8. Computer-Controlled Machining - Week 7¶
8.1. Assignment¶
Computer-Controlled Machining
group assignment do your lab’s safety training test runout, alignment, fixturing, speeds, feeds, materials, and toolpaths for your machine individual assignment make (design+mill+assemble) something big (~meter-scale) extra credit: don’t use fasteners or glue extra credit: include curved surfaces
8.2. Group Assignment¶
8.2.1. CNC Machines¶
In our lab, we have following:
Makera Desktop Cravera
Stepcraft
Shaper
8.2.2. Safety training¶
keep flex low -> longer milling mit sticks out, more flex -> as short as possible, as long as necessary (flute length)
keep a distance and safety gear!!
FIRE!!
hair, weary cloth -> 20k RPM of mill -> HURTS BAD
stuff (work-piece, chips, MILLS!) flies away -> especially mill -> carbide -> hard -> kick-back -> work-piece throw
no gloves!!
8.2.3. Machine Properties and Parameters¶
We used the Carvera in our Lab to learn about those parameters and test them out, because this is the one we will use the most.
8.2.3.1. Conventional(CW) vs Climb-Cut(CCW)¶
Conventional : Movement direction of the workpiece and rotation direction of the mill are opposite.
Consequences:
not that good edges but the machine does not need to be that stiff
Climb-Cut : Movement direction of the workpiece and rotation direction of the mill are the same.
Consequences:
good edged but machine needs to be stiff
much force needed for cuts
8.2.3.2. Run-Out¶
Run-Out : The movement the Drill/Mill does around its rotation axis, which is never perfectly concentric.
Measuring
Using a dial-indicator we clamped on something rigid:
we gently touched the mill with it
set the scale to zero
and rotated the mill and looked in the changes of distance
Outcome
In our Case, event at the tip of the machine, we couldn’t measure a real distance. The distance we measured were just errors resulting from a not stiff mounting
8.2.3.3. Alignment¶
following 2 Properties are important:
8.2.3.3.1. Steps per mm¶
Meaning in the distance given in my G-Code should be the same as the actual distance the CNC travels.
Measuring
Just move in all available axis a distance and measure the actual traveled.
G1 X0 Y100
G1 X100 Y0
8.2.3.3.2. Perpendicularity to bed¶
The mill should be perpendicular to the bed for optimal milling results, but in reality, there is always a little angle between mill and bed.
Measuring
For a rough measure, use a right angle. Better: Attach an dial-indicator on the Z-Axes (Mill-axes) and drive around the bed and if the needle does not move, it is very perpendicular
8.2.3.4. Fixturing¶
Fixturing the workpiece is very important because of the possibly strong force that acts on it.
Also vor positioning Alignment Bits are very useful, because they are measured in and guarantee, that the workpiece is in the right orientation.
Possible Methods:
Clamps (T-slot, Nuts, Mitee-Bite)
think about overlapping!!
avoid using!!
Screws
Vacuum Table
Tape
8.2.3.5. Speeds and Feeds¶
Very dependent on the material and used mill. Should be determined with LISTENING.
feed-rate : Movement of the mill cutting in the material
should not be to high -> resulting in the translative force into the material to be to high -> moving it
should not be to low -> more friction of the mill and material -> burning/melting
Speed/RPM of Mill
not to slow -> increase in translative force resulting in the movement of the workpiece
not to high -> increase of friction resulting in burn/melt
8.2.3.5.1. How to find the optimal Speeds and Feeds¶
Use the machine defaults and test it with the mill on the material and LISTEN
If it melts/burns:
stop, clean
turn RPM down
manually step, if chips are produced, good
use reasonable feed-rate
Look at the chips and adjust the speeds accordingly
increasing the feed-rate (same RPM) results in longer chips
increasing the RPM (same feed-rate) results in shorter chips
Chips should be in a reasonable length and the machine should not sound horrifying
chips should be:
not too short -> material gets pushed more then chips are produced -> high rubbing -> short chips stick to the mill
not too long -> chips get tangled in the mill -> no good
8.2.3.6. Up-cut vs Down-cut Bit¶
up-cut : Flute direction like a drill, resulting in an upwards movement
Consequences:
gets chips out but no clean edge
down-cut : Flute direction opposite to a drill, resulting in a downwards movement
Consequences:
pulls chips down but clean edge
cross-cut : does both (split in flut direction), on the tip up-cut and down-cut on the rest of the flute, but usually more expensive
8.2.3.7. Step-down and Step-over¶
Step-down : Different depth-steps into the material to not mill the whole workpiece in once
usually the diameter of the mill
Step-over : The radius/diameter the mill is moved into the material/overlaps with the last path
8.2.4. Milling the Fixture Test Comb¶
I use Freecad for modelling and also for milling with the Carvera, and Fusion for the CNC from CNC-Multitool.
