Week 7 - Group assignment - Computer-controlled machining
Assignment
Test runout, alignment, speeds, feeds, and toolpaths for your machine.
Design
We made a design file together during the explanation of the CNC machine. The design is made in VCarve pro. We use a piece of wood with a thickness specification of 18 mm. But never trust your wood supplier. We measure the thickness. It varies between 17.5 - 17.7 mm. We choose to use cut depth 17.6mm. This is the thickness of the cut depth.
We make two squares of 150 x 150 mm and two stars.
Square 1: outside line
Square 2: inside line & pocket
Star 1: outside line + pocket
Star 2: inside line
Result
Square 1: Outside line
Cut should be 150.00 mm. The actual measurement is 151.15 on one side of the square and 150.78 on the other side of the square.
Square 2: inside line & pocket
Inside cut should be 150.00 mm on both side of the square. The actual measurement is 149.57 on one side of the square. The other side 150.15.
Star 1, Star 2: outside line + pocket, inside line
The inside line star won’t fit into the outside line star. We did put dog bones in the points of the star. But we forgot to put dogbones on the corners of the star. Therefore there is not enough room for the press fit.
Runout
Runout is a rotation inaccuracy which occurs when the tool is no longer aligned with the main axis.
A drill will be x mm but the path it cuts will be bigger because it operates slightly wobbly. We have a 5 mm drill bit. So the cutout from both sides should be 2x5 = 10mm. We measured in the horizontal a 10.03 gap. And vertical 9.66 mm. This can’t be right because the 5 mm drill should at least remove 10 mm. But you can see that the inner square can not be entirely pressed to the side of the outer square. So the impossible measurement of 9.66 mm is due to this "unscientific" method.
The inside line square will not fit into the outside line square. We forgot to put dogbones on the corners. It would have fitted if we had applied them. The inner square when placed vertically in the middle of the outer square did fit.
For the pocket square we used a last pass to make a straight line. A pocket is made by sending drill back and forth. Because it is round it will not make a straight line but will have many bites in it. The last pass should make a straight line. This worked really good.
Test speeds
This test is done by milling at different speeds and look at the result. You do not want to go to slow because that will take much time. Too fast will not result in a pretty cut.
Test toolpaths
The toolpath is the traveling path of the machine. We tested the Pocket toolpath and the Profile Toolpath.
The Profile Toolpath will just mill the lines
The Pocket Toolpath will take out the material in between the lines
We also tested with the different endmills (sizes and flutes), the move speed (how fast the bridge moves over the X-Y axes), and the spindle speed (endmill rotation speed)
Drill speed: We didn't see much difference but 18000 was slightly better
Old drill v new drill: The new drill was much better
Move speed: the faster setting makes a less straight cut
Flute : The edges made by the 2 flute drill are less rough than the 1 flute drill