, my nut doesn't come close to screwing on.

Example: 12mm x 1.5
I cut my OD to 0.543"
I set Mach up Simple Threading Wizard for a OD or XStart to 0.543" and a Pitch of 0.0591"

I then calculate my depth (0.0591 x 0.866) = 0.0512" so
0.543" OD - 0.0512" = XEND of 0.4918"

In order to get a nut to screw on, I end up with XEnd around .465" or so.

Where am I going wrong?

[valmet58]

you should multiply by .613435
go see this site http://shopswarf.orcon.net.nz/unc.html

[rwskinner]

I very much appreciate the reply but it didn't help me understand anything. As a matter of fact, that makes the depth shallower not deeper. I'm having to go much deeper for the nut to fit.

[valmet58]

first of all: how are you cutting this thread? are you using a canned command like g76?

[valmet58]

12 mm is .472 inch . in your question you post .543 inch
so to start with your stock diameter is too big..as a rule of thumb you can take
the diameter of the wanted bolt less 1%.that would give you the required stock diameter to cut your threads..

[rwskinner]

Sorry: 14mm x 1.5

Yes, MachTurn using G76 in the Simple Threading Wizard.

[valmet58]

can you give me the g76 line that you typed .
the complete line g76 ...........................

[rwskinner]

Ok, Let me start from a different angle.
Now, I'm using a carbide, 60* V cutter with a 0.015" nose radius. Instead of my HSS that I used before.

I did the math like this for a 14x1.5 external thread.

Major: 0.545" O.D.
Pitch: 0.0591"

So Depth = 0.0591 * 0.866 = 0.05112"

I then cut my thread until the tool was at 0.493" or a depth of .0511"
Using my thread measuring pins, the pitch diameter was right in the middle of Min/Max Pitch Diameter and nut fits perfect.

So why did this work out great with a 0.015" radius on the nose when the formula was for a Sharp Vee ?

What I'm attempting to learn is the math for tip radius compensation, but in order to prove it, I have to have a valid starting point = Sharp Vee Tip.

When I would do this manually on the lathe, I could start doing test fits right near the end. On the CNC, well, if I don't enter the proper X-END for the depth of the thread then it's all too late when it's done.

Richard

[rwskinner]

I see where I'm screwing up I believe. I'm calculating Thread Depth on ONE side, and since I'm entering the Final Diameter, I have to double it.

So my Major Diameter - (thread depth x 2) will be my final root diameter.
Using an example of a 1/2" - 13 bolt.

Major = .4985
Pitch = .0769

0.0769 x 0.866 = 0.0666 x 2 = 0.133
0.498 - 0.133 = 0.365"

Final Diameter at the root will be 0.365" with a sharp V.
Final Diameter at the root will be 0.425" with a tip radius of 0.015"

Yeah. Hopefully this is right !!!