Thread: How Do You Pick the Correct Rod Dia for Threading?

OK, so I'm just a hobbyist in this field, so please forgive me if I say something terribly stupid or use the wrong words.

For years, I've been putting threads in holes whenever I need to. That's pretty simple as I can find charts that tell me what size of drill bit to use for a given desired thread. However, for the first time, I need to add some threads to the end of a rod. I have yet to find any material online referencing the correct rod size to start with, other than some vague warnings on this forum not to use a rod that's "too big" because it's hard on the die.

To give some specifics, I need to thread the end of a rod to 5/16-18. I assumed this would require starting with a 5/16 rod, but this seems not to be true. I measured the un-threaded part of a 5/16-18 bolt and the bolt is clearly not 5/16 thick. So I went hunting for rod the same thickness as the bolt at McMaster-Carr and couldn't find any.

So how does this all work?

Here is a chart I dug up. The nominal size is shown in the basic column, and you would be interested in the class 2A thread data. You will notice that class 3A threads have a more precise tolerance band, but I am assuming that class 2 threads will be sufficient for your application. I don't think you will find stock that can be bought already within these tolerance bands. Either turn the Major Diameter on a lathe, or buy fasteners and modify them as needed.

http://hexagon.physics.wisc.edu/rese...ds%20Chart.htm

In general you'll find that thread O.D.s will measure under thier stated diameter. They will typically be .010 under depending on the size.

Example: 5/16-18 Major Dia. .3113 to .3026 for a class 2A.

You can skim it if you want, but if you hit it with a file after threading to deburr, you'll be there anyway. As long as your pitch diameter is correct. You'll be OK.

Machinery's Handbook is a handy thing to have for reference, depending on the amount of machining you'll be doing.

Tom

So, basically, I need to go out and find a rod who's dia is between 0.3026 and 0.3113?

Looking that that chart that aerodark dug up, Major Diameter Basic is basically what we're going to call the thread while Major Diameter Min/Max are the range of sizes of the rod the threads must be cut into?


Hitting McMaster, in 316 stainless, they don't have rods in that size range, they drop direct from 5/16 down to 1/4. Am I missing something here? Aka, I searched for "metal rod", do I actually need to be searching for some other specific word that means "rod stock for threading"?

There's no single exact size that the OD needs to be. Even for Class 3 threads, major and minor diameters are given in ranges. This is because the crests of a thread play a minor role, being so fragile. You can knock the top 25% off of the "V" and have no appreciable impact on the thread's strength. In practice there's another good reason for clipping the tops of the V's, and that's because the valleys in the mating part are certainly not cut to a sharp V owing to fragility of the tip of the cutting tool, and any grunge and bits will accumulate in the valleys and foul a very "peaky" thread.

I'm spec'ing a 12mm x 1.75 thread right now. Max major diameter is 12.00mm. So, if I had exact 12mm rod, I could go right to cutting the threads, and fit within the spec. Practically though, I might rather shoot for the middle of the range to ease the machining, especially if using a die.

It's kind of interesting though that when we talk about a certain thread like 1/4-20, the 1/4" part of it is rather colloquial -- it's the pitch diameter that really matters, and a 1/4-20 obviously doesn't have a pitch diameter of .250".

Regards.

Finegrain

For external threads, the major diameter of the thread (5/16 in you case) is the diameter of the stock to start with. Unless you are using pprecision ground stock such as drill rod, you will find that the stock is not right to size, it is usually slightly undersize. This is fine, it helps make up the allowance (or clearance) on the major diameter of external threads and the minor diameter of internal threads that enable the two to go together without undue friction (resistance) for the class of fit desired. If your 5/16" rod is .005" undersize, this should be just fine. Chamfer the end slightly to enable your die to start easily. Start cut with the tapered side of the die. Use cutting oil and back off the die every turn or two to break the chips. I have found that the measured major diameter of most commercial fasteners are about .008 to .010" under the stated major diameter - this works fine for a Class 2 fit (free fit), which is the most common. If your stock OD is about this size, everything is fine. If not, you can either turn it down slightly (or hit it with a coarse file in the lathe) or use a fine file on the OD of the threads after you cut them.

More info on thread systems and classes of fit can be found in a Machinist's reference book or even in a good general machine shop text. You should read up on this info as it comes in handy, especially if you need a closer fit than a Class 2.

Mark

No, you can use 5/16 dia. rod.

Keep posting to this thread (post) as your questions arise.

Tom

You just use the "nominal" bar size, in this case 5/16. Many bolts start with undersize because they roll the the thread on which increases the diameter like knurling.

Originally Posted by williaty

Hitting McMaster, in 316 stainless, they don't have rods in that size range, they drop direct from 5/16 down to 1/4. Am I missing something here? Aka, I searched for "metal rod", do I actually need to be searching for some other specific word that means "rod stock for threading"?

I've never seen anyone stocking "rod stock for threading". Theoretically f you use a high-quality die, you can just use the nominal size rod and the die will give you a fully-formed thread with appropriate truncation, although it'll be harder to run the die. Same for a tapped hole, although the consequences of a hard-to-run tap can be way worse than a hard-to-run die .

