Thread mills technically violate bottom heights

[programming2C78B]

I noticed this because its in a blind hole with an undercut.

[seth.madore]

How odd, send me the file please?

[programming2C78B]

I can, but it's happened on at least 3 different ones this week. 

emailing now. 

[seth.madore]

Oh, this is being caused by Helical Leads. I don't believe it's a bug, more like "intended behavior".

[leo.castellon]

IMO, that shouldn't be allowed. If the user sets a bottom height, then there should never be an occasion when it goes deeper than the user selected bottom height. Most people are using thread mills in harder material, so if the tool bottoms out in the hole, the user is out an expensive thread mill and possibly an expensive part. 

LeoC

[seth.madore]

I agree, which is why I've opened up CAM-60529 to investigate the issue. I suspect it's always been like this but never really picked up on.

[programming2C78B]

Helical Lead:

Regular:

Where does .00285 even come from? 11.5 tpi tool, 0.75" od

Im wondering now if Helical leads are even necessary - not much info on what they do online except for this post where they have the same issue (it also sounds like OP isn't aware of how to properly set his bottom heights)

https://forums.autodesk.com/t5/fusion-manufacture/feature-request-thread-milling-distance/m-p/114285...

[seth.madore]

Yes, I would recommend using helical leads, as you will likely have a false cut on the lead in/out move if you don't do helical. Minor issue for single profile threadmills, more of a concern for full-form. You could also just do a zero radius lead out move so it goes from a G1 link to a G3 cutting move, but it's possible Wear/Control comp might complain to you (at the control).

The .00285 is the calculated distance required for the tool to lead in/out on the pitch of the thread for the sweep angle needed

I chatted with a developer yesterday afternoon, and while this behavior is undesired, it's likely intentional (that's not to say we can't improve it). All toolpaths (3d toolpaths for example) are understood to have retracts and lead/links that go outside the boundaries of Top/Bottom height. It's the actual cutting that is constrained within those limits, not the links.

[leo.castellon]

I can understand that the 3D tool paths would violate the Bottom height as those are model aware and those are "generally" external of the model tool paths. But a thread milling operation is generally performed in a blind hole where there is no room to move around in. I could see if Fusion wants to add a helical move in a vertical direction for a lead in, but make that in Z+, not Z-. It would be preferrable to have a quarter or even half a turn less thread engagement then having the tool hit the bottom of the hole and break the tool. We make quite a few parts in 17-4 and 15-5 in the higher hardness specs and it would totally suck to break a $100-250 thread mill. My employer would not be a happy camper if I kept breaking thread mills.

LeoC

[marcus.toepke]

I am of the opinion that this is also the case with the Heidenhain control if you use the machine cycle. This is normal due to the Helix approach.

[jeffescott]

It occurs to me that the thread must always end higher, than the bottom of the tool, since thread mill form is an equilateral triangle.

I made a generous allowance for this by cutting the boss deeper than the thread.