Question about Cusp Height Calculations

[Anonymous]

I have a question about Powermill concerning the calculations of cusp heights with ball milling tools.

It seems to be using a different formula than standard when calulating cusps, and I'm wondering why that is.

A normal formula for calculating cusp heights would be:

c = cusp height (scallop height)
d= diameter
s = sidestep / stepover

If you input d = 10 and s = 10 you would get a c = radius, that makes sense (10/2 = 5 = radius). This is not the result you get in Powermill. 

It's comes out as 8,535534, which is.. impossible? You can't get a cusp height that is higher than the radius of the tool, it would technically be infinite?

If you put s = 1 on the same d = 10 tool a normal formula would produce a c-value of 0.0250...

Where as if you enter sidestep = 1 in Powermill you'll be presented with a cusp height of 0,0502...

I did some testing of trial and error to see if I could match the caculations that Powermill uses and I've figured out how it calculated cusp heights with values of 0 < s < r , r being d/2 and s being the sidestep used. This is what I believe is being used to calculate cusp heights in Powermill:

c = cusp height (scallop height)

d= diameter
s = sidestep / stepover

Using this formula I'm able to produce the same results as Powermill as long as 0 < s < r :

Powermill Formula (?):

Normal Formula:

If you manually set the sidestep and don't care about the 'calculated value' of the cusp this isn't a problem. However if you want to calculate sidestep based on cusp height (which is something Powermill supports), then you'll get an incorrect value:

Powermill Formula (?):

Normal Formula:

Why does Powermill calculate this way? Is this the formula that Powermill uses?

[M_Hennig]

That was a whole lot of math that I did not double check you on, but I will say your correct, Powermill does not calculate the cusp correctly.  A very simple example of this are the corner toolpaths, make a multipencil toolpath with a cusp of .0001, then make an along corner with the same cusp. You will get two drastically different stepovers, and this should not be. A cusp is a height of remaining material, regardless of tool diameter OR toolpath strategy. It would seem we are the only 2 guys on planet earth to have a gripe about this, because this has been the behavior since day one.

[Anonymous]

This was just my example concerning ball nosed mills. If you really want to boggle your mind over something concerning the cusp-calculation;

Use "Offset Flat Finishing" (instead of the "3D Offset Finishing" I used in OP) , you'll get the same results concerning ball nosed mills as I concluded in my calculations, however what really throws me off with all this is that you'll still get cusp-heights in Powermill using an end mill.. And we're not talking miniscule levels we're talking full on millimeters. 

10mm End Mill with a sidestep of 2mm apperantly gives you a cusp height of 1,414mm.. based on what?! Where would the cusps even be?! A flat mill giving cusps of the same factor as the diameter on a flat surface? You learn something every day.

[Anonymous · Accepted solution]

Relayed the information to Autodesk and got an answer I accept.

I will not delve further into it here.

[Jaanyaar]

@Anonymous 
This issue is a question for me too. Please may you update it with your new info? Thanks in advance.

[Anonymous]

Of course Jaanyaar, I will update you on the issue. Was not my intention to be cryptic about it.

Powermill uses a formula with more variables, presumably to give a more accurate representation of real usage.

This formula is thusly understandable confidencial so I've not inquired further about the nature of it. Basically it tries to compensate for an (average abstract) surface angle.

A flat surface, which has a normal cusp height formula

An angled surface produces another cusp height.

I've requested an update for milling strategies focused on flat finishing to not include this abstract angle compensation to provide more accurate cusp height calculations. I have faith that now that this issue has been highlighted it will be adjusted in a future update.