# How necessary is having modular / Diametric pitch capability on your lathe...



## ArmyDoc (Dec 22, 2020)

If you don't have it on your lathe, can you still make a worm gear, gear hob etc?


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## Jim F (Dec 22, 2020)

Isn't that a cutting tool parameter ?


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## ArmyDoc (Dec 22, 2020)

I don't know.  I'm trying to decide on a lathe, the ones with universal gear boxes have inch, metric and Modular and diametric pitch settings.  I'd never heard of the latter two, and when I looked it up it said it was used with making worm gears.  I'd seen a video on making gears, but it was on a horizontal milling machine.  I found another that was making a worm gear on a lathe, but he used a tap as the gear hob.  Final video I saw made a hob on a lathe, but didn't say what the settings were.  It's a case of not even knowing what I don't know, and not knowing where to start...   

Lathes with universal gear boxes cost 50% to 100% more... hard to know if that's worth it, since I don't know how/when you use the modular/DP settings, other than apparently they are used when making gears.


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## Jim F (Dec 22, 2020)

My understanding is Modular are metric gears tooth shapes and DP are Imperial.


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## ArmyDoc (Dec 22, 2020)

Jim F said:


> My understanding is Modular are metric gears tooth shapes and DP are Imperial.


Not so much shape as pich.  Modular are measured in milimeters and Diametrical are measured in inches.  Both are related to pi...   But I don't know how they are used, or if there are alternatives if your machine doesn have those settings.


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## benmychree (Dec 22, 2020)

Module and diametral pitches can be accomplished on ordinary lathes that can accomodate change gears (there are lathes that cannot mount change gears)  I made a set of change gears for my 19" Regal LeBlond lathe that creates the capability of cutting metric pitches and diametral pitch leads, I have not cut any DP leads so far, but do occasionally use the metric pitches.  The necessary tables of change gears came from a LeBlond manual.


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## savarin (Dec 22, 2020)

This would make an awesome "How to" video for someone who knows "How to" make these along with easy descriptions of all the terminology.


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## Lo-Fi (Dec 22, 2020)

Module and diametral pitch are terms to describe the size and spacing of gear teeth, with dp usually describing gears of imperial measurement and module metric, though is entirely possible to express a module as dp or dp as module.

Pitches for threads, which is what you're going to be using a lathe for, are described using the number of teeth per inch - _similar_ to diametral pitch for gear teeth - for imperial threads, with metric being specified by how much one turn advances the thread, or "lead". 

With a change gear lathe, it's possible to make any thread you can imagine regardless of the leadscrew pitch, providing you have or are prepared to make the required gears. With a thread cutting gearbox, you often have some options for cutting threads of the type that your leadscrew isn't, though metrics on a lathe with an imperial leadscrew are often approximations only, and vice versa. The smallest gear that can correctly convert inches to mm is 127 teeth - quite a thing to fit in a small lathe. There are some smaller gear combinations that result in a ratio close to 1.27:1, and these are often used, but 127 being a prime, nothing spot on. Mostly the inaccuracy is academic for hobby purposes.

We're in odd positions either side of the pond with standards and what equipment is available. Here, most work is metric, though most old machines available are imperial. I'm so glad my mill is metric - I'm no great fan of the (sorry) imperial system - but my lathe is imperial and that needs to change as I work in metric. In the US, you're still using inches in a largely metric world, but most machinery is still set up for inches. I guess it depends what you want to do?


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## benmychree (Dec 22, 2020)

I also have a change gear device for my B&S milling machine that can divide table movements in diametral pitch and circular pitch, it was made mainly for spacing gear rack teeth, but I have also used it to space graduations on several brass blacksmith's rules that I have made. this can also be done with a universal dividing head geared to the table screw, but it takes a lot of cranking; 10 turns of the crank for 1/16".  B&S in their book on milling and milling machines has a table on various spacing movements using combinations of change gears and crank movements.


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## Jim F (Dec 22, 2020)

Lo-Fi said:


> Module and diametral pitch are terms to describe the size and spacing of gear teeth, with dp usually describing gears of imperial measurement and module metric, though is entirely possible to express a module as dp or dp as module.
> 
> Pitches for threads, which is what you're going to be using a lathe for, are described using the number of teeth per inch - _similar_ to diametral pitch for gear teeth - for imperial threads, with metric being specified by how much one turn advances the thread, or "lead".
> 
> ...


