Computer-controlled machining¶
In our fablab we have a Versatil 2500 SB cnc machine:
| Property | Value |
|---|---|
| Model | Versatil 2500 SB |
| Enclosure | Fully enclosed, milling process only starts when enclosure is closed |
| Safety Measures | Hearing protection required during operation |
| Workpiece Mounting | Vacuum table (through MDF sacrificial plate) + wood screws for small plates |
| Tool Changer | Automatic, holds up to 8 tools |
| Working Area (XYZ) | 2500mm x 1250mm (+1000mm swivel arm) x 170mm |
| Space Requirement | 3000mm x 1650mm (+1000mm swivel arm) x 1600mm |
| Table Height | 950mm |
| Portal Clearance Height | 260mm |
| Weight | approx. 550 kg |
| Drive | Hardened ball screw, 10mm pitch, self-adjusting nut to reduce backlash |
| Linear Axes | 2 ground rails + recirculating ball slides per axis |
| Stepper Motors | 230V, 4A, 900Ncm (X, Y, Z axes) |
| Accuracy | Max. positioning error: 0.1mm, technical resolution: 0.00625mm, repeatability: +/- 0.025mm |
| Speed | 24,000mm/min (positioning) |
| Spindle Speed Calculation | Dependent on cutter diameter (e.g., for 6mm: ~23,885 rpm, for 8mm: ~17,914 rpm) |
| Feed Rate Calculation | Dependent on spindle speed, number of teeth, and tooth feed (e.g., for 6mm: ~2,420 mm/min, for 8mm: ~2,600 mm/min) |
| Fusion 360 Settings | Spindle speed slightly different, feed rate significantly higher than calculated (likely optimized for softwood) |
CNC Machine Safety Training¶
For the safety check on our CNC machine, we first inspect our equipment. We wear hearing protection, safety glasses, and safety shoes. When turning on the CNC machine, everyone in the room is informed to put on their hearing protection.
The nearest fire extinguisher is located and its position is checked to ensure quick access in case of an emergency.
Before turning on the machine, we make sure that there are no objects on the milling surface that could interfere with operation.
The emergency stop buttons are checked at all relevant positions to ensure they are operational.
The compressed air system that holds the workpiece in place is activated. We then verify that the workpiece is securely fixed. If necessary, it is additionally screwed onto the spoil board. We also check that the workpiece lies flush on all sides.
The extraction system is turned on, and all protective windows are closed. The machine will only start if the safety covers are properly locked, which adds an extra layer of safety.
The milling tools are checked to ensure they are in the correct positions. The machine is then calibrated before the milling process begins.
During the milling process, the operation must always be monitored. The control unit is kept in hand at all times. When the milling tool first enters the material, the extraction system is briefly lifted to check that everything is functioning correctly. It is then lowered again.
These safety measures ensure a secure and efficient operation of the CNC machine.
Testing tolerances¶
To check the dimensional accuracy of our machine, we designed a tolerance test, which you can find in the Download section at the bottom of this page.
It features two plates. One with 12mmx40mm pockets and slots with increasing tolerances from 0mm to 0,5mm. And one with the corresponding tenons.
Unfortunately we did not think about completely clearing out the slots so besides the usual tabs holding the whole piece in place, we had to get rid of these bits inside the slots as well.
To make things worse, the milling operation did not seem to go all the way through at some points because we set the bottom height offset to -0,1mm, which left a kind of skin at the bottom of the slots.
We later set it to -0,3mm for all following setups.
Here is what our plates looked like after removing all the unwanted stuff.
We began measuring the horizontal and vertical distances in the slots and pockets.
This gave us an average deviation of -0,039mm, which fits well within the maximum positioning error of 0,1mm the manufacturer claims.
After measuring we quickly put the pieces together and found that 0,1mm of clearance produced the best fit without being too tight.