# Another single point threading question, sorry guys.



## Moderatemixed (Nov 22, 2019)

The background..... a quick history. I decided to make a drawbar to hold a new to me Albrecht Drill Chuck in my Mill. Day 1, 6 hours, 3/8” thread (via Die) perfect, 1/2” thread (via Die) not straight. Morning Day 2, 3/8” thread (via Die) perfect, 1/2” thread, light single point, finished by Die, not straight again. Afternoon, 3/8” via single point, perfect. 1/2” via single point perfect, except......

The material was reduced from .500 to .492 Threads are perfect but the finished diameter is .464 and the nut is loose fitting. So what happened? How do I account for loss of diameter when threading? Thoughts anyone? Yes, this is my first time single point threading but how did I mess up SO badly. 

Regards all, and thanks in advance. 

Cheers. 

Derek. 


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## benmychree (Nov 22, 2019)

Is it an adjustable die, or was the die off center?  For nice threads, I would nearly always single point to finish size, but a little undersize on OD rarely makes any difference in thread fit; for long threads I use a Geometric die head held in a aloris holder.  Button dies are a poor proposition on lathe work, even with a well fitting holder that positively centers and guides the die.


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## GL (Nov 22, 2019)

I'll jump in, but others may also.  First guess, your diameter reduction is you went too far when single pointing.  Maybe you went to a chart depth of cut.  There are two ways to get there, get close and thread your chuck on (sneak up until you get the fit you want), second get some thread wires and measure them.  The first yields matched parts, the second yields a "standard" thread.  Following up with dies yields a thread of the die tolerance, if they are not straight, you are not holding the die perpendicular to the lathe axis – lots of die holder videos out there.


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## Moderatemixed (Nov 22, 2019)

I think from doing some reading that the answer may be that I went too deep with the thread cutter. I’ll measure the minor diameter and get back to the string. 


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## darkzero (Nov 22, 2019)

Check to see if your threading tool is ground to a 60° taper. If it is, it shouldn't reduce the major diameter when you get to the proper pitch diameter. It would only reduce the major diameter if taken well past the spec pitch diameter as it will be cutting down the crest. When properly done, the crest should not be touched by the tool so the major diameter that you initially machined would stay the same.

Also, if your are using the modified flank method for infeeding, check that your compound is set to just under 30°, 29.5° is most commonly used. Note the diagram below. If you have a smaller import lathe, most have a limited scale protractor that only goes up to 45° or 50° so if you set the mark on the compound to 29° on the protractor it's wrong & can cause the same issue you are having, it needs to be set at 60.5°. If you did have it set this way it would probably be more obvious as the threads would look funny.


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## Moderatemixed (Nov 22, 2019)

The lathe is an Atlas 10F. I do have a tail stock Die holder so perhaps I should be starting with single point and then finishing with the Die. The dies I used on drawbars 1 and 2 were fixed jaw and I think that I simply started the threads crooked. I measured the minor thread diameter and it is 17.5 thou (.0175) undersized but the major diameter is .4630 29 thou (.0290) undersized. Suggests to me that as I cut the threads deeper, the cutter was peeling OD material at the same time. 


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## brino (Nov 22, 2019)

I'd also suggest practicing your thread cutting on scrap material until you get all the kinks worked out.
It will be worth it!
Keep at it.
-brino


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## benmychree (Nov 22, 2019)

Forget about all the formula crap; start threading on a full size OD and cut until the thread is nearly sharp, then try a nut and take light finish cuts until the nut or gauge fits, then file off the crests to remove burrs, take one more cut to remove burrs from filing; the thread is finished!


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## MrWhoopee (Nov 22, 2019)

benmychree said:


> Forget about all the formula crap; start threading on a full size OD and cut until the thread is nearly sharp, then try a nut and take light finish cuts until the nut or gauge fits, then file off the crests to remove burrs, take one more cut to remove burrs from filing; the thread is finished!



