# Trying to make sense of the threading charts



## better-lathe-than-never (May 5, 2022)

Figured I would post this question in a separate thread, even though it's related to my other thread on fixing the toolpost - that's also in this section. 

I'm trying to understand the threading chart on my Jet BD-920N.  I've studied the manual and watched a couple of good videos on this subject.  With that bit of knowledge I'm trying to understand what this chart is telling me about the gears in the back of my lathe.  I need someone to confirm my understanding that the "a" and "b" gears in the picture on the right are the corresponding gears on the left with the arrows - if not, I'm gonna need you to straighten that out for me first.

Given the way those gears look I suspect that my lathe came setup by the previous owner for 'mm' metric threading.


----------



## RJSakowski (May 5, 2022)

I believe you are correct on both accounts.


----------



## Bi11Hudson (May 5, 2022)

Your interpretation is correct as far as I can tell. For a novice at threading, one must understand what threading actually is. I have a Grizzly G1550 which is *almost* identical to the Jet BD920, so I will present my thoughts from experience rather than theory. The lead screw is a 16 pitch, 16 TPI, screw. Therefore, if you want to cut a 16 TPI thread, the cutter must move 1/16 inch for one revolution of the spindle. The gear train is adjusted, gears changed as needed, to accomplish this as needed for the particular thread.

To cut metric threads, the gears are adjusted to move the lead screw in (sub)millimeters per revolution. This is accomplished by inserting a ratio of 127:120. The 127, a prime number, is the only conversion from imperial to metric. The 120 tooth gear could be a 100 tooth as well, although the 120 tooth allows divisions by 3. There are many gear combinations that yield "almost" metric threads, many to a factor of four decimal places. With the Modulus 1 gears, 25.4 DP, the 127 tooth gear is small enough to be usable.

When calculating the gearing required for a particular thread, the quick change (QC) box must be included in the calculations. As a starter project, the 127 tooth gear is removed, forming a spacer or buffer but not a portion of the reduction gears. Using a piece of plastic, 1/2 PVC is highly recommended, as a convenient material to practice. Set the machine up for 16 TPI and rotate the spindle by hand, watching the lead screw. The depth of cut is slight, just a few thousandths for each pass.

When cutting imperial threads (16 TPI, et al) the threading dial may be used. For cutting metric threads, the half nuts are never released. The machine is stopped, the cutter is backed out, and the machine reversed beyond the starting point. I do this for both imperial and metric simply because, as an old man, muscle memory for one process is easier to remember than two.

Once you are comfortable with cutting threads, imperial or metric, the range of small threads can be easily doubled on the down side, though not shown on the threading chart. Doubling on the upper side, coarser, is not recommended for such a small machine. The slowest speed, ~130 RPM, is a little too fast for threading close to the chuck. I built a 3:1 reducer that I will post in the archives, _some day_. But for the novice, it is simpler to rotate the chuck by hand. Twenty some years ago, there was a move on a board dedicated to these machines that advocated several methods for this, the most notable being a small steering wheel bolted to the spindle pulley.

Cutting Left Hand threads is also no big deal. The BD920 is not equiped to reverse the lead screw, although a tumbler reverse can be easily built. There are several iterations available on the Web. It is not an advanced project, but does require a little "finagling".

.


----------



## better-lathe-than-never (May 5, 2022)

Bi11Hudson said:


> When cutting imperial threads (16 TPI, et al) the threading dial may be used. For cutting metric threads, the half nuts are never released. The machine is stopped, the cutter is backed out, and the machine reversed beyond the starting point.


Thanks Bill -

I plan on doing exactly what you described above for practice and probably the real run too - it's fewer things to worry about for me at this stage of learning.  The machine gear pics I posted above is how the machine came configured from the previous owner.  I need to re-configure it for Imperial threads cutting eventually, so I'm trying to understand how to change all the gears using the chart provided.   With my assumptions confirmed I can now plan my next steps - just hope I don't break any teeth doing it... I'm on a bad streak lately.

