# SM1340 Change Gears



## RobinHood (Mar 31, 2017)

My Standard Modern 13X40 lathe is all metric. I want to be able to cut inch threads as well. It did not come with the change gears. Called SM to see if they are available: YES. Great, how much? $$$$$$$$s! OK thanks, I'll pass for that price. Off I went to make my own. This time I had my mill and universal indexer. So I made a few gears, the most important one being 127T transposing gear because it is the lowest common denominator for the 254mm/1in ratio. I have enough room on my lathe for such a big gear as long as my change gears were all DP24s , 14.5 PA.

Oh, wait, I have no arbour for my gear cutter; so I made one out of an old lawn mover spindle



 

I used mild steel plate, 1/2 thick, and rough cut all the gear blanks with an appropriately sized hole saw and turned to size after. Except for the 127T one, which I rough cut on the band saw into an octagon shape and turned to size on the lathe.

My first gear was the 45T in the set - if I mess up, then he would be the least amount of work to redo.



 

Here is another view



 

And here is the final product with some of the calculations I did to create it



 

As Tom Lipton says: "Rinse and Repeat" for the others. The 127T gear is special since 127 is a prime number and there is not way for either "direct" or "indirect" division of the 360* circle. But since I have an Universal Dividing Head with all its change gears, I was able to use the "differential indexing" method to divide the 360* circle into 127 equal parts.

Here is the set-up



 

And a view of the dividing head's change gears. In a nutshell here is how "Differential Indexing" works mechanically: the indexing plate is on a shaft which is linked via the change gears to the dividing head's spindle. As you rotate the indexing lever the appropriate # holes along the indexing plate, the dividing head spindle moves. Also - through the gears on the back - the shaft upon which the indexing plate sits moves; therefore the indexing plate moves - hence the term "differential" (that is what's different from "direct' or "indirect" indexing). Kind of hard to describe; easier to grasp when you see it in action and quite a clever thing. Kudos to the engineer that came up with it!



 

After many hours of cutting, here they are. I ran them on my tool & cutter grinder for a nice surface finish - because I had no surface grinder at the time. All were engraved with the respective # teeth on the pantograph.

A close-up of the surface finish



 

Here is the set-up for the engraving process using the Deckel Pantograph



 

And all done



 

And two in use on the SM1340 (here they were not surface ground or engraved yet).


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## PeterT (Mar 31, 2017)

Very impressive!
Now I'm intrigued - legend has it that Asia lathes like my Taiwan 14x40 & similar flavors were 'heavily based on' (some say made casting patterns for) Standard Moderns. Can we see some pics of the lathe sometime? I've never actually seen them up close personally. 

So did the threading lead screw & half nuts also have to be changed out to cut IMP threads? I've never totally understood this, if it can be dealt with in change gears alone, a function of the lathe, or only certain pitches available, or some are close but not exact...


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## RobinHood (Apr 1, 2017)

I have heard that as well; there are certainly a lot of similarities...

Here is one after we just got it home, still on the trailer



 

A close up of the business end



 

Taper attachment and Acu-Rite DRO scales



 

After some cleaning...



 

Here she is, all done, including a little goose neck LED work light (the pic is really too close, but that's all I have right now)



 

*
Metric/Inch conversion (btw, one can also go Inch/Metric, the principle is the same, the ratios change)
*
The lead screw, half nut and the feed box all stay the same.
Below is a table from the manual that gives me the change gear "B" (the ones I made) I have to mesh with the transposing gear "A" (the 127T one I made) and the feed box setting in order to cut the required inch thread.





This table could be expanded to allow for Module and Diametral pitches as well - just would need to make more change gears "B".

As far as accuracy of the threads: they are theoretically 100% accurate because of the 127T transposing gear. There are exactly 254mm in 1 inch or 127mm in 0.5in. The 127T gear runs on the same shaft coupled to the 48T gear (see diagram above), so they both turn at the same rpm. Any time you have two gears on the same shaft coupled, you create/change the ratio of the whole gear train. Any idler gears in a gear train do not have any influence on the overall ratio. In the diagram above, the 45T and 60T idler gears are just "space fillers" and could be replaced with a chain and the ratio of the gear train would stay exactly the same. The change gear "B" sits on the input shaft to the feed box and will mesh with the change gear "A". The overall ratio of the whole gear train is determined by the 24T head stock gear, the coaxial 48T/127T transposing gear, and the feed box input shaft gear (aka change gear "B").

Ignoring the 127T transposing gear for a moment, the basic gear train ratio is a follows 24T (headstock) to 45T (idler) to 60T (idler) to 48T (feed box). This can can be simplified to 24T/48T = 0.5. When we put the 127T gear into the set, on the same shaft as the other 48T grear (see diagram), we change the ratio to 127/0.5 = 254 ===> how convenient, there are 254mm/1in.

They did the rest of the math for us, phew, and gave us a nice little table, see above...

Sorry, this got a little long. It's much easier to understand when you see it in action.

So as long as my metric lead screw does not have any/too much wear, any length of SAE thread can be cut accurately.

I do have to keep the half nut engaged at all times, when I cut Inch threads. It's like on a SAE lathe cutting metric threads: leave the half nut engaged and use spindle fwd/rew to move the tool - if one goes by the book.... But I don't...

