I took that to mean positional accuracy down to 0.025mm as well. Personally, I think they’ll fix it before release.
Fix the spec or fix the mechanics?
I don’t think 0.025mm accuracy is achievable with belts and V rollers unless it has closed loop control.
.001" is both the inches-per-line and the positional accuracy.
–dan
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Really? So you have a closed loop control system? Open loop stepper motors and belts would be nowhere near that accurate. Even the thermal expansion of steel would give a bigger error than that over 500mm with a 3C temperature change.
You certainly don’t need closed loop to have 1-thousandth accuracy. How big are you imagining the pulleys to be and how many pulses per revolution do you think they’ll be limited to?
I think you are confusing resolution and accuracy. Yes it is easy to get that resolution, for example with 20 tooth GT2 pulleys you get 40mm per revolution. A 200 step motor with 8x microstepping would give a resolution of 0.025mm. However the accuracy of a typical stepper motor is +/-5% of a full step. That would make any particular position vary by +/- 0.01mm from its correct place to start with. Any friction would cause some backlash reducing the accuracy and so would thermal expansion of the belts as the distance travelled depends on the belt pitch which would change if they were stretched on a steel chassis.
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I don’t feel like having this conversation. Sure, any system that does not compensate for thermal expansion will lose accuracy when the temperature changes. The motors (steppers or servos), linear bearings (V groove, rail, rod, ways, etc.), and method of motion transmission used (belts, screws, rack and pinion, anything else) is completely irrelevant to this fact.
I’ll concede that I do not think parts will be coming off of Glowforges with an accuracy down to 0.001". How would you feel if the claimed positional accuracy was ±0.002"?
Comparing the top of the cut to the bottom will probably reveal a discrepancy of a thousandth. In fact, a taper of just 1º in 1/8" material will be off by ~2 thousandths. No need to look any further than that.
Yes ±0.002" would be OK but not what I paid for. However the photo @takitus posted looks like the machine has alignment errors about 3 times the kerf when engraving.
@dan, are you saying the engraving errors are not down to positioning inaccuracy?
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While it is an internal linear actuator, one of the bipolar steppers I use in my CNC coil winder has a resolution of .00012" per step. So on the mechanical side of a stepper resolution of 1 thou’ shouldn’t be out of the range of possibilities. Can’t comment on the belt drive aspect.
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All I know is, the apparent positioning issues in the image above are concerning. At about 2 months before shipping I would expect fundamental issues like cut positioning to be put to bed. However, I do not operate a manufacturing company so I will have to trust that everything is on track, or that the gap I noted is a problem with the source artwork.
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Camera accuracy sets positional accuracy. Since the macro lens picks up the Glowforge logo on the top of the cutting head to know where it is.
So, regardless of thermal expansion or any other factor… the machine will know exactly where the cutting head is, and then can move it based on maximum resolution.
Building a machine with enough resolution to achieve the stated accuracy is definitely possible, you just can’t achieve that much accuracy in practice unless you can measure and compensate for things like backlash and thermal expansion.
They could use the macro camera to implement closed loop control (i.e. take the number of steps the controller thinks is necessary, use camera to check position, adjust as necessary) but since it only has a resolution of 0.05mm they will struggle to achieve an absolute positioning accuracy of 0.025mm without doing something quite clever or making use of an additional, more precise position feedback system.
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Yea having a resolution of 0.025 mm is not hard but having an accuracy of 0.025 mm over 600mm I.e 1 part in 24000 is hard to believe. I think it would need to be closed loop with position sensors that are immune to thermal expansion.
I’m sorry if I was unclear. I said that there were three separate causes for the various observations, all relating to low resolution mode, and that improvements are forthcoming.
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How did the low resolution lead to the comb appearance on the vertical edges?
I’m going to sit tight on the details of that since explaining would reveal some cool engineering around engraving optimization that we don’t want to share.
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