I was planning on using brass rod I found in the scrap pile and just run some threads on that.    PVC is a good idea - I'll bet I have some laying around too.


----------



## Mitch Alsup (May 5, 2022)

In addition to "never release the half-nut" metric threading, there is another method::

You "pickup" (engage) the half-nut on the reverse stroke, stop, reset zero, add some cut, then cut forward, release-and-backout-and-stop as one motion. Once the lathe stops, you switch to reverse, turn the lathe on and pickup the half-nut as the number passes on threading dial. With a bit of practice, this enables threading to a shoulder, backing out the cutter (being more important than stopping the lathe) and turning the lathe off.


----------



## Bi11Hudson (May 5, 2022)

Save the brass for something useful. Good brass is hard to come by. PVC pipe is cheap, and readily available when you need it. Plus, you might get some ideas making threaded joints in PVC. The only issue with converting to imperial is removing the 127 tooth gear from the gear train. Learning to cut imperial threads is much easier, if for no more reason than using the dial. You will want to work longer pieces to start with, between the chuck and a tailstock center. Keeps the cutter away from the chuck. That does matter, a crash with either threading or power feed can cause serious damage. In that regard, there should be a plastic gear somewhere near the spindle. It is plastic strictly as sacrificial for such a situation. Once the basics are confirmed muscle memory, it can be replaced with a metal gear. It is the gear where I installed the tumbler for doing left hand threads.

A manual is available from Grizzly, https://cdn0.grizzly.com/manuals/g4000_m.pdf

The G4000 is basically the same machine as my G1550, built in mainland China instead of Taiwan. The quality is debatable, but most of the parts will fit. The chuck thread is M39 X 4 as opposed to 1-1/2 X 8. There are a few other differences, but except for screw threads really make no difference. The manual is a little better than what I got with my G1550, The gears are the same, Modulus 1, although shafts and keyways may need dressing a little. Fine sandpaper should suffice.

Mitch made a comment above that does help, *once you are up to speed*. I wouldn't try to decipher it until a basic concept is grasped.

.


----------



## better-lathe-than-never (May 5, 2022)

Mitch Alsup said:


> In addition to "never release the half-nut" metric threading, there is another method::
> 
> You "pickup" (engage) the half-nut on the reverse stroke, stop, reset zero, add some cut, then cut forward, release-and-backout-and-stop as one motion. Once the lathe stops, you switch to reverse, turn the lathe on and pickup the half-nut as the number passes on threading dial. With a bit of practice, this enables threading to a shoulder, backing out the cutter (being more important than stopping the lathe) and turning the lathe off.


That sounds too complicated for me at this point.   I need to crawl first... before I even walk.


----------



## fitterman1 (May 6, 2022)

"a" = studgear
"b" = leadscrew gear

And always be aware of the position of the "b" gear. There is a spacer there so you will have to change its position when you change between metric and imperial threads.


----------



## better-lathe-than-never (May 6, 2022)

Here are my first threads on a PVC pipe - learning the basic hand operations at low speeds.  Very satisfying.

Observations so far:
1) The current settings cut very find metric threads - my final threads will be much bigger (18tpi).
2) The machine doesn't stop instantly... it coasts to a stop: left or right making it more difficult to start and stop in the same spot even at these low speeds.  
3) My thread cutting bit is very small (1/4'' I think) - so I may have to do another thing I'm not very good at:  grind a larger tool.  I'll see if I can purchase it locally from Western Tool Supply - just to eliminate that factor.

Next, I'll have to tackle the difficult part:  reconfiguring the gears for Imperial thread.  Dreading that part, actually....


----------



## Bi11Hudson (May 6, 2022)

With a 60 degree threading tool, the depth of cut is minimal on such a small machine. The largest thread I have ever cut was around 4 TPI, actually metric, but that coarse. The thread was cut on a 12 inch swing machine, in PVC plastic. Conceded, an internal thread, a female coupling. The spindle threads on both of my machines is 1-1/2 X 8 TPI. In *all *of these cases, I used a 1/4 inch cutter at less than 3/16 depth of cut. The 1/4 inch tool was *more* than sufficient. The only advantage of a larger tool is rigidity in tough to cut materials. For a novice learning on PVC plastic or brass, or even *mild* steel, there is no need for a larger cutter. The down side is that with a large (1/2" or larger) tool, there is also a larger tool holder which can interfere with the chuck jaws cutting close to the chuck and/or to a shoulder. Keep your dollars in your pocket. . .

