- November 29, 2015 at 8:24 pm #4457
I’ve got everything up and running fine, just have a few questions/problems with skipped steps (primarily in one axis, though it can also happen in both).
1. I’m using 4x NEMA17s with A4988 stepper drivers off a 12V ATX power supply, the X and Y driver VREFs need to be at least 800mV before I get smooth movement. Is this VREF normal for steppers running in parallel? I noticed a few other posts that mention 500mV for X-Y VREF, but not sure if this is the same for parallel motors.
2. How snug should the conduit pipes be between the bearings? I’ve noticed a few skid mark tracks on the cross pipes from bearings, but they don’t snag or stop when I push them back and forth. I’ve even sanded the pipes down a bit to increase smoothness, do they need to roll freely when the steppers aren’t connected?
3. I’ve tuned down my acceleration and xy jerk to nearly half of what’s posted on the Marlin firmware (about 600 for accel and 4.0 for xyjerk), are there any other settings to help reduce resonance-related belt skipping?
This is a great design, hopefully I can fix the layer offset problem and get started on printing the clamps for my DW660. Thanks for the help!November 29, 2015 at 9:10 pm #4460
1)I don’t use the A4988’s becasue they don’t seem to handle heat as well, but the vref should be twice what it needs to be for 1 stepper. That is dependent on your stepper so I can’t tell you what to set it at. You are going more for as high as the steppers and driver can handle not the bare minimum to get it moving.
2) Start as loose as possible and only snug things up if you notice an issue. They will self smooth after a few uses. You can also just model a circle as big as you build area and let it run that for a while to get things smooth.
3) you should not get belt skipping. Have a look at the beta values, I spent almost 2 days on these settings, they should be very conservative. Although it is set for 32nd stepping so you might need to find these values on your own using the sripts to make it easier.
http://www.vicious1.com/forums/topic/need-some-beta-testers-for-some-new-firmware-stock-machines-only-please/November 30, 2015 at 8:04 am #4463
Wow super fast reply, thanks I appreciate the help! I finally printed my first error-free ABS model last night, after upping my VREF to about 1V for both the X and Y axes.
1. Although I’ve got a fan blowing directly on the A4988s, I’m planning on buying some DRV8825 drivers soon to help improve movement. 32nd stepping should make designs twice as accurate as 16th steps right? Also, I’m using 20t pulleys and I might move down to 16t for better performance, but I don’t think they are the reason I was missing steps.
2. A helpful note for people like me who couldn’t find 1.25″ bolts for the middle assembly bearings: You can use 1.5″ bolts however they will scrape up against the plastic when tightened and bend inward, creating a Vulcan death grip on the conduit pipe. I’ve found it’s better to leave them a little loose (or just use proper sizes).
3. I might’ve found another culprit for those skipped steps: lack of speed bearings. I couldn’t find any in local stores and decided to loosen the fender washers to help the belt bearings slide better, however this causes the roller motor mounts to wiggle at higher speeds and potentially jump over belt teeth. After tightening them down, the bearings might not move as freely but it’ll keep the motors and belts from bouncing around.
TL;DR Note to self (and others with skipped steps): Don’t skimp on the parts! Use what’s listed, follow the guide, and the hardware should work nicely.November 30, 2015 at 8:48 am #4465
After it has been running a while feel the driver heat sinks and the steppers they can be really warm but hot might indicate too much current and need to be dialed back a bit.
1)In theory 2 times as accurate but really do difference, I feel they allow you to run things with more current = more torque. 16T pulley I believe also increase the torque by 20% over the 20T. I did spend a little time making my choices 🙂
2)Agreed 100% I did my best to choose readily available materials and hardware world wide. Not an easy design task….
3)A very likely cause. There is a printable version, should help. http://www.thingiverse.com/thing:1105029
let me know how its progressing please.December 2, 2015 at 1:46 pm #4540
Here’s an update on my issue with skipped steps. It seems to only occur on the x-axis now, especially when doing the final pass of a 3-ring perimeter in curved pieces like the 660 lower tool clamp shown in the photo. Basically it does two perimeter passes fine and on the third pass it speeds up for some odd reason and skips one mm to the left every layer. I’ve tightened the belts as much as I can but there is still some play in them on both axes. These were also printed with an acceleration setting of 500 and xy jerk of 4.0, and I even tried turning down the speed to 80% and even 50% throughout the print to try to reduce skipping but it still skips.
Any thoughts to why it would only occur on these pieces and not rectangular ones? I will try printing the circular bearings you mentioned and see if that skips as well.
Attachments:December 2, 2015 at 9:00 pm #4558
Ok a (hopefully) final update, I think I’ve finally solved the skipping steps problem. Tightening the belts is a must to reducing skipped steps, as well as using the printed bearings previously mentioned. It helps if the belts have some sort of belt tension adjustment like the MPCNC corner belt clip or this. After tightening up the belt, lubricating the conduits, and setting the proper VREFs I’m finally getting consistent prints!
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