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Sportandmiah
05-21-2010, 11:17 PM
I am making some brass screws for a project, with various sizes. But what size rod should I use when using a die to cut? Is there a chart or formula? Example: what size brass rod do I use to cut a 8-32 screw using a die? Or what diameter do I turn the rod down to in order to cut a 4-40 screw using a die?

Dr Stan
05-21-2010, 11:32 PM
Machinery's Handbook will give you this information. It's and excellent resource for all kinds of machine and machining information.

You may also want to look at: http://www.marfas.com/machinescrewtapping.shtml

the major diameter is also the body diameter. You can go .002 or .003" undersize, but I highly recommend that you do not make them oversized.

Rich Carlstedt
05-21-2010, 11:35 PM
Screw sizes start at 0 like for a 0-80 thread
That size is .060" and is the benchmark for all "number threads"
Each number above that is found by adding .013" to the size smaller.
So a 1-72 is .073(.060 +.13), and a 4-40 is .112" ( 4x .013=.052 + .060)
Now this is the max diameter of the thread. When cutting with a die, its a good idea to be a few thousandths smaller. My 4-40 screws would be .110
Rich

PS when going smaller than 0, you subtract the .013 and put a zero in front
So the next smaller threads are 00-90 (.047") and 000-120 (.034")

Sportandmiah
05-22-2010, 08:25 PM
Excellent info, thanks.
I also think I overlooked the obvious: If I wanted to make a 8-32 screw and had a sample one on hand, I could just measure it's outside diameter and turn my brass rod to it's size, right?

Carld
05-22-2010, 09:49 PM
Yes, you could do that.

Sportandmiah
05-22-2010, 10:55 PM
Cool, thanks!

gwilson
05-22-2010, 11:56 PM
The diameter of the blank rod should be such as to generate about a 70% thread IIRC. If your blank is the size of the finished thread depth,I think you may find that the die will not start cutting threads on it,or might wring off the rod of it does get a purchase on it.

Someone correct me if I'm wrong. For most "offhand" type work,I just reduce the rod little by little until a die will thread it.

Dr Stan
05-23-2010, 10:32 AM
I prefer to stay within 2 or 3 thou of the major diameter and then put a chamfer on the end of the screw to help the die start. Usually I'll just hit it with a file for 1/2" or smaller threads.

tyrone shewlaces
05-23-2010, 10:51 AM
A little trick or two for using dies on the lathe I'll add to the list:
Aligning the die to the rod is important to insure the cut threads will be on the same center axis of the rod. If you start it crooked, it will just keep on cutting at the crooked axis, which may still function depending on how ghetto the application is, but straight is always better. For smaller threads, it doesn't take much crookedness to make you start over, so this may help.

1) I'll chuck a piece of rod in the tailstock which has a flat faced end. This will provide a guide to keep the die perpendicular to the lathe axis. You simply center the die on the material as you would normally, then bring up the tailstock so the faced rod lightly bears on the back face of the die. Now you can keep a bit of light pressure on the die with the end of the rod & tailstock while you rotate the die to cut threads, and it will help to keep the die aligned and not crooked. This results in nice, concentric threads.

2) When making something from scratch material, you can make it a bit longer (like 3/16 to 1/4 - nothing critical here), then turn the extra length down to the minor diameter so it just barely slips through the die. This little stub of extra material will align the die to the material so it starts straight. When you're done cutting threads, you just turn or grind off that extra stub of material.

gwilson
05-23-2010, 11:31 AM
I use the tailstock face,too.