If you're single-pointing on the lathe, then just skim the nominal sized rod to whatever major diameter you decide on.

Regards.

Finegrain

Alright, thanks for your help guys. Now all I have to do is sit and wait on McMaster to give me my stuff!

OK, so its not that easy. I'm going to order the die I need from McMaster when I order the rod stock. However, McMaster has both hex and round dies (this is a choice my local hardware store doesn't have). What's the difference?

Thanks again.

Putting aside complicated modern specifications it has been pretty much general practice over the years for ordinary work to truncate the threads by 1/8 th of the nominal depth. In other words the top 1/8 of the thread triangle is chopped off. Given that both male and female threads are operated on this gives an effective thread depth 75 % of nominal, well above the level at which strength starts to drop off noticeably (60 % I believe). This level of engagement gives a nice free running thread and is easy to cut, especially where tapping small sizes is concerned. The rate of tap attrition apparently rises rapidly above 80 %. Due to the relative slackness there can be alignment problems and a tendency to loosen where things are temperature cycled. Less truncation and tighter fits are specified to nail these problems.

So far as I am aware rod sizes can't be bought unless mixing imperial and metric gets you close.

Generally fastener thread specifications refer to the diameter of the plain shank on a bolt and, incidentally, give the top diameter of a fully pointed triangular thread form or curvy topped Whitworth.

Clive

Originally Posted by williaty

OK, so its not that easy. I'm going to order the die I need from McMaster when I order the rod stock. However, McMaster has both hex and round dies (this is a choice my local hardware store doesn't have). What's the difference?

Thanks again.

Round dies are typically adjustable so you can tune the pitch diameter. Yet another parameter to worry about . I've actually wondered about this a bit myself -- can you assume that, as purchased, the the adjuster is set to give the "nominal" pitch diameter, or do you have to dial in such a die before use?

Regards.

Finegrain

I was taught a long time ago that hex dies are rethreading dies. They are used to clean up existing threads that got messed up in some form where they would not screw into a mating part. They are not to be used to cut new threads in unthreaded material. You would want to use a round die for that.

Mark

I like the rule of thumb that says the O.D prior to threading should be nominal size minus 1%. So, for 5/16" thread (.3125") we should be looking at subtracting .003125" from the nominal to give us a desired O.D. of .309" and some change (hey, it's just a rule of thumb).

Reducing the diameter of the bar stock prior to using a hex or round die would probably make the operation go a little easier. When "single pointing" it probably makes sense to reduce the diameter first as well. When I'm using my Rubometric die head on my chucker, I don't bother. I let the full form cutting tools take care of everything. All I do is tweak the adjustment to give me the pitch diameter I want.

I have always used the tables in the machinist handbook for major diameter (go to nominal of the range), then turn the rod in a lathe to the appropriate size before threading. No formula or calculations necessary. Works every time. I have also assumed that hex dies were for repair, round adjustable dies for new threads. Adjust it to a master to set it. RJT

Also make sure to get a HSS die. Carbon steel doesn't hold hold up to a lot of threads on tough materials.

It is the simplest and most practical solution to buy 5/16 rod if you want to cut a 5/16-18 thread with a die. Be aware that there are different materials available. You did not say what material you want to use. Steel is the most common material, but 12L14 steel is much more easy to thread than 1018 steel or drill rod, for instance. And you will find a cutting fluid will make any material easier to thread than trying to do it dry.

You mentioned that the unthreaded part of a 5/16-18 bolt was much smaller than 5/16. That is because the threads on modern bolts are rolled, not cut from solid. The OD of the rolled thread is cold formed and becomes larger than the unthreaded OD of the blank bolt.

Rethreading dies are indeed always hex shaped. But they are also the same size hex as the standard nut for that size thread. So a 5/16-18 rethreading die is 1/2" across the flats. They are used to re-form damaged threads on bolts, and cannot cut new threads. The hex dies that are standard die size, like 1" across the flats, are for cutting new threads on solid bar.

Dies for cutting threads can be either solid or adjustable. Hex dies are always solid. Round dies can be solid or adjustable. There are several ways that adjustable dies are made. Some have adjusting screws built in to the die. Others need a special die stock that has a pointed adjusting screw to expand the die in addition to the two side screws that can compress the split die. Die stocks for solid dies need only one locking screw. You can buy die stocks that hold both 1" hex and 1" round solid dies. If you buy a Sears or Ace or similar set of taps and dies, it will probably have solid carbon steel dies, whether round or hex. If you buy industrial quality high speed steel dies, they will probably be round adjustable, so be sure you have the right kind of die stock for it.

Larry

For a noob like me, is there any way to just look at a hex die and determine if it's a cutting or rethreading die?

Originally Posted by williaty

For a noob like me, is there any way to just look at a hex die and determine if it's a cutting or rethreading die?

I already gave you a clue. Measure the distance across the flats of the hex.

Once you have seen one, you will also see the obvious difference in the way it is made. Also, rethreading dies only have the fractional inch size and maybe an NC or NF stamped in the face, which is quite cramped for containing markings. Cutting dies have larger hex sizes than the same thread size rethreading dies, so there is room for full thread size and pitch information and the maker's name.

Larry