Workshop
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
Boy! Whata’ Drag
Here are the pictures of both pair of Y axis bearings. The drag I suspected wasn’t on the outside case edge but right in the middle of the shields. I most likely started this problem when I was experimenting with end thrust. I have replaced these but not tested the new bearings yet. I’ll do that this weekend.
I found a source for some 1/16″ thin solid thrust washers if I have to use them. They are available in several types of material and the center hole is very accurate.
One shield shown here is actually dented. I don’t know what caused that but it had to be a very hard stop or tapping on the bearing. This was also on the Y axis screw shaft that was factory supplied turned slightly undersized (loose rather than slip bearing fit.)
I also note the scraping is not 360 degrees on the bearings. That indicates some shaft/bearing misalignment. That is a big clue.
The lesson learned is to keep the preload as close to zero as possible. Mine were and are now set where I can turn the holding screws at their heads and the nylon lock nuts will turn if they are not held. I have had them a bit tighter when I first set up the machine, which I soon discovered is incorrect. A thin thrust washer may have prevented the digging in scrape. I’ll also make sure the end cover (pressure) plate holding screws are adjusted equally side to side (a part of bearing alignment).
Oh No! Again
Broke a coupling again, one of the new ones. So the new coupling isn’t any stronger than my original design. This time it was in the X axis where there has never been a problem. I replaced a perfectly good original style coupling with the new one and the trouble began.
This kind of “random error” coupling failure has been driving me crazy, especially now in a place where it has never been an issue. It is a good example of how powerful the stepper motors are at low speeds. They can break things well above their holding torque ratings.
What I think happened this time is I may have changed the preload on the X drive screw end bearings when I replaced the coupling. Those bearings are straight radial bearings not designed to take end thrust loads. After I replaced the new failed coupling, I could hear a slight binding (groaning noise) near the middle of the axis nut travel. I spent hours fine tuning the X axis and double checking every alignment. The problem was I could not tell what component was generating the sound. The steppers themselves make a buzz type noise when rotated, even when not powered.
Finally I backed off on the axis bearing cap to as loose as I could make it without end play. I would say as near zero end load as can be set. The groan went away and the X axis started running extremely smooth with no complaints of a binding noise.
Now I am thinking my oversize screws may drag on the case or outside rim of the bearings when under any type of end loading. The original design was to use 1/2 inch screws. I am using 5/8 inch on X and Y. There could be some… Continue reading
PD400 in Action
I know some readers just like to look at action shots. So I have thrown a few pictures in here for your enjoyment. This is the Proxxon PD400 lathe and I am making the standoffs shorter that I use on the HB2 router. I installed the new couplers on the HB2 this weekend and I discovered I can bring the screw shaft and the motor shaft closer together using the new couplings.
I needed to remove 3.18mm from each end of the standoffs then extend the threads a bit deeper into the aluminum standoff. That was about 1/4″ overall for you non metric holdouts. 🙂
It was short work and the HB2 is purring along just nicely. So you see the PD400 does get a little workout from time to time. Oh, and it didn’t hurt that the lathe was about 2 feet away from the HB2…
Metric Thinking
I have been selling some metric tools (Proxxon) and actually using them too. I have discovered it is actually quite easy to work in either metric or SAE (inch) standards. There is no evil in either. I (almost) hate to admit I enjoy metric.
Of course the U.S. general prejudice to metric stemmed from our indoctrination, from what I now view in retrospect, to be a very lame educational system of the time. Post WWII there was a push for the USA to go metric. The requirement was to teach young children to exactly convert through (then) seemingly complex formula from one system to another. Remember, there were no calculators in those day. I could derive the equivalent answer but it made no sense why we would want to do this. The same thinking was crammed into our brains about temperature conversion.
In those days a student was not permitted to question the process but only to do as instructed.
As in learning a new language, it is very cumbersome to convert every word from one language to another. You only become efficient when you start thinking in the other language without the conversion. That is how metric should be understood.
I learned Morse code as a radio amateur. All radio operators will tell you that you do not become proficient until you stop counting dots and dashes and start “hearing” the sounds of letters. The really good can hear words. The very best hear Morse as a conversation.
When using metric hand tools I do not think of what millimeter is equal to in inch measurement. A good mechanic looks at a bolt and can say it is ½ inch or 12 mm, not stopping to think they are almost the same. When cars started using a lot… Continue reading