7. Computer Controlled Machining - Students¶
Clearance Test¶
For this week’s CNC assignment, I designed and cut a test piece to determine the optimal fit for a 0.5” wide dogbone joint. Dogbone joints are commonly used in CNC milling because a standard end mill cannot cut perfectly square inside corners — the dogbone shape adds a small circular relief at each corner, allowing flat sheet parts to fit together cleanly.
This is the test made in Fusion.
Test Design¶
The test piece features seven slots, all designed for a 0.5” wide tab. The center slot is exactly 0.5”, and the remaining slots vary in width to test different clearances:
| Slot | Width (in) | Offset from 0.5” |
|---|---|---|
| 1 | 0.40 | -0.10 |
| 2 | 0.42 | -0.08 |
| 3 | 0.44 | -0.06 |
| 4 | 0.46 | -0.04 |
| 5 | 0.48 | -0.02 |
| 6 | 0.50 | 0.00 |
| 7 | 0.52 | +0.02 |
| 8 | 0.54 | +0.04 |
| 9 | 0.56 | +0.06 |
| 10 | 0.58 | +0.08 |
| 11 | 0.60 | +0.10 |
The slots decrease from 0.50” down to 0.40” in 0.02” increments, and increase from 0.50” up to 0.60” in 0.02” increments. This range tests both press-fit (undersized) and loose-fit (oversized) tolerances to find the sweet spot for our CNC machine.
Why Dogbone Joints?¶
When a CNC router cuts an inside corner, the round end mill leaves a radius equal to half the bit diameter. This means a square tab won’t fit into a square pocket. Dogbone joints solve this by adding a small circular cutout at each corner of the slot — the circle matches the end mill diameter, allowing the square tab to seat fully into the joint.
Purpose¶
By testing this range of tolerances, we can determine:
- Which slot provides a snug press-fit — tight enough to hold without glue but still assemblable by hand
- Which slot is too tight — the tab won’t fit or requires excessive force
- Which slot is too loose — the tab slides freely with visible gaps
- The actual kerf and deflection of our CNC machine, which affects how closely the cut matches the designed dimensions
This information is critical for designing accurate joints in future CNC projects, ensuring parts fit together as intended on the first cut.
You can download the dxf file by clicking here
Aspire¶
Dogbones were added to each corner inside the clearance test so a piece with 90 degree corners can properly fit in each slot. Dogbones were added in aspire not fusion.
Profile Settings
These are the only settings that were changed, all the other settings were left as default.
Vcarve Settings
A 60 degree Vcarve bit was used to cut the letters because they were small and close together. These are the settings that were used.
These are the toolpaths that were calculated.
Cutting on the ShopBot¶
Because we were unsure of our vcarve settings, we decided to test the numbers.
It took a few tries to get the numbers right, we adjusted things like flatdepth.
After we got the numbers right, we cut the whole test itself.
This is our test after being cut out.