Week 07 - Computer-Controlled Machining

Goals

• make (design+mill+assemble) something big (~meter-scale)
• group assignment: do your lab's safety training +test runout, alignment, fixturing, speeds, feeds, materials, and toolpaths for your machine

Week's explorations and achievements

• I had no experience designing furniture
• I had no exeperience using a large CNC milling machine such as the Shopbot Buddy.
• I practiced using Fusion 360 for parametric 3D models
• I designed my first ever piece of furniture, a desk to fit in my appartment which could also be used for my piano keyboard.
• I created the design in Fusion 360 using parameters for most of the dimensions.
• I exported the sketches to be able to cut them either with a laser cutter (for tests) or a CNC machine.
• I learned how to use the Shopbot buddy CNC milling machine, complying with the lab's safety rules.
• We did some tests with various mills and settings for the group assignment
• I learned how to prepare outline and pocket jobs in VCarve, in particular adding dogbones and T's to take in account the mill diameter and shape.
• I machined all my desk's parts with the Shopbot Buddy
• I assembled the desk's parts and enjoyed the great result in my apartment!

Group assignment

This week's group assignment was about testing settings of the CNC machine and document safety rules.

See the group assignment page

Furniture design in Fusion 360

Ideas, sketches and dimensions

I decided to design a desk where I could temporarly put a keyboard on. I want it to feature a small shelf to put some decorations or store some objects (small aparment, everything needs to be optimized!).

Here are the requirements for my desk:

• To fit in my tiny livingroom/bedroom
• To support my keyboard when I'll want to play it
• To have a shelf wide enough to support the fancy HAY baskets
• To be at a correct height for writing on the desk: a bit higher than my current keyboard stand
• Not to big visually
• To be cut with the Shopbot buddy

It should fit here, between my couch and my bed %)

The keyboard has been temporarly lent to me but while I host it I'd like to be able to use my desk as a keyboard stand. It's a Yamaha NP-30, its dimensions are: 1244mmx100mmx259mm, and it belongs to my friend Anne 8-)

I thought of a desk with two triangle shaped legs (making four legs). I want the shapes to be simple and rounded.

Design choices

Assembling

My initial goals:

• not to use fasteners
• being able to assemble it without glue
• use pockets to have the less possible visible impact

Sturdiness and steadiness

As I know nothing about furnitures deign I had to improvise a bit and ask for my instructor's feedbacks.

• I thought of putting 2 horizontal bars under the the board
• Or a triangle supporting the board

• Add extra foldable legs to support the front of the board when I'll play the keyboard

• Finally I just put a horizontal bar between the two back legs, for parallelism

• And used thick enough materials (22mm thickness), both for sturdiness and to have long enough tenons

CAD parameters

The most important parameter to define is of course the material thickness. I defined two thicknesses, a leg thickness and table thickness as I thought at the beginning that I'll make thicker legs. The shelf was intended to be made with the same thickness as the table, and same for the bottom bar I added at the end.

I defined some other important parameters, as the general height, the board depth and the shelf depth, the board height, and some secundary parameters.

Another value important for me but which I didn't defined them as parameters was the distance between the leg and the front of the board, in order to place on the keyboard on it. I couldn't make a desk wider than my keyboard as I have little place at home. I checked this distance by measuring it aftewards with the Inspect menu.

Final model

I didn't encounter any major difficulties, but I have to admit that I was sometimes a bit confused in the operations order.

For a similar project, my advices would be: * Draw some rectilign sketches, then extrude them and finally chamfer them * Create the pocket/ tenon sketches and extrusions before mirroring them

The operations / functionnalities I used were really straight forward:

• Create a sketch
• Draw a line in the sketch (rectilinear)
• Modify dimensions
• Create an offset plane
• Extrude, either with a parametric value (tenons, legs), or to an object (shelf, board, bottom bar), or a negative parametric value (pockets). Be careful to be consistent when you create a new component or choose the join option.
• Chamfer (I could have use parametric values here also, but I entered absolute values such as 20mm and 40mm)
• Mirror features
• Project shapes
• Constraints: collinearity, middle

Learning desk

Exporting sketches to .dxf

As I learned during the week 2, the cleaner way to export your sketches in dxf is to:

• create a new sketch on the corresponding face
• project the part outline (shortcut P)
• rename that sketch
• right-click on it then export in .dxf

Lasercut tests

I checked the overall design was fine by first laser cutting the parts in MDF. From the first design I thought I would modify the proportions for something higher and with a smaller depth (I really did good about the depth because I would have really struggle with the limit dimensions of the Shopbot with a 600mm deep board!).

0.1 scaled lasercut model in 3mm MDF

My second laser cut design is really close to my final one, except I changed the two bars below the table for a single bottom bar between the two back legs.

0.2 scaled lasercut model in 6 and 3mm MDF

Machining wood with the Shopbot Buddy CNC

Cutting maximum dimensions : 610mm x 1219mm x 127mm

Softwares: VCarve to prepare the toolpaths, Shopbot to monitor the machine

Files formats: DXF to import paths, CARV to save VCarve project, SBP to save your toolpaths and send them to the Shopbot

Safety rules

• Compatible clothing
• Don't use it when you're alone
• Safety equipments: safety glasses and ear protection
• Always watch / ear / smell what's going go during the whole machining, keeping the stop button in your hand

See extensive safety rules in the group assignment page

Tools

Depending on the operation, the material composition and its thickness you'll use different tools.

