Trying to add a Model A apron to a SB 9" model C

[Tim9]

I have a 9" South Bend Model C I am working on. I have not set it up yet and don't even have the motor mounted on it yet. That said, I really want to have the power cross feed of the models A and B, so I already purchased a model A/B Apron. I planning on cutting a keyway in the feed screw which will drive the worm gear mechanism.
I think this will work fine for now and I'll deal with adding a Quick Change Gearbox at some point in the future but for now I am trying to hold off on purchasing the gear box.
The problem I have right now is that I did not purchase a carriage assy yet because I think I can cut the teeth in the cross feed screw that is presently on the carriage. I blued the screw and and screwed it in the carriage/apron assy and you can see where it stopped and scraped on the pinion gear....So I think I have enough metal on this screw for the teeth of the gear. I'm guessing South Bend made just one screw back in 1939 and for the A & B models they would then grind the teeth in the screw. Does that make sense, especially since I can't bolt the new model B Apron to the old carriage due to the screw collar getting in the way of the pinion gear assy on the apron.

So...Here is my question. I read up a little on gear and diametrical pitch. My apron gear which meshes with the cross feed screw is a 20 diametrical pitch. The screw collar which will become the gear to mesh with this apron gear has an OD of 0.610 inches. How many teeth should I put on this cross feed screw. From looking at some of the pictures it looks like 10 teeth, but I am not sure nor am I sure how I can figure this out.

 

[Mark_f]

I have made those gears for the cross feed. You could make a cutter for the purpose but it would be better to purchase a gear cutter for the proper DP and number of teeth, then cut the gear on the mill.

 

[Tim9]

I have made those gears for the cross feed. You could make a cutter for the purpose but it would be better to purchase a gear cutter for the proper DP and number of teeth, then cut the gear on the mill.

Thanks for replying Mark. I'll do just that and buy the involute cutters for 20 diametrical pitch, but my problem is that I am not sure how many teeth are in the gear. I don't know if it is 10 teeth. I think it is, but don't know for sure and was wondering if anyone knows which formula I should use to figure the number of teeth. I know it is 20dp and also has an outside diameter of 0.610 inches. I was hoping someone knew off hand the number of teeth in that cross feed screw or if they knew a way to figure the number of teeth I should cut in the 0.610 collar of the screw.

 

[Tim9]

I thought I just figured it out. I found this formula on the Martin Gears site. PD=NT / DP or Pitch diameter = Number teeth divided by the diametrical pitch. Does this look correct ? I'm open to all comments here.

For what it's worth... I don't know the actual Pitch Diameter. All I can do is use the Outside diameter and then assume the outcome would be a little less.
So...that would be 0.610 = number teeth / 20 ........ number teeth = 20 x 0.610
that comes to 12.2
And since I'm measuring the OD and don't really know the Pitch Diameter I can assume that Pitch Diameter must be .60 inches because 12 = 20 x .60

But....Am I way off base here. If the Pitch diameter was .55 then .... .55 = number of teeth /20.... then 20 x .55 = number teeth....and I come up with 11 teeth
And again.....If the diametrical Pitch was .50 then 20 x .50 = 10 teeth.

I'm at a loss. Is there a way to figure a diametrical Pitch when one knows the OD ?

 

[Mark_f]

DP = (N+2)/OD

DP = PITCH DIAMETER
N = NUMBER OF TEETH
OD = OUTSIDE DIAMETER

 

[Tim9]

DP = (N+2)/OD

DP = PITCH DIAMETER
N = NUMBER OF TEETH
OD = OUTSIDE DIAMETER

Thanks Mark...That should get me to where I can pull this off. Thanks again.

 

[Mark_f]

Here are some handy gear formulas.

GEAR FORMULAS

PD=PITCH DIAMETER

OD=OUTSIDE DIAMETER

DP=DIAMETRAL PITCH

N=NUMBER OF TEETH



DP=N/PD

DP=(N+2)/OD

PD=N/DP

OD=(N+2)DP

N=PD * DP

 

[Mark_f]

I have an extra screw assembly with the gear. If you want , tomorrow I can measure the length of the gear and count the teeth.

 

[Tim9]

This is doing me some good. I forgot that I have a book from Martin Gears. Its their big blue hardcover book with all the gears and lots of info. I found a page in there which is rules and formulas. Anyway....It says...To find Number of Teeth when you have an outside diameter and the Diametrical Pitch, MULTIPLY Outside Diameter by Diametrical Pitch and then Subtract 2.

And yea Mark, your formula is the same thing. It's just sad that I was having an absolute brain fart and questioning my math. Anyway...Its what I originally thought... 10 teeth. I'll go to Ebay and see it I can find a Involute gear cutter for 20DP

Since I know my DP is 20 because I have a gauge and measured it.....Then 0.61 x 20 = 12 and then subtract 2

So...Its 10 TEETH

 

[Tim9]

And thanks again Mark. I'm kind of confident now that it's 10 teeth so don't put yourself out.
Mark, I really appreciate the offer but the formulas say 10 teeth and the pictures I saw looked like 10 teeth so I'm going to go with 10 teeth. That said...If you can put your hands on it with out any effort...Then counting the teeth would reassure me even more. YET...If its any trouble or you have to dig through boxes, then please do not put yourself out. I'm pretty confident that it must be 10 teeth.