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



## Tim9 (Dec 1, 2017)

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.


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## Mark_f (Dec 1, 2017)

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.


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## Tim9 (Dec 1, 2017)

mark_f said:


> 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.


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## Tim9 (Dec 1, 2017)

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 ?


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## Mark_f (Dec 1, 2017)

DP = (N+2)/OD

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


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## Tim9 (Dec 1, 2017)

mark_f said:


> 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.


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## Mark_f (Dec 1, 2017)

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


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## Mark_f (Dec 1, 2017)

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.


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## Tim9 (Dec 1, 2017)

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*


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## Tim9 (Dec 1, 2017)

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.


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## Tim9 (Dec 1, 2017)

SPUR GEARS....RULES AND FORMULAS


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## Mark_f (Dec 1, 2017)

Make sure you also get the right number cutter. The Chinese ones are usually numbered backwards.

If you haven't already ...... Search the net and download Shop calc. It will figure the gear info automatically with your known info. It is a great program and free.


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## Mark_f (Dec 1, 2017)

Tim9 said:


> 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.


It is in my spare parts drawer. I removed it because I made a new screw when I made the large dial assemblies for my lathe. 10 teeth does sound correct.


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## Tim9 (Dec 2, 2017)

Thanks again Mark. 10 teeth it is.


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## 4GSR (Dec 2, 2017)

Tim,

I hate to bust your bubble, the cross feed screw I have here in my hands has 12 teeth and a OD of .687".  

My old model C cross feed screw has the OD turned down to about .500" so gear teeth could not be cut on the screw.

May want to check this one more time.

Ken


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## Mark_f (Dec 3, 2017)

Ken is correct. The gear is 12 teeth. The diameter is .687. ( my old shaft measures .685 diameter).


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## Tim9 (Dec 7, 2017)

Thanks for the update Ken and Mark. 12 teeth and .687 it is. I'll take it from there figure out my next move. I'm still working on setting up the lathe and have to mount the motor.


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## Tim9 (Mar 30, 2018)

OK....Forgive me the the long post to follow but I just wanted to update my original post about the upgrade to a model B from a C. This has been a very long road, especially since I am just starting this hobby and have limited knowledge and limited tooling. The tooling is another issue as I was tooling up before I even had finished getting my mill working.
   Anyway, the main issue here was that my SB 9C was fairly well worn. My cousin gave it to me and its a big improvement from my Atlas 618. That said, the Atlas saved my butt a couple of times because its nice to have a second lathe available plus I was missing the change gear needed to thread 24t threads.
  FWIW....the main reason I reached the desire to upgrade to a Model B was that my model C apron was flat garbage. The hand wheel to move the carriage literally has about an EIGHT of an inch slop up and another 1/8 down. I'm talking almost 1/4 worn. Its a wonder it worked at all. 
  So, I found a good used model B apron on Ebay for about 100.00 bucks plus shipping. Good score IMO
Then I tried to braze a bronze bushing to the old cross feed screw.


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## Tim9 (Mar 30, 2018)

Milled a keyway slot in leadscrew. But then when I went to bolt everything up...The Apron was rubbing on the bed of the lathe. So I had to mill part of the bed at the bottom lip.


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## Tim9 (Mar 30, 2018)

So...it worked, but not great. That gear on the crossfeed was warped/bent. And it binded on every revolution. So...time for an upgrade to larger dials and I figured I would try to cut an Acme Left hand 7/16" screw. Yes...it would be a challenge. Cut the crossfeed/dial housing and then brazed together.


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## Tim9 (Mar 30, 2018)

Here's the spline blank cut and my screw housing, dials plus  with some bearings i already had.


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## Tim9 (Mar 30, 2018)

Next....grinding the Acme screw lathe tool and cutting the acme screw. I must give a shout to Tex-Va who posted a wonderful write up on Pract Machinist. Thread: Making a new Cross Slide Acme Screw
Without that write up...I wouldn't have even tried to do this at this stage of my abilities. Had to make a follower rest plus added a slotted crossslide


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## Tim9 (Mar 30, 2018)

Loctite and pinned the screw to the spline.


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## Tim9 (Mar 30, 2018)

Had a bit of trouble with making the dial. Only have a rotary table which I purchased off of Ebay. Its a Palmgren and although the price was good since it was new in the box, now I know why. It is way over-rated. The old Palmgren may have been OK...But this has been a let down. Anyway, I easily divided the 5's and 10's and engraved the lines with a carbide bit on the mill. But for the 100's lines...I had to improvise. It came out OK...But my numbering usings stamps really sucked IMO. I'll have to re-make that part of the dial at a later date. In any case....I'm up and running and its working pretty good and much better as far as slop and backlash is concerned.  Can't upload last pic yet since haven't taken one of the installed numbered dial.


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## Tim9 (Mar 30, 2018)

Anyway... even though it’s not on the level of many others here, I’m pleased with the outcome. FWIW if I could rename this thread, I’d call it lipstick on a pig. But I live in a machine tool desert for small bench lathes. Mostly oilfield and ship repair stuff show up on the second hand market


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## Mark_f (Mar 30, 2018)

I'm impressed!  Good job.


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## Tim9 (Mar 30, 2018)

mark_f said:


> I'm impressed!  Good job.


Thanks Mark. That means a LOT coming from you....Because sir, you are a true master craftsman IMO. It shows in every project you post.


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