Quote:
Originally Posted by Al Skierkiewicz
While it is possible to produce better accuracy in the CIM in a closed loop configuration and with higher reduction ratios, there is a diminishing return on investment. This example does not take into account the accuracy of angular position within a 100:1 transmission and assumes it to be infinitely accurate.
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I agree the 100:1 transmission will not be infinitely accurate.
However:
As per the link above using a 3/16" end mill in a 20,000 RPM router you want a feed rate that will require 900RPM from the stepper.
I can post the output at 900RPM for a PK268M-02A but the result will be clear that it will miss steps at that sort of feed rate. It just does not produce enough torque at that RPM. I know I tried it.
On the other hand, I can gear down a CIM 2.5:1 and get to that feed rate at maximum power.
The feed rate does matter. It impacts the tool life and the quality of the result.
That was my reason for the link offsite there's a target 'sweet spot' wandering from it means compromises.
Obviously people manage this trick ignoring the 'sweet spot' to make the steppers work but at a price.
It reduces the tool life and it can reduce the output quality.
Cutting without any coolant at all or even wax will make things even worse.
All that being said your point about cost is extremely valid.
As I take the money I would have spent on CNC parts and buy motors and motor controls that FIRST approves of.
I often realize that I am consuming $100 a motor I could have used to run a CNC axis.
As I buy the FIRST legal batteries I often consider the cost I consumed that could have been the power source.
As I buy the appropriate wire and components I realize that I am now consuming my machine costs there as well.
The encoders to close the loop. Yes I could well avoid those on a FIRST robot but plenty of teams use them.
So once I buy all that stuff to comply with FIRST rules why not reuse it?
My point here is not to be 'better' than the Bridgeport or Haas mill.
From a rigidity perspective that is unlikely to happen.
My point was to make the ever growing pile of FIRST stuff year after year into something more than a dusty demo.
Well that and when someone makes an error they burn up a CIM or wreck other parts I can get from the pile.
Even if the quality suffers. These robots are not going to Mars or doing chest surgery.
The quality will still be way better than somebody with a vise and an unguided die grinder.
Besides I'm pretty sure it would be really nice to know just what sort of accuracy we can get from a CIM on a linear stage for purposes other than CNC anyway. Considering it's a finite element of the FIRST kit of parts.
Even if you had to make a gear box with 2 or more speeds that is well within the scope of FIRST these days.
Maybe you have a 100:1 ratio for accuracy and another for 100ipm.
Let us consider what my current build just for the motors and electronics costs:
Gecko G540 - $320 (includes step morphing)
PK286M-02A - $60 each x 3 = $180
Logic power supply = $0 - Got this otherwise would be $30
48V power supply = $0 - Got this otherwise would be $90-200
Spindle motor system = $180
So the cost of these cheap steppers: $500 and that doesn't include the power supplies since I have them.
Cost of parts from old robot:
PDB = $0
CIMs (probably x2 or x4) = $0
Batteries (you probably have at least 2) = $0
Wire (bet many of you bought the spool) = $0
Electronic motor controls (you had as many as you had CIMs) = $0
Lugs (bet you bought boxes of these) = $0
Parts missing from that $0:
Encoders? (you might have these already)
Gear boxes
Other interface hardware
As long as that 'Parts missing from that $0' does not exceed $750 - $800 you probably saved money not considering the spindle.
Considering the spindle add $180 to that $750 or $800.
So potential cost savings? As high as $1,000.
Oh and if you break something in the middle of the night on Feb. 1st?
You just need to find a part you probably have already.
Need several machines? Multiply.
Make plans that work for this and more teams build them?
Consider it a self-multiplying donation: that's $1,000 or less saved for each team that builds.
Multiplying the effectiveness of the existing donations from others.
Here's the thing. My stepper driven base with linear bearings and lead screws are already being assembled.
I have 3 Chinese encoders from the other topic.
As long as the gear boxes attach to the NEMA 23 mounts and don't block the assembly that is the real testing.
So this is hardly academic.