8. Computer controlled machining¶
Group assignment
Test runout, alignment, speeds, feeds, and toolpaths for your machine
Milling machine¶
We tested two machines for this week. A Stepcraft and a Shopbot.
Stepcraft¶
| STEPCRAFT-2 D.840 | Value |
|---|---|
| Clamping surface (x,y) | 612mm x 920mm |
| Working space (x,y,z) | 600mm x 840mm x 140mm |
| Programmable resolution | 0.005mm |
| Maximum speed | 3000mm/min or 50mm/s |
| Spindle | Stepcraft MM-1000 |
| Weight | 31 kg |
| Interface | USB or parallel |
| Software | UCCNC |
Tests¶
The milling machine came as an assembly kit, so it required a few hours to put toghether, and a fwe hours to calibrate, align, and adjust.
Testing the stepcraft840
Initially, we noticed an accuracy error in the machine cuts. The cut piece was smaller than the design. So we made several dogbone tests while varying the tool diameter inside the software.
The tool we used is a 6mm flat end mill with 3 flutes. With this kingd of tool fusion360 advise a 1400 mm/s feed rate.
After several tests while varying the feed rate into UCCNC software during a job, the machine sound was the best for 1080 mm/s on plywood.
The stock we used is 8mm thick and we were able to cut through it in 2 passes.
These were our settings:
| Parameter | Value |
|---|---|
| Cut z | -8 mm |
| Passes | 2 |
| Travel z | 25 mm |
| Feed rate | 1400 mm/min to 1000 mm/min |
| Spindle speed | 12 000 |
 |
We first tried to use a 6mm tool diameter. After measuring the cuts with a caliper we realized that the exact value is 5.4mm. But, still, the test couldn’t fit into the dogbone.
Eventually the test part fitted in the dogbone with a tool diameter fixed to 5,05mm.
We were not able to find the reason of that deviation.
New test settings:
| Parameter | Value |
|---|---|
| Cut z | -8,5 mm |
| Passes | 2 |
| Travel z | 25 mm |
| Feed rate | 25 mm/s |
| Spindle speed | 9 000 |
 |
Best cut so far / workable.
Our material was 8.5mm, so we designed our test bone to be the width of 9mm and we determined that the difference between the design and the cut was of 17%. But still we don’t know the reason for this.
| ShopBot PRSalpha | Value |
|---|---|
| Clamping surface (x,y) | Full size |
| Working space (x,y,z) | 2490mm x 1270mm x 200mm |
| Step resolution | 0.0127mm |
| Maximum speed (x,y) | 15,24m/min |
| Axis drive system | Rack and pinion |
| Linear cutting force | 68 kg |
| Software | Shopbot |
Tests¶
We were granted access to a Shopbot machine, so we made new tests. We spent three days testing and even when the machine seemed to be working fine, and the measures of the cut parts were correct, the quality of the cut was horrible every time.
The Stepcraft was cutting very nicely, so we took the same material, end mill, and parameters to the Shopbot. Still the cut was not good.
These were our settings for the tests with plywood:
| Parameter | Value |
|---|---|
| Cut z | -11,2 mm |
| Passes | 4 |
| Plunge rate | 30mm/s |
| Feed rate | 25mm/s |
| Spindle speed | 12 000 |
| Then, we changed all the values trying to get a good cut. | |
 |
These were our settings for the tests with mdf:
| Parameter | Value |
|---|---|
| Cut z | -11,2 mm |
| Passes | 4 |
| Plunge rate | 30mm/s |
| Feed rate | 25mm/s |
| Spindle speed | 12 000 |
| Then, we changed all the values trying to get a good cut. | |
 |
Even when the dimensions of the “cut” piece seemed accurate, any test we made on the Shopbot failed in getting a nice cut. We tested many different parameter values.
After two days we satrted looking at other possibilities. At some point, we tested the spinning of the splindle… That was it! It was going counter-clockwise. It seems that when the machine was being moved, somebody misconnected a couple of cables.
After switching those cables, the machine started cutting nicely.