• small diameters for some finer operations
• downcut mills for pockets or better upper surface state
• regular mills (upcut) for better down surface state

There are a lot to know and to experiment but I only machined MDF wood and plywood.

Good practices

• follow strictly the safety rules
• as soon as you have placed and secured your milling tool engage the spinning
• always use the aspiration except if you machine soft metal
• always use the appropriate milling tool
• always do some tests to see if the feeds and speeds settings are correct
• do some assembly tests if you have to assemble parts
• always start by checking if you have some opened paths in VCarve to join
• add some tenons to hold your piece if you cut an outline trough the material
• add some dogbones if you have to assemble parts

Steps to follow for machining

1. Evaluate the dimensions of your work and choose an appropriate board. Take in account the fixtures you'll use.
2. Import your design in VCarve and set all your operations in VCarve, including choosing the tools for each operation. You might need to adjust your design by creating dogbones and T's in VCarve.
3. Save your first operation's path. Name it without special symbols!
4. Turn ON the Shopbot machine.
5. Check the absolute X and Y:
   • check that there are no obstacles
   • click on the icon in the red dialog

6. Install the milling tool

   • take out the aspiration
   • if there is already a mill inserted, take it off with the two keys
   • pick your milling tool and the adequate mandrel
   • place the mandrel
   • when you feel some resistance, insert your milling tool
   • tighten it securely with the two keys: the big one below in your left hand, the smaller one in your right hand and above, then bring the two keys one towards the other to tighten securely
   • put away the tools: the big key is tied to a small key to engage the tool, you'll then insert it and turn right to engage
   • double check it's engaged

   

7. Set the zero on the sacrificial board's surface:

   • check that the aspiration's holder part is up and secured. If not lift it and tighten the screw.
   • using the yellow dialog, put the mill above the sacrificial board (after checking there are no obstacles: you might need to lift the z.)
   • install the metal plate on the sacrificial board below the milling tool, and grab the milling tool with the aligator clip
   • click on the red dialog's icon to do the z zero: be ready to replace the plate or to interrupt the move by keeping the red stop button in your hands
   • when it's done, take off the aligator clip and the metal plate and place them back

   

8. Lift the head in the Z direction: be careful not to go above the Z-maximum level indication!

9. Install your board
   • Fix it with the clamps
   • Be smart and take in account the maximum course of the machine, and the places where it will travel: you don't want your clamps to be on the head's way (in particular the voluminus aspiration tube holder)
   • Check that your operation's dimensions will fit in
10. Set the relative X and Y:
    • Using the yellow dialog, put the head where you want your origin to be. We always use the bottom left corner as a starting point for our operation.
    • If needed you can lower the mill to see more accurately where it's pointing at
    • Double check that the dimensions will fit in with this origin start
    • Click on the button Zero Axes in the yellow dialog

11. Put back the aspiration

    • Shift x and y where you'll be comfortable
    • Raise z (not too much)
    • Release the aspiration's holder by loosening the screw
    • Put back the aspiration tube in its holder
    • Lift it back and secure with the screw

    

12. Go back to the relative X and Y: enter the value 0,0 in the yellow dialog and click on GO TO. This is just to check that this was done properly.

13. Check in VCarve that the operation you're about to launch is correctly set up (in particular the correct milling tool). If you feel safer that way you can resave the path.
14. Open the path file and preview it:
    • Load the file
    • Select the preview mode
    • Click on Start
    • It's emulated in a new window. If it's the file you expect you can carry on
15. Put your safety equipments: safety glasses and ear protection
16. Tell someone in your lab that you're about to start machining
17. Ultimate safety check:
    • The key is in the position ENGAGED
    • The aspiration is in place
    • I am wearing the safety equipments
    • I told someone in the lab I am about to start machining
    • I have the red button in my hand
18. Start machining
    • Select the move/cut mode
    • Click on start
    • Be ready to press the red button anytime
    • Validate
19. Now be very careful of all unexpected sounds and keep an eye contact with the machine during all the process Don't hesitate to press the red button if you have any doubts!

Cutting my desk project

Material

I used 22mm thick plywood boards. I used board scraps as I could, but I'd say that in total it represents 2 boards of 1200mm x 900mm

Tools and process

Choice of mills

Downcut and regular (upcut) mills both have their avantages and disadvantages. A interesting compromise, experimented by the Sorbonne's last year student Chloé Laurent is to first use a downcut mill to cut 75% of the thickness then switch for an upcut mill to cut through the rest of the board. This will give both a nice surface state on top and on the bottom of the board.

