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

IMAGE IMAGE

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

IMAGE

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

IMAGE

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
  1. 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

  2. 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

  3. 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.

IMAGE

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).

IMAGE

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.

IMAGE IMAGE IMAGE IMAGE

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.

IMAGE IMAGE IMAGE

Dog-Bones and Bridges are a Dressup and can Be found in the CAM menu under ‘Path Dressup’.

IMAGE

Then to Export, you choose Post Process, look at the G-Code and save the file.

IMAGE

8.2.4.1.2. Milling

The Powerswitch of the Carvera is on the back side.

IMAGE

Then Start the Carvera Controller software and use the top left button to connect to the it (USB).

IMAGE

If the CNC was previously halted, you need to unlock the machine, so the motors can move again.

IMAGE

On the left side is the software-controller to drive the axis.

IMAGE

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.

IMAGE IMAGE VIDEO

Then we set x and y to zero on the workpiece where we wanted to start the milling.

IMAGE

Then we need to upload the file to the machine and then select it again for milling.

IMAGE IMAGE

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).

VIDEO

Then we reset the Z value to the measured one and started milling

VIDEO IMAGE IMAGE

So for this wood, the good fit was at 14.7mm.

8.2.4.2. Custom CNC AC2513F from CNC-Multitool

IMAGE IMAGE IMAGE IMAGE

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

IMAGE

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

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