Changing out the threading gears is usually a dirty operation. Having a few rags handy is helpful. And is no big deal. Just swapping a gear with a spacer. The 120/127 pair is used as an idler in imperial gears. As long as the same gear is used on both sides, it has no effect on the ratio, just a spacer. Another consideration you need to remember. There are "ranging" gears that may need to be changed as well. The "Norton" or quick change box has, if I remember correctly, 3 ranges of threads, depending on which gears are used on the leadscrew train. You mentioned a very fine metric thread, it is likely you will need to set the middle range as well.

.


----------



## better-lathe-than-never (May 6, 2022)

According to the chart on the front of the machine, to achieve 18 tpi I have to configure the machine like this:






a=30
b=30
Lever = 2

I don't think I have to mess with the top 40-80-40 gears - going to leave them alone.  I'll start with "a" and make it 30 which will then mate to 127 idler which connects to "b" that I will set to 30 as well.   According to the chart - IF I understand it correctly.

That's the initial plan anyway.  Thx -


----------



## better-lathe-than-never (May 6, 2022)

Bi11Hudson said:


> In *all *of these cases, I used a 1/4 inch cutter at less than 3/16 depth of cut. The 1/4 inch tool was *more* than sufficient. Keep your dollars in your pocket. . .


That's good to know - I'll go with what I have then, taking light cuts.  I don't have to finish this thread on the stud either... I can finish it with the round die of the same TPI if necessary - I just need a proper ellipsis cut into the stud to guide the die.


----------



## fitterman1 (May 6, 2022)

Before you actually cut the thread, mount a dial indicator somewhere so you can check saddle movement per revolution of the chuck. If you do this accurately enough you can actually get a very precise measurement. Divide 1 by this measurement and you will know exactly how many tpi that setting has or how far out you are. Its my way of double checking a new / unknown machine.


----------



## better-lathe-than-never (May 6, 2022)

fitterman1 said:


> Before you actually cut the thread, mount a dial indicator somewhere so you can check saddle movement per revolution of the chuck. If you do this accurately enough you can actually get a very precise measurement. Divide 1 by this measurement and you will know exactly how many tpi that setting has or how far out you are. Its my way of double checking a new / unknown machine.


OK.  That's after I do the transmission 'surgery'...


----------



## fitterman1 (May 6, 2022)

"surgery" will only take you 5-10mins once you get the hang of it.
Make sure you have some clearance between the gears when positioning them so they don't run noisy and there are no high spots. I work from the bottom up when I do this.
Then i set the qcgb and check the pitch after that. Go for the lowest speed you can choose, and go for it.


----------



## Bi11Hudson (May 7, 2022)

"General Practice" is often a misleading term to a novice. I refer to the following as general practice simply for lack of a better word. 

(1) When setting the "banjo", the manual recommends passing a piece of paper between the gears to set spacing, hence backlash. I use currency paper, a dollar bill, simply for consistancy, the note paper I keep handy varies quite a bit in weight.

(2) There is a technique used even by experienced machinists called a "scratch pass". The cutter is set so slight as to scratch the surface of the work, then checking with a thread gauge. Even well experienced people use this technique "just to be sure" as one little mistake or forgotten step in the setup will bite your a$$. It *WILL* bite, usually when you least expect it.

(3) The threading chart shows the 127 tooth gear in the setup. It can as easily be the 120 tooth, a 100 tooth, or any other. It is simply an idler gear and has no purpose other than as a spacer. It only matters in spacing the banjo.

.


