The Y axis for the movable gantry cnc machine was originally designed to be driven by GT2 timing belt.
2.5d engraving showing elliptical ball instead of round ball
Another problem that I faced with belt-driven movement was the fact that it was difficult to maintain consistency with the belt tension. Although I tried to make the adjustment of belt tension and its locking in position as simple as possible, I found that I did run into problems of either it being too tight or too loose. When it was too tight, the motor could encounter too much load, which could lead to motor slippage and lost steps. Conversely, if the belt was too loose, the linear movement transmitted from the motor gear might also generate some lost motion, especially during reversing movements. In addition, the belt could also stretch after some time and this again would require me to repeat the same estimation of optimum tension.
Prior to this, I had temporarily solved a problem of two opposing tooth faces of the belt getting caught with each other because the space available in between them was very tight. I just placed a thin piece of steel plate between them, but really, this was not an ideal solution. Actually, the space for placing the belt drive mechanism in the cnc machine Y-axis was quite limited.
With all the overwhelming reasons above, I decided to convert the Y-axis to be lead screw driven instead of belt driven. So, its back to the drawing board.
The main component had to be ordered – lead screw 8mm diameter 600mm with anti-backlash nut. However, when the lead screw was delivered, I found that it was slightly bent. I almost half expected this because I bought it online. The anti-backlash nut was perfect.
Next, came the task of preparing the shaft to be coupled with the motor. As I did not own a lathe, I had to prepare a jig so that I could use my hand grinder to grind the lead screw end to fit a motor flexible coupling in a controlled manner.
I made use of my movable bed cnc machine to fabricate ball bearing housings so that I could make use of them to maintain the lead screw center line while I turn the lead screw by hand. I also fabricated some sort of crude screw jacking mechanism to assist me to move the lead screw laterally against the grinding wheel so that I can skim off the outer periphery bit by bit and thus gradually reducing the shaft diameter.
As I mentioned before, the lead screw was slightly bent. To avoid whiplash and other problems, I decided to try to bend it back as straight as I can. I think it had become like this through the handling during shipping and so I could bend it back again cold.
My plan was to determine the spots with the most deflection, and to bend it back in the opposite direction. From my observation, whenever there was a bend, the round shaft would find its most stable position and rest on it when the largest footprint was formed between the bend and the rest of the shaft. If there was no bend, the shaft would simply roll and not stop.
So I made another jig to help me bend the shaft back in the opposite direction. I had to be careful not to damage the threads on the lead screw, so I made all the contact points out of wood. In order to help me apply a spot load by hand, (I don’t want to over bend) I fabricated a piece of cam handle so that the load could be applied sideways at any spot that I wanted.
After this, it was checking for the high spot, clamping the shaft in place, applying the load in the opposite direction, removing the shaft and checking again. I did this for a few times until I was satisfied that I could do no more. I did not want to overdo this as this could result in the shaft being bent too much in the opposite direction and I still won’t have a straight shaft.