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11-22-2010, 07:02 PM #1
chart for rod diameter for threading?
This question may seem very elementary to many here, but I've been trying to find some kind of chart that will tell me what the starting diameter of a rod should be for threading with any given size die. For example, I learned the hard way that a 1/4-20 die really struggled when I tried to thread a full 1/4 " diameter rod with it. Apparently, the starting diameter needed to be smaller. Any tips as to where I can find a chart that might cover this for the popular size threads? ( 6-32, 8-32, etc...)
11-22-2010, 07:07 PM #2
11-22-2010, 07:59 PM #3
The starting dia. for any thread is the nominal size of the thread. You put a chamfer (lead) on the end to start. Cutting a thread free hand is not easy and requires some experience.
The main thing to do is keep your threading die at 90 deg. to your work. Also look at the die from each side. One side will have a longer starting chamfer. That is where you start.
Practice! But is is best to have lathe, vertical mill or a drill press to help you get started.
Good luck and use cutting oil.
11-23-2010, 02:20 PM #4
If you look at Machinery's Handbook, you'll see that the major diameter tolerance allows the diameter to be a little smaller than nominal. For average work, making the outside diameter about .005" smaller than nominal works well. This reduces the torque required to turn the die.
11-23-2010, 02:25 PM #5
In addition to the advice already offered, if your die is adjustable, make sure it is fully open when you make your first pass. You can close it up a bit and take a finishing pass afterwards.
11-23-2010, 02:56 PM #6
I think I'd also add, as an amateur, that the quality of the die makes a lot of difference, and yet it's not generally obvious (to me) from looking at it. I've collected a lot of dies over the years and they really are very variable. I've got dies with reputable names on them which I wouldn't trust to put a thread an a piece of brass, and one particular 5/8" die nut which puts a beatiful thread on a piece of (reasonably) hard steel at full diameter.
So, what's the secret from the Professionals?
Bobw liked this post
11-23-2010, 09:25 PM #7
11-23-2010, 10:24 PM #8
Send the parts out and have them rolled?
11-24-2010, 07:14 AM #9
There are also threading dies and re-threading dies. I think this is or once was indicated by the shape, round or hex. Make sure you have the right one. My experiences with modern dies have been less than wonderful. Single point or a die head ($$) is way better.
03-05-2016, 03:24 PM #10
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03-05-2016, 08:29 PM #11
03-06-2016, 04:39 AM #12
1) .003 to .006" undersize is not unusual. larger number for coarser threads
2) not all thread dies have same starting taper. almost exactly like starter tap, middle or plug tap and a bottom tap. often i find a larger thread die like 1.5" has more starting thread taper than a 1.0" thread die.
3) i find it is easier to cut most of thread on a lathe and then just finish threads with a thread die. cutting just last .005" with a die is like reaming a hole.
03-06-2016, 05:43 AM #13
As for the chart, it starts at 1/2". Everything that you need on threading is in Machinery's Handbook. Also, it has pretty much all the sizes, standard or not.
03-06-2016, 06:17 AM #14
As also has been mentioned then for a standard 2A tolerance on UNC threads the max diameter is a little under nominal diameter.
These might be useful as to OD tolerances:
6-32 max 0.1372 min 0.1312
8-32 max 0.1631 min 0.1571
10-24 max 0.1890 min 0.1818
12-24 max 0.2150 min 0.2078
1/4-20 max 0.2489 min 0.2408
Hope you have a good micrometer
Nothing against Machinist Handbook but for threads then I suggest buying ASME B1.1. As far as thread information goes ASME beats the handbook hands down.
Oops noticed post #11 after I posted and also how old this thread is.
03-06-2016, 06:47 AM #15
i have cut fine threads up to 127 tpi by using front of cutter to push or form threads before
what happens is since metal is pushed around the outside diameter gets bigger and often a wire edge or sharp edge forms at top of vee. when you use a dull thread die i believe it pushes metal around a bit and the outside diameter can change. that is
..... you might start the threading .005" under size but when done threading you can easily measure less or it might be .000 to .003" under size after threading.
some early fine pitch threading dies or plates actually cut not threads but formed the threads by pushing metal around. thus you had to start with under size outside diameter. very very similar to using thread forming tap. you start with bigger tap hole size and after threads formed if you measure you will measure hole size closed up a bit
by the way if you think thread forming is odd or new that is how henry maudslay first created threads over 200 years ago. it actually is a very very old thing forming threads in soft ductile metal
03-07-2016, 04:49 PM #16
Multiply the threads per inch X 13. Add 60. This is the stock size for threads less than 1/4-20.
2/56 thread= 2x13=26+60=86 .086 is the stock size for 2/56 thread
3/48 3x13+60 =.099
4/40 4x13+60 =.112
5/44 5x13+60 =.125
This formula works up to but not including 1/4/20
There is another formula for larger threads,I have it written down in the shop.I'll post it tomorrow.I think if you do a search ,you may come up with it yourself.I wrote down formulas for metric as well.also in the shop.
03-08-2016, 02:41 AM #17
03-08-2016, 12:38 PM #18
Where can you buy steel rod under size? I can see a turned dia for threading to be 0.002 to 0.004 undersized, but threading standard rod should be used as it is. May be it comes 0.001 to 0.002 undersize - fine. Nobody will take foot long sections of 1/2 inch dia rod and turn it down. A good die will have no problem cutting a thread on a rod 0.500" in dia. I have never has a problem with running a 1/4-20, 5/16-18, 5/16-24, 3/8-16 or any other die over a full size dia rod.