Re: Cutting a SB Change Gear with a Straight HOB!



Thanks a lot for that.  I understand it is an approximation, but thought I would give it a try.  Why not try to make a part for a South Bend Lathe, with a South Bend Lathe.  

I did read the Helicron reference you included below.  Unfortunately I did not absorb the calculation chance due the the change in the PA (the fine print).  Based on my drawing of the Rack Hob, and a drawing of a SB 32 tooth gear, I knew I was wrong on my depth.  Based on a comparison of the two drawings, I guessed at a depth of 0.239.  Based on the formula you included below for the PA=14,5, the actual depth should have been 0.233.  We'll see how things turn out. 

I will re-read the reference below.

Thanks a lot for the insight,


On Wednesday, November 10, 2021, 07:04:06 PM CST, Mike Byrne <mgbyrne3@...> wrote:

Your depth of cut is not appropriate for you gear geometry.


I believe your reference cites rack hob depth of cut as 3.41/DP which assumes a 20 deg pressure angle

and a gear tooth whole depth ratio of 2.25/DP.  That is not your gear (nor his by the way).


For your SB gear, you need to specify DP, PA, and WDR. 


Back to basics:  Hob DOC = (pi/4/tan(PA)+WDR-1)/DP    (see reference mentioned below)


for 20 deg PA and WDR of 2.25 one gets 3.408/DP


for 14.5 deg PA and WDR of 2.157 (more likely for a 14.5 PA gear) one gets 4.194/DP as the rack hob tooth depth.


Note this is for theoretical depth of cut for a "V" shaped cutter.  Depth would be increased to reflect a cutter

tip radius.  The challenge is get a depth of cut that produces equal tooth and gap at the pitch diameter when cutting

the gear.



                the rack hob works for any number of teeth for the given DP

                the gear tooth produced is not an involute but rather an approximation using linear segments

                more linear segments can be cut by offsetting cutter and rotary indexer but it becomes tedious

                       with a manual mill and manual rotary

                if the video you cited produced gears that meshed, it suggests there considerable wiggle room

                    in cutting shop made gears


On reference for derivation of rack hob geometry is

Read the fine print to notice the dependence on tan(PA) AND WDR.

Join to automatically receive all group messages.