CNC
HB2 Bearing Replacement (X – Axis)
Here is how I trimmed the Axis screws for the added thickness of the thrust washers (bearings). They are really being used as washers to keep the screws from impinging on the bearing seals.
I thought I might have to protect the screws when in the lathe chuck but I discovered clamping firm but lightly did no damage. Also the screw nut never gets that far down to each end of the screw. I generally use aluminum cut from a soda can for protection when needed.
I was surprised to see the “juices” leaking from the far end bearing The grease didn’t show while assembled.
The thrust washers turn with the screw shaft. The close up picture shows the washers do not ride against the bearing seals. They actually contact the flange on the inner ball bearing race. So in my opinion they are working perfectly as intended. Note the high quality of the thrust washer finished edge.
All axis screws are barely end loaded. Just barely on the loaded side of touching with zero end play.
After doing all the investigation and testing and more reading, I remembered a drive screw specification I studied long ago that affects how fast a drive screw can or should go in RPM. It all has to do with the end bearings support, screw diameter, unsuported length and preventing vibration. The best set ups are double bearings at both ends for rigidity. The lessons I learned here on the HB2 are the bearings ARE the most important factor for a good running machine. However, I doubt I need double bearings on each end. HB2’s screws theoretically could run over 3K RPM as they are. I’m not going there!
I have settled in… Continue reading
Euphoria
I think I need to make one point clear from my previous post. The ball bearings are still taking all thrust loads present. The thrust bearings (fancy name for washers) are there to spread the load evenly and transfer it to the center race of the ball bearing. I looked for washers with very accurate center holes and found them in the form of thrust bearings. I even considered making my own washers as precision washers are hard to find in small quantities. Thrust washers are not hard to find in small quantities. The thrust washer also protects the bearing seals from the damage seen on the first set of bearings. And lastly, there is one more surface for rotation when it happens.
The best thing I did was my own machining of the ends of the screws. I now know for sure that the thrust faces are smooth and true. The scratches on the first bearings really bothered me how they got there. They were not there when the bearings were new. I now have a 1/16 inch thickness of protective leaded bearing bronze between the screw steel and the ball bearing shields on both ends of the screws.
The proof is the Y screws are now running much smoother and even faster. The “clean up” did what I thought it would do.
The strange thing is when I do something like this, I sort of get a “high” from having solved a problem. Drugs are a lazy persons way to a high. Much better to gain euphoria the old fashion way, earn it! 🙂
An Improvement!
I modified the two Y screws on HB2 this evening. I removed the axis screws completely from the machine and chucked them (one at a time) into my Proxxon PD400 lathe. I cut back the 0.250″ diameter shafts at each end an additional 0.063″ – 0.065″ to make space for the 1/16″ thick 1/4″ x 5/8″ hard bronze thrust washers. I also made sure the screw shoulder was absolutely flat to the washer and square to the shaft.
I reassembled all four Y axis ball bearings (new) with a hard bronze thrust bearing (Alloy 932, sae 660) between the shaft and the ball bearing. The flat thrust bearings (McMaster-Carr 7814K11) cost $2.52 each so they were not cheap. I chose them because they are rated to take sudden loads without wear or damage. Exactly what happens in a rapidly and constantly reversing CNC machine.
The improvement in the Y axis operation is amazing! It runs like a totally different machine. I can do 150 IPM rapids (750 RPM) without weird sounds. I don’t trust it that fast so rapids are now set back at 125 FPM (625 RPM). Still louder than the X axis at the same speed but so wonderfully smooth sounding again.
I’ll take some pictures of the X screw rebuid to show what I did. That change out is next. It will be the same detail as how I did the Y screws. The addition of thrust bearings is certainly worth the slight effort and cost.
I may stop thinking about the motor resonance unless that is what’s limiting me to under 600 rpm. It’s really a non issue as I don’t cut anywhere near those speeds.
Good Feeling
I received the thrust washers for the HB2 yesterday. Great quality and FLAT washer type bearings. I am so used to seeing and using stamped washers with a crown and rounded edges and imperfect holes, it is a simple pleasure to see a perfect square edge. Why do I feel good about such a simple part? I guess it is just the precision of a well made part and the machinist in me.
It’s All Resonant?
I am having some thoughts that the HB2 stepper problem may be part of the stepper resonance phenomena. The two Y axis motors on separate channels but running in tandem may be creating a mechanical resonance between them. Kind of like tuning both engines to the same RPM in a twin piston engine aircraft. There is a “beat note” low frequency resonance between the engines as they approach the same speed. (I am a pilot.) The same effect occures in a twin engine boat.
The same holds true when tuning a CW frequency in amateur radio. Another frequency (BFO –Beat Frequency Oscillator) is offset about 600 Hz so the signal can be heard. In older radios it was possible to tune the BFO to the exact (beat) frequency and when very close you would hear “wow, wow, wow” beat frequency of a few Hertz beat. (I am also a ham radio operator)
That beat can get fairly strong and become resonant. I think this may be contributing to the stepper resonance sensitivity. The issue is the two Y axis steppers HAVE to run at exactly the frequency so they could be adding to each other’s mechanical feedback. Then suddenly one stepper falls into its resonance “hole” and quits, just stalls. All my crash stalls have been on the tandem Y axis. It doesn’t happen very often so it is hard to prove.
This is all wild and just slightly educated speculation. I can’t find any real data on the Internet about tandem drive resonance but will say it has to be there. Large machines may be able to absorb such resonant frequencies or damp them. My tandem Y stepper steps have ALWAYS sounded louder than the other single drive axis and I don’t mean just because of the sound… Continue reading