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Bevel Parallel Gear Blank Roll and Offset

Direction of Blank Roll and Offset cutting gears for Shay Locomotive

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Robert Blodinger23/04/2019 06:34:29
7 forum posts

I am cutting Bevel Gears per Ivan Law Book for Parallel Depth gears.

I just finished the first pass of three.

Need to apply the Blank Roll now of 2.25 degrees.

Using my rotary table set up on the Mill, I now have table set to 360 degrees, and just finished milling the first pass, having cut the slot between tooth 40 and tooth one. The required blank roll is 2.25 degrees.

When I rotate the gear clockwise by 2.25 degrees, I will be bringing the cutter to cut more of the right side of tooth 1.

So to now apply the offset which is .023 inch or .58 mm do I raise the cutter which will cut more of the right side of tooth 1, or lower the cutter.

In other words I am moving both cutter and roll to cut more of the right side of tooth 1 and less of the left side of tooth 40.

I have cut bevels before but been a while and cannot remember and Law sure does not give an answer other than pictures which has arrows going both way

The gears looks good and odnt want to ruin it with a worn turn.


JasonB23/04/2019 07:03:51
17826 forum posts
1949 photos
1 articles

Page 110 shows which way to move but basically you want the middle of the cutter to still line up with the ctr of the cut at its inner end so if your roll rotation makes the groove lower then lower the tool, if when you roll the gear the groove is above ctr then raise the tool.


Edited By JasonB on 23/04/2019 07:07:28

Robert Blodinger23/04/2019 07:32:21
7 forum posts

Thanks for the fast response and understand. Another question. I have a dividing wheel on my rotarytabale of 28 holes. The table has a 1:72 turn ration, so each rotation moves the gear 5 degrees.

I need a roll of 2.25 degrees. 5/28 = 1.78571429 x 12=2.143 degrees or am I better to go 13x2.143 = 2.32 degrees, closer but larger.

Other than that to be exact requires that I pull off the dividing plate and put the vernier back on, but that is very troublesome because I have a lot of gears to make.

JasonB23/04/2019 07:50:50
17826 forum posts
1949 photos
1 articles

I don't think it will make a lot of difference, not sure what size your gear is but if say 2" dia then it is only a thou off the tooth at its outer edge. If you recalculate the offset to this new angle then that should be reduced.

I suppose you could work out the roll for the other gear and go the opposite way so you either have big gaps with big meshing teeth or small gaps in this wheel with larger teeth on the pinion.

Edited By JasonB on 23/04/2019 08:40:31

Robert Blodinger23/04/2019 08:56:35
7 forum posts

The gear is 1.627 in on the large end. The Pinion is .814in on the large end. I guess I will try it at the smaller number on the Gear and then make the pinion and modify it if it does not roll without binding. Actually, come to think of it, the gear is 40 tooth and the pinion is 20 tooth. The blank roll is 4.5 degrees on the pinion and I expect that I can divide that directly since it is half of the 40 tooth 9 degrees spacing but I will have to sit down and calculate whether it will work out to either the 28 hole or 15 hole circle of the division plate.

Thanks again, good forum here.


John Pace23/04/2019 10:33:35
176 forum posts
168 photos

On page 111 of the gear cutting book about three quarters down the first
column it explains the principle to get the blank roll.
For example to get a thirty tooth gear using your 72 :1 table
and using a 60 hole division plate= 72 x 60 = 4320 holes
for one complete revolution of the rotary table ,divide by
30 = 144 holes or 2 turns and 24 holes.
The blank roll would be 36 holes in the 60 plate.
The important point made in the book is that the dividing plate
must be able to be divided by 4.
You could of course if the plate is only divisible by 2 make
the cuts in the gear in 2 passes missing out the initial centre
gashing cut ,same page second column.

I see from your later posting that you have a 15 hole plate ,
the easiest solution is to use this and make a 60 hole plate
and from there the 20 tooth gear =72 x 60 = 4320 holes
for 1 revolution of the RT, divide by 20 = 216 holes or
3 turns and 36 holes ,the blank roll = 54 holes.
For the 40 tooth gear = 72 x 60 = 4320 holes for 1
revolution of the RT, divide by 40 =108 holes or
1 turn and 48 holes ,blank roll = 27 holes.

The sizes of the gears at the large end do not influence
these calculations ,trying to work out the rotation of the blank
roll in degrees will only lead to mistakes.


Andrew Johnston23/04/2019 10:39:28
5400 forum posts
621 photos
Posted by John Pace on 23/04/2019 10:33:35:.

The sizes of the gears at the large end do not influence these calculations

Correct, unlike normal bevel gears the parallel depth method uses the small end of the gear to set the design parameters.


Robert Blodinger23/04/2019 18:45:44
7 forum posts

John, thanks for the good math. Yes, I saw page 111 and gave up trying to figure out the correct size for a plate. I will go back now and make a 60 division plate.


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