Actually, I don't leave them engaged in either case: when I thread SAE, I open the half nut lever at the end of the cut and watch how far the thread dial moves after I stop the lathe (I don't have a brake or clutch on this machine, so the spindle rolls over a few revs before it comes to rest). I reverse the lathe and re-engage *when the same mark on the thread dial comes by *(that is important!). Joe Pieczynski on YouTube has a great explanation of the process. I also always try to thread away from a shoulder if at all possible (left or right hand thread). Again, see Joe Pie's video.

Hope that helps explain things a bit.


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## Alexander (Apr 1, 2017)

Great post. I only ever tried diferential indexing at sait during my apprenticeship. It is time consuming to set up and tricky to hit exactly the right whole all 127 times. You must be very tallented. Great explination of how it all works.


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## PeterT (Apr 1, 2017)

Great explanation on the threading gears. And nice lathe recondition. Well, I'd say 'some' things are similar to mine, but a lot of things are quite different & (yours) looks beefier in many respects.
- re taper turning, what's the procedure or underlying mechanism that disengages the cross slide nut / lead screw assembly? I've heard some you unscrew a nut & others have a telescoping screw.
- looks like DRO encoder on RH of carriage? If so, how do you lock the cross slide?
- what is the circular looking housing on RHS of apron? I thought maybe threading indictor gear assembly, but it looks more substantial in the pic


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## RobinHood (Apr 1, 2017)

Thanks Alexander for the compliment.

I knew this was going to be the critical gear, so I left it almost until the very end. I wanted to get to know my Universal Dividing Head a bit first. I then went through all the 127 cycles and did not cut anything - just marked each position on the gear blank as best as I could with a Sharpie. Inspected the blank to make sure I did not mess up and only then cut the actual teeth the next day. It's repetitive work and one has to concentrate for sure. I figured if machinist did it in the past in a production setting, I should have a fair chance at getting it right in a more relaxed, hobby environment.


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## RobinHood (Apr 1, 2017)

1) Taper Turning (see picture below)

The picture shows the whole taper attachment removed off the rear of the saddle (it is normally bolted rigidly to it). You can see the cross slide screw. Note on the very right of the screw there are splines that telescope in and out of the dial/handle assembly. Moving left, you see the lead screw enter the taper attachment pin; inside this pin is where the lead screw's thrust and support bearings sits. The lead screw is captured axially in this pin and always moves with it. The hex bolt sticking out of the taper attachment body is used to lock/unlock the taper attachment pin-lead screw assembly to the taper attachment body. The cross slide usually covers the lead screw and part of the taper attachment body. The cross slide nut engages on the lead screw.

During normal turning operations, you tighten the hex bolt and the pin-lead screw assembly does not move axially. The cross slide wheel turns the lead screw and the nut follows it, moving the cross slide. The taper attachment slide plate (the part that has the angle scales on it in the pic) is also left unlocked and the whole thing just follows the carriage parallel to the ways.

When you want to turn a taper, you unlock the hex bolt. You lock the taper attachment slide plate to the lathe ways. You set the angle of the taper using the taper bar. You can see it sitting on top of the slide plate; it's the long slender piece. It is bolted to the slide plate with two bolts, one in each T-slot. Underneath the taper attachment pin you can see a rectangular block - that is the taper bar follower. It is mechanically attached to the pin. As the carriage moves, the taper attachment slide plate remains stationary and so does the taper bar. But since the taper bar is at an angle w.r.t. the lathe ways, the taper bar follower moves along as well as axially and takes the pin-lead screw assembly with it. Since the lead screw can telescope in the dial/handle assembly, the cross slide moves with the same angle as the taper bar is set at. Any in-feed is done as per usual using the hand wheel.



 

Cross Slide Dial/Handle assembly.

The cross slide lead screw splines fit inside the shaft extending out from the gear. That assembly always remains axially stationary and is fixed to the carriage.



 

2) The DRO encoder is on the right side of the cross slide. I do not have a cross slide lock on this lathe. I would have to remove the scale and tighten up the gib strip to lock it if I ever needed to.

3) The circular/cylindrical attachment is indeed the thread dial. It has a number of gears in it that are stacked vertically. Depending which metric thread you cut, you engage the correct gear by moving the whole unit up or down on the carriage. There is a table in the manual that tells you which position is needed for which thread.


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## PeterT (Apr 1, 2017)

Thanks for detail & pics of the innards. I (foolishly) did not get taper attachment on my King 14x40 at the time, just naively assuming it could be purchased later & bolted on. But that's not the case. The taper attachment version has a different lead screw, nut assembly & possibly the cross slide casting itself. I'm not sure its if its same as yours, but I suspect similar disengagement procedure. Seems like majority of new Asian lathes have taper attachment integrated right off the bat. I've had my cross slide apart & think I could retrofit it, but it wouldn't be trivial. I'd consider doing an upgrade if say the screw assembly wore out. OTOH by then might be worth considering an 'upgrade'.


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## RobinHood (Apr 1, 2017)

RobinHood said:


> There are exactly 254mm in 1 inch or 127mm in 0.5in.


Not really: I had a brain fart. Should of course be 25.4mm in 1 inch or 12.7mm in 0.5in.

And same mistake again here:


RobinHood said:


> how convenient, there are 254mm/1in.


Should again be 25.4mm/1in. That is a factor 10 times less than 127/0.5 = 254. That factor 10 is made up in the feed box to make it all work.

Sorry for any confusion!


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