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Pfauter P900 Differential Formula

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cnctim_s

Plastic
Joined
Apr 29, 2021
Hey guys,

It has come to a pretty grim time for us, the operator who usually setup and run our Pfauter machines from the 60s has tested positive for Covid-19 and is in a very bad state right now - he's retiring in February so he´s most likely not coming back to work when he gets better.

I have run the machines before when we just did cylindrical spur gears so I have a basic understanding of how to operate the machine.

The boss asked me to atleast give it a shot at setting up the machine for a gear with a helix angle of 15 degrees.

I can´t seem to wrap my head around how to calculate what gears to add to the "differential feed".

My boss told me that the old operator used to have a Pfauter manual with alot of Index & Differential gear setups, however - this manual was accidentally sent off with a machine we sold.

Anyone out there who could point me in the right direction to correctly calculate this type of setup?

I have the machine constant for the differential which is - 8.952466

I have found a formula that was written by hand on a piece of paper which looks like this:

(8.952466 x Sin11Degrees) / Module 6.5 which results in 0.262801699 which he has separated like this:

0.262 | 801 | 699

How do I calculate what change gears to choose for this formula? I´m guessing the separation is to to be able to calculate a gear set which comes in as close as possible?

I hope I didn´t bombard you all with this question - which is my first post aswell.
Have a great evening you all,
cnctim_s
 
I suspected as much, I’ll reach out to the company who bought the machine from us! Thanks for the heads-up!
 
Screwmachine said:
Have you tried contacting Gleason? You never know, they might have the literature (they absorbed Pfauter).
Click to expand...
You can afford to do that, you live in a place with health insurance. No idea what cardiac arrest goes for in the US these days but I know I can't afford it.

Gleason likes their stuff ... a lot. There's some dealers in Germany who might be better. These guys have been around a long time, sell big stuff : Teco Germany | Werkzeugmaschinen GmbH

Or even fleabay. P900 is fairly common, someone should be willing to make a copy for a couple hundred dollars.

It's worth the effort to get the manual back. It'll also tell you about stuff like hob shift (Pfauter had an option for continuous) and all the places for lube and how to adjust the worms for backlash and other stuff you really need.
 
We had a Module brand German made hob.It had a differential and hob shift,verticle slide 14" face and 14" diameter capacity.The only thing I have left is the manuals about 4 of them. I hated to throw them out but when I go they probably get trashed. I was hoping that some one could use them and they are available for the cost of shipping. The chapters on set up and calculations would probably apply to most similar machines just the controls would be different.
 
Given your above 8.952466 Diff constant- it appears the formula for your differential gear ratio is the following (8.952466 x sin(helix angle)) / normal Module. In your example the resulting ratio is 0.26801. You can calculate the change gears to produce this
in a variety of ways, but is normally a 4 gear ratio of (driver/driven)x (driver/driven) with one driver and one driven keyed together on a common shaft. There are 4 gear ratio calculators on line or various numerical methods to do this, but the actual ratio you arrive at must be accurate to the 4th or 5th decimal place. We have several books of gear ratio's that make it very easy to determine the change gears for a given ratio,(the best on by far was published by Pfauter). I believe I have the diff constant for a P900 at the shop and will report back. Also, there should be lever in the change gear compartment to engage and disengage the differential mechanism. Further, depending on your gear to be cut changing from a left hand helix to a right hand helix will require the insertion of an idler gear to the diff gear train. Last thing, and I'm not 100% certain on this but engaging the differential may double or cut you index ratio in half, so be aware.
 
Dan from Oakland said:
Given your above 8.952466 Diff constant- it appears the formula for your differential gear ratio is the following (8.952466 x sin(helix angle)) / normal Module. In your example the resulting ratio is 0.26801. You can calculate the change gears to produce this
in a variety of ways, but is normally a 4 gear ratio of (driver/driven)x (driver/driven) with one driver and one driven keyed together on a common shaft. There are 4 gear ratio calculators on line or various numerical methods to do this, but the actual ratio you arrive at must be accurate to the 4th or 5th decimal place. We have several books of gear ratio's that make it very easy to determine the change gears for a given ratio,(the best on by far was published by Pfauter). I believe I have the diff constant for a P900 at the shop and will report back. Also, there should be lever in the change gear compartment to engage and disengage the differential mechanism. Further, depending on your gear to be cut changing from a left hand helix to a right hand helix will require the insertion of an idler gear to the diff gear train. Last thing, and I'm not 100% certain on this but engaging the differential may double or cut you index ratio in half, so be aware.
Click to expand...

