# Thread: Involute shaft

1. Senior Member
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## Involute shaft

Bear with me this could take a week to do from start to finish as will be explained in the post but I hope to show it with pictures and text.

Got a job in last week to do some short shafts with an involute spline on the end, even got a drawing that told me the OD, how many splines and how long- wow - but nothing else.
0.650" OD, 0.875" long and 12 splines.

Involute splines are just gears but only half the depth and usually have a 30 degree pressure angle and occasionally 25 degrees.
Because of this gear calculations work the same - to an extent.
splines are usually denoted by both sizes of the make up, like 16/32 DP.
This means the teeth are 16DP but the depth is equal to 32 DP

To calculate the DP you take the number of teeth, add one and divide it by the outer diameter, for a normal gear it's number of teeth plus 2 and divide. splines are plus 1 because the depth is half of a normal gear.
So in out case 12 + 1 = 13 / 0.650 =20 DP so this is a 20/40 DP spline.

Run these figures thru the gear generator and we get this.

Note 12 teeth, 20DP, and the important bit the depth factor has been entered as .5 to allow for the depth of a 40DP spline / gear.
This then gives us a true involute of the complete spline which can be saved as a DXF file.

Once this is brought into CAD and zoomed in you can draw two straight lines, one from the root of the tooth to halfway up the involute and another from there to the outer tip.

Now if two new lines are drawn midway on each line and at 90 degrees to the existing lines they will converge on the centre of a circle that passes thru all three points, standard geometry 101.

Draw a circle to check and repeat on the other side.

This gives us four important pieces of information to make a cutter, the size of the circles, 0.277" and the distance apart 0.300", the width of the gear cutter blank at 0.119" and the depth the buttons need to be infed, 0.051"

Next job is to make what is usually termed a button cutter.

Just a piece of flat bar with two holes drilled in at 0.300" centres and tilted at 5 to 7 degrees to accept two top hat buttons made of Silver steel or drill rod , hardened and tempered to light straw for cutting steel.
These are then surface ground on the top face to sharpen them and give a cutting angle.

This button tool is used to prepare the cutter blank, more of which, with pics in a later post.

.

2. Senior Member
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I don't think that I've ever seen a button cutter-in 3D or in action! I for one am eagerly waiting for more spline teasing!

3. Senior Member
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Thanks for posting this.

I'm not likely to ever need to do anything like it it myself but it's very interesting all the same and I really appreciate that you have the patience to share your knowledge.

4. Senior Member
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Originally Posted by John Stevenson
you can draw two straight lines, one from the root of the tooth to halfway up the involute and another from there to the outer tip.

Now if two new lines are drawn midway on each line and at 90 degrees to the existing lines they will converge on the centre of a circle that passes thru all three points, standard geometry 101.
.
John, You lost Me . I only made it to geometry 100.5 not 101

Steve

PS and please don't call Me a clumsy bass turd.

5. Senior Member
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Dec 2003
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Great Stuff Sir John!!

I'll most likely never use it, but it sure is interesting, I also want to say thanks for your time and efforts in posting this.

best

rollin'

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Aug 2004
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## half depth splines

Where i used to work we had JIS splines that were the half depth you described, but the PD on the spline was actually at the OD per JIS standard as well as using a 20 dsegree pressure angle.

Is that possibly the case with your splines?

If so, it makes the spline thicker at the top of the spline and the curvature less on the spline faces.

7. Senior Member
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Originally Posted by Jpfalt
Where i used to work we had JIS splines that were the half depth you described, but the PD on the spline was actually at the OD per JIS standard as well as using a 20 dsegree pressure angle.

Is that possibly the case with your splines?

If so, it makes the spline thicker at the top of the spline and the curvature less on the spline faces.
I think when John makes the shafts it will be up to the customer to adapt their parts.

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Im intrested in this 'button' method.. However a nagging feeling has struck me...

How did you align circles to involute curves? Should'nt they be.. involute not circular?

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I dont think i get the spline generation but i like the gif, cool animation
in time i may yet understand
mark

10. dp
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Originally Posted by Black_Moons
Im intrested in this 'button' method.. However a nagging feeling has struck me...
The button method is well described in Ivan Law's little red book of gears - I consider it essential reading.

http://www.amazon.com/Gears-Gear-Cut.../dp/0852429118

Here's Johns' own version: http://www.metalwebnews.com/howto/gear/gear1.html

And once you have the gear cutter you need a reliever: http://www.youtube.com/watch?v=kJ8kyC_bpHs
Last edited by dp; 05-23-2010 at 12:49 AM.

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