Automatic Tool Changer for Carvera Air

A hardware and firmware retrofit

Overview

The Carvera Air is a capable desktop CNC machine, but it ships without ATC. For multi-tool jobs, every stop means a manual swap, inconsistent offsets, and broken workflow. I wanted to fix that, so I designed and built a full ATC retrofit inspired by its bigger sibling, the Carvera, but with more tool slots.

This project is built on top of the Carvera Community Firmware. With help from the repository maintainer, support for ATC-equipped Carvera Air configurations was added upstream. I continue to run a custom fork while testing, but all design files are open source and available on GitHub and the community Discord.

Sensor Selection

The ATC needs to know three things at all times: is the tool in the slot or not, is the drawbar engaged, and which slot is the spindle positioned over. I evaluated photoelectric and inductive proximity sensors for tool presence detection, but ultimately landed on a laser diffuse sensor due to its small spot size. The smallest tool I run is 3.175mm, and neither inductive nor standard photoelectric sensors were reliable at that diameter. The laser sensor handles it cleanly. The sensor keeps track of whether the tool is in the slot or not, handling the pickup and drop routine.

Finding Available I/O

The Carvera Air's control board isn't well documented, so I traced available digital inputs and outputs myself, cross-referencing the community firmware source, probing test points, and mapping what was unused. I found spare I/O that wasn't exposed in the documentation and claimed those pins for the ATC sensors and the motor. If there weren't any extra I/O pins, I would reuse already existing ones, like the door sensor, laser on/off, light output, etc.

Mechanical Design

The tool changer mechanism had to fit within the Carvera Air's footprint without major structural changes. Key design constraints were drawbar force, tool rack indexing accuracy, and clearance during a tool change cycle. I went through several iterations before it was reliable enough to run unattended.

Lever Range

All angles reference the back plate as 0°, not the lever's own relative position. To convert any value to lever-relative position, subtract the start angle (0.55°).

Position	Angle	Description
Start (Clamp)	0.55°	Lever fully pressed - maximum clamp force applied
Neutral Clamp	1.0° – 1.5°	Natural spring-back resting position while clamped
Spring Range	up to 16.8°	Low force region - spring is the dominant force
Drawbar Pull Range	16.8° – 61.4°	Drawbar is being pulled, beginning to open the collet
Unclamp (Hold)	61.4° – 77.2°	Fully unclamped - lever stays without input
End of Travel	77.2°	Hard end of travel

Force vs. Angle

Note: High uncertainty (~20%) - measurements taken by hand with a scale. Values are directional only; do not use at face value.

Angle (°)	Force (kg)
20.0	1.37
25.0	2.80
27.0	4.00
35.0	3.00
50.0	2.60
53.0	3.20
55.5	3.80

Maximum force required: 5.80 kg (occurs within the Drawbar Pull Range)

The force curve peaks around 27° before dropping off - I'm pretty sure there is a cam system built into the lever shaft.

Napkin Calculations

Tool Rack

There is a spring under the pods to allow some compliance. I sourced the pods locally in Washington from a spindle rebuilding company. The only other place that sells them is Alibaba.

Firmware

I forked the Carvera Community Firmware and built the ATC logic on top of the existing architecture for testing and proof of concept. Thanks to Wario for updating the community firmware for ATC support for the Carvera Air, so it will work for anyone in the future that does this mod.

Results

A full tool change cycle with more tool slots than its big brother, the Carvera.

concept verification

First Test Run

Pickup verification