7. Computer-Controlled Machining
This page documents the group assignment for week 07 of Högni, Jóhannes and Ólöf.
Group assignment:
- Complete your lab's safety training
- Test runout, alignment, fixturing, speeds, feeds, materials and toolpaths for your machine
- Document your work to the group work page and reflect on your individual page what you learned
Overview and Approach
We did the group assignments in two different locations.
Ólöf worked with the Shopbot at Fab Lab Austurland.
Högni and Jóhannes worked with the Shopbot at Fab Lab Ísafjörður.
Notes on Health and Safety
It is important to take great care, when operating the milling machine.
- Always wear safety glasses and hearing protection.
- Make sure that clothing or hair cannot get caught in the machine.
- Locate the emergency stop buttons and familiarize yourself with the safety features of the machine.
- Don't leave the machine unattended and be ready to stop it if something happens or an unauthorized person enters the hazard zone.
- Tools and pieces of objects can break and come flying at people.
- Never use your hand to reach into a powered machine.
- Be careful when removing material to avoid cuts or burns.
- Make sure the machine is well maintained and in good mechnical and electrical condition.
- The person who is going to operate the machine also needs to be well balanced and ready to concentrate on the machine; watching, listening and smelling to make sure everything is as it should be.
- Small sparks can ignite fire. Be aware of all friction and overheating.
Below we show a photo with arrows pointing towards the important emergency buttons and mains switch at the Shopbot at Fab Lab Ísafjörður.
Fab Lab Ísafjörður
We performed several tests with the Shopbot using 12 mm birch plywood for all of them. We used a new 1/4'' upcut milling bit with two flutes.
Högni made the kerf comb test and used VCarve.
Jóhannes made the speed/feed test, the flexure test and the joint test and used Fusion 360.
Note
The dust collection shoe was broken on our Shopbot, so we had to live with dust in the room and on the workpiece. We used the vacuum hose after milling to remove the dust and chips.
Runout
We tried to measure if there is runout at the spindle. We had a dial gauge and probed the shank of a new 1/4´´ milling bit. There was no runout measured, so it is less then 0,01 mm.
Fixture and Z-zeroing
We fixed the stock piece with wood screws into the sacrificial MDF sheet below. For zeroing the Z-axis with the tool, we used the aluminium plate that comes with the Shopbot.
Speed and Feed
We milled several slots into the plywood in two two passes. The rotational speed was 10000, 14000 and 18000 rpm and the feed steps were 500, 1000, 2000, 3000, 4000 mm/min. There was an audible difference between the cutting operations and also a slight difference in the finish of the slots. However, none of the cuts was totally unacceptable.
Note
Later we realized, that we should have considered the grain direction of the top plywood layer and make sure to conduct the test both lateral and longitudinal.
The best results appeared to be with a medium speed and feed - 14000 rpm and 2000 mm/min.
Here is the Fusion design file.
Kerf Comb
Joint test
We decided to test regular tee joints and milled two parts according to our own drawing.
The real thickness of the 12 mm plywood was measured 11,7 mm.
The parts fit together in various ways, but it didn't seem to make a lot of sense, when comparing with the offset values.
When measuring the side lengths of the square piece we discovered something very odd.
The deviation of the piece was X +0,35 mm and Y +0,15 mm. If any deviation, we would have expected to be negative (because of runout), not positive!
This meant, that because of the shape of the piece and alignment of the slots, the width and length varies, which explains the strange fitment.
Our instructor had recalibrated the Shopbot in 2023, when he was attending the FabAcademy and documented about it.
So we assume this needs to be done to be able to succesfully finish this week's assignment. The deviation seems to be related to (uneven) wear in the rails/gears.
Recalibrating the Shopbot
As mentioned above, it was necessary to recalibrate the Shopbot.
Here is the correction factor calculated by diverting the expected with the real value:
Next the old machine values are corrected by multiplying with the new correction factors:
X: 0,9965122072745391 * 97,7487 = 97,40777279521674
Y: 0,9985022466300549 * 97,7487 = 97,60229655516725
After milling the test piece again, it measured perfectly 100,0 mm x 100,0 mm so that was a success.
Flexure
We tested flexure by milling different slot patterns, allowing the crosspieces to twist and therefore create a curved shape.
The size of each flexure area in the test is roughly 240 mm x 200 mm.
The table below states the dimensions of the flexure pattern 1 to 3 (from left to right) in millimeters.
| Flexure | 1 | 2 | 3 |
|---|---|---|---|
| Slot length (c-c) | 44,25 | 102,5 | 95 |
| Slot width | 7 | 7 | 10 |
| Spacing horizontal | 8,75 | 7 | 10 |
| Spacing vertical | 7 | 7 | 10 |
Note
Flexures are flexible, so you need to plan the toolpaths wisely and take extra care fixing the stock.
Then it was time to remove the fixture and feel how flexible it actually is.
Here we used clamps to hold the bends for the photo. We didn't want to break the flexure right away, so the shown flexure is achievable with reasonable force.
The first flexure with the short slots was least flexible with an estimated bending radius of 250 mm.
The second flexure was most flexible with an estimated bending radius of 90 mm.
The third flexure was in between the two other with an estimated bending radius of 150 mm.
Here is the Fusion design file.
Fab Lab Austurland
The Shopbot
Shopbot PRS Alpha
We have a Shopbot PRS Alpha in Fab Lab Austurland and it's mainly used for woodworking.
Joints and deviation
Joints and deviation
I made a file to measure how joints would fit and if there was some deviation in cut parts. I work in a vocational school and I used leftovers from a house building project. The material is called Oriented Strand Board (OSB). The problem with this material is that it is very uneven. When I measured it the numbers went from 11.80 to 12.20mm. I decided to use the average; 12mm.
Here you can see what the material Oriented Strand Board (OSB) looks like. It is very rough, but the pieces turned out fine.
Joints and deviation
There was almost no deviation. The numbers look pretty good! 100mm drawing measures almost exactly as 100mm.
The joints fitted well and it was good to be able to test different sizes before doing the individual assignment.
Importance of zeroing x-, y- and z-axis
Zeroing axes
It is important to zero all axis for safety reasons. By zeroing the x-, y- and z axis you know where the beginning point is an you can make sure that the Shopbot will work in the right areas. When you set up your job in Vcarve you have to choose if you want to zero the z-axis on top of your material or if you want to zero it on the machine bed. The material I was using, Oriented Strand Board (OSB), is very uneven and for that reason I chose to zero the z-axis on the machine bed.
Here you can see the alligator clip that is used to set the z-axis.
Emergency stop buttons
Zeroing axes
Before you start the Shopbot, be aware of how to stop the machine if necessary. We have two emergency buttons that we can press.
Changing endmills
As you can see, the tool that is used for changing endmills is fastened to the machine with a key so that the machine cannot operate while changing endmills. Very good for safety reasons.
