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## Disparity in Cusp Height math in Parallel

12 REPLIES 12
Message 1 of 13
263 Views, 12 Replies

## Disparity in Cusp Height math in Parallel

Am I missing something here or is the math wrong in fusion?

We're going after a /45 finish, which is 0.000045".

In fusion, it is giving me this for the 1" ballnose, a .0095" stepover

http://theoreticalmachinist.com/Surface_Finish_Calc.aspx tells me this: a .0134" stepover

and https://www.villamachine.com/ball-endmill-surface-finishing-calculator-how-to/ gives me this; a .027" stepover. I suspect they confused radius and diameter in the equation

and finally, https://www.desmos.com/calculator/er9xnxrsps gives me .0134

12 REPLIES 12
Message 2 of 13

45 RA, RMS? Microinches or micrometers?

Agreed that I'm seeing differing values depending on the calculator. What's your math for 45=0.000045 cusp height?

Message 3 of 13

45 RA, RMS? Microinches or micrometers?

Agreed that I'm seeing differing values depending on the calculator. What's your math for 45=0.000045 cusp height?

The callout is 63μinch but since it's a 4+hr surface program, we will add some safety margin into it and aim for 45. 32 would just make the part take a whole shift.

I converted it with https://www.unitconverters.net/length/microinch-to-inch.htm

Also curious how you dictate feedrate on here. Usually I make it equal to the stepover without issue, but the MFG of the ballnose says we can run this thing at .025" ipt! I know with chip thinning, and the overlap it'll be smoother then that but feels wrong on paper (for now).

Message 4 of 13

And then this calculator (with your .0267" stepover) gives us a 38.6μinch result 🤔

https://www.widia.com/us/en/resources/engineering-calculators/end-milling/ball-nose-surface-finish.h...    (same calculator is used by Kennametal, since Widia=Kennametal)

Message 5 of 13

Which this calculator also puts me at ~38 or so

https://www.heritagecutter.com/MillingCalc/pages/3DCalc.html

Message 6 of 13

So, even if we go conservative with the .0134, is there a reason Fusions is still off by 41% natively? I'd rather run the first one at .013, and if the finish really does come back as /25 then we can run the 2nd with a higher stepover.

For a part this size, maybe I'll look into making a test block 1/4" wide and seeing the actual finish achieved.

Message 7 of 13

Yeah, I don't have an answer as to why it's off so far, sorry

Message 8 of 13

Do you think the equations would be fixed? The last site I used shows the math on calculation.

Message 9 of 13

Waiting to hear back from the relevant development team as to what and how their math is doing and returning for values. I do know that we're calculating on a 45 degree slope (which my foggy memory (from trade school) seems to recall as being important)

Message 10 of 13

Yes the 45° slope is important

0.006mm cusp height on a horizontal plane:

45°

30deg

So the fusion calculation is only accurate at 45°

for any other angle that deviates a lot from 45 degrees you need to do your own calculations to find the optimal stepover.

this will depend on the toolpath, for parallel the stepovers are in the XY plane so you need to use the Horizontal stepover

For Contour the stepovers are Z level so you need to use the Vertical value.(at 45° these are the same)

You can derive the horizontal and vertical components from a Horizontal calculation using trigonometry

If i use your values(converted to metric) is gives the same results a Fusion for a 45° slope

0.241mm = 0.00948"

Message 11 of 13

Interesting. Do you mind doing the calc for a radius of 5.965"? It is a convex surface so I assume "male"

Message 12 of 13

That app generates all the coordinates when you calculate for a Rad, more for manual plotting with a DRO on a manual mill.

as the radius is variable slope it doesn't give a  constant stepover value you can enter into a toolpath.

Some toolpaths like Bend, Flow, scallop and geodesic maintain a constant stepover regardless of slope. I haven't really dug into how exactly that's calculated based on the 45° formula Fusion uses

Message 13 of 13

Here's a graphing calculator that's been setup with the algebraic expressions for cusp and stepover
https://www.desmos.com/calculator/poyj4vuzkd