Noob 4-axis questions

1. ## Noob 4-axis questions

I've been running a 3 axis (X/Y/Z) CNC machine for a few years, so I'm reasonably comfortable with basic 3 axis CAD and CAM. For a specific application I'm considering building a 4 axis machine

With reference to the images below, I'm looking at what is essentially a CNC lathe (parts held along the X axis, spun as axis A), with a spindle that can travel up and down (axis Z) and along the length of the object to turn (axis X). I'd like to also be able to rotate the spindle around the Y axis (Axis B). The spindle centre would remain in the XZ plane - i.e. it cannot move forward/backward on the Y axis (essentially like a 5 axis machine, but where Y would always be 0).

I'm assuming https://www.cnczone.com/forums/uncat...70276-cam.html might be relevant in terms of choosing CAM software, but as a "rotational axis noob" I'm trying to get my head around how the machine would be set up in the CAM software - as it would need to understand the centre of rotation of the spindle (axis B), the tool protrusion from that centre, and (I assume) some data on the dimensions of the spindle body and gantry, in order to avoid collisions. E.g.. "if I tip the spindle by 90 degrees so the cutter is horizontal, can I reach the bottom of a bowl without the machine colliding with the workpiece"?

Anyone care to educate me how this is done? It's all much easier in the 3 axis world!

2. ## Re: Noob 4-axis questions

You might be overthinking this. Usually when setting up a 4-axis job, you center Z and Y on the center of the A axis, with X zeroed to the left side, so you don't run into the rotary table as long as X values are positive. If you're tipping the spindle assembly, that requires adding a 5tj axis; it's not something you normally do in 4-axis machining.

3. ## Re: Noob 4-axis questions

Originally Posted by awerby
You might be overthinking this. Usually when setting up a 4-axis job, you center Z and Y on the center of the A axis, with X zeroed to the left side, so you don't run into the rotary table as long as X values are positive. If you're tipping the spindle assembly, that requires adding a 5tj axis; it's not something you normally do in 4-axis machining.
In this instance my "4 axis" refers to XZAB; rather than the usual XYZA. I guess you could just think of it like a full XYZAB 5 axis machine; but where Y is always 0 (and cannot move).

The reason why I'd wanted to have the B axis (to tip the spindle) was so that the cutter could be placed perpendicular (the normal) to the surface being cut. For simple spindle work, having the cutter vertical would likely be fine, but if you wanted to, say, engrave a pattern into a sphere, you'd want the cutter to be perpendicular to the surface at all times.

Where the centre of the B axis is (i.e. at which point the spindle tilts) and the projection of the cutter, would be critical to CAM; otherwise you could end up crashing into the edge of a bowl when trying to engrave the inside - it's that setup I'm trying to understand. Surely you'd need to tell the CAM software exactly where the centre of the B axis was; as it would need it to understand what'll happen to the end of the cutter when it tips?

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