compound dial help!

[BillC]

I'm trying to make a new dial for a lathe with a 8 tpi screw which will need 125 marks i think its impossible with a 40:1 dividing head. in the book "dividing" by Harold Hall he shows a set up with a 50 tooth gear for the indexing and a gear 75/30 to give a ratio of 15:6 and this will achive the 125 divisions to make the marks,,how does 75/30 make 15:6 to 1??,,also this dial has a O.D of 1.375 so 125 marks around that makes it hard to read will going bigger O.D. make a diff?? thanks in advance for all the help.

[garyphansen]

No help here just a question. Why not mark it at 250 divisions and make it direct reading? Gary P. Hansen

[BillC]

thats a good idea but I'm still with the problem of dividing 250 and will a bigger O.D. be a problem? thanks for your input Gary.

[Oldbrock]

you need a plate with a 125 hole circle. If there is room on the outside you could drill a 125 circle a la Gingery. Have had to do this twice in the last year and it works just fine. Peter

[garyphansen]

Well if I wanted to divide 150 or 250 divisions this is how I would do it. I lack a dividing head but that would be no problem. I would do it in my lathe, I would cut the biggest circle that would fit out of plywood and mount it on the back of my headstock spindle. Then I would divide a circle in a CAD program and print it out in a size that would fit the plywood circle and glue it on the plywood. A threading bit mounted sideways in the tool post makes a good scriber to mark the graduations. I will go looking for a photo. Gary P. Hansen

[garyphansen]

This is the only photo I could find that is related to my above post. In the photo I an scribing lines in a thread dial but the principle is the same. You could set up an indexable carriage stop to cut the short lines and the longer lines and make them stop at the same place. Gary P. Hansen

[djc]

I think its impossible with a 40:1 dividing head.

If you have a 25 hole plate for your head, it is possible by using 8 holes. See:

http://www.atmsite.org/contrib/JSAPP/divide/divhead.html

for how to calculate the figures, but briefly 40/125 = 2x2x2x5/5x5x5 = 2x2x2/5x5 = 8/25.

In the book "dividing" by Harold Hall he shows a set up with a 50 tooth gear for the indexing and a gear 75/30 to give a ratio of 15:6 and this will achive the 125 divisions to make the marks; how does 75/30 make 15:6 to 1?

The 50t gear and the 30t gear must be on the same shaft [driver] and keyed together (i.e. not be able to rotate independently). The 75t gear needs to be on the same shaft [driven] as the workpiece to be marked. The 30t gear needs to drive the 75t gear.

One full turn of the driver gives 30/75 = 2x3x5/3x5x5 = 2/5 of a turn of the driven. If you index every tooth of the 50t gear on the driver shaft, that gives 1/50 x 2/5 = (1/25x2) x 2/5 = 1/125 of a rotation of the driven shaft.

[BillC]

Wow, you guys are great, DJC, Gary thanks for the info, DJC you are right the 50 gear is the indexer with the 30 gear on the same shaft and that meshes with the 75 gear but does that come out to 15:6 ratio?? if so how forgive me if I seem like a math tard cuz i am,,, and will going to a bigger O.D. matter???.on the dial it self??

[Evan]

Is it the cross feed screw that is 8 tpi? If so I would replace it with a 10 tpi screw and then make the dial with 100 marks. I can't imagine trying to keep track of the number of 1.25 turns when feeding in and out on the cross. Bleh.

[garyphansen]

Evan a lot of larger lathes have 8 TPI in the cross slide. My 1909 14" LeBlond does. If he replaced the screw he would also need to replace the nut. Gary P. Hansen

[J. R. Williams]

125 divisions

Time to dig out your "Machinery's Handbook" and look up dividing. To obtain 125 divisions use a 39 hole plate and move 13/39 for each division. This requires a dividing head for compound dividing that can use auxillary gears with teeth of 24, 40, 24 and 44.

JRW

[Charlie Rose]

Bigger OD

[dial has a O.D of 1.375 so 125 marks around that makes it hard to read will going bigger O.D. make a diff?? thanks in advance for all the help.[/QUOTE]

If you have room for a larger OD dial then do it.A larger dial will give you wider spaces you'll still have 125 spaces.
The spaces on your 1.375 dial will be .0345" wide if you make the dial 2.00" dia. the spaces will be.0502". Wide spaces are good for your eyes.

[Oldbrock]

Of course djc I thought 40/125 and didn't divide, 8 holes on a 25 circle would do it. If I'd written it down I'd have seen it. Peter

[BillC]

Thanks for all the help,,,so if i get a 25 hole plate i would just turn the handle every 8 holes??,,to get 125 marks if i want 250 marks would i just turn every 4 holes??

[Evan]

Evan a lot of larger lathes have 8 TPI in the cross slide. My 1909 14" LeBlond does. If he replaced the screw he would also need to replace the nut.

Yep. Is that a particular problem? If a new dial is needed it seems like a good time to change to something that makes life a lot easier. Modern materials mean that a 10 tpi cross feed will last just as well as an old 8 tpi screw. In the past a lot of things were designed to accomodate the engineer, not the machinist. Still are sometimes.