Issue with z homing touch plate consistency

Hey back again. I have done many cuts with this machine and love it, but have an issue I am hoping someone can point me in the right direction.

Setup: SKR Pro v1.2 (with the LED voltage issue that has been remediated)
Firmware: Everything stock from the v1e approved git repo
v1e Tiny Touch Plate with it’s supplied wiring
correct z rods # of leads… forget the total but had another thread where we fixed that problem lol

I noticed that when I z home and the bit hits the touch plate, sometimes, there is a delay which causes the z home to be off by almost -1mm. It pushes into the plate and pushes the machine up just a hair before it registers. Sometimes it doesn’t. Sometimes it is .5mm, sometimes 1mm.

I have moved the magnet clamp to the bit itself and also the collet. Doesn’t seem to make a difference. I have used the clamp w/o the magnet directly on the bit. That seemed to help.

I read a post about enabling this setting. Nothing is running at the time of z homing. No vacuum, obv not the router, nothing.


Any clues?

In some cases I have seen a similar effect, where the trigger occurs late and the Z depth is lower than I would expect, and it is not repeatable.

At the time I set it aside and went back to paper because I wasn’t ready to diagnose the issue. My guess is a poor contact but I should dig into it more. I can try a simple repeatability experiment.

I wonder if in the firmware there is some polling frequency to check the trigger condition.

I had two signs i printed back to back and one was about .5mm deeper than the other which was noticeable to the customer. so i really gotta figure out how to get this thing consistent.

I see in some marlin threads stuff like this:

Currently the stepper ISR calls endstops.update to get the current state of the endstop pins. This allows it to stop within only a few steps of hitting an endstop, whereas if we polled less frequently (for example in the temperature ISR, which runs at ~977Hz) then things like bed probing might be less accurate. It would be interesting to see whether such a loss of probing accuracy would be significant, since probing is usually done at a pretty slow feed-rate.

-------curious if there is a setting that can be updated to poll more frequently

I suspect that it is a hardware issue of making poor contact and not a software timing issue. Homing is basically the same thing and it has never had problems like this. Maybe the carbide has trouble making electrical contact, or it’s dirty.

There is an issue with a few SKR Pro boards wrt endstops. This issue is the only thing I can think of that matches your symptoms. Assuming this is the cause of your choices are:

  • Replace the SKR Pro board
  • Disconnect the LED for the endstop(s) by removing the LED or cutting the traces.
  • Add pullup resistor(s).

Most people who have this issue elect to add pullup resistors. These can be added by soldering the resistor directly to the SKR Pro board, or by adding the resistor to the wiring connection.

Here is the first topic where the issue was identified.

Here is a topic where the resistors are soldered to the board.

Here is a topic where the resistor is added to the wiring.

As for other causes, you might try flipping the plug at the control board to reverse the polarity. This shouldn’t make a difference, but there was one topic where was supposedly did.


If this is an SKR board issue, changing this setting might mke the issue worse.


can anyone provide insight into these?


marlin documentation has them set to HIGH but doesnt explain the diff between high/low

Thanks for the reply. I do have the pullup issue and have a thread where i documented a fix which i have deployed.

i was thinking contact issue, but everything is wired right and m119 always shows correctly open/closed.

it’s just a weird delay.

i am going to try swapping the polarity as well. that is something i didnt think of and appreciate the idea.

Given how close the voltage is to the trigger point, correct M119 readings are not a reliable way to be assured the pin is working correctly. Is it possible you have a cold solder joint or you did not use small enough resistors? I see you used 1.5K resistors, maybe try a 1K resistor? You could swap one of your adaptors between another endstop to see if that makes a difference.

the poopy thing about this is i am willing to buy another SKRv1.2 board, however, we don’t know if they have the dang pullup problem. i dont want to re-wire everything only to find out i have the same issue!

Clean your endmill

Seriously- any dust or even oil on there will inhibit the conduction path.

And put the magnet on the bit, not the collar.

I would occasionally get the same effect, maybe 1:15 probes, never on a first run though.

Since I instituted those two thing, I’ve done probably 100 z-probes with no failure

Hope it helps

“Or it’s dirty”

Winner winner!!

When I started making sure to clean my bits during tool changes, this behaviour went away. Prior- it was 1:15 probes. Since, Ive done ~100 probes with no failure

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well… i went out there to put a pullup resistor on the z and the board said printer not found.

since this board has so many hacks w/ the pullup resistor stuff i am just going to bite the bullet, order another, and re-wire everything. sigh.

In one case I’ve seen this before the issue was that VCC was connected to ground on the endstop plug. If you disconnect the pullup resistor, does the board then work? Do all three power lights show?

ok so…

replaced the board with a new one. still has pullup issues. fixed all six steppers with resistor.

found the real problem of z… connection to tiny touch plate wasn’t secure so sometimes it would read and sometimes wouldn’t.


was doing leveling and square. upon level I did m666 z0.25 followed by m500 and went to move z up… suddenly my z2 was moving the opposite direction making clunking sounds. so z1 moved up and z2 moved down. what the heck? wires didn’t change. they are all hot glued in. I reset eprom and power cycled a few times and magically it’s going in the correct direction again.

any ideas why such a thing would happen?

Welcome to “Things we have heard about with the SKR Pro 1.2.”

One person that I know of has noted that the “Dir” pin on one stepper driver was not soldered. That board, the motor always turned the same way, regardless of what was commanded. An intermittent connection could do what you saw. The driver not quite being seated in the socket could.also do that. That would make the M666 a coincidence.

I would power down the board, re-seat all of the stepper drivers, and try again. If the M666 command does the same thing, then look at software. If it keeps working OK, keep an eye on things,.and ifnit happens again, maybe inspect the bottom of the board, particularly solder joints along the polulo stepper driver pins.

thanks Dan. looks like these boards are a crap shoot. guess my next item is using a different board. any suggestions.

Can you use the new jackpot?
Also I had some funny issues with my z probe but it’s because of a win7 and repetier host thing.

Honestly, no. The odds of getting a bad one are actually very low. Ryan mentioned it as about 1%, so the gambling analogy is more like getting the “00” at a roulette wheel.

I mentioned in another thread, but I have 2 of these boards and zero problems. One runs my MP3DPv3 and another is being a test bench board that Im currently experimenting with RepRap Firmware on. Once Im satisfied eith the test, I want to move the MP3DPv3 to RRF. (Haven’t actually worked on it in months tho.)

By contrast, I have an MKS board that needs the pullup resistor boost, and have had bad solder joints (cold solder) on a genuine Arduino Mega 2560 RAMPS stack. (Both of these are supposed to be exceedingly rare.)

Fact of it is, that even the most reputable of these manufacturers for 3D printing stuff still have their factories in China, and get Chinese QC. The SKR boards are very popular (even mlre so when supply chain issues meant they were the knly ones available through V1 shop) so there are loads of them out there. We don’t hear about people who have no problems, so the problems seem larger and more numerous.

If you have a bad board… well, every deck of cards gets a Joker or two. You can draw another card, or draw from a different deck…

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I must be super lucky bc I got two bad from two different sources. last source said boards were about four years old so I figured it likely is a good one. oh well.