Sunday, 24 April 2022

Fusion 360 post processor handling of internal and external turret tools

What up?

Phil Trueman posted a query on the Centroid forum about rear mounted tooling. His machine is very similar to mine in having a turret that requires CCW spindle direction for external tools. That got me thinking, as I had no idea how this is handled, yet it's something I'll need to bottom out myself sooner than later.

The turret in my machine presents the external tools from the rear (ie X coordinates are negative) and expects the spindle to rotate CCW when machining. Internal tools such as boring bars move in positive X coordinates ie they operate "in front of" the spindle with CW spindle direction. Not all turrets are like this - it depends if the tool is "upside down" in the turret. In my case, external tools are mounted "the right way up" like this:

If the tools were mounted "upside down", the spindle would turn CW. The reason for NOT going this route is that the cutting forces would tend to lift the turret / cross slide / carriage. Some machines are designed for CW spindle of course - but not this one.

One downside of this scheme is that the tool tip needs to be presented 3/4" above the turret face. That's 19mm, so a standard 20mm toolholder will present the tip 1mm above centre height. Sure enough, when I measure the height of the toolholders that came with it, they have metric 20x20 shanks that have been ground down to 19mm height. Something to bear in mind when I come to fitting tools to the turret.

The other slight downside is that the "handing" of the toolholders is flipped by moving them behind the spindle. So the default tool for RH turning in this scheme is actually a conventional LH part.

I struggled to figure this business out, as it's not something I've encountered before. The Bantam doesn't have a turret, so all machining is with CW spindle direction and +ve X coordinates.

Show me!
I managed to get the tool set up correctly and the spindle direction set to CCW within the tool library definition for the threading tool (as used in my first thrading trial). But no matter what I tried, I couldn't get the tool to reside on the -X side of the spindle. Note the X axis direction shown here - flipping it within the operation setup options doesn't help, as it gets realigned when you regenerate the toolpath. Hmmm.

What's the answer, fatty?
Turns out the X coordinates is flipped within the post processor. Remember this message?

Here's where that critical content gets used:

This is also where you get the spindle direction and the "hand" of the toolholder.

But finally, you get to post the g-code with the correct (negative) X coordinates. That's the only difference between the default "turret 0" and "turret 103". 

There doesn't seem to be any change in the way the tool is called up when you compare g-code generated with -0 and -103 turret options, so hopefully the "T6600" command is all we need to operate the turret. Of course, I've not got anywhere near trying this for real yet but hopefully it will work out when I finally get there....

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