Diametral Pitch change gears for a South Bend Lathe

[steamer]

Hi,

I'm off for a few days from work...and I wanted something that would let me putter I the shop, but still make progress on project I have been drawing up.

Like Steve and Arnold, I have a vise mounted dividing head that I am drawing up, based loosely on the GHT versatile dividing head.   Changes will include a center height change to suite a South Bend 9, and the incorporation of a 1 1/2-8 thread, and direct mounting 3C collets, so I can use my lathe spindle tooling.

Here's a couple of pictures of what I have laid out so far





You will notice that the worm gear appears to be a spur gear...and it is.  A 60 tooth SB change gear specifically.
To make a worm that will engage it, the lead must have a linear pitch, that matches the circular pitch of the gear.   Additionally, the worm will need to be mounted on the helix angle to match so that it engages the gear.   ( 3.14 degrees in this case.)

Now to do this, I need to set the lathe to cut the appropriate linear pitch.   If you read Ivan Law's book, we need to calculate the     

linear pitch = 3.14159  /  Diametral Pitch

3.14159/18 = .1745 inches.   Or 5.729 threads per inch.!

To do this we need to add a pair of gears to the train, with a ratio of PI or Pi/2....    ( 3.14159  or 1.5707)

For convenience,   I'll be using Pi/2.    If I put that in the train, and set the box to turn 9 threads/inch, Bingo!.......9 x 3.14159 /2 = 5.729 threads per inch!......


But it's not quite that simple.........I'll type more!

Dave

[steamer]

OK....now it gets weird........


The input gears to a SB 9 are a 20/56.  That is a 2.800 to 1.    Odd aint it?!     They gear it down to gear it back up.

So this ratio needs to be compensated for ....and I thank Kitno45 over on PM for this one.

So the gearbox has a inherent ratio of 2.800 to one.    We need to get it to be 1.5707.

If we take out the 20/56 gears shown below and replace them with something else...we should be there.   There is an 80 tooth idler in between.  This idler has no affect on the ratio. 

 

OK so 2.800 x number = 1.5707......that's the question.    What do I replace the 20/56 gears with that will give me that?

Well the number  or ratio that gives me that is 1.5707/2.800 = .5609...or inversely.....1.782.

Well.....that is very close to a 23/41  of .56097.

So I need a 23 and 41 tooth pair to replace the 20 and 56.    Once I do that, the gear box is set to cut 9 in the usual way...and I'm there!.

Additionally, it will cut for 20dp  set to 10....and 16 DP when set to 8....ect.

Next we calculate the dimensions for the gears.

Dave




 

[cfellows]

I suspect that you could get away with 6 threads per inch and wouldn't really notice the difference.

Chuck

[steamer]

Hey Chuck

You would be off about .010" per tooth....not a lot...but not insignificant...and as I'm going to use this to index with....I can make 2 gears and get it right on.

OK

The SB is 18 DP 14 1/2 degree pressure angle.
They are 3/8 wide, and have a 0.5625" bore.

The OD is given by  OD=  N+2/DP   

N=23......OD = 25/18 or 1.3888

N = 41.....OD = 43/18 or 2.38888

The depth of cut is .120"...
The cutter for the 23 tooth should be a number 5, and the 41 should be a number 3.

Here's a picture of the blanks turned oversize, a stub mandrel to turn the OD and cut the teeth on,  along with the 60 tooth gear I will be using to index with.



I'm using 12L14 steel for these....more than adequate enough for the application.

I have a #3 cutter, but Not the #5....though I have a #6 , and I'll be using that as I know the other gears in the train are 62 years old, are somewhat worn as a result and I suspect not that particular about a little tooth form variation....

Dave

[steamer]

Ok.....a DOOOOOOH moment

Checking the top row of the threading plate she'll cut 5 3/4!
That will be an error of about. 00067" /tooth!
That"ll do.....I'll put these blanks away regardless....just in case.

Dave

[Don1966]

Looking good Dave, do you think you will be able to finish it in a week? Oh! By the way I have an extra worm spindle bracket casting if you want it. I can drop it in the mail Monday.

Don

[steamer]

Thanks Don, but I think I'll need to make this one.    It won't be done this weekend...and I don't need to get it done...I'm just going to relax...do what I feel like doing....if I don't feel it....I'm not doing it.   I intend to spend some time at the RC track with my son tomorrow....then maybe some shop time.

Dave

[Kim]

Fascinating Dave...  I'd have never thought to make your own set of gears for this.
I'll be watching and learning as you make progress on your relaxing project

Have a great time at the track tomorrow!  These are the best of times! And spin some cookies for me! 
Kim

[steamer]

Thanks Kim!.....I'll just try the 5.75 threads per inch.    See how it goes...maybe I'll try a sample worm and see.

Dave

[Anthony]

Thanks Kim!.....I'll just try the 5.75 threads per inch.    See how it goes...maybe I'll try a sample worm and see.

Dave

Dave,

I happened on this thread via the screwcutting clutch thread. I expect you've already cut your thread for engagement with the 18 DP gear but, if you can collect the gears, you might try:
28 stud driving 23 followed by 20 driving 31 followed by 20 driving 56 on the gearbox, select 4.5 TPI on the gearbox. Requires two change gear bushings.

Precision is better than 2 parts in 100,000. 20/23 x 28/31 = 0.785413744, divide by pi = 0.250004959, divide by 0.25 = 1.000019839. Method is select TPI value 1/4 that of DP value, in this case select 4.5 TPI for cutting 18 DP thread.

Hope this is useful, either now or in the future.

Anthony

[ncollar]

Dave
I know this is a very old topic but one question. Did you do anything with the dividing head set up and if so did you do a thread on it?
Nelson Collar

[steamer]

I got as far as cutting some stock, and then RC racing kicked in full bore......I'll get back to it....

Dave

[smithdoor]

In a pinch just use 5  3/4 TPI
This is only 0.0036" off 5.729
Save time and money making or buying the change gears
Change gears are also off by 0.00125"

Dave

Hi,

I'm off for a few days from work...and I wanted something that would let me putter I the shop, but still make progress on project I have been drawing up.

Like Steve and Arnold, I have a vise mounted dividing head that I am drawing up, based loosely on the GHT versatile dividing head.   Changes will include a center height change to suite a South Bend 9, and the incorporation of a 1 1/2-8 thread, and direct mounting 3C collets, so I can use my lathe spindle tooling.

Here's a couple of pictures of what I have laid out so far





You will notice that the worm gear appears to be a spur gear...and it is.  A 60 tooth SB change gear specifically.
To make a worm that will engage it, the lead must have a linear pitch, that matches the circular pitch of the gear.   Additionally, the worm will need to be mounted on the helix angle to match so that it engages the gear.   ( 3.14 degrees in this case.)

Now to do this, I need to set the lathe to cut the appropriate linear pitch.   If you read Ivan Law's book, we need to calculate the     

linear pitch = 3.14159  /  Diametral Pitch

3.14159/18 = .1745 inches.   Or 5.729 threads per inch.!

To do this we need to add a pair of gears to the train, with a ratio of PI or Pi/2....    ( 3.14159  or 1.5707)

For convenience,   I'll be using Pi/2.    If I put that in the train, and set the box to turn 9 threads/inch, Bingo!.......9 x 3.14159 /2 = 5.729 threads per inch!......


But it's not quite that simple.........I'll type more!

Dave

[steamer]

smithdoor

See earlier post....

Dave