----------



## better-lathe-than-never (May 9, 2022)

After a weekend of graduation parties and mothers day events I'm back to the lathe project:

This morning before work I took apart the lead screw gear and the stud gear.   Turns out I didn't even have to mess with the two stud gears because they were already what I need for Standard threading anyway.   Still, I didn't know that at the time and began unscrewing the stud/shaft partially - that's when I noticed that the stud/shaft is BENT - and it was not going back in there straight at all - that shaft and gears on it had a very noticeable wobble.   So I took it out all the way to inspect it... sure enough: it's bent, and also noticeably worn - I can't use it in this way.

Not sure how well it will show in pictures (the one on the right shows it more clearly).  It's not that noticeable until you start screwing it in.

Another day, another setback....  will have to order a new one.


----------



## better-lathe-than-never (May 12, 2022)

I ordered the wrong part...  Feels like I'm digging a deeper hole.  I'll try to straighten out the bent shaft and see if I can do it right - it doesn't need that much.   Measuring the thread major it reads .302'' and 20tpi.   Does that translate to 5/16'' 20tpi?   I'm thinking I'll drill a hole in a scrap piece of metal and tap it to that size so I have some leverage to straighten out that shaft.

The yak shaving continues...


----------



## WobblyHand (May 12, 2022)

Asian lathe?  Likely to be M8.  The pitch on M8 is 1.25mm which is equivalent to 20.32 TPI.  Minimum major diameter is 7.760mm = 0.3055" which is quite close to what you measured.  

5/16" min major diameter is 0.3026".  But 5/16" common sizes are 18 TPI and 24 TPI, don't believe there is a 5/16-20 screw that is commonly available.

My guess, it is M8-1.25.


----------



## better-lathe-than-never (May 12, 2022)

By George, you're right.   That is a very good fit:  M8 1.25 and I have a tap like that...  which is already encouraging.  Many thanks.


----------



## Bi11Hudson (May 12, 2022)

5/16-18 V M8-1.25 is a good example of where confusion can occur. I suggest that you use a glass (magnifier) for examining Imperial or Metric threads. Mx-1.25 from 25.4 V 20 from 25 is so close that a slight light leak on the guage is the only givaway.
.


----------



## better-lathe-than-never (May 12, 2022)

Those numbers sure are super close, Bill - I'm learning some painful lessons here but with the help here I think I'm starting to dig myself out (at least I keep telling myself that).


----------



## better-lathe-than-never (May 12, 2022)

Success - I took a 1/2 rod, drilled and tapped an M8 1.25 thread to match the shaft's crooked threaded end.   I threaded on the rod and mounted the shaft in the lathe.  Used a dial indicator to find the high spot and used the rod as a lever to gently bend it down until it was close to 0.   Checked it all around - it's good now.  Made quick work of it.   Whew.  

So now I feel like I've climbed out of the 9th circle of Hell up to at least lever 6... we're goin' up.  I can now assemble the gears and start cutting test patterns again, start matching them with the thread gauge, etc.


----------



## Bi11Hudson (May 13, 2022)

"And remembering 50 years back on a ship in the South Pacific that was a thousand miles from spare parts."

From another post, it is the whole point to any craft that is hobby status. The day I don't learn something is the day I will dig a hole in the back yard and crawl in. That is what seperates us from monkeys, for lack of a better term. Making do with what we have. .  . And just think, once you have threading down, making a couple of spares for when it happen next time.
.


----------



## better-lathe-than-never (May 13, 2022)

I put the machine's gears back together, with the proper configuration for the Imperial thread cutting, according to the chart:





I inserted a think plastic label about the thickness of a dollar bill in between the gears to give them some slack.  I put the belt back on the lowest speed and carefully turned the machine back on after that.  I noticed that the clutch gets activated and starts clicking.

When I start the machine with the belt tension removed and slowly engage the belt, there is no clicking initially, but if I should apply a bit of load on the spinning chuck, then the clutch will start clicking.  Not sure if that's normal or if it indicates that something is a bit too tight now.


----------



## Bi11Hudson (May 13, 2022)

There is an overload clutch on the drive side between the motor and the spindle. Mine got loose, I think the spring went soft but so long ago that I don't remember. That is where the 'clicking' is coming from. It may be something too tight or may be the spring too soft. This is where your judgement comes into play. Rotate the spindle by hand and 'feel' for a tight spot. 

