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Threading on the manual lathe

PracticalMan

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By Tom Lipton

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Screws and screw threads hold millions of things together. Nearly as many types and forms of threads exist as there are products that use threaded fasteners and connections. Equally, there is much confusion and misuse of threads in general among those who aren’t “gearheads.”

From the machinist’s point of view, cutting threads is a satisfying experience. When you’re done, hopefully, you have two parts that mate together with a level of precision and smoothness not found in run-of-the-mill, hardware-grade fasteners. I have always enjoyed cutting threads on the manual lathe and have learned a few tricks over the years.

<img class="frame-19 " src="http://www.ctemag.com/aa_pages/2012/1208-StayingSharp-web-resources/image/Ch05.Fig084.Lipton.DSC_opt.jpg " alt="Ch05.Fig084.Lipton.DSC_1302.tif ">
Courtesy of All images: T. Lipton

Align your threading tool against a freshly faced end or against the side of the chuck.

■ Align your threading tool against a freshly faced end or against the side of the chuck. The little arrow-shaped alignment tools you see are a pain and are only good for gaging hand-ground tool bits.

■ If you do a lot of threading on a manual lathe, invest in a tool that accepts inserts. The inserts are precisely ground and easily changed. One insert cuts dozens of thread pitches.

■ I learned how to thread on the lathe using the compound infeed method. Contrary to popular belief, the compound set doesn’t have to be at half the thread angle. By using what’s called “modified-flank infeed” and changing this angle, you help alleviate threading problems in difficult-to-cut materials.

■ Another advantage to threading with the compound is you don’t have to keep track of the dial position. The cross-feed dial is always zeroed after each pass, so you have less to remember, such as whether the last pass was at 0.030 " or 0.050 ". The main disadvantage is your Z-axis position changes as you feed in. This is usually not a problem on external threads, but it can be on internal threads that end against a shoulder.

■ Try the following strategies when you are ending threads and the part designer has not specified a thread. When I want to do something with the groove that gets cut at the end of the thread, I usually use the threading tool and traverse a small relief at the end. It saves a tool change and looks OK. If I want a nicer look, I switch to a radius tool. Just be sure the relief is a little smaller than the thread’s minor diameter so the mating part will thread all the way to the shoulder.

<img class="frame-20 " src="http://www.ctemag.com/aa_pages/2012/1208-StayingSharp-web-resources/image/Ch05.Fig091.Lipton.DSC_opt.jpg " alt="Ch05.Fig091.Lipton.DSC_1511.tif ">

Keep a complete set of nuts on rings, with one ring for coarse threads and the other for fine threads.

■ Use a large DOC on your first pass during threading. The point is small; in the first couple of passes, the area of the tool tip engagement is also small. Taper your DOC as you get deeper. On the last pass, feed straight in with the cross-feed at a light 0.001 " spring cut. This cuts on both tool flanks and removes chatter and tool marks from the thread.

■ I can never remember which line on the threading dial to use with which thread pitch. If you’re lucky, it will be marked. When in doubt, just use the same number or line each time. Always use the same number when cutting multiple-start threads.

Do internal threading from the inside out with left-hand tools. You will get less chatter and see what’s happening down the bore. You will need left-hand threading tools, running the lathe in reverse. Remember, it’s easy to pull a rope; it’s really hard to push one.

■ When you have a choice, fine threads are easier to cut and need fewer passes than coarse ones. The shallower depth on difficult-to-cut materials might save your bacon.

■ For quick and easy day-to-day threading gages, I keep a complete set of nuts on rings in my toolbox for fitting threads. One ring holds coarse threads and the other holds fine ones. When you thread, be sure to run the nut the full length of the threads. When left to their own devices, machinists tend to cut threads tighter than necessary.

■ Mating materials in threaded connections are important. If you must use the same material for male and female threads, do yourself a favor and put a few molecules of thread lubricant or antiseize on them before you crank them together.

<img class="frame-22 " src="http://www.ctemag.com/aa_pages/2012/1208-StayingSharp-web-resources/image/Ch05.Fig093.Lipton.DSC_opt.jpg " alt="Ch05.Fig093.Lipton.DSC_1387.tif ">

A thread file is ideal for straightening the annoying half thread fade at the beginning and end of an external thread.

■ If you do happen to get male and female threads wedged together in an intimate embrace, a simple trick to separate them is to quickly warm the female portion to 100° F or so, using a propane torch. A quick shot of penetrating lube before you twist might save the work.

■ When measuring threads, a dedicated thread micrometer is handy and quick to use at the machine. But for the highest accuracy, use the three-wire thread measuring method. This method is more accurate because the wires present a true parallel surface for measuring. If it’s good enough for the gage makers, it’s good enough for me.

■ A piece of modeling clay or window glazing putty can help hold pesky thread measuring wires. Better yet, buy a set of the plastic holders that fit the micrometer spindle.

