# Modular Pitch 1 Gears & Making a Worm to Suit



## Antman (Jan 24, 2010)

Hi Gearheads,
  The change gears on my lathe are given as having a Modular Pitch 1. Is this some standard for the shape and size of the gear teeth? If so, where do I find these specs? Having had some success in screw cutting in the lathe, I want to try something a little more ambitious, using these gears and a worm to make a vertical and horizontal indexing arrangement. I believe a worm uses the same tooth form as a rack. Would it be possible to grind a tool to match the gears and make a worm?
  I am hoping for some expert advice, and the encouragement to go for it.
  Thanks,
     Ant


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## John S (Jan 24, 2010)

Modular pitch is the metric version of DP which in turn is the imperial pitch.

If you are happy working to DP them 1 Mod = 25.4 DP

There are two common pressure angles for gears 14.5 and 20 degree DP uses both and a lot of change wheels are the earlier 14.5 standard because of grandfather rights, they have been making the gears for so long it hard to make the change.

Module doesn't suffer from this and most, being modern are 20 degrees.

Cutting a worm is exactly the same as cutting a thread with a straight sided tool, in fact an Acme thread is a dead ringer for a 14.5 gear IF the pitch is correct.

Mod is even easier to cut worms for because you need to cut a 1mm pitch thread, that's it.

John S.


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## Antman (Jan 24, 2010)

Whoa there John, either you are mistaken or the inscrutable Chinese who built the lathe and wrote the book have tossed us a red herring. Well, 1mm pitch gear teeth is kinda small and I know my gears are bigger.

 Decided to measure the gears approximately (where do you measure a gear, somewhere in the middle of a tooth? ) and it turns out that an 80 tooth gear has a diameter of something like 80mm. So 80 teeth on a circumference of

      pi X 80mm  (is this how mod gears are defined?)

 gives a pitch of (pi)mm or 22mm/7 , which should be easy with a 2mm leadscrew and 33T gear.


 Ant


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## Blogwitch (Jan 24, 2010)

Ant,

You will find that John is correct.

I have a largish Chinese machine that states it uses a mod 1.25 pitch for the change gears. I didn't believe it at first, until I checked it out with a 1.25 mod gear hob, and it definitely is as it says.

As shown in the first pic here

http://www.homemodelenginemachinist.com/index.php?topic=7954.msg83982#msg83982

If memory serves me correct, mine are a larger tooth form than the 20DP Myford change gear, which if calculated across I think works out to somewhere around mod 0.8.

Blogs


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## Jasonb (Jan 24, 2010)

Have to agree with Ant the pitch should be pi x Mod in this case 3.142x 1

A 1mm pitch worm would be too fine, just try offering a M6 thread to the sid eof your MOD 1 Gears and there is no way they will mesh.

Jason


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## Blogwitch (Jan 24, 2010)

I think things are getting a little crossed here.

John is perfectly correct in his interpretation between Mod and DP sizes, and is what I showed in my explantion of size of change gears.

I think the confusion arose in the way he described a method of making a worm wheel. I don't think that was a direct relationship to what was being discussed between the two different standards. Maybe John can sort that out.

I would just like to point out that John has many years in gearmaking, and I would be quite happy to know as much as what he has already forgotten.

Just to prove your point of explanation is correct, I have braved howling gales and hurricanes to take a couple of piccies in my shop.

These measurements are not spot on, just nearto's. The first pic shows the tooth pitch of the Mod 1 cutter and the second one the Mod 1.25.


Blogs


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## tel (Jan 24, 2010)

> If memory serves me correct, mine are a larger tooth form than the 20DP Myford change gear, which if calculated across I think works out to somewhere around mod 0.8.
> 
> Blogs



20 DP = 1.27 mod


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## John S (Jan 24, 2010)

Sorry my mistake, I was rushing out the door to go buy some collet chucks.
It is 1 x Pi = 3.142mm pitch

Should have reread my post before pressing send, anyway it proves some people [ not me ] are awake today. :big:


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## Blogwitch (Jan 24, 2010)

Tel



> 20 DP = 1.27 mod



I bow to your superior calculation.

