I have decided to use the same Z-axis design for both the movable bed cnc machine and the upcoming moving gantry cnc machine. In this way, I do not have to buy another spindle for the new machine, but can use it interchangeably for any one of the machines.
I can then save some money for other output devices like laser or drag knife. I am quite keen on trying out laser, as it is capable of cutting very finely.
Making the Z-axis housing turned out to be quite a challenge because it consisted of many components that had to be aligned in a very constricted space.
First, there was the stepper motor that was attached to and drove the lead screw. The stepper motor came with a long shaft. I had to work around this to drive the lead screw through a flexible coupling. I did not want to shorten this by cutting it, so I built the assembly to suit.
So the length of lead screw had to be measured and cut accurately to fit the housing. Like before, I had to cut the lead screw rod to the correct diameter so that it could fit into the end bearing. I did this by manually grinding on an angle grinder with the help of a jig.
Getting the motor-lead screw assembly to be accurately positioned in a vertical plane was a challenge, because it would mean adjusting the position of the bearing bore – something which I was not ready to do at this time. I hoped that the measurements marked out for drilling and the drilling itself was accurate enough.
Next, I needed to construct a holder for the lead screw nut. As per my previous and current design, this would be just a piece of plywood with another piece glued to it at right angles. The lead screw nut would be bolted on one piece while the other piece would hold a linear bearing housing. Again, hopefully, it would be accurate enough.
After this, there would be a linear bearing piece that would slide through 2 parallel round rods. Making this sliding piece able to slide smoothly involve making the round rods parallel to each other.
This meant that the holes where the round rods were secured need to be exactly vertically in line. In addition, the z-axis tool travel should be moving exactly in a vertical direction, up and down. This meant the round rods had to be both perfectly parallel and perfectly vertical.
This was almost impossible to achieve with my crude drilling equipment, so I had to enlarge the holes when adjusting the positions of the round rods. I think I would have to come back to adjust the whole assembly after everything was in place.
To make matters more complicated, the movement of the lead screw nut and the linear bearings should be perfectly parallel, otherwise some of the moving parts will get squeezed at some point.
The above described the complications of the moving mechanism.
Now for the attachment mechanism of the spindle. Following the existing design installed at the moving bed cnc machine, the housing for the spindle would have 2 bolts for attaching to the moving mechanism of the Z-axis. Again, I was using the holes that were already in place at the linear slide housing.
I wanted the housing spindle to be readily detachable so that I can easily transfer it from one machine to another. Here, I was not sure in the existing movable bed cnc machine, how I managed to attach the nut for bolting the spindle holder in place. I suspected it might have been glued in place by super glue.
For the new machine, I tried several times using super glue and epoxy resin to glue the nut in place, but was not very successful. Despite the claims that the super glue could withstand 4 tonnes per square inch of pressure, it could not stand my light tightening of the bolt. I was not confident at all that the nut would stay in place after some repeated use. It would be very difficult to extract the bolt from the nut, if the latter worked loose and turned.
Then, I decided to use the existing movable bed cnc machine to mill a hexagon pocket into the linear bearing housing so that the nut could be inserted and would stay in place.
It was a good idea except that I had no experience in milling on aluminium material of which the linear housing was made of. On top of this, because the aim of the milling was to ensure the nut stayed in place, the latter must fit accurately into the pocket in the aluminium housing. The corners of the hexagon pocket had to be quite sharp. This meant I had to use a small diameter milling bit like 1 mm in order to achieve this.
But I was up to the challenge! Machinists had been doing micro milling on hard materials all the time. I looked up the internet and found out how to calculate chip rate, feed speed, plunge speed for aluminium running on my existing spindle speed of 6000 rpm and adjusted my g-code accordingly. I did my first attempt at milling an aluminium work piece with fingers crossed.
At first the hexagon pocket was cut very nicely. However, towards the end of the milling process, the 1 mm milling bit broke off. That was my greatest fear. What had gone wrong?
Looking through the internet for information, I found out that milling tool bits often failed because of too high feed speed, lack of lubrication, use of bits with too many flutes, too high plunge speed, chips not removed, machine run out, machine flexing among others.
Analyzing the situation, and believing that my spindle and machine was in good enough condition, I decided that the easiest step that I could take would be to prevent the aluminium from getting so hot as to seize up on the tool. It’s by either spray mist cooling, lubricating or air blow cooling. I decided that I could do lubricating easily with penetrating oil. I had the equivalent of WD40 and I could try this.
Well, I lubricated the cutting area with penetrating oil spray and it worked! The aluminium piece was cut until completion and the 1 mm milling tool still remained intact. This was quite a feat, since I found out that the tool was actually very fragile. I broke one just by accidently touching it lightly with the vacuum hose while vacuuming the moving bed immediately after the cutting job was completed.
Now, I made it a point to remove the tool from the spindle immediately after the cutting job was completed. Just to avoid breaking a tool.
Spurred on by the successful cuts, I made another 2 cuts in another linear bearing housing. This would be a spare in case the movable bed cnc machine needed this. The lubrication really did the trick. The 1 mm diameter milling tool did not break during the cuts.
However, as you can see from the video, the chips were more messy, but it was worth the cleaning up because the cutting tool could be used again.