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-2/840

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

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.