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KiloBravo
08-09-2010, 08:17 PM
I had to thread a steel rod the other day for 1/2 - 13
Major Diameter .5000
Minor Diameter .4056

So, for practical purposes do you guys start with a plain 1/2" steel bar and just got to depth or do you take a little off like 10 thou or so ?

Do you do anything different if you are using the lathe or a Die ?

I used both and was in a hurry I actually started with the lathe and finished with the Die. I have an SB Heavy 10 and a crappy Chinese Tap and Die set, which I would like to replace at some point.

The problem was probably the crappy Die but I want to here how others cut threads. I ended up taking the bar down to .4900 and using the die.

I much prefer to use the lathe but had other issues.

Thanks,
Kevin

squirrel
08-09-2010, 08:22 PM
I had to thread a steel rod the other day for 1/2 - 13
Major Diameter .5000
Minor Diameter .4056

So, for practical purposes do you guys start with a plain 1/2" steel bar and just got to depth or do you take a little off like 10 thou or so ?

Do you do anything different if you are using the lathe or a Die ?

I used both and was in a hurry I actually started with the lathe and finished with the Die. I have an SB Heavy 10 and a crappy Chinese Tap and Die set, which I would like to replace at some point.

The problem was probably the crappy Die but I want to here how others cut threads. I ended up taking the bar down to .4900 and using the die.

I much prefer to use the lathe but had other issues.

Thanks,
Kevin
Maybe clean up the surface with a little emery cloth and let it rip.

wierdscience
08-09-2010, 08:34 PM
Common threads I'll turn to .495 and then have at it.

More precise threads I'm using a full profile insert so it doesn't matter.

Mcgyver
08-09-2010, 08:38 PM
I I used both and was in a hurry I actually started with the lathe and finished with the Die.

Perfectly acceptable methodology.....the die sees light use, the thread form is perfect and chasing the lathe cut thread ensures its perfectly aligned

to your Q, if i'm turning the piece, I'll turn a couple of thou under the major diamater. If rough and ready, meaing the accuracy/finish of the existing bar stock is good enough, just chuck a piece of 1/2" and have at it

gregl
08-10-2010, 12:11 AM
For single-point threading in the lathe, an old-timer's rule of thumb is to reduce the o.d. by one percent. Helps keep the crest of the external thread from bottoming out against the mating internal thread. Probably won't pass NASA specs. or satisfy those who lie awake at night thinking about thread theory, but it seems to work.

strokersix
08-10-2010, 08:35 AM
For general utility work I'd go .495/.490. Makes the final sizing with the die much easier. Reduced thread bearing area due to small OD is probably irrelevant and I'm pretty sure there is no practical strength difference either.

You'll have to decide based on your application.

And a quality die will work a lot better too but you already know that.

hawgwrench
08-10-2010, 09:04 AM
This is the way I thread most of the time.Granted,it probably wont satisfy purists,but it does work.Seems that I read somewhere that theres no real gain after about 60-70% of thread,so if you take off a few thou off the major you'll still have 70% thread....however,I would think the larger the thread diameter the less critical this is.Your on the right track with the good quality dies...the cheapos are fine for light chasing or aluminum,but carbon steel will give really crappy results with a poor quality/dull die.

Toolguy
08-10-2010, 09:21 AM
Carbon steel taps and dies go bad quickly, often in one use. You want to get HSS or High Speed Steel. Single pointing on the lathe and chasing with a thread die is an excellent way to finish the threads to a standard repeatable size and thread form. Once the threads are single pointed, I will flat the tops with a file to remove the knife edge and burrs, then chase the thread again or run the die over it.

Carld
08-10-2010, 09:28 AM
To answer the other part of your question I don't plunge cut, I take .010" with the compound for the first few passes then take .005" or less as the thread gets deeper. I use a chart that gives the in feed of the compound for the finished depth but it is just a guide and can't be trusted for real.

There are machinists that cut threads with one pass but you don't get a nice thread that way.

Sometimes I cut half the depth and finish the thread with a good HSS die.

I always use a test nut to check the finished size of the thread. I only use thread wires for a class fit thread or special reasons.