8.2.4.1. Makera Carvera Desktop¶
8.2.4.1.1. G-code generation¶
For our Test-wood (15mm depth) I designed a small comb with distances between 14.9mm and 14.4mm in a 0.1mm difference ( I forgot to add 15mm ans also values above -.- but in the end it was the right scale for this wood). We tested the reel wood on the bigger CNC.
Also I also modeled the workpiece and created an assembly I then used for the G-Code generation. Then to create a path, I first changed the units of the Project (and in general) to mm/min (for the RPM).
Then I switched to the CAM Workspace and started a job, there I first set the processor to mach3,mach4, which is needed for our Carvera. Then in setup I used the plane of the assembly as the sock (so an ‘existing solid’, also possible is using a box, I used that at the CNC from a friend of mine). Also important but can be done later is to create and adjust the used tool. There you need to set all distances and flute number of the tool. Also important is to set the Speed (Spindel RPM) there Freed (H(orizontal) and V(ertical))!! Also note the tool ID is important for the Carvera, because there are a total of 6 tool slots the machine can switch between.
Next I started a ‘Profile’ tool-path and selected the shape I want to mill around in the assembly. Important there is the start and final Depth. Also important to note, wee model, that Z==0 on the bottom of the workpiece, which inherently constrains the tool to move into to bet. Also here the step-down, Safe Height and Clearance Height (travel distance to bed of CNC) is configured.
Dog-Bones and Bridges are a Dressup and can Be found in the CAM menu under ‘Path Dressup’.
Then to Export, you choose Post Process, look at the G-Code and save the file.
8.2.4.1.2. Milling¶
The Powerswitch of the Carvera is on the back side.
Then Start the Carvera Controller software and use the top left button to connect to the it (USB).
If the CNC was previously halted, you need to unlock the machine, so the motors can move again.
On the left side is the software-controller to drive the axis.
Then we used the probe end to find the bed zero. Therefor, because we used a different mill, we needed to change the end of the mill so it fits into the CNC Head.
Then we set x and y to zero on the workpiece where we wanted to start the milling.
Then we need to upload the file to the machine and then select it again for milling.
Then we just milled in the air to test out the G-Code (here I found out, that I forgot to set the RPM in FreeCAD).
Then we reset the Z value to the measured one and started milling
So for this wood, the good fit was at 14.7mm.
8.2.4.2. Custom CNC AC2513F from CNC-Multitool¶
8.2.4.2.1. Outcome¶
tabs where bigger then needed, 3mm sufficient
Fixtures where nice and tight at 17.7mm
8.2.5. Tips¶
don’t forget dog-bones for fixtures
use minimal dog-bones -> place middle point of circle on a line 45deg. in the corner with its radius to the corner
plan screw in CAM - Programm
make a model in laser-cut
test-print joints
plan screw in CAM - Programm
For fixing:
(tape)
vacuum table (not for small parts)
tabs
straight flute also available
high chip load
easy to sharpen
the more cutting edges, the bigger the mill
posidrive vs philips head screw -> posidrive the sides are parallel -> philips the side is conical!
small parts screw down
clamping is very important -> clamp to far out -> wood may flex -> clamp to far in -> machine may drive into it -> take into account
wood is probably not exactly flat and has tension in it
screw
on tension -> may bend if cut -> more tabs, more screws
caliper -> don’t save money on them (helios-preisser in Germany)
chips on mill -> use something like WD40 on the mill against alu-particle -> also mill conducts heat better
8.3. Individual Assignment - Make something BIG¶
8.4. Notes¶
8.4.1. Freecad¶
change units (mm, mm/s, etc.) left in the project itself when selecting the project
selecting material
8.4.2. FerdiAcademy¶
@ CNC-Multitool
8.4.3. Material¶
plywood
2500mmx1500mmx18mm
quality 1-2
6 mm milling bit
0.01 mm steps can depend for a good fit
wood not same thickness everywhere
z=0.0 auf der Unterseite (Empfehlenswert)
8.4.4. MAKE SOMETHING BIG¶
Andrew Klein - twin turbo wise - in-klein
magnetic vice
https://www.youtube.com/@sliptonic/videos
8.4.5. Mills an Drills¶
extra mill for roughing (with rills in flute)
depend on material cut
HSS (high speed steel) mill -> alu and wood, plastic
carbite mill (dark-ish) -> harder (steel, etc.)
flute length -> wegen abnutzung
strong ridgit
not just tip, as loong as needed
flute number
radius
as much material as possible
stability