■ Thread files actually work. They are great for straightening the annoying half thread fade at the beginning and end of an external thread. CTE

About the Author: Tom Lipton is a career metalworker who has worked at various job shops that produce parts for the consumer product development, laboratory equipment, medical services and custom machinery design industries. He has received six U.S. patents and lives in Alamo, Calif. Lipton’s column is adapted from information in his book “Metalworking Sink or Swim: Tips and Tricks for Machinists, Welders, and Fabricators,” published by Industrial Press Inc., New York. The publisher can be reached by calling (888) 528-7852 or visiting Industrial Press. By indicating the code CTE-2012 when ordering, CTE readers will receive a 20 percent discount off the book’s list price of $44.95.


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A thread file is ideal for straightening the annoying half thread fade at the beginning and end of an external thread.

Thread file . . . wow, what a great tip! :)
---ortho
 
Using nuts as gages is a bit risky, because you never know at what dimension they are within the tolerance. So you can make a thread that fits one certain nut, but not some random nut.

I know a shop that was too cheap to buy a gage and they had no other means at hand. Thus they used a nut for "inspection". After using that nut, they even returned it to the place where they bought it.
They got all parts returned.


Nick
 
I am just starting to learn machining. I bought a mini-lathe and would like to make some screw clamps for my woodworking hobby. I believe that this will require both a right and a left hand thread on each rod. Does anyone have any experience with or ideas on something like this?
 
Why do you think you'd need a left handed thread?

Gordon

If you google "Jorgensen" woodworking clamps you may get an image of what he was talking about. These clamps have double steel screws and wooden handles with one left-hand and the other right-hand. This allows the wooden jaws to clamp at angles as well as parallel to each other.

Brian
 
Which "Mr. Know it all" are you addressing? :)

Every time I hear that expression used it's always been negative. Was this the exception or ...............?

Gordon

Like happens many times in these forums - there is somewhat of a difference as to how people look at things.
From a professional standpoint we tend to sometimes overlook the fact that some people just know how to help themselves
using what ever is available. It works for them and that is what counts. Others working in the same environment will of course
be very thankful for the tips received.
Now for the professional - you could not manufacture threads by using a nut as a gage. You would not last very long -guarantied.
That is why we use thread gages and thread plugs in addition to 3-wire measurement. Very expensive for the home shop guy.
The cheapest way to make a fairly good thread without having to buy expensive gages would be to make the male first to 3-wire measurement and use it as a plug gage. If one hole is newly tapped than the male can be made with 3-wire measurement and in most (not all) cases you would be OK. I do not want to take anything away from anybody. They are doing the best with what they have. But some facts can not be altered and must be adhered to if called for.
 
I learned how to thread on the lathe using the compound infeed method. Contrary to popular belief, the compound set doesn’t have to be at half the thread angle. By using what’s called “modified-flank infeed” and changing this angle, you help alleviate threading problems in difficult-to-cut materials.
PracticalMan - could you please explain how this is done? Thank you.
 
Those clamps are usually called "Parallel Clamps" and are standard equipment for Tool and Die Makers. Most have sets starting at 1" opening and go all the way to what ever is needed for the kind of work being done (12" and more). Usually 5 - 6 sets (2 clamps) are require and for the most part they are self made.
 
As a toolmaker I feel qualified to comment and perhaps simplify something about screwcutting using a single point tool on a manual machine. Setting up the gearing and engaging the drive is obvious, the only variables are compound or plunge cuts. With compound you would set the tool post to half the thread angle. The advantage of this is the tool will use just the leading edge to cut metal as it is plunged, thus less binding but more importantly the plunging increments are set by moving the tool post so no forgetting where you are. The plunge method used both edges of the tool but can yeald a better finish, better on softer materials but being you would commonly use the cross slide to plunge the tool you always need to remember what your last cut was. Where possible, and particulally on older machines it is best to engage the feed permanently for the process and use the reverse gear to go back, this way you won't have issues with pitch errors.

Personally I always use the plunge method. Don't know if any of you agree with me but thats how I do it.

Regarding an issue of using a nut as a guage, nothing wrong with that so long as it is made as loose as the bolt it would go on, but remember, if using a single point tool to form the thread it will not form the radius to the thread tops, this can make a difference to the 'fit', which is where a 'Die-nut' is usefull.
 
I was taught many years ago to set the compound angle 1/2 degree less than half of the thread angle so with each cut you are scraping the trailing thread surface. I've done it that way for 40 plus years. BTW I love cutting threads of any kind.
 
