Sally Williams @ Fab Academy 2015

Assignment: 

• Make something big


Design

Last week, a dear friend of mine asked me "So what's your project this week Sal?!". When I told her the brief was captured in three words "make something big", she said "Any chance of a new garden bird table?" I thought it was a nice challenge.

I had witnessed many times, magpies and crows swooping and stealing bird feed from the smaller more vulnerable garden birds on her bird table and so I wanted my design to offer some element of protection for them. Something that would enable them to move freely (due their size) and make swooping difficult for the larger birds, whilst not prohibiting the view from the house window.

After an initial idea and sketch, I set about cutting some cardboard by hand so that I could see the form as soon as possible in space.

  

I then set about tracing the cardboard rib as a vector drawing in Affinity Designer:

   

I tried to draw the vector paths in both Affinity Designer (£39) and Inkscape (free). As I am new to both applications, a large part of the week was trying to create these paths and refining design and structural detail as I went along. Given my design was quite tall with minimal in structural support, my tutor pointed out that a lock joint would help to give strength.

Affinity has some very powerful and sophisticated tools for recognising geometrical relationships and snap-to tools. Moving to Inkscape after Affinity feels like a downgrade, then again, Inkscape is free.




If I had more fluidity in 3D CAD I would have definitely tried to progress further using Rhino than I managed to do.




A 3D model of the design in space would certainly help me identify any potential design flaws. I imported the rib into Rhino to see the structure in space and be able to more easily slice and select 'planes' for intersecting pieces, but in the time available I was not able to progress quickly enough, so I focussed on preparing the form in 2D to create a pdf for the Shopbot Milling Machine.

HEALTH and SAFETY

Health and Safety is the most important aspect of working with the Shopbot machine. The tool itself is surprisingly unguarded and moves across X Y Z axis, so its critical to be mindful at all times. 

When handling materials, gloves should be worn to protect from splintering. Goggles must be worn to protect eyes from the possibility of loose pieces flying off the bed, or worse, the tool itself shattering. The milling action creates a deafening noise compounded by an extractor fan, so ear defenders are also must. Although we do have an extraction fan in the lab, I could feel the irritation of the dust particles so I also chose to wear a mask.

I am under there somewhere.

 
Setting up and preparation for Milling

The material I had chosen to work with was 12mm ply. In actual fact the material varied between 11.0 and 11.4 and this had to be accounted for in the dimensions of all joints; an average of 11.2mm was applied. The Computer Numerically Controlled (CNC) Machine that we had at our lab was a ShopBot machine. The software used to create the toolpaths was Partworks.

Once the material was placed on the bed, it became apparent that one corner of the piece was slighlty warped. This made clear the importance of laying the material as flat and sound to the bed as possible, so 7 points were chosen to screw the material solidly to the bed. I had to refer to the layout of the design to ascertain where to best place the screws - ie, in the waste sections and where they would be most effective, considering any weak areas and those prone to vibration.



The machine was now ready to have the X, Y and Z axis 'zeroed' (- the origin offset is never used in Partworks) .
The machine was powered on and the tool column could now be moved by left-right on the PC keyboard to bring the middle of the tool bit to the origin of the bed / material. Finding the X and Y axis is straight forward and done by eye alignment along the length of the material.

To determine an accurate zero for the Z axis and to avoid damaging the material (which is soft relative to the powered drill bit), the Z axis zero origin is detemined by use of a metal plate of a certain depth (known and previously calibrated in Partworks), which is placed solidly on the material (away from the edges) and under the drill bit (choose a clean undamaged area). A metal clip is attached onto drill - this establishes an open electrical circuit. Upon selection of Z-zero on the PC control panel, the tool tip automatically travels down and detects the right contact with the metal plate (closing the electrical circuit - check for green light in the control panel). This process completes the Z-zeroing 



Checking and amending the toolpath in PartWorks (ready to mill)

Partworks (the software that controls the shopbot) is where final checks and refinements are made to the pathwork before calcualting  the final toolpaths to mill.

Dog and T-Bone fillets

Most joints are square in form. However a milling tool is circular. The 'Dog-Bone' and 'T-Bone' fillets allow toolpaths to mill into corners of slots and grooves, allowing mating parts to fit flush. This also allows the limiting radius of the cutter to be removed from the bottom of slots. This ensures the strongest possible engagment and fit. Choice depends on the needs of the design and whether the joint can be visible or needs to be hidden for aesthetic reasons.

      






When I transferred my pdf file to Partworks, all the vector lines were closed as required, which is good. However there was quite a bit of work to do. For example, we found duplicate line work which needed to be deleted. Also, in some places, the click-on-path feature in PartWorks to assign a dogbone fillet in a corner would not work in every position. In some places, new traces had to be drawn over existing lines and then the fillets would work. In some cases, by reducing the radius of the cutter in the dogbone settings, the fillet would also work, as the software would not allow the fillet to extend beyond the width of the slot.

In future I would choose to go through this process outside the shopbot PC environment to make sure it was truly ready to mill.

Toolpath set up

There are 3 toolpaths to choose from. Profile, Pocket (for scooping out) and Drilling.
I chose 'Profile' toolpath; outside; climb. Tool was going to be 6mm downcutting tool, which would give the best finish in this set up. Settings are cpatured in screen grab below:




Depth of cut passes were edited, taking into account the depth of the material, and the amount of passes calculated by Partworks. This is optimised so that the best finish is obtained on the last pass of the mill, ie it gives best result when the tool has  around 50% of its pass in the material to be cut and 50% of the tool  passing through the sacrificial layer. This helps to avoid tearing on the last pass.

Adding tabs

In order to keep the material being cut to remain stable and safe under the power of a 18,000rpm end mill, tabs are added to the toolpath. The terminology is misleading, because, in fact the reverse is happening; 'adding a tab' means that the end mill toolpath skips a few layers (around 4mm sq area) in the toolpath, leaving a link to the main body of the material so that it does not fly off.

  Example of a tab
 
Tabs can be placed evenly distributed around the pieces being cut, and in particular where there may be increased weakeness, risk to fly off or potential tendency to vibrate and cause damage to the finish.



Toolpath summary:




Mill cutting the plywood

I was very happy with the quality of the cutting, it was clean and sharp.
However there was still an hours worth of sanding to do around all the freshly cut edges.
 
  

The photo above shows the detail of the locking joint used, that would give further strength to the design, as well as clearance from the ground. The bottom edge of this piece would provide a third foot to lift the table off the ground (as three contact points gives the greatest stability). Before, leaving the lab, I tested the joints to ensure that the tolerances of the assembly were workable. There was good firm and solid engagment in the clearance between the pieces.



However, as I did not want to put any unneccesary stress or torsion on the structure (in transit in my car), I took the flat pack, carefully protected and packed, to my friends garden to assemble in situ.



To help ease the joints together, I used grease in the form of lipbalm smeared on the edges. This definitely helped the assembly push-fitting process and reduced the stresses so that any fragile joints could be protected from fracture.

   

View 360 degree movie here
Download design file here

Pdf here