A case of my gob being engaged before my brain

Bogs


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## Antman (Jan 24, 2010)

Hi Gearheads! 
  Ive been doin some web searching and thinking mostly, about positive things when it hit me, I know a definition when I see one. Dont you think its cool, an engineering standard, off the shelf, almost internationally used, is defined in terms of an irrational number.


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## tel (Jan 25, 2010)

I better 'fess up Bogs, t'weren't my superior calculation, but that of this handy little program I got on here.


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## ianjkirby (Jan 25, 2010)

Antman  said:
			
		

> Hi Gearheads!
> Dont you think its cool, an engineering standard, off the shelf, almost internationally used, is defined in terms of an irrational number.



Hi Antman,

 Its purely coincidental that an irrational number is used in a standard. Keep in mind that even an irrational number may be resolved to an unachievable level of accuracy.

 If I may add to the detail of the discussion a bit;

 DP stands for "Diametral Pitch", and is defined as the number of teeth divided by the pitch diameter of the gear, usually measured in inches. Thus, a 20 DP gear of 20 teeth is 1" pitch diameter, which is measured at approximately half tooth depth. A 20 DP gear of 40 teeth is 2" pitch diameter, and so on.

 The definition of "Module" gears is the inverse of DP, ie the module size is defined as the pitch diameter divided by the number of teeth, where the pitch diameter is measured in mm, and the resultant answer for Module has the units of mm.

 Thus module gears are inherently metric, and DP gears are inherently imperial. They may be related by the expression DP =1/M, provided that a factor of 25.4 is appropriately applied to make the outcome "neat"! It is most unlikely that DP and module gears can be made to run together, unless one of them involves a 127 tooth gear, and this is why this gear is necessary to cut metric gears on an imperial leadscrew, or vice versa.

Regards, Ian.


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## John S (Jan 26, 2010)

ianjkirby  said:
			
		

> Hi Antman,
> 
> 
> 
> ...



You are getting sided tracked here in the making or use of them as opposed to the design of them.

A 1 Mod gear is the same as a 25.4 DP gear, there is no difference, true a 25.4 DP gear doesn't exist in theory but in practice you can fudge any of the calculations to finish up with a gear for a special application.

If you look inside a modern car gearbox it's more than likely there are no standard gears because the designers have altered the calculations to give stronger wider teeth or suchlike to increase strength / reduce noise etc.

Twp practical examples are early Harrison / Colchester lathes that used 16 DP gears, later ones used 1.5 Mod which is 16.9 DP making it hard to spot the difference.

Micron gear hobbers use 1.25 Mod but there are plenty of them out there using Myford change wheels as a stop gap for missing ratios, the Myford works out to 1.27 Mod which in practice isn't noticable.

John S.


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## Jasonb (Jan 27, 2010)

And if you can live with a small error 8tpi ( in Metric) comes very close to Pi 3.175mm vs. 3.142 a difference of 0.0013" so this could be used to cut your MOD 1 "hob"

Antman one other thing that has not been mentioned from your original post is that although worm & wheels can be had in MOD sizes the wheel is cut at an angle not straight accross like your existing gears so won't mesh with a worm. This angle will change depending on the number of starts on the worm.

Jason


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## Antman (Jan 30, 2010)

Thanks all who replied to my thread.  Jason, I did start thinking along those lines, that the worm threads are at a helix angle, and now I have a clearer mental image about what I think will get me a worm to mesh with spur mod1 gears.

My lathe is metric and has a 2mm leadscrew, so a screw with a pitch of (pi)mm is easy using 66T and 42T gears keyed together (pi is a fleas whisker from 22/7 ). I am very new to machining having only started August last year so am not using small imperial sizes of fastener yet, though when I do I hope to go metric, so dont use inches for anything.

But if I skew the worm out of the plane of the spur gear

If I had enough gears in my box of gears I could make a pitch of 3.15mm ( 315 = 5 x 7 x9 ) on a diameter of about 40 mm with a helix of around 4 degrees and skew it by the same amount. 22/7 is doable and close.

If someone can suggest a drive train for 3.15mm, Id be eternally grateful for that 0.01mm. Ha ha. Lathe has 2mm leadsrew, 40T spindle gear.

I have some gunmetal tube stock, OD 40mm , ID 17mm 

       Ant


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