There is an interlock between the power feed and threading feed.(half nuts) That's not a distinct likelyhood, just a possibility. On a new (to you) machine, the 'feel' of the spindle and gearing will be 'iffy'. Being able to bring the gear train in or out by sections will be an assist here.

Trying things first with the gear train out will point to the area where the tight spot is. Then uncouple the quick change box by letting the selector hang loose. Take things in sequence to get a feel for the machine.

.


----------



## better-lathe-than-never (May 13, 2022)

I played with the gears some more trying to isolate the tightness.   I disconnected the 127 tooth gear from the stud gear and the problem still remains, so it's nothing to do with the threading parts.   Maybe that spring is soft - I don't know, but I'm suspecting it now.   That whole power pulley is wobbly... there is a lot of play on that shaft me thinks.

I'll have to look at the diagram to see what all is in there and how to get to the spring.   Gonna tread lightly - don't wanna create any more problems than I already have.

The work around is to start the machine with the belt disengaged and then engage it slowly - works every time.


----------



## better-lathe-than-never (May 15, 2022)

Started making my first cuts in mild steel (12L14 cold): 18tpi, speed 130 - the machine runs ok -  have to take small cuts:  .005 to .003 - just taking my time and trying to get the hand coordination figured out.  I keep the half nuts locked all the time, and run the machine FWD and REV each time.  Starting to get a hang of it, though I still make mistakes frequently.

I get quite a bit of chatter, though even at the lowest speed - maybe because I'm feeding straight in, not at an angle.  Maybe that's the next adjustment I can make to see if it helps reduce the chatter.


----------



## Bi11Hudson (May 15, 2022)

While not an absolute necessity, many people feed the cutter straight in, most use a 30 (+/-) degree feed to keep the working face of the cutter working on one side. There are several considerations for threading, not the least being to never stop rotation with the cutter engaged. Usually a groove cut at the nominal depth of the threads as a place to allow the spindle to "run down" after the pass. This is particularly important if you use carbide tooling, the tip breaks easily.

Keep in mind, for my purposes, the depth of cut is 0.002-0.005 thou. More _might_ be appropriate for larger threads, I cut small screws. 1/4-20 to me is large. I do cut larger threads, but it's rare. Much of my work is in the range of 2-56. A final finish pass to clean things up when you have the proper size. A good learning tool is to cut to a nut or a die to get a feel for the last couple of passes. 

There are many people more appropriate than me to answer questions on threading. It is a subject as complex as the lathe itself. One thing I have found, again my_ personal_ perspective, is that 130 RPM is the high end of speed for threading. That's why I built a 3:1 reduction gear long years ago. Larger machines have a system of back gears to this end, the smaller machines have other methods for keeping speeds down. A very common version from years back was to use the hand rotation. The ultimate to me was a small steering wheel bolted to the spindle pulley. That allowed threading even medium hard steel at a controlled pace. My recommendation for hand rotation was for more than plastics. I became part of that discussion around the time I built the reduction gear so I never pursued the idea. 

Once you have the machine working to (more or less) your satisfaction, it would probably generate more responses by starting another thread asking for input on threading. There are many different subjects about a lathe, literally the "queen of the shop". A good starting point for the novice is at 



 . Quinn has many videos on the various shop subjects. She starts here, with links to further expand your knowledge. I follow (loosly) her web site still. There are many details where I feel a need for other approaches than my own. 

.


----------



## better-lathe-than-never (May 15, 2022)

Bi11Hudson said:


> One thing I have found, again my_ personal_ perspective, is that 130 RPM is the high end of speed for threading. That's why I built a 3:1 reduction gear long years ago.


Bill - sounds like a nice solution for our lathes - can you share design drawing or at least a photo of what one looks like?


