7. Large format CNC (computer controlled Machining)¶
In this week our assignment requirement was to create something with the CNC machine that has at least one dimension to be more than 700mm in addition to being a press fit design that could be assembled.
This week group work assignment was done by Batool check the link
Design process¶
initially I had a thought of creating an adjustable height table that allows me to use it in different chairs and as a mechanism I thought of having legs with holes that joint to the base with some pins and this was the imagination drawing.
but after consultation with AbdAlGhafoor and Faisal I tried changing the design again as the joint between the base and the table would be weak and I found a design that would have better structure this is the reference picture:
the only issue was the joints where the two pieces will connect as the material that I am using is extremely fit into the joints and it is not easy to disassemble and reassemble the pice over and over again and by time the joints might get loose therefore I chose to make a normal table but with a Z shape legs to be able to slide it under the chair.
I made sure to have good support structure while being able to sit comfortably and be able to get my legs through.
In addition, I was very sure to have good structure when it comes to the joints so I made them big enough to not break and made sure to not have big holes in the sheets to no lose it’s strength and not to break while assembling. I made my support pieces based on the reference integrating the support into the table surface itself and into the table legs locking it in place vary tightly while keeping in mind the strength of the wood structure.
the final Cutting design turned out to be like this
as you saw in the final sketch picture the details are everywhere the extra construction lines and the dimensions are everywhere so to not face any issues with the CNC machine we create a new sketch with those steps we first extrude the shapes we want and then after that we press "P" on the keyboard we select one of the faces and we hold "ctrl" and select the rest of the shapes
You should assemble the final design after doing the extruding to make sure that all the pieces fit together.
this is the final 3D design
this is the design final design assembly:
download the STL file here
Download the DXF file here
Download the editable file in Fusion360 here
Machining process¶
After sending the file to the laptop that is connected with the CNC machine the first thing we need to do is create the dogbone fillets but what are they?
Dogbone Fillets are a fantastic solution for the CNC industry because they let vertical plates fit into horizontal plates without worrying about the cutter’s residual radius. The size of dogbones required will vary depending on the part’s thickness and size.
The software that we use for the CNC machine "VCrave pro" has the dogbone fillet ready as an option so you just select the size of it and click on the joints you want to fillet
This is how the design turned out after rearranging in VCrave pro
Speed and feed rate and depth of cut¶
after arranging the pieces in the software and creating dogbones it is time to adjust the speed and feed rate at which the machine will be cutting the material but you would ask what are those?
Speed: Is the speed at which the milling bit is going to be rotating at. It could vary based on the material used in addition to having the feed rate in mind.
Feed rate: Is the speed at which the bit will be moving across the sheet that you are cutting.
These two must be selected carefully after testing as if the speed of the machine is not in balance with the feed rate it could put pressure on the milling bit and might cause it to break and fly off which is extremely dangerous or it might cause damage to the material you are cutting.
Adjusting speed and feed rate
This picture have been taken from Reynaldo Garcia’s websitecheck it out
Check out my friend Faisal’s website to see their feed rate/speed test
When it comes to depth of cut we set the machine to cut more than the sheet width which is 12mm and we choose 14mm just to make sure that the machine cuts through it with no doubt as the sacrificial sheet wears down.
the wear and tear of the sacrificial sheet comes to cause a problem ahead when it comes to engraving as it has an uneven surface resulting in uneven engravings.
A possible solution to this might be changing the sacrificial sheet or even doing a resurfacing of the sheet after having a high level of wear
next step is to bring in the plywood sheet and place it in the machine and it has to be screwed in place to not move
After stabilizing the plywood sheet we set the Z axis zero
when starting the cutting process this shows up and shows the path of the machine
After that everyone steps back to start the cutting
after finishing the cutting we have to remove the wooden sheet scrap and remove any left screws or nails from the wood to not get anyone hurt
Then the process of sanding the pieces begin we used a foil to smooth down the fine details and the sanding machine for the larger areas
After sanding all the pieces it is time to assemble the table and we do that by using the rubber hammer to press the pieces together
I needed help from my colleagues to press the more complex pieces Abdullah wasn't the only one to help Faisal and Hasan and Yaqoob also contributed thank you all
What I came to find out after finishing the assembly is that the joint sizes that I used were too tight which was the exact thickness of the plywood sheet. these settings were supposed to only be used on the MDF material rather than the plywood especially when there are some complex joints with multiple fits in the same piece which was my case. based on what I experienced I would recommend at leaaast 2mm extra in the joints (1mm for each side) and using the sanding machine is a must as it is extremely time consuming, in addition to paying attention to the bending of the plywood sheet as well as it might cause errors.
Hero shots¶
These are the final results and how the table turned out:
Some problems that were faced¶
A major issue that was faced was having a bend in the plywood sheet. When screwing down the sheet on the machine most of the bend is irrelative because it is stuck down to the sacrificial sheet but as soon as we take the pieces out we notice slight bends in the pieces which affect the accuracy of the fitment.
This error made me struggle so much with the fitment of the assembly and made me use excessive force to fit the pieces together which resulted in some cracks.
a small crack at the top of the table
a crack on the side
the bottom layer got the most damage because I had to fit it on my own and it was probably the most bent piece out of all the pieces
Overall¶
this week was extremely fun yet it was very exhausting but my situation was better than others and i believe that is because of how I approached the design and how I always asked for consultation from the instructors. doing this made me avoid fatal mistakes that would result in redesigning or changing the whole idea.
and as an advice if you do not have experience with wood work or sanding specifically I would rather go for the MDF material rather than plywood as it needs a lot of work in this regard.