Thank you very much for the answer! I posted on my Instagram asking if someone had a manual I could purchase or look at to get a feel for the formula.

Turns out people were more than helpful and I have 2 guys making copies of both the manual and also a book called "Change Gear Tables" by Friedrich Werner Becher which supposedly has all the info I would need.

Love it when people share knowledge, so if anybody else reads this thread and is wondering about something similiar - hit me up and I'll help out the best I can!

Have a great weekend everyone,
cnctim_s
 
cnctim_s said:
... also a book called "Change Gear Tables" by Friedrich Werner Becher which supposedly has all the info I would need.
Click to expand...
The Pfauter book is huge, it has like 100,000 four-gear ratios. Even the Barber-Colman one was 10,000 ratios and an inch thick, nice guy to copy that for you :)

Anyway, when you get to the point of choosing a ratio, it won't match exactly. They never do. Pick the one that's going to make the helix tighter - gears always unwind in heat treat. In fact, if you are doing a batch it's best to do one first, heat treat it, measure it after, then compensate for the change in your helix settings.

The other thing is, with a diff you can use the hobber as a cumbersome lead tester. Figure out what you think the helix is, install the diff gears to match, mount a lever-tpe indicator on the hob head with the point about in the middle of a tooth and run the slide up and down. By measuring how much you are off in how much travel, you can zoom in on the exact helix.
 
EmanuelGoldstein said:
The Pfauter book is huge, it has like 100,000 four-gear ratios. Even the Barber-Colman one was 10,000 ratios and an inch thick, nice guy to copy that for you :)

Anyway, when you get to the point of choosing a ratio, it won't match exactly. They never do. Pick the one that's going to make the helix tighter - gears always unwind in heat treat. In fact, if you are doing a batch it's best to do one first, heat treat it, measure it after, then compensate for the change in your helix settings.

The other thing is, with a diff you can use the hobber as a cumbersome lead tester. Figure out what you think the helix is, install the diff gears to match, mount a lever-tpe indicator on the hob head with the point about in the middle of a tooth and run the slide up and down. By measuring how much you are off in how much travel, you can zoom in on the exact helix.
Click to expand...

I was wrong on the title of the book, the title is "Pfauter Wechselräder Tabellen von F. Becher und A. Körner"

It has alot of amazing stuff in it, and I just received the hardcopy by mail.

If anyone ever needs any help or needs a page from the book, don't hessitate to contact me!

Thanks for the helpful comment about zooming in on helix angles, this will surely come handy some day!

Have a great day,
cnctim_s
 
What blows my mind is how helpful everyone is regarding gearhobbing - seems like most people want to pass on the knowledge since there's not alot of people out there who wants to learn it.

I get that it's a much more messy trade than CNC-machining but damn it, it's really fun and interesting!

Have a good one,
cnctim_s
 
I have a Pfauter p900 manual (I have no experience operating the machine, though). I think this manual is from a different era or is for a variation of the p900 you have.
My manual is for hobbing diametral pitch gears, so it has '25.4' written into the formula.
It shows the axial-differential constant as 75/4 x 25.4; if it were for module gears (I believe), it would be 75/4.
I think your axial-differential constant is 225/8. After division by pi, it is 8.9524655489191; this agrees with your number.
8.9524655489191 x sin(11°) / 6.5 mod = 0.26280168577421 again agreeing with yours.

171651985.png


These are the change gears you may be looking for, selecting from 20-97 tooth gears. I would be interested to see how these match up with the gears from your new book.

Also, does your Pfauter have a tangential differential constant? My book shows 1/4 for it (referring to the hobber connected to this book). 0.079577 after division by pi. This is for auto hob shifting. This may have been an option and yours may not have it.

And another thing to watch for is multiple index constants:
171651992.png

This shows 16, 32 and 48 depending on gears e:f and e:f gears depend on the number of teeth being cut (and the number of starts in the hob). This could have been an option, too.

I went back and squeezed the change-gear selection to the midrange. This would have the same error as above, but I entered the decimal number in a less rounded form.

171652798.png
 
Last edited:
Index gears always need to be exact ratios. Do not use an approximation as it will introduce often times significant lead errors. Learn how to factor your whole number fractions to arrive at your change gear tooth numbers. Since index ratios are always the result of whole numbers, this is not that difficult.
 
Oops!

I went back and checked the manual. Pfauter wasn't as generous with the supplied change-gears as I thought. Instead of 20 through 97, it is as shown by the checkmarks below.

171652889.png
 
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