To add a bit to your post, a 127/100 transposing gear for a SB9 and 10k, and probably the Atlas/Craftsman is 7" dia.
You have to remove the gear cover to use.
This is mine that I had 3D printed.


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## RJSakowski (Dec 22, 2020)

Circular pitch, p, of a gear is the distance between teeth along the pitch diameter and is related to the diametral pitch by p = π/d.p. Circular pitch of a gear is the equivalent of lead or pitch in screw threads.  

Diametral pitch is limited to whole numbers, and since the circular pitch, which is what would be of interest when cutting a worm gear, is π divided by a whole number, the change gear ratio must necessarily be an irrational number.  Gears by their very nature must have rational numbers which means that to cut worm gears on lathe with a conventional lead screw, an approximation must be used for the gear train.

Looking at my chart of all possible threads that can be cut on a Grizzly G0602, it appears that there are a number of diametral pitch worm gears that could be cut to better than .1% accuracy using the stock change gears.  If I add the 127 tooth transposing gear to the list, I would expect that a number of others would pop up.


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## benmychree (Dec 22, 2020)

In my experience, all the worm gears that I have encountered have bee circular pitch, easy to gear up on most any lathe to cut the worm; the gear is quite another matter, and nearly always, the gear is the part that wears out, not the worm.


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## ArmyDoc (Dec 23, 2020)

RJSakowski said:


> Looking at my chart of all possible threads that can be cut on a Grizzly G0602, it appears that there are a number of diametral pitch worm gears that could be cut to better than .1% accuracy using the stock change gears.  If I add the 127 tooth transposing gear to the list, I would expect that a number of others would pop up.



Ok, so let me put it a different way.  If I were to purchase a lathe like the PM 1340GT or 1440GT, which do not have a universal gear box,  and I wanted to make something from scratch that was driven by a worm gear, could I do it?   It sounds like I wouldn't be able to make all the possible worms and gears out there, as I would if I had a universal gear box, but that I would likely be able to make some, or even several.



benmychree said:


> In my experience, all the worm gears that I have encountered have bee circular pitch, easy to gear up on most any lathe to cut the worm; the gear is quite another matter, and nearly always, the gear is the part that wears out, not the worm.



As I understand it, the gear would usually be cut with a hob, not directly on the lathe.  But the hob is made from the worm.  So, if you could make the worm, you could make it into a hob and then use that to cut the gear?


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## Lo-Fi (Dec 23, 2020)

If you're talking about worm wheel cutting, then yes, the tool is made the same way as the gear with some flutes and relief cut in. 

Yes, you'll be able to to it. Like anything threaded, what pitches you can achieve depend on the change gears and/or gearbox.


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## RJSakowski (Dec 23, 2020)

ArmyDoc said:


> Ok, so let me put it a different way.  If I were to purchase a lathe like the PM 1340GT or 1440GT, which do not have a universal gear box,  and I wanted to make something from scratch that was driven by a worm gear, could I do it?   It sounds like I wouldn't be able to make all the possible worms and gears out there, as I would if I had a universal gear box, but that I would likely be able to make some, or even several.
> 
> 
> 
> As I understand it, the gear would usually be cut with a hob, not directly on the lathe.  But the hob is made from the worm.  So, if you could make the worm, you could make it into a hob and then use that to cut the gear?


You should be able to cut any worm gear with a change gear lathe provided that you have the right combination of change gears.  It will not be possible to find an exact combination as can be don for cutting metric threads on a lathe with an Imperial lathe.  The inch is defined as exactly 25.4mm which can be achieved by the ratio of to whole numbers, typically 127/100 sd exact metric thread can be cut.

A ratio which contains pi as one of the multipliers can't ever be achieved by the ratio of two whole numbers.  However by choosing the right combination of gears in a change gear train, you can come up with an approximation that is close enough for all practical purposes. 

Different lathes use different strategies for achieving the change gear combinations to cut a particular thread.  The change gear set is chosen by the manufacturer to cut all the common threads within a particular range.  The change gear set for the G0602 provides 1280 possible combinations, not counting those if the 127 tooth transposing gear is used which would triple that number. 

The classic approximation of 22/7 for pi gives a ratio that differs from the true value by .04%.  This is easily attainable with common gear ratios, e.g., 44/14, 66/21, etc.  A closer approximation would be 201/64 which is accurate to .03%.   201/64 = 3*67/2*32 or 67/32 * 45/30.


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