Hobbing a "Grob" Spline

[BlaineW]

I am trying to hob a "Grob standard" spline into a part.

It is a very obscure type of straight sided tooth form that isn't used very often. We have a cutter that successfully formed the spline on a mostly manual machine several years ago. We are now trying to make it on a newer and more sophisticated machine.

Many of the typical involute variables that this machine needs to program don't exactly exist in these grob standards. (Diametrical pitch and pressure angle for example. I've talked with the manufacturer of the cutter and they gave me numbers to use, but of course wouldn't tell me where those numbers come from.

For whatever reason, I can not seem to create a straight sided tooth...I always get a slight involute or my teeth aren't parallel like they are supposed to be. The critical measurements are the chord thickness across two teeth..and the root diameter, and I can never manage to get both.

I would really appreciate any ideas anyone has on how I can accomplish this. I expect I'm missing something very simple.

FYI...here is a site for grob information...

http://www.grobinc.com/coldrolled/std_spline_shafts.htm

We're making the 2250-24-2 type

Thank you

 

[unclesyd]

The Grob spline is rolled and I've never seen one cut. We used a lot of them and we got both parts from Grob. I know we were unsuccessful when trying to buy a broach for one of their splines.
Have you talked to any of the cutter manufacturers.

 

[mfgenggear]

BlainW

I drew this spline in cad. the key was 24T & the
cord dim of .440-.439 .

my results is

Tooth space is 30 deg included
tooth is 15 deg included

This splined can be hobbed with no problem.
there is most likely an error with setup or the hob.
I suspect hob wear :|

we run strait sided or 80 deg included splines with no problem. how ever the imputs are fudged.

Sector = angle in rads x radius (nominal size)=circular pitch

Pi / Circular pitch = DP (Diametral Pitch)

for example this circular pitch = .2945243 @ 2.250 dia.
there for its DP = 10.66666

PA (pressure angle )would be half of the included angle 7.5 deg.

Instead of a dialog program, "if the Hobbing Machine will
let you" use G code.

give this a try
Hope this helps

 

[BlaineW]

Ok mfgenggear, sounds like you might actually know a little about these grobs so let me fill you in on a little more detail.

We have a vendor that supplied this part using our cutter when our original machine broke down. They just enter in the angle, the number of teeth, and the depth and the parts look exactly like they are supposed to, so they weren't able to help me much.

On this new "used" machine we got, I've been able to make involute splines with few problems. On these grob types however I can't seem to get a straight tooth.

The control requires a lot more input then what our vendor puts in, so I'm pretty sure that something I'm entering is screwing it up. I basically have to enter in all the information related to both the cutter and the hob...and the control I assume calculates all the necessary data to cut the part.

I talked to the cutter manufacturer, and they told me to use a DP of 10.502784 and a low pressure angle, something like 10 degrees, but weren't particularly interested in telling me where those numbers come from unless I was interested in buying more cutters.

No matter what DP, pressure angle, or anything else I change seems to fix the fact that I don't get parallel or straight teeth...and since I know the cutter works, I'm completely stumped.

Thanks for the input.

 

[BobM3]

Does your CNC control generate G code based on your inputs? If so, can you view the G code? That would tell you what each axis is being commanded to do. Then compare that to what was done on the manual machine.

 

[tomwalz]

Might try posting in
practicalmachinist.com

Thomas J. Walz
Carbide Processors, Inc.

http://www.carbideprocessors.com

Good engineering starts with a Grainger Catalog.

 

[mfgenggear]

BlaineW

Start from the basics.

How is the part being clamped. It must be rigit and not move.

What is the lead angle of the hob,
To obtain a straight lead the swivel of the hob in the machine must compensate for the lead angle?

Is the part & cutter being Indicated w/I .0005 or less TIR?

how is the lead (parallelism) accross the teeth of the part after it is cut?

use pins "which clears the rootof part " to verify P.D. runout of .001 max.

Are you able to verify .0015 max Total Index Error of the part teeth.

Try these settings

DP = 10.66666
pA= 7.5 deg.


Hope this helps

 

[mfgenggear]

BlaineW

any luck with your Grob spline?