This is how it's usually done. We kept a set of test nuts (shop made, not grade 2) for general threading. Hobbyists seem to get all wrapped up in the precision of the thing. Working in a job shop, just get the damn thing threaded and finish the job.


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## benmychree (Nov 22, 2019)

MrWhoopee said:


> This is how it's usually done. We kept a set of test nuts (shop made, not grade 2) for general threading. Hobbyists seem to get all wrapped up in the precision of the thing. Working in a job shop, just get the damn thing threaded and finish the job.


Amen!!!!!


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## mmcmdl (Nov 23, 2019)

I follow along with many of the threads relating to threading sometimes in dis-belief . If you're working on DOD contracts , sure , you need ring gages , comparators , pitch mics etc . As stated above though , for non government work ( g jobs ) , cut your major diameter to nominal size and thread till the crest is not sharp but close to it . Fit it to a nut and be done with it . As Bemmy says above , a geometric die head is the way to go if you do a lot of threading , and as mentioned on other threads , not all dies are meant to cut threads , some are meant to repair damaged threads .


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## Moderatemixed (Nov 23, 2019)

It’s funny, because I am just learning this I assume that “real” Machinists in “real” shops are consulting the Machinery’s Handbook and working to ten thousandths. I’m not exposed to this day in and day out as many of you are, and so what you guys take for granted or what is just known by being immersed in the industry, those of us on the outside simply don’t know. I’m an Airline Pilot, there are hundreds of “industry norms” that I take for granted that may seem like utter magic to the “lay person”, but to the rest of my colleagues is business as usual. 

My thanks to the bunch who have chimed in, and I will focus less on building to aircraft specifications and rather closer to what should be expected of a basement hobby shop. Just to give you guys a bit of a laugh however, and I will say that all the while that I was making my measurements I had in the back of my head that something was “not quite right”. I’ll remind you that in my line of work, this sense is something I have come to rely on and has saved my bacon more often than not. So to the confessional..... I did a bit more digging in my Machinery’s Handbook. I was looking for a nice easy chart with max and min for specific threads. Somehow (in aviation terms I lost situational awareness) and skipped the whole Unified Screw Threads section for my guidance values (ROOKIE MOVE) and chose “Interference Fit Threads” tables. While the numbers were close, they were wrong. 

I’m flying all week with an anniversary at the end of the week but as soon as I can I will be heading to the metal supply to pick up some more stock and I’ll follow every suggestion within this thread. Thanks again guys, and to those who have made a living of this, take the time to remember how much you do in fact know, how really cool your industry and your abilities are, and how much we the basement shop guys and galls respect you. 

Best regard to you all.

Derek. 


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## WCraig (Nov 23, 2019)

Moderatemixed said:


> The lathe is an Atlas 10F. I do have a tail stock Die holder so perhaps I should be starting with single point and then finishing with the Die. The dies I used on drawbars 1 and 2 were fixed jaw and I think that I simply started the threads crooked. I measured the minor thread diameter and it is 17.5 thou (.0175) undersized but the major diameter is .4630 29 thou (.0290) undersized. Suggests to me that as I cut the threads deeper, the cutter was peeling OD material at the same time.


It is not like I have a tonne of experience, but...

Re threading with a die, I find it helpful to put a small chamfer on the end of the stock.  Just a little more chamfer than the size of the thread.  This helps centre the die on the stock to cut that first thread straight.

Re single-pointing, could you post a picture of your compound set up and threading tool?  Perhaps something needs to be tweaked.

Craig
(BTW, I had a nice chat with a friend of yours yesterday--Mark from Burlington.)


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## RJSakowski (Nov 23, 2019)

I would suggest deburring before trying the test nut.  A burr will give the false impression that the thread isn't fully cut yet.


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## MrWhoopee (Nov 23, 2019)

Moderatemixed said:


> . Just to give you guys a bit of a laugh however, and I will say that all the while that I was making my measurements I had in the back of my head that something was “not quite right”.



