I liked the idea of engraving on plastic cd’s because firstly, they were attractive even by themselves, with reflective coatings and rainbow colors produced from the light interferences.
Secondly, I had lots of them because nowadays, many companies like to distribute financial reports, or marketing brochures in the form of files burnt into cd’s.
Thirdly, I had acquired many cd’s from the previous era when computers were not as powerful as now and when softwares, and other files were usually stored in the form of cd’s. Many of these obsolete softwares were cluttering my drawers.
Fourthly, my testing of engraving on a cd was quite successful.
During the previous engraving session, I had programmed the center of the cd as the home location. That was a logical choice for any round object. But unfortunately, the center hole of the cd was rather large, i.e. 15 mm diameter. So during that time, I was just estimating the exact center of the cd as accurately as I could, although it did not make much of a difference in the appearance of the final product as the estimated center was quite accurate.
But I did needed to put a white paper underneath the cd at the hole in order to make a mark of the estimated center with a ball point pen. That mark was to be the center of the cd.
In order to be more accurate, I decided to make an template disc with an accurately marked center point so that I could place it on any cd anytime and I would be able to determine the exact center instantly.
What I had in mind was to make a disc of the correct dimensions on plastic. I happened to have a piece of plastic which I thought would make a good disc. I suspected the material was polystyrene and it was a thermoplastic.
Thermoplastic melted and stuck to the milling tool bit
My first attempt to cut the material was disastrous. The plastic melted onto the tool, and the cut on the work piece was not clean. I also found out that the diameter that I wanted was not correct.
So I had 2 problems: plastic melting and dimensions wrong.
I tackled the 2nd problem first, because I reckoned that I needed to learn how to create a graphic and then interface it with the machine in order that I could produce reliable, accurate parts later. I had to master this skill first.
Thermoplastic cut melted
This was because any cutting tool that I used would definitely have a thickness. In my case, the current end mill bit was 3.175 mm (1/8″) in diameter. This meant that in order to cut my disc, I needed to create a circle 3.175 mm larger in diameter than my wanted diameter of 15 mm. That was assuming that the tool path would be in the center of the tool diameter. I later found out that this assumption was correct.
I did some experimentation with the software tool paths, stroke sizes, alignment, tool library selection, orientation points, feed rates, plunge rates and other settings and was finally able to produce correctly what I wanted to create.
However, the results of the experimentation could not be observed until I could solve the 1st problem – the melted plastic. This was because the resulting diameter was obscured due to the melting of the plastic. So at that same time that I was working on the software settings, I also did try to solve the problem with the melting of the plastic.
Milled to correct sized diameter
Reading up on internet, the recommendation found was to increase the feed rate. I did this up to a certain extend, but had to give up when my motors started to stall.
There was no improvement. The plastic still melted onto the tool and the work piece melted as before.
Milled to correct sized diameter with water coolant
I also tried to program my g-code such that the machine would cut shallower and also pause after every round. My idea was to give the plastic some time to cool down before I started the next round of cut. For the first two rounds, cutting at depths of only 0.2 mm each time, these actions resulted in clean cuts and I was quite encouraged.
However, the plastic started to melt again when I did the next few rounds. Perhaps I was too impatient and did not wait for the plastic to cool sufficiently long enough before I started again.
Milled circle showed the correct diameter
In desperation, I tried to cool by water. The work piece was in the form of a shallow box, and so I was able to pour some water onto the work piece and immerse the tool and the work piece in about 2 mm depth of water.
Well, that did the trick. I got a clean cut and my problem was solved.
But I did not like the idea of using water for cooling because my machine was made principally out of plywood and fiberboard and water would cause untold damage to these if I were not careful.
Well, maybe this could be my last experimentation with thermoplastic. But the tinkering around and the lessons learnt was indeed valuable. On second thoughts, I could also fabricate a shallow container for machining items that needed cooling jobs like these later on if required.
Meanwhile, let me try machining other types of materials first.