Diameter for Threading.

I actually have a nice set of dies from the mid 1980's, MATCO variety. They are not sized to cut class 3 threads, even starting the taper side. If I don't have a major diameter around the bottom of class 2 or top of class 1, the die has too much to cut. This basically extrudes the metal between the threads and you have a much bigger diameter after threading. If your threading with dies you just need to experiment to see what your dies like best.

For those who like threading on the lathe, I would wager you probably don't want to make a class 3 thread if you only have a class 1 tap. :

Kel,

Everything is sharp and setup properly, but the aluminum is lumpy looking, like regular swarf but with a nasty case of acne. Almost like scooping ice cream. The cut still looks clean though. I am using a pre sharpened bit that is flat on top. Should I grind this bit that is supposedly made for the job and put a rake on it? Is it better to cut under power?

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If you're talking about the swarf, that would be pretty normal from my experience for single-pointing, especially on coarser threads if you're using direct infeed off the crosslide. If you use your topslide set to 29 1/2 degrees (I presume you are doing 60 degree thread form - UNF/UNC/Metric) this will be slightly less so, but single pointing on coarser threads can still be a pretty "brutal" process as you are removing a chip from a relatively large "irregular" contact area resulting in the ice cream scooping. If you mean the thread on the workpiece itself is looking like that, then I'd recommend you take another pass at the same infeed setting. For me this works very well, especially if I'm close to final dimensions on the thread; I sometimes do up to four passes on the same infeed setting for the last couple of thou; this smooths out the thread a lot.

I don't recommend that you put a rake on a pre-sharpened bit, as it actually changes the effective angle of the thread. With some care, it is possible to hand-grind a bit with rake for thread cutting, but the side-angles on the tip has to be adjusted a bit. OK, having said that - it is minimal, and I'm guilty of having added some rake without bothering to adjust the side angles; for a lot of the work we do as modelers, no-one will ever know! (Except that everyone now knows I do it :big For coarser threads, or where you have to match existing threads, this has more of an impact - for example, even though I cheat, I've been meticulous on matching the 12TPI 55 degree Whitworth thread of my lathe's spindle nose when I made my collet chuck, as well as the bit used for cutting the ACME thread form for the rotary table worm and gear cutter.

I don't know what lathe you have, but hand-cranking is just dandy if you're comfortable with it. It's a great way to start single-pointing, as you get to feel how much force it needs to turn the chuck and in turn you can start compensating for the depth of the cut; the deeper you go on a thread, the smaller your infeed should become.

There are many differing opinions on the methods for single point thread cutting; you have to find the way that suits your habits through some trial and error.
From my own learning experience I initially found it best to start with small infeeds and many passes - it takes time but produces less heartache ; later on I started getting a feel for how much my lathe and toolbits could stand to take heavier cuts and still get good results. I prefer HSS toolbits for threading; I have a couple of tungsten tipped bits lying in my collection with their sharp tips broken off :

Cutting fluid is IMHO very important though - even for threading with a die. The only metals I cut dry is brass and cast iron, and I even found I get improved finishes on single-pointing brass if I use some of my favorite cutting fluid for the final passes.

One thing's for sure though - even after cutting quite a couple of threads, it is always a treat to see that thread slowly emerging from a workpiece! - At least for myself and quite a couple of other HMEM members ;D

Regards, Arnold

mklotz said:

Some time ago I drew a detailed diagram of a 60 deg thread form and worked out the formulae for virtually every dimension of interest associated with this thread form. I've attached this to this post
....
The nominal diameter of the stock should be reduced by this amount before threading.* I believe that what Shred has done in his table is calculate the resulting diameter for common bolt sizes by subtracting U from the nominal diameters. (Shred, please correct me if I'm off base here.)

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That's exactly what I did. I like tables because I can stick them up in the shop near where I use them (this one is right over the lathe headstock) and refer to them instantly... "hmm... 7/13-24... that needs to be .456..." and away I go (rather, I usually take anything close on the larger side unless I'm being picky).

Lately I've been writing the formulas down on the table in case I encounter an item not on the table. I've got a shop calculator handy, but it's still a few moments of punching keys and then it turns itself off before I want to refer back to it

thank you Shred very useful table.

kcmillin said:

I am having difficulty trying to find the correct diameter to make stock to get it ready for threading.

Is there a rule of thumb here, like a certain percent less that the major diameter?

I always have to make parts smaller than I think to get the die to start.

Also, can this be applied to making threads on the lathe?

I guess my question is, Is there an Index for threading like there is Tapping?

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Kel,

The designated major diameter is (almost always -- there are a couple of very strange exceptions, but they are extremely uncommon) the maximum major diameter of the screw thread. The minimum major diameter depends on the type of thread and its fit class. Acme threads have a fairly straightforward calculation set (see: Acme Thread Data Sheets posted at http://www.scribd.com/Lew Merrick for the equations). 60°-V threads get a bit more complicated. There is a tolerance based on the maximum major diameter of the thread and an allowance calculated on both the circumference and tension area of the thread that create the minimum major diameter. This is why an attempt to plot the values from the tables in Machinery's Handbook or FED-STD-H28 generally fail.

I live and work in that part of the universe where compliance with standards is unrelenting. The odds are very great that your work will never be held to such standards. The general rule of thumb reduces to using a minimum major diameter of 95% of the maximum major diameter for major diameters greater than #10 (.190). Increase that to 97% of the maximum major diameter for major diameters #10 and smaller.

Additionally: (1) I highly recommend making a tool holder that allows you to use modern insert threading tools as they have really improved over the past few decades; and (2) that you get a set of thread measuring wires and use them to check the pitch diameters of your threading work. These will save you a world of heartache.

I rarely use dies because I love single point threading on the lathe.

Material that is the nominal size will result in a Class 3 thread, as Greg mentioned. I am going to share this thread https://www.homemodelenginemachinist.com/threads/diameter-florida lottery-for-threading.9016/ to my kith and kin who want to know about that. That is 100 percent or pretty close to it. I've worked on a few jobs where such was required, but not many.

In hobby machining, a Class 1 fit is entirely acceptable for practically any thread.
A thread of 75 to 80 percent is usually sufficient.
If the crests of your final threads have sharp tips, your OD was too large.

Rick