This is one of the most important things you can learn, both in machining and life in general. If that little voice says something is not right, LISTEN TO IT! That little voice knows whereof it speaks. Virtually every time I have ignored it I have come to regret it. Take the time to double check, call the client, question that irritable, self-righteous engineer. Whatever you do, DO NOT IGNORE THAT VOICE!


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## markba633csi (Nov 23, 2019)

First time threading there is often something that goes wrong- either one cuts too deep (as you probably did) or the tool post angle is wrong or there is something wrong with the cutting tool, or you lose your place on the leadscrew.  Atlas lathes are rather springy in terms of their rigidness which adds to the difficulty/challenge.  Practice, practice, practice and you will get there
Mark
ps check your Zamak gears frequently for embedded debris and damaged teeth-they must run smoothly so as not to transfer imperfections to the thread you are cutting- It's happened to me


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## mikey (Nov 23, 2019)

Derek, I really like your attitude. You are going to find that many of the most experienced guys will pass on practical advice in a way that may sometimes come across as condescending. I'm almost sure that it is not meant that way; they are trying to help, just like most of us here are. In this case, they are right ... sorta'. Most stuff we make in a hobby shop (or even in a job shop) doesn't need to be made to fit specs so we can cut and fit to suit our needs. We don't need to use thread mics or wires. We cut the threads on one part and then cut the threads of the other part to fit that first one and we're done. I know you already know this so let me give you the stuff I think you're really after, knowing that you're trying to understand how screw cutting works.

Screw cutting is actually very detailed and complex but in practice it isn't that difficult to understand. There is just a lot to know, and much of it can be found in a good table like the one I am attaching. The chart gives you all the practical information you need to cut threads, including the OD you need to turn your part to in order to cut a given thread. This OD depends on the class of thread you want. Most of the time, a class 2 fit is all we need. Sometimes we need a precision fit - a class 3 - and you will see that the range of diameters needed for each class changes. You choose the class you need and then turn your part to fall into that range of OD listed in the chart. Say you're turning a 1/4-20 thread:




Let's pick a class 2A (A means a male external thread) fit. Look at the numbers in the Major Diameter column. That Min/Max is the range of OD's your rod must be turned to in order to produce a class 2 fit, so you turn your rod until the OD is somewhere in the range between 0.2408 to 0.2489". Then you cut your thread. Now, in order to know how deep to cut you can calculate an estimate by using the formula 0.750/TPI, where TPI is the number of threads; in this case, 20, so 0.750/20 = 0.0375". So, you track your depths of cut as you cut your threads and well before you hit that total of 0.0375", you begin to check the thread pitch diameter with thread wires or a thread mic. The Pitch Diameter column will give you the range of diameters that will give you a class 2 fit. When you have cut deep enough for the pitch diameter to fall in this range, you're done.

Note also the last number, the Minor Diameter. That number is useful for cutting a thread relief. Just cut the relief about 0.005" deeper than the listed figure and about 2-3 threads wide.

As you can see, there is a lot more to cutting to specs so doing it this way is done only when it matters. For most things in a hobby shop, it won't matter; hence, the advice you got to just cut it until it fits. However, you should know how to cut it to fit when it does matter to you and I hope I made this a bit clearer for you. If not, say so and we'll go at it until it IS clear.


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## Moderatemixed (Nov 23, 2019)

To Mickey and all...... thanks ever so much for the valuable input, suggestions and guidance material. I will follow this thread up in about a week, hopefully with successful result. That said my sincere thanks for all of your time, and for giving a darn regarding the newbies. Best regards to you all. 

Derek


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## mikey (Nov 23, 2019)

Every single one of us started exactly where you are. You are simply new to this, not stupid. It will come so be gentle with yourself.


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## Tozguy (Nov 24, 2019)

Derek, if you can keep track of all those gauges and switches in the cockpit and still land those machines on the runway then single point threading on a lathe will get easy for you real quick. I think back to my initial thread cutting efforts and get a good laugh. Now I would rather single point cut threads to get a custom fit instead of using a die even when it is not necessary. Thread charts are no mystery anymore and it has been a pleasant flight to get here.