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Last edited:
re 2) Internal threads can be measured too and thread wires aren't the only was to me

An informative thread that could be even more so with a little extra information.

re 1) There probably aren't many that haven't had this experience but if the thread pitch diameter is measured and found to be a safe distance from the outer limits this problem will become as rare as a blue moon. Threads have a surpringly large pitch diameter tolerance and a lathe should have no problem staying well within these tolerances even on the smallest threads. When mating parts "jam" this is invariably because only thread gauges are used for inspection and setting up and a "tight fit" is regarded as being "excellent". However "perfection" is when both mating threads (external and internal) are in the middle of their respective tolerances. To achieve this measurement is unavoidable which brings me to the next point.

re 2) Internal threads can be measured too and thread wires aren't the only was to measure external threads. It is true that thread wires are used by gagemakers and are more accurate than thread micrometers. There is a simple explanation for this. On a thread gage threads are ground and this, among several other things, means that the flank angle is more accurately controlled than on a component thread cut by a tool. the flank angle tolerance on a gage is much less than that on a component thread and thread wires will "sink" further into a thread with a flank angle on the plus side and not so far (as it should do) on a flank angle less than nominal. There are expensive and inexpensive methods of measuring internal threads (and of course external). Which is preferred boils down to what degree of accuracy is required or essential. An elphant rifle isn't necessary for hunting rabbits.

re 3) That's pretty much covered in 2) except to add, and I've used thread wires more times than I can count, they aren't the easiest of methods and not the fastest either. Making a small mistake in the "calculation" to find the correct pitch diameter just means one more possibility for error.

For heaven's sake don't regard this as criticism of the OP - just additional information :)

Gordon
For heaven's sake don't regard this as criticism you said,
re 2) Internal threads can be measured too and thread wires aren't the only (was)? to measure external threads. WAY ??? gagemakers? elphant?


as far as measuring with the wires, In a pinch, I sometimes use grease to hold the bottom 2 wires in place between the threads while the top wire rest between the thread, that way seems to make it it easier for me to mic threads. Just a opinion, and you know how opinions go. saves me from hunting in the chip pan for the wires. Thanks Ron :)
 
I am just starting to learn machining. I bought a mini-lathe and would like to make some screw clamps for my woodworking hobby. I believe that this will require both a right and a left hand thread on each rod. Does anyone have any experience with or ideas on something like this?

In reply to Dale1944:

You have chosen a terrifically challenging project to start learning with! If you look closely at a production woodworkers clamp, you will see the threads are not just both left and right handed on each screw, but they are Acme threads, and they may even be multi-start. They are fast acting. I don't have one in front of me at the moment, but I do have a bunch of them in my shop if you'd like more details.

Your lathe may have trouble with cutting the threads, if it's too "mini". It's a pretty deep cut. You will need to rig up a solid follow rest, to support the shaft as the tool travels along, because the spindles are so long and thin. In production I think they are made with a special die, one (actually two, one right and one left) that I doubt you can buy off the shelf.

The nuts could probably be cut with a single point tool, and it would be a heck of an exercise. Again, it's a heavy cut, and the boring bar will be thin and prone to chattering, or breaking. Even for one clamp, it would probably be worthwhile to make your own taps to thread the nuts with, also a good exercise. By the time you've made the spindles, cutting the threads on the taps will seem easy. They would have to be tool steel, and heat treated, and there is some tricky machining on them to make them cut, even well enough for just a couple of holes. I'd recommend a pair for each thread, one a little undersize (the "roughing" tap), and one the final size (the "finishing" tap).

This is not a practical way to make a large number of clamps for your woodwork enterprise. And it's not a beginner machining project, though if you pursue it to conclusion, you will certainly no longer be a beginner.

The similar, but usually much smaller, all metal toolmaker's handscrew clamps like Starrett makes are a more conventional project for learning machining. They have standard threads, both right hand only. Starrett's are probably heat treated, or at least case hardened, but plain mild steel ones would be pretty useful.

Another practical alternative would be to make all wooden reproductions of the earlier style of woodworking handscrew clamps. The Jorgensen style clamps are more versatile, but the old wooden ones worked just fine for cabinet makers for I suppose hundreds of years, and still would. You'd have to choose a small enough size to fit in your lathe, but making wooden threaded shafts on a metal lathe with a single point tool would be a great beginner project. Then you'd move on to making a steel tap (just a little bigger - wood threads need some clearance so they don't jam). The tap would work OK (in wood) without being heat treated, though that would be a good thing to learn as well. I'd recommend finding a good old one, and copying the dimensions, threads, and wood type exactly.

You could start a whole thread dedicated to this project...

Thanks for listening,

Chuck
 
A thread file is ideal for straightening the annoying half thread fade at the beginning and end of an external thread.

Thread file . . . wow, what a great tip! :)
---ortho


I have had thread files in my box for many years. Actually, the only reason I had them is because my Dad was a machinist and I wound up with a lot of his tools when he passed away. Even though I had the thread files, I never found a good use for them unless I used the to clean up a damaged thread that was in a machine without having to disassemble it.

As far as using a nut for checking threads for accuracy, it depends on whether it is a Class I, Class II or Class III thread that I was making. I was a machinist for over 40 years before becoming disabled. Most of the time I didn't use anything to check the threads unless it was a Class I thread. Another thing I will mention. I always found that the faster I cut the threads (surface speed) the better quality of the thread I was able to obtain. Usually, anything under 3/4" I was turning between 200-295 RPM. I learned to thread on a machine that had to be slammed from forward to reverse, backing the cross slide off and zeroing the micrometer dial without ever stopping the machine, forward or reverse. That was the only way it could be done on that machine. In other words, the only time the machine stopped was when the thread was finished.
Like others have mentioned on this thread, I also love cutting threads on a lathe.
 








 
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