Feedback for my machine

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17 Sep 2021 14:14 - 17 Sep 2021 14:17 #220895 by Jonas
Sorry for not being able to explain better. English is also not first language.

I am trying to design a small (650x600 mm footprint) desktop machine for milling aluminum. I think I want to closed loop. At least I do not want to design for it to add later if it can get better with closed loop without breaking the bank totally (since it is a hobby). Will be using Aliexpress/Stepper online stuff since otherwise one ball screw can cost $600.

Having hard time to choose what to use. I have read the posts on linear scales and see that people have tried different things. Wondering what the community has as input for choosing one of the below.

1.     Open loop stepper and driver. Cheapest but lacks feedback. This will maybe be as good as point 2 if we know that motor does not skip steps?

2.     Closed loop stepper driver system. This will give me information on where the motor is. Maybe no gain if we know that system in point 1 does not skip steps.

3.     Open loop stepper and driver and linear scales. Same “cheap” stepper and drivers (as in point 1) and add linear scales for feedback. Gives information where the table is instead of the motor, thus a little more accurate than point 2.

4.     Closed loop steppers (or steppers from point 1 with rotary encoder) and “normal” drivers. Steppers send feedback to LinuxCNC (through Mesa) and not the driver. Basically, the same as point 3 but measure where motor is instead of the table. Easier to mount than point 3 but “less” accuracy. Cheaper than point 2.

5.     Closed loop stepper driver system with linear scales. This will be most expensive. Is there a advantage of two feedback systems? Will one be used between motor and driver and then the linear scales will go to LinuxCNC (through Mesa) and make adjustments.

1 is of course the cheapest. But 4 seems cost effective and easier to mount rotary encoders instead of linear scales. Next step in cost would be to go for 3, little more money but harder to mount.
Last edit: 17 Sep 2021 14:17 by Jonas.

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17 Sep 2021 21:55 #220952 by tommylight
Replied by tommylight on topic Feedback for my machine

Normally i would ask how much you want to plop into this, but if using ballscrews at 20mm or over i would spring for some proper servo stuff, be it from Leadshine, Delta or preferably Sanyo or Yaskawa.
People cut aluminium with wooden CNC's so ....
On the same boat as i need something similar and i do have a lot of weird stuff around the shop, but thinking of doing two of them, i just have one problem, no place to get the screws cut and turned properly.
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20 Sep 2021 17:37 #221211 by Todd Zuercher
Many screw suppliers will custom cut/grind your screw ends to your specifications for a small up charge. (Often more than well worth the cost.)

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20 Sep 2021 20:03 #221228 by tommylight
Replied by tommylight on topic Feedback for my machine
Yes they do, last time i asked several years back they wanted 380 Euro for machining only 3 of them, not to much when you consider the price of the screws.
The issue is i have 2 of 2505 at 2.4 meter length and a 60cm 2505 that are not machined, so cutting them i can do, turning them on a lathe i can also do but it will not be usable as the lathe has 0.2mm off-center, the best i can do is have a friend turn them at 0.1mm, but still not good.
Goes to show that saving a bit here costs a lot later .... :)

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24 Sep 2021 15:38 #221505 by Roiki
Replied by Roiki on topic Feedback for my machine
Last time I did that, it was 40 for the screw and 100 for the machining. Per end. :P
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26 Sep 2021 14:45 #221637 by Twizzlermill
You could think about driving the ball screw nut instead of the ball screw, then the quality of the machining doesn't really matter.
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28 Sep 2021 07:09 #221728 by machinedude
Replied by machinedude on topic Feedback for my machine
to get back on topic here :) the feed back loops need to have a way to correct for error and work together. i would think of it like this. closed loop steppers or servos are only a semi closed loop systems when that's all that you are using. the encoder only picks up what is going on with the motor. so with a servo or closed loop stepper motor it does nothing to correct for things like backlash or a screw winding up under load. that's where the duel loop feedback system picks up the slack with a linear scale on the load.

the best thing to do is keep things as tight as you can so the system is as stable as you can make it. but a duel loop system is fully closed as far as the system goes.

i have plans to try this type of system out just because it's a different approach to precision ground screws and high end components. the duel loop method seems to have some potential to make a more accurate machine with less expensive parts. i just like to see how stuff works first hand. so what i see as a learning project might not fit into everyone's master plan.

if the duel loop system works well i don't see why it could not be used for rotary motion as well? time will tell :)
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28 Sep 2021 12:23 #221746 by tommylight
Replied by tommylight on topic Feedback for my machine

You could think about driving the ball screw nut instead of the ball screw, then the quality of the machining doesn't really matter.

I did a quick test of rotating nut some years back, it went at 60m/m with a Nema23 motor and 2505 screw, wobbly but bloody fast! :)
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