Seeking pro's and con's while I design my first CNC build
- Kylehopestolearn
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03 Apr 2019 22:15 #130239
by Kylehopestolearn
Hi all. Yes I'm new to this subject. I have been gathering information for about a month now. I want to say the videos on Youtube from Joe Hildreth (Webpage: myheap.com , YouTube: www.youtube.com/c/myheap) really helped me organize the info I have gathered and gave me structure to follow moving forward. My plans are to build a 3 axis 4' x 4' machine. A 4th axis in the future but I'll deal with that after. I plan on having a variable height table top as I want the ability to work on materials 1/2" thick to a rough cut log 2' thick. From what I gather this isn't a problem as long as the frame doesn't flex from weight or table top movement between jobs. I plan on welding the frame from metal so that's just a matter of engineering.
Onto the LinuxCNC questions.
1. Jitter - Most of what I've read involves video tests to stress the system. LinuxCNC doesn't stess the video much right? I understand it uses the CPU clock to run the motion and any video that consumes cycles on the cpu can be bad but really the Jitter value is just a value needed when the cpu is under heavy load right?
2. Linear Motion - Are there pro's and con's to mechanical gearing vs microstep's? Mentally I would love to build into my machine switches to change microstep settings and have different config files for LinuxCNC so I can modify resolution vs speed issues depending on the current use case. I plan on using screws, type to be determined.
I am doing my best to see things in pro's and con's as I notice so many camps of thought and practice in regards to CNC. I have yet to really define my use case as I haven't even built one yet but I do see soooooo many possibilities. Any advice or direction is very welcome. Thanks
Onto the LinuxCNC questions.
1. Jitter - Most of what I've read involves video tests to stress the system. LinuxCNC doesn't stess the video much right? I understand it uses the CPU clock to run the motion and any video that consumes cycles on the cpu can be bad but really the Jitter value is just a value needed when the cpu is under heavy load right?
2. Linear Motion - Are there pro's and con's to mechanical gearing vs microstep's? Mentally I would love to build into my machine switches to change microstep settings and have different config files for LinuxCNC so I can modify resolution vs speed issues depending on the current use case. I plan on using screws, type to be determined.
I am doing my best to see things in pro's and con's as I notice so many camps of thought and practice in regards to CNC. I have yet to really define my use case as I haven't even built one yet but I do see soooooo many possibilities. Any advice or direction is very welcome. Thanks
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03 Apr 2019 23:27 #130244
by andypugh
It you single-step and gear down then ever step will definitely result in motion. (unless it disappears in backlash or stretch in the drive-train). At high microstep ratios ever step might not result in a move. You have a magnet balanced between two coils, one pushing and one pulling. When you make the field of one ever so slightly weaker and the other ever so slightly stronger the actual torque delta between the two positions might be too small to cause movement. In fact it might take a few steps, but then the motor will make up all the backlog and might slightly shooth through. You will often read that microstepping reduces the motor torque, but this is not the case. The torque of a single step might be pretty small, but after a full set you have exactly as much torque as you would with single stepping.
(And if you consider a hypothetical system that only just has _exactly_ enough torque to move when single stepping, but you are running 256x microstepping then you would have to send 256 steps to see any movement at all. But that's a hypothetically bad design and no real machine will be quite that bad.
Replied by andypugh on topic Seeking pro's and con's while I design my first CNC build
Interesting idea, it ought to work just fine.I plan on having a variable height table top as I want the ability to work on materials 1/2" thick to a rough cut log 2' thick. From what I gather this isn't a problem as long as the frame doesn't flex from weight or table top movement between jobs.
It isn't entirely about load. Some peripherals can get in the way of realtime (in effect they get priority over our realtime code, and delay our execution). Video hardware is one common culprit, which is why running glxgears (to exercise the OpenGL system) is often recommended.1. Jitter - Most of what I've read involves video tests to stress the system. LinuxCNC doesn't stess the video much right? I understand it uses the CPU clock to run the motion and any video that consumes cycles on the cpu can be bad but really the Jitter value is just a value needed when the cpu is under heavy load right?
Yes, but it's complicated.2. Linear Motion - Are there pro's and con's to mechanical gearing vs microstep's?
It you single-step and gear down then ever step will definitely result in motion. (unless it disappears in backlash or stretch in the drive-train). At high microstep ratios ever step might not result in a move. You have a magnet balanced between two coils, one pushing and one pulling. When you make the field of one ever so slightly weaker and the other ever so slightly stronger the actual torque delta between the two positions might be too small to cause movement. In fact it might take a few steps, but then the motor will make up all the backlog and might slightly shooth through. You will often read that microstepping reduces the motor torque, but this is not the case. The torque of a single step might be pretty small, but after a full set you have exactly as much torque as you would with single stepping.
(And if you consider a hypothetical system that only just has _exactly_ enough torque to move when single stepping, but you are running 256x microstepping then you would have to send 256 steps to see any movement at all. But that's a hypothetically bad design and no real machine will be quite that bad.
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03 Apr 2019 23:28 #130245
by andypugh
Replied by andypugh on topic Seeking pro's and con's while I design my first CNC build
Oh, yes, and you need to decide between servos, steppers, closed-loop steppers, linear motors, servo-hydrauilc actuators, piezo-electric actuators...
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