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 Learn how to improve your lathe part-off toolpath in Mastercam — add chamfers, avoid tool collisions, and control spindle speed as parts break free.

Improving the lathe part-off toolpath in Mastercam

The cutoff operation in Mastercam works fine right out of the box — pick a point, choose a tool, accept the defaults, and the part parts off. But a default cutoff leaves a few things on the table that are worth addressing before that part ever sees a machine.

See what's actually happening with "keep both pieces." By default, once a part is cut off in Verify, it disappears from the screen. Turning on "keep both pieces" in the toolpath parameters' stock update section keeps both the stock you're holding and the piece you just parted off visible, so you can actually inspect the result.

Deal with the sharp corner. Parting off straight to zero stock leaves a sharp corner that usually needs a manual deburr or a second machine operation. The cutoff operation's corner geometry section lets you add a chamfer (width and angle are both adjustable, with optional blend radii) directly in the toolpath — no extra geometry required.

Leave stock on the back face when needed. Long cutoff blades can dish in or out under uneven load, leaving an uneven face. The "back face stock" setting lets you leave a defined amount of material (e.g. 50 thou) for a clean-up pass later, without having to move your cutoff point.

Avoid collisions on larger chamfers. Once chamfers get bigger, the relieved geometry of a typical parting insert can create an undercut, causing a collision as the tool retracts. The clearance cut option solves this by adding a straight plunge that clears the corner before the chamfer move runs, defined by an entry amount and depth relative to your cutoff point.

Watch your RPM as the part breaks free. Constant surface speed (CSS) at a fixed SFM will spin faster and faster as diameter decreases — right up until your spindle's max RPM clamp kicks in. For a large-diameter part, that clamp can mean the part breaks free at very high RPM, which is loud and potentially unsafe. The secondary feed rate and spindle speed option lets you define a new feed and RPM cap at a specific radius, so the cut slows down before center without sacrificing efficiency for the rest of the operation.

Leave a nub for low-quantity work. If a part would be damaged by dropping when it parts off, you don't have to cut all the way to zero. Stopping short by a small amount (the right value depends on blade width and material) leaves a thin, rigid nub that holds the part in place during retraction but is weak enough to snap off by hand — a good option for one-off or low-quantity runs, though not practical at production volume.

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