# Large Spur gear design

1. Feb 14, 2010

### ssakcaj77

Hi, Im new to this forum so forgive if i am mis posting or multi posting. I am in the process of designing a 44 inch gear to cut on my shop bot cnc machine. I have been up and down calculations but i end up with tiny teeth and can not find the right formulae to calculate an oversize spur gear. Any thoughts ?

2. Feb 14, 2010

### pantaz

What exactly are you trying to calculate? What do you mean by "tiny teeth"?

3. Feb 15, 2010

### ssakcaj77

I am trying to build an oversize gear with large teeth. Given a Pitch Diameter(D) of 44" I am trying to find how to base the number of teeth (N) in order to calculate Diametral pitch(P)

Pitch Diameter is D=N/P

Number of teeth is N=D x P

Diametral Pitch is P= N/D

Based on that, we have D but I am not sure what to do about knowing what the propper amount of teeth would be or do ya just pick em and go with it.

I picked a number and went with it for fun and when i was done calculateing addendum, dedendum etc I ended up with teeth that were around 1/2 big. It would work, but i need larger teeth for this project.

I also need to check to see about the radius used to make the sides of the teeth. I read that there is a chart of info on this . Need to find my machneries handbook!

4. Feb 15, 2010

### Doug Huffman

Pi is irrational, where do the left over digits go?

5. Feb 15, 2010

### ssakcaj77

doug, what the hell ya want me to do with that statement ?

6. Feb 15, 2010

### Doug Huffman

I am traveling at the moment and for the week. I don't have my library handy. ISTR I seem to recall that there are formulas in The Machinery Handbook for setting up gear cutters. The extra digits of irrational numbers are a typical problem for real world problems.

7. Feb 15, 2010

### ssakcaj77

im just trying to find my copy of MHB right now

8. Feb 15, 2010

### pantaz

QTC has their http://www.qtcgears.com/Q410/Q410Cat.htm" [Broken]. The spur gear calculations are universal.
http://www.qtcgears.com/Q410/Q410Cat.htm [Broken]

Last edited by a moderator: May 4, 2017