I thus used three different mills:

• 6mm width downcut mill with 2 flutes: to cut the outline of my parts through the 16 first mm of the board
• 6mm width regular mill with 1 flute: to cut the outline through the rest of the board
• 3.175mm width downcut mill: to create the 20mm pockets for my assembly. I chose this smaller diameter to have smaller dogbones around my pockets

Zero on the sacrificial board

My instructor advised me to always zero on the sacrificial board and not on the top surface of the plank, to be sure to cut all the way through but not too much, so that's what I did for my first parts. Of course, you have to enter this information in VCarve when you define the board's dimensions and thickness.

But while cutting my third part (the table board) I realised that my plank was too big to set the zero on the sacrificial board for the second tool! For this operation I thus had to switch for a zero on the top surface, which went okay but not excellent for the down surface. I had to come up with another way to make my two last parts. Here's the trick:

If you have to change the milling tool in the middle of a job but your plank is too big to set the zero on the sacrificial board once you positionned and secured it, create an outline somewhere in your board outside of your outlines (a "useless" part of the board). This outline must be larger than the metal plate you're zeroing with. You'll then cut this outline all the way through first, and you'll be able to zero the tool on the sacrificial board each time you'll change the tool.

I then had four toolpaths to run in the following order for the right leg:

• 22mm deep outline with the DC 6mm mill to have access to the sacrificial board for the zeroing
• 16mm deep outline with the DC 6mm mill for my parts outlines
• 20 mm deep pockets with the DC 3.175 mill
• 6mm deep outline with the UC 6mm mill (starting from 6mm above the sacrificial board) with tenons

For the last leg I could do the zero in a hole left by the first leg, so I didn't have to go through the first step

Feeds and speeds settings

Path adjustments: dogbones and T's

Since my object requires tight fitting for the assembly features, I cannot cut directly the tenons and pockets as I designed them in fusion. Indeed, the round mill has an important kerf (approximativiely equal to its diameter) and cannot create some sharp corners. To obtain sharp shapes, you have to tell the mill to step back in the material then enter from a bit further. An optimal way to do so is to create dogbones.

You can add them directly in VCarve by setting the mill diameter then clicking on the corners you want to place your dogbones on. We did a test during th group assignment, and I reproduced a test (but with the correct material and the 20mm pocket) with the dogbones. I cut my first part (the bottom bar) with 3.175 dogbone corners on my pockets and 6mm dogbone corners on my tenons.

Here you can see screenshots of my test parts where I added dogbones in relevant corners.

Afterwards I thought that the 6mm dogbone corners weren't very aestheic so I changed them for T's, hoping that they will still be sturdy enough.

Tests

I did some test parts (by saving a copy of my freecad project, then creating sketches, then drawing large rectangles, then Extruding > Cut in both direction through all)

Machining!

For the leg part remember that we have four toolpaths. The first one is an outline with a 6mm DC mill through all the material (0mm to 22mm) for the trapeze shape, with tenons. I don't include screenshots here. Reminder: the trapeze shape will just allow us to do the zero on the sacrificial board at each mill change.

Then the leg outline 16mm deep with the same 6mm DC 2 flutes mill and no tenons. If your zero is set on the sacrificial board, you'll have to enter 0mm for the starting value (D) and 16mm for the depth of cut (C).

For the pockets, don't forget the dogbones in the corner, adapted for a 3.175mm mill. Then set a 20mm deep pocket toolpath for a DC 3.175 mill 2 flutes.

Finally create a second toolpath for the leg outline, this time for an upcut 6mm mill (we only had a 1 flute one), starting from 16mm (D) and with a depth of 6mm (C). Don't forget to add some tenons to keep the part in place!

work in progress

For the second leg, I wanted to use the same plank to avoid waste. That needed some careful origin setting! I put the plank upside down and check with the first leg how I should positionned the second job. I fixed the board, draw a relative origin, then checked numerous times the measures.

Now I had to reproduce that in the software. I kept the first job as a reference.

• I turned it upside down as this was how I saw the outcut.
• I duplicated it
• I mirrored the duplicate version (in order to have the pocket on the wrong face of course!!)
• I turned it in order to have the orientation allowing me to optimize the material
• I resized my document to the available material
• I placed the new job very close to the document origin

Be very careful to always manipulate grouped shapes!!

Of course if your CNC has larger maximum dimensions you can machine the two legs at the same time, but that was not my case (max job width 610mm).

Here they are, all my parts!

Assembling

I did very little post-processing, just some rough sanding on the edges that were not cut so beautifully. I assembled without glue, since it's not in its definitive location. It fits well as is!

Hero shots

I assembled it in the Fablab, then disassembled it to bring it back home.

Feels like home!

Fits nicely in its tiny spot

Keeping it tidy is the real challenge

Seems great to write your documentation here

The keyboard fits perfectly!

Enough space to place the sheet music

Very convenient to read chords on the computer!

Its probably not the steadiest but yet I feel empowered by this very first Shopbot project, and a big one! If you're interested in making your own desk, you should maybe consider addind a rectangular plank in the back or angle irons to make sure that the orthogonality is preserved.

Files

• Fusion 360 Parametric file

• DXF Cut files for 22mm thick wood

• Bonus: the .svg cut files to make your 0.2 scaled model with the lasercutter. Use 3mm and 6mm thick MDF.