Uneven stepdown in Surface Finishing

[Anonymous]

Hi

When using Surface Finishing to machine an involute gear tooth form, I get a bigger stepdown for the first half on the one side. Both flanks are symmetrical so the stepdown should be the same both sides.

Has anyone had similar problems or got any ideas on what is causing this and how I can fix it?

Thanks Peter.

 View from top.

view from side

[5axes]

Depends of the geometry of your surface and perhaps also of the "Surface units" parameter used in your settings . But without a sample part, difficult to give you an answer.

Note : It looks like more as a Bevel Spiral Gear or a Hypoid Bevel Gear and in this case I don't think that we can say that both side are "symmetrical"

[Anonymous]

Thanks for the reply.

Yes it is a spiral bevel. One side is concave one side convex but the cross sectional profile is the same and should produce a similar stepdown. I have tried with straight bevels as well  and get the same result. model attached.

[Anonymous]

From what I can see, it looks like it may have to do with using the cusp and angle of the surface. As the angle gets smaller or larger your step down/over changes to keep the cusp the same while following the surface.

[5axes · Accepted solution]

You can try something like : Surface Units : parametric  and Stepover 0.006 to get an identical  distance between each paths. 

It's not unusual to get this type of result by using this strategy. Where so see only one surface in PMILL in reallt it's a multipatch surface ( 32 degree 3 in the U direction and 6 patch in the V direction)  so the first path correspond to the first patch of the surface . And as it is longer than the other patches and toolpath  looks like non homogenous. 

So if you have a look to the limits of this first surface segment, you will find the end of  the area with the larger distance between paths. 

[Anonymous]

Thanks for the help, changing the surface units to parametric has evened it out nicely.

Just for interest something else I have just discovered.

I have two surfaces that I can work to, one is the single tooth surface generated by the gear software used to cut the model of the gear and the other is the gear model itself. I prefer to use the single tooth surface as it is slightly bigger than the gear model so the tool machines just past the edge of the work piece leaving a better finish.

If I use the surface of the gear model I don’t have this stepdown problem. Both surfaces should be identical but for some reason or other PowerMILL is reading them differently.