----------



## fitterman1 (May 15, 2022)

I think "better late than never" has chatter problems due to two issues.
First one is speed which can be rectified by either mechanically gearing down or changing the drive motor to dc or vsd control. Can be expensive but necessary.
Second issue is rigidity of crosslide and compound assemblies.
Are the gibs correctly adjusted?
How much compound slide overhang?
Cutting tool geometry may also rear its ugly head too.


----------



## Janderso (May 15, 2022)

Bi11Hudson said:


> Your interpretation is correct as far as I can tell. For a novice at threading, one must understand what threading actually is. I have a Grizzly G1550 which is *almost* identical to the Jet BD920, so I will present my thoughts from experience rather than theory. The lead screw is a 16 pitch, 16 TPI, screw. Therefore, if you want to cut a 16 TPI thread, the cutter must move 1/16 inch for one revolution of the spindle. The gear train is adjusted, gears changed as needed, to accomplish this as needed for the particular thread.
> 
> To cut metric threads, the gears are adjusted to move the lead screw in (sub)millimeters per revolution. This is accomplished by inserting a ratio of 127:120. The 127, a prime number, is the only conversion from imperial to metric. The 120 tooth gear could be a 100 tooth as well, although the 120 tooth allows divisions by 3. There are many gear combinations that yield "almost" metric threads, many to a factor of four decimal places. With the Modulus 1 gears, 25.4 DP, the 127 tooth gear is small enough to be usable.
> 
> ...


Bill is scary smart, articulate too.
For an old timer, you still got it!


----------



## Bi11Hudson (May 15, 2022)

My 'contraption' was built using the planetary gears from an automotive automatic(GM??). Essentially replacing the spindle pulley. . . It was posted for years on my web site, GoDaddy changed software a couple of years back and I am no longer able to (re)learn their system. I still have the PDF of the article and will one day post it to this site as an archive. Been waiting (hoping?) for an invite to do so. But mostly I'm not as 'functional' as I once was. The project was literally built from scrap, and most anyone with half a brain can eventually devise one once the seed is planted. Just takes a loonnngggg time.

Since I built the original there has been a commercially produced one, sufficiently different to avoid any patent/copyright issues. It replaces the clutch you had trouble with. I don't believe it is as strong as my design, but is commercially viable. Not sure where to get it, I just remember seeing one somewhere. The major difference is that I use gears as the final spindle drive, the commercial version uses the OEM(?) belt. I feel that gears are stronger at the lower speeds involved.

The article dates from around 2000, give or take a couple of years. It was posted to several sites at the time, may still be around somewhere. Our machines today are somewhat archaic, the newer versions have variable speed (DC) motors with high tech drives. My machine dates from the mid '90s, before the G1550 became the G4000. Your Jet is of the same class, although Jet kept the same design for longer  A treadmill motor was a common conversion, may still be. The problem with a DC controller is the torque falls off radically at half speed or lower. It doesn't matter so  much for threading, but there are other uses of slow speed where it does.

*EDIT*: An afterthought, some links to threading





 | Blondie Hacks





 | This Old Tony

I had one for Joe Pie but managed to overwrite it. . .
.


----------



## better-lathe-than-never (May 16, 2022)

Was finally able to do the threaded rod using the single-point-threading technique after all the mechanical gymnastics.   5/8'' at 18tpi - chased the final thread with a round die for good measure.   It's a good fit to the test base I made earlier.  Overall pretty pleased with the outcome.


----------



## fitterman1 (May 16, 2022)

Well done


----------



## Bi11Hudson (May 16, 2022)

better-lathe-than-never said:


> Bill - sounds like a nice solution for our lathes - can you share design drawing or at least a photo of what one looks like?


It took a while to find which archive drive had the file resident. There are a few photos at the end. It seems it is only 25 odd pages long, when I spoke of 40, it must have been for a different file. Feel free to pass this around, it is not copyrighted. The only restriction is that if the design is used for a commercial product, I want my cut. The PDF is an older version, there is no encryption. The contraption is not the most usable, especially for commercial work. But quite suitable for casual(hobby) work. The speeds listed in the chart are for the G1550, they may vary a small amount for other brands.

.


----------

