Why is my estimated machining time so far off, and how do I set machine acceleration for better estimates in?

Why estimates miss

The estimate starts from the feed rates on each tool. A real machine rarely holds the programmed feed, because it has to speed up and slow down at every corner. V-carving and 3D work are full of short moves, so they run far slower than the feed rate suggests. Before 12.5 the only correction was a single Scale Factor multiplier. Version 12.5 adds Machine Acceleration, so the estimate can work out the speed the machine can actually reach on each move.

Set it up

1. Open Machine > Machine Configuration (also reachable from the Tool Database, the save toolpaths form and the Estimating Machining Times form).
2. Select your machine in the Machine List.
3. In the Acceleration section enter a value for each of X, Y and Z. Each axis can be different, and the documentation notes the Z axis is commonly slower than X and Y.
4. Pick the unit that matches your source: mm/s², inch/s², mm/min², inch/min², m/min² or feet/min². Using the wrong unit is the easiest way to get a wild estimate.
5. For a rotary axis set Angle in deg/s² or rad/s², or leave the unit empty if you have no rotary.
6. Click Apply, then open the Estimating Machining Times form, choose that machine in the Machine dropdown, and set Rapid Rate to your machine's rapid speed.

Where to get the numbers

The documentation says to get acceleration values from your machine supplier. On many hobby controllers the same numbers are stored in the controller's own settings, so read them from there or ask the manufacturer. Do not guess.

What it does not do

The acceleration values are used only for the time estimate. The documentation states they have no effect on the G-code output. Your machine will not move differently.

Pro tips

• Keep using Scale Factor as a fine trim. Log estimated against actual times for a few jobs and adjust. The documentation suggests many users keep one factor for simple 2D work and another for 3D or V-carving.
• If your control software shows its own time estimate, the documentation says that one should be more accurate, because the controller knows exactly where it accelerates.
• See the entry on estimating machining time and creating a job sheet for the rest of the form.