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## ttabbal (Nov 24, 2019)

While I think I sometimes get off in the weeds on a topic, learning how the details work can be useful for me. I rarely cut threads to spec, but knowing how means that I better understand things and have an easier time making normal threads.

It's a bit like accuracy. As I do things for hobby reasons, time isn't a big issue. So sometimes I try to aim for tighter tolerance than I need to improve my ability to hit them, and measure them. It's a way to practice with less chance of blowing the part. Sometimes I miss, but am still within the tolerance I actually need, but I'm improving. It's been a bit of a trip from thinking 1/8" was "small".


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## Shiseiji (Oct 4, 2020)

Old thread, but I wanted to add an inexpensive, easily tested, practice I've found. In the US, 3/4" PVC schedule 40 can be used to practice 3/4" 14 tpi Straight Pipe for Mechanical Joints, _Loose-Fitting Mechanical Joints for Hose Couplings_ (NPSH), the fitting found on common propane/MAPP gas canisters. The PVC OD is ~ 1.055, the ID is ~.81" The minor diameter is.925", the pitch diameter is min .9822, max .9956. "Opisies"/practice runs are easily parted off with either HHS or an "E" type insert tool. The thread is easily  tested with the propane nozzle. For what it's worth . . . 

Ron


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## savarin (Oct 4, 2020)

Probably bad practice but I start single point at 90 degrees, finish with die then use a fine wire wheel in an electric drill to "smooth" the threads first one way then in reverse.
I find this gives me almost polished threads.
I started doing this when I cut a heap in hot rolled bar that left the crests like saw blades.


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## fixit (Oct 5, 2020)

darkzero said:


> Check to see if your threading tool is ground to a 60° taper. If it is, it shouldn't reduce the major diameter when you get to the proper pitch diameter. It would only reduce the major diameter if taken well past the spec pitch diameter as it will be cutting down the crest. When properly done, the crest should not be touched by the tool so the major diameter that you initially machined would stay the same.
> 
> Also, if your are using the modified flank method for infeeding, check that your compound is set to just under 30°, 29.5° is most commonly used. Note the diagram below. If you have a smaller import lathe, most have a limited scale protractor that only goes up to 45° or 50° so if you set the mark on the compound to 29° on the protractor it's wrong & can cause the same issue you are having, it needs to be set at 60.5°. If you did have it set this way it would probably be more obvious as the threads would look funny.
> 
> View attachment 306320


I understand the 29.5 degrees but the tool position confuses me.  Should it not be perpendicular to the workpiece??


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## SLK001 (Oct 5, 2020)

fixit said:


> I understand the 29.5 degrees but the tool position confuses me.  Should it not be perpendicular to the workpiece??



Yes, the tool needs to be set with a thread gauge (a tool with a 60º cut in it that is held against your work and then you align your tool tip into the cut).  Set the 29.5º angle of the compound first, then align your tool to your work.  Then proceed to thread.


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## erikmannie (Oct 5, 2020)

I understand that cut threads are never allowed on aircraft (as the sharp edge can act as a stress riser). Rather, all threads on an aircraft must be rolled, and, as such, never made in a home shop. I could be misinformed here.


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## derf (Oct 10, 2020)

For a drawbar, I wouldn't use cut threads. Because it will be cycled many times, use a rolled thread. I make all my drawbars with a grade 8 bolt attached to the drawbar via a socket and welded or brazed.


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## Joeman77 (Oct 10, 2020)

MrWhoopee said:


> This is one of the most important things you can learn, both in machining and life in general. If that little voice says something is not right, LISTEN TO IT! That little voice knows whereof it speaks. Virtually every time I have ignored it I have come to regret it. Take the time to double check, call the client, question that irritable, self-righteous engineer. Whatever you do, DO NOT IGNORE THAT VOICE!


 That is not until you've proven it wrong beyond the shadow of a doubt, twice!


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## epanzella (Oct 11, 2020)

I never use dies. Just single point every thing. That said, are you using a split die? Cranking down on it could make it cut undersize.


