Smooth idler kit?

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Smooth idler kit?

Postby BenJackson » Sun Sep 18, 2011 9:58 pm

After cutting a few things meant to press-fit together I have noticed an inconsistency in my Y travel. It's particularly noticeable if you cut something like a dinosaur puzzle in 3mm acrylic: The fit of the slots will vary depending on orientation and location.

Just observing the Y idler and the Y drive pulleys I think this is due to wobble. Is there a kit for this? Does it only change the idlers or does it do something about the Y drive shaft?
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Re: Smooth idler kit?

Postby dirktheeng » Sun Sep 18, 2011 10:17 pm

Ben,

I'm noticing the same thing and I have the smooth idler setup and precision shafts on my setup. There isn't a bit of wobble in my setup. I am not convinced that the problem is not backlash in the drive pulley/belt. I think it can varry by about 10 thousanths or so. Also, did you check your table for flat? Are you absolutely sure that your beam is aligned perfectly? Those could also be culprets. I am actually thinking that beam alignment could be a big player too. How do we know that it is aligned within 10 thousanths?
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Re: Smooth idler kit?

Postby BenJackson » Sun Sep 18, 2011 10:47 pm

I engraved a ruler along Y and there's a periodicity to the error that's close to the ~36mm pulley diameter. Also, the periodic error is about 0.5mm which is very close to the expected worst-case error from a 0.25" pulley mounted on a 0.2175" (1/4 20 major diameter) shaft -- about 0.8mm.

I suppose I should re-do the tests I did with 10mm and 20mm shapes using 36.576mm (1 pulley diameter) and half of that. Error should be minimized for the 36mm and maximized at 18mm. In fact, checking my notes on the test cuts I did before, the maximum error occurred in the 20mm Y axis cuts, which is close to the 18mm worst case.
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Re: Smooth idler kit?

Postby dirktheeng » Mon Sep 19, 2011 12:05 am

BenJackson wrote:I engraved a ruler along Y and there's a periodicity to the error that's close to the ~36mm pulley diameter. Also, the periodic error is about 0.5mm which is very close to the expected worst-case error from a 0.25" pulley mounted on a 0.2175" (1/4 20 major diameter) shaft -- about 0.8mm.

I suppose I should re-do the tests I did with 10mm and 20mm shapes using 36.576mm (1 pulley diameter) and half of that. Error should be minimized for the 36mm and maximized at 18mm. In fact, checking my notes on the test cuts I did before, the maximum error occurred in the 20mm Y axis cuts, which is close to the 18mm worst case.


Ok... periodicity is probably egg shape. To get rid of all of that you have to put precision shafts on the Y axis too... I can't remember if you did that or not.

I should engrave a ruler on mine too and check. I have noticed that my cuts varry by as much as about 10-thousandths though too which is close to 0.5mm I haven't figured out where that is coming from either. It is completely possible that the plastic drive pulleys are not completely cylindrical or coaxial... they didn't look like they were a very precision made component. I know the outside isn't co-axial with the shaft. I can see that by eye, but I never measured the actual pulley.
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Re: Smooth idler kit?

Postby r691175002 » Mon Sep 19, 2011 9:13 pm

Unfortunately, real precision costs a lot of money. There are many potential sources of error and they all need to be fixed.

I would start by making the bearing and stepper mounts more rigid - at least on the 1.0 laser there is a fair deal of flex. I would also use dual groove bearings or even a preloaded pair of AC bearings on each pulley (avoid radial loads on the stepper shaft). That way one could properly tension the belts without flexing the mounts (I think they need 15-20lb but I haven't checked). Finally, you need to make sure the pulleys are actually accurate and get some good couplers to avoid backlash and windup.

I suspect at that point 0.001" might be possible but getting the extrusions flawlessly aligned would be a pain.
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Re: Smooth idler kit?

Postby bdring » Tue Sep 20, 2011 1:52 pm

Sorry for no posts for a while. I was completely off the cell phone and Internet grid for a wedding in rural Vermont.

I have a new Y drive system designed that I will be testing soon. I will be getting rid of the threaded rod and modified coupler. I will have a 5mm to 1/4" coupler and a 1/4" smooth shaft. I found one that fits the bearings. My couplers seem to be lost in shipping right now though.

I also think that the MakerSlide might help too by reducing the weight of the gantry.

Edit: couplers have been located and should be at my house in a day or two.
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