MK3 constantly losing home position  

Re: MK3 constantly losing home position

Are you by any chance pressing the big X button to stop the action?

When powered off - how easy is it to move the X-Axis manually left to right? Any spots along the way that tend to grab or stick?

The rods look very dry. Did you pack the bearings with grease when you assembled the printer?

Re: MK3 constantly losing home position

Thanks for the reply.

You may be onto something in regards to moving the unit along the X axis when its powered off. I definitely run into more resistance when I reach about 3 inches from the left and right side. The center 8 inches is very smooth. What could cause This?

You other questions are answered below.

I'm not using the X to stop the prints. All I need to do is move the x-axis in the settings menu and the home location is lost.

I didn't use any grease during assembly. I'll clean that up. I can't see how that would only affect the home position during printing stoppages though.

Re: MK3 constantly losing home position

Your x-axis is running into enough resistance during leftward motion that it exceeds the Stallguard threshold for sensorless homing. This is happening even before x hits its physical end stop. So, the machine thinks it has reached the home stop before it actually reaches it.

Something is making it harder than normal to move the x-axis. Things that can cause this in descending order of probability...

- Excessively high x-belt tension. This increases general friction against which the x-motor assembly must work. If tight enough, will create false positives for Stallguard sensing during x-motion.

- Bad bearings / unlubricated bearings. The bearings only come with preserving oil, not final lubrication. Unfortunately, assembly instructions never tell user to clean out the preservative and lubricate by either packing with grease or using light machine oil. (Don't mix the two BTW)

- Twisted x-assembly - fairly unlikely

- Extruder cable bundle hitting an obstruction

- Unusually low torque or out of spec motor - This is unlikely in a stock 1.8 degree motor, but you could swap x and y motors (and their EINSY connections) and see if the problem goes away. X-axis sensorless homing is more finicky than y-axis.

Re: MK3 constantly losing home position

I'm in the proces of resetting the belt tension. Something interesting to note is when the x belt isn't attached the left-right movement along the x axis is smooth as butter to both edges. I'll update once I have the belt tensioned properly. It was showing as 295 which oddly enough indicated slightly looser then optimal.

Re: MK3 constantly losing home position

when the x belt isn't attached the left-right movement along the x axis is smooth as butter to both edges

Is the drive pulley position on motor shaft centered with the belt? The belt ideally does not rub against pulley flanges. I find it best to set the x-motor pulley position while the extruder is moved all the way left. That lets belt position on the extruder have maximal influence.

While you are at it, check that the x-idler is spinning freely.

Pay ZERO heed to the reported slackness numbers, they only indicate resistance to motor rotation - and pretty poorly at that.

Re: MK3 constantly losing home position

The numbers in the selftest are worthless indicators of belt tension. Also, belt tension should have little to do with bearing stiction. You'll need to remove the rods and pack the bearings with grease to get a significant improvement. Dry bearings don't roll well; and a roll test must be done power off.

The other X-Axis possibility is if the rods are bent, e.g., they were used to lift the printer. Pretty much everything else is lack of lubricant.

You can test the theory by adding a few drops of sewing machine oil to the X-Axis rails - see if that helps. If it does help, then plan to do a maintenance cycle and pack the bearings.

Re: MK3 constantly losing home position

Did you build your printer?

One thing that is not made clear is that the X,Y and Z axis must be able to run smooth and without any sticking or stiffness.
The bearing must be cleaned before fitting to the rods and then greased or oiled, either is fine. Unless you have the pre packed bearing which i dont beileve Prusa come with. Read a use guide for Misumi bearings here https://us.misumi-ec.com/pdf/fa/2012/p1_0265.pdf

Then make sure the rods are parallel. Each axis (Z nuts not attached) should just easy push with no force. if the X axis sticks it will give a false stall point to the controller and think its hit its limit. Belts dont need to be very tight, just ehough for the tooth of the belt to not pull over the slit in the drive pulley.

I have a problem with my first mk3 when it travels to its furthest position on the x axis as it is a little sticky there and prints can fail wit crash detection. I need to strip it and align the rods

Re: MK3 constantly losing home position

Just a thought, is the idler wheel able to turn freely? there is a temptation to tighten the bolt too much, causing the idler wheel to bind on the side cheeks of the end piece!

regards Joan

Re: MK3 constantly losing home position

Just a thought, is the idler wheel able to turn freely? there is a temptation to tighten the bolt too much, causing the idler wheel to bind on the side cheeks of the end piece!

regards Joan

Re: MK3 constantly losing home position

RESOLVED

So I've resolved the issue thenaks you all the feedback you guys have given. As I was retensioning the belts I noticed that when the rear casing was off (the black piece with 5 screws) the extruder moved the full length of the x axis fine. I then began reattaching the casing by slowly drawing in the five screws. Ultamitely the four corner screw posed no issue and could be tightened properly but the center screw was where the issue was. When tight the resistence returned but when lossening by 3/4 rotation the resistence dissapeared. I certainly did not think I was over-tightening but apparently that was the cause.

Again, thanks a ton guys for all the help. Saved my bacon!