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
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
What will it cost to make this for me?
I receive requests from designers and people with ideas asking if I can make something for them. Most of the time I say no, because I have enough projects of my own on which I would like to be working. There are also some designs that are beyond my means, usually because it is too large or requires special tooling and materials. (I wish I could charge the designer for the new tools!!) Some if not most of the designs also suffer from knowledge of how things are made on machine tools. The sketches and drawings show holes where they can’t be drilled or unnecessary and difficult areas requiring multiple setups.
These requests show that there is a need for prototyping services and these inventors and idea people have problems turning their ideas into products.
But prototyping is not as simple as sending out an unproven design for bids. (Yes, I know it is done.) But I am talking about solicitations from hobbyist and small time inventors who have never worked with a prototype or even in a machine shop. A good design is one that can also be made as inexpensively as possible on standard machines and tooling. That seldom happens on the version #1.
What I am saying is the folks who approach me don’t realize their design may need a lot of cooperation (face time) between the designer and the maker. Of course I am not talking about a bar of aluminum with two holes drilled into it. The designer can do that himself. I see the hard stuff, like machining threads on a very thin tube and the tube is thinner than the thread depth. (Yes, I have seen this.)
Outsourcing prototyping is not inexpensive. Building a prototype may cost 100 to 1000 times what… Continue reading