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Movable Bed CNC machine 24

I always wanted to do something with my old CD’s.

I must admit that I have a certain fascination for CD’s because the close lines on the surface causes interference of light which then breaks up into rainbow colours. By themselves, they are already beautiful.

But if I could add in some engraving into them, they would be even more attractive.

I had been blessed by God, and I would also like to become a blessing to other people.

St. Michael's Prayer on display

St. Michael’s Prayer on display

So I thought that the idea to engrave a prayer or a psalm or a proverb onto the CD’s so that people could readily have access to them would be a good idea. Furthermore, it is part of recycling and is good for the environment.

One day, I found a bundle of leaflets attached to the windscreen of my car. When I read them, I found that they contained a prayer to St. Michael. It was a powerful prayer. St. Michael is the Head of the guardian angels and is known to help and protect people against the snares of the Devil.

The complete prayer run like this:

St. Michael the Archangel, defend us in battle.
Be our defense against the wickedness and snares of the Devil.
May God rebuke him, we humbly pray and do thou,
O Prince of the Heavenly hosts, by the power of God,
thrust into Hell Satan, and all the evil spirits,
who prowl about the world seeking the ruin of souls.

I reckoned only good will come out of this prayer, and more people should know it.

So started my experimentation on engraving the words of the prayer into the spaces of the CD’s.

Outline of text

Outline of text

It was not as easy as it seemed. Firstly, I had to find out how to engrave single-lined text. In most softwares, texts when converted into paths for engraving, become outlines. So the text would turn out to have borders and each text would be an outline.

I managed to solve that by using Hershey Text. But there were some limitations on how I could use them. I could only use them as objects. Therefore, I could not make them follow a circular path.

Next came the task of fitting the text or graphic into the spaces of the CD’s. As you can see, the CD consisted of a center hole, and another circular area around the hub. These areas could not be used for engraving.

At my cnc machine, I have installed clamps which would be used to clamp on the outer edges of the CD’s. Clearly, I need to leave some space for me to clamp them on in place.

St Michael's prayer engraving

St Michael’s prayer engraving

The remaining surfaces could be utilized for engraving.

I made a template of the actual size of the available engraving space and used this to help me place the text and graphics in position. I made the reference home position of the cutting tool as the center point of the CD. I had already made a disc to place at the center hole of the CD’s. This helped me get the starting position correct.

The engraving of the CD’s were not exactly smooth sailing. After one or two successful attempts, problems cropped up.

Miss steps on engraving 1

Miss steps on engraving 1

The text became slanted towards the left. And because there were many words in the engraving, it took almost 45 minutes to complete. Things became worse when steps were missed and some texts got displaced and were engraved not in their correct position.

I did some troubleshooting and analysed the situation to find out what could have possibly gone wrong.

Missed steps on engraving 2

Missed steps on engraving 2

Looking at the slanting of text towards the left, I deduced that there were some delay in the X-axis. I suspected that the lead screw for the X-axis was having some backlash which could cause the delay in response.

Ideally, I would need a steel spring to force the lead screw nut against the lead screw so that the threads would always be in contact despite changes in direction. In other words, there should not be any free-play. But I did not have any steel spring available.

anti-backlash with rubber band

Anti-backlash with rubber band

I decided to make do with rubber bands, pulling the nut instead of pushing it against a spring. It was a quick solution and easily implimentable.

After a few attempts, and adding more rubber bands to increase tensions when needed and oiling the contact surfaces, I finally did reduced the backlash.

Next, to tackle the missed steps.

Heat sink on easy driver chip

Heat sink on easy driver chip

My computer was overheating and I think this had contributed to the missed steps. In my last article, I did mention that I tried to clean up the CPU heat sink without success, but was able to replace the computer. Replacing the computer might have partly solved my missed steps problem, but that was not all of them. I also managed to attach heat sinks onto the IC’s of the stepper motor drivers just to prevent them from overheating.

Missed steps on engraving 3

Missed steps on engraving 3

During the process of troubleshooting, I did try to configure the speeds, and acceleration of the axes. At one point, I made the speeds fast, but later found out that I missed many steps because of this. But I love the speeds. They were pretty impressive and very fast.

St Michael's prayer engraved on CD

St Michael’s prayer engraved on CD

Well, to avoid the missed steps, I had to reduce the speeds. That was a shame, but it was necessary.

Yet after I had done all these, I still find missed steps. That was when I deduced that the cutting depth of my work piece was too much.

Reducing the cutting depth solved the problem. Seems like the resistance of the plastic material of the CD was quite substantial. They could cause the motor to stall and created missed steps.

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