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## Cooter Brown (Oct 12, 2020)

I just used a Carbide insert threading tool yesterday to cut a 7/8-20tpi and now I'm absolutely hooked it's the only thing I will use anymore....


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## Larry$ (Oct 12, 2020)

I use the KISS approach. Sharp tool, forget the 29.999* stuff, plunge straight in, deburr when you get close and try a nut, or if you are going for interchangeable, use the one wire system and measure with the micrometer. Save the other two wires for when you loose one. I didn't invent any of this. It makes sense to me and best of all it works. I also don't like to thread toward the shoulder so run the tool upside down, which BTW allows me to run at higher speed. But you can't do that with a threaded chuck. Thank you Joe Pie.


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## machPete99 (Oct 12, 2020)

I made a drawbar recently, using 4140PH. I single point threaded it about 90% of the way, then used a die in a tailstock die holder to finish it up.
It came out pretty good, much better that the one that came with the mill.

The tailstock die holder was another project of its own, the guide part being made from an older MT shank reamer that I got for cheap.


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## erikmannie (Oct 12, 2020)

Cooter Brown said:


> I just used a Carbide insert threading tool yesterday to cut a 7/8-20tpi and now I'm absolutely hooked it's the only thing I will use anymore....



I quickly developed a strong preference for the carbide 60 degree form tool. Not that there is anything lacking in HSS!


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## Shiseiji (Oct 14, 2020)

I  have a Logan 820 10" lathe that obviously pre-dates carbide in a hobby shop, with a current max rpm of  ~1400  with the non-stock pulleys it came with. Not really different than what stock gave. So far in my very limited attempts, I've used both carbide and HHS. I've had my best results with HHS, using all three techniques I've found in various places:

The typical American technique with the compound at 29.9 degrees
The English technique with the compound parallel to the bed and 90 degree plunge cut with occasional compound increases to relieve some of the right side pressure. This is the technique covered in the workshop series _Screw Cutting on the Lathe_.
A 90 degree plunge cut.
I find I like the English technique. If everything is going well, it's a simple plunge. If I see some issues with the right side of the thread, I can play with the compound to clean it up. 

I'm able to afford a set of AR Warner HHS indexed threading tooling to not add "must get a good grind" along with my other frustrations. I'm wound tight, and know it  I like the HHS a I don't need the speed and pressure required for the best carbide work though I have had decent results with 1/4" carbide, not so much with 1/2".

Just my $.02 noob experience.

Ron


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## e189552 (Oct 17, 2020)

erikmannie said:


> I understand that cut threads are never allowed on aircraft (as the sharp edge can act as a stress riser). Rather, all threads on an aircraft must be rolled, and, as such, never made in a home shop. I could be misinformed here.


Almost all threaded fasteners in aircraft are rolled threads.  That being said, I have threaded thousands of parts by single point threading, on everything from C-5's to F-16's.  Materials ranged from high strength steel to aluminum to titanium, both internal and external.  Measured with the 3-wire method (external), checked with GO/NO-GO ring or plug gauges.


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## Larry$ (Oct 17, 2020)

An easy way to pick up an existing thread, particularly when doing metric on an imperial lead screw, set the compound parallel to the bed. You can then easily adjust the tool to match the existing thread just by moving the compound. Not that I've ever messed up and lost my position when metric threading. But I have used this technique to recover. Thank Joe Pie for it.


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## Tozguy (Oct 18, 2020)

Easier still is to just loosen the tool in the holder and locate the point in the threads by hand. Then tighten the set screws while holding the point of the tool in the bottom of the thread.


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## epanzella (Oct 19, 2020)

Tozguy said:


> Easier still is to just loosen the tool in the holder and locate the point in the threads by hand. Then tighten the set screws while holding the point of the tool in the bottom of the thread.


^^^ That's the way I do it. Don't forget to to take out the endplay first. I just run the lathe in the direction of the thread and kill the power to take out the backlash.


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