Toolpath Simulation

Overview#

Simulation is the cheapest crash you will ever have. Fusion's Simulate command replays one or more toolpaths against a model of the stock, showing material being removed, and flagging problems such as gouges, collisions between the holder and the part, rapid moves through material, and leftover stock. A toolpath that simply looks right as lines on the model can still gouge a wall, leave a ridge, or drive a holder into a clamp. Simulation exposes these before metal is cut.

Autodesk describes simulation as two aspects: animation (the tool moving along toolpaths and interacting with stock) and verification (background checks such as collision detection). It covers milling, drilling, turning, inspection probing, cutting and DED additive processes. Layers of checking, increasing in realism:

1. Toolpath preview: the toolpath lines drawn on the model. Good for positions and heights, but says nothing about material.
2. Simulate without a machine: a quick, basic simulation of tool movement, stock changes and collision detection. It suits milling, turning and cutting setups and does not use information from the machine or post.
3. Simulate with a machine: available for milling setups with a CNC machine. It uses kinematics from the machine definition and post processor and, if the definition has a linked machine model, shows the machine moving. More accurate, but slower to load.

A fourth layer exists outside Fusion: backplotting or dry-running the posted G-code in a separate verifier or on the machine in air. Simulation inside Fusion checks the toolpath; it does not check what your post wrote. See the post-processing page.

Where to find it#

• Milling > Actions > Simulate, or Actions > Simulate with Machine when a machine is attached (the Turning, Fabrication and Inspection tabs have Actions panels too).
• Select the setup or individual operations in the Browser first (hold Ctrl to multi-select); operations must be generated.
• Toolbar search (S): type "Simulate".

The Simulate dialog opens and the Simulation contextual environment replaces the other Manufacture tabs. Click Exit Simulation when finished.

Step by step#

1. Make sure every operation you want to check is generated (no out-of-date icons). Select the setup or operations in the Browser.
2. Click Actions > Simulate. Set the Viewpoint mode in the dialog.
3. Use the player controls at the bottom of the canvas: go to start, previous operation, previous move, play, pause, next move, next operation, go to end. The speed slider changes play speed (further from centre is faster; left is reverse). Clicking the timeline jumps to that point; dragging on the canvas scrubs the simulation.
4. Turn on Display > Stock and choose a Colorization option and Material.
5. Verification starts immediately; issues are marked on the timeline in red (errors) or orange (warnings). Open Information > Issues to review them chronologically; clicking one jumps to it.
6. Use the Info and Statistics tabs for tool and operation information, number of detected collisions, start and final stock volume, and estimated machining time.
7. Fix issues in the operations, regenerate, and simulate again.

Simulate dialog and toolbar#

Accuracy#

The Accuracy slider sets how closely the stock shape is represented. Lower accuracy is faster but coarser and may hide small gouges; higher accuracy takes longer to calculate. Reduce it while iterating on roughing and raise it for final verification of fine finishing passes.

Reading the simulation#

Colorization and comparison#

Use the Comparison colorization to judge the stock against the finished model: set the Leftover Stock offset (the skin you expect to remain) and the Tolerance. Blue marks material not yet removed, red marks overcut, green is within tolerance. This is the most useful check for 3D surfaces after finishing. Stock to Model gives a point-by-point shortest distance.

Issues and collisions#

Fusion detects and reports:

• Collisions within the machine: between the machine and the stock, the machine and fixtures, and between machine components.
• Overtravel of machine axes beyond the physical limits.
• Retract and reconfigure moves, where the simulated toolpath is automatically modified to avoid an overtravel.
• Rapid moves of the tool or table that cut into the stock.

By default, to improve performance when simulating stock visually, collision detection is not performed during the animation phase; it is still fully performed during verification. Use the Machine Definition's Kinematics tab to exclude pairs of machine items from collision checks to speed up machine simulation. A Process Issues tab lists problems with how machine simulation is operating. When simulating without a machine, enable Preferences > General > Manufacture > Optional Features > Stock Simulation Improvements.

Typical responses (general guidance): for a holder collision use a longer tool or a different stepdown or order; for a rapid through stock raise the heights or fix linking; for an axis limit change the WCS or tilt solution.

Tip: Set the Issues dialog's Settings (or the Stop on Collisions checkbox) to stop playback at an error, then step back move by move to understand the sequence.

Stock handling#

Simulation uses the Setup's stock. If From preceding setup is selected, the simulation of the second Setup starts with what remains after the first, provided the first was generated. A Setup with an incorrect stock gives misleading results: if the stock is larger than reality, the simulation shows cuts in material that is not there (cycle time overestimates and cutting at rapid appears fine). If the stock is smaller than reality, the simulation shows air where you will really cut, and rapid moves that look safe may plunge into uncut material.

When you have a cast or oddly shaped blank, model it and use From solid so the simulation is faithful.

Machine simulation#

With a machine attached to the setup (see the multi-axis page) and a linked machine model, Simulate with Machine shows the machine moving. Use Display > Machine to control its visibility. Benefits:

• Verify the rotary axes reach their positions and stay within limits.
• Detect collisions between machine components, the stock and fixtures.
• Visualise the actual part orientation as the machine would have it.

It is most reliable if the machine model is accurate and the post and machine definition agree on axis conventions. For simple 3-axis machines it adds little over simulating without a machine.

Cycle time and statistics#

Select a generated setup, NC program or operations and click Milling > Actions > Machining Time. The Machining Time dialog shows estimated times, and you can change Feed Scale, Rapid Feed and Tool Change Time to see their effect on the overall time. The simulation Statistics tab also reports an estimated machining time. These are estimates only: real machines slow down on short segments because of acceleration and look-ahead (general guidance). Use them to compare strategies, not as a production bid.

Setup sheets summarise each tool and operation for operators.

Options reference#

Tips and pitfalls#

Tip: Simulate after each operation while programming, not only at the end. Catching a wrong height after one operation is far less work than untangling it after twenty.

Tip: Look at the stock on its own, then switch to Comparison colorization. Two views, two different insights.

Tip: Check the first and last moves of each operation. Many crashes happen on entry, exit, and between operations, which people skip when watching the middle of a pass.

Warning: Simulation does not know that your vise is a different size than modeled, or that you moved a clamp. It checks only what is in the document. Keep the model of the setup in sync with the real world.

Warning: A clean simulation does not validate the post. Rotary angle conventions, tool numbers, work offsets and unit codes are all post responsibilities. Review the G-code.

Warning: Low accuracy can hide small gouges. Verify finishing operations at high accuracy.

Practice exercise#

Take the pocket-and-island part from the adaptive page. Intentionally make two mistakes: set the Setup stock top offset to 0 and shorten the tool's length-below-holder to 15 mm while the pocket is 18 mm deep.

Simulate with the tool holder displayed and Stop on Collisions on. Note the messages, fix both mistakes, and re-run.

Expected result: the first run reports a holder collision at the pocket floor (and shows the facing operation cutting nothing); the corrected run is clean and the Comparison colorization shows no remaining stock above your tolerance on the walls and floor.

Related#

• Setups and stock
• Post processing
• Pocket and adaptive roughing
• 4-axis and 5-axis milling