SteveD

Looks like your G54 Z position is 40mm lower than G53. I.e. When Z is at G54 0 the machine Z is at G53 -40.

G38.2 uses G53 values. G38.2 Z-20 F25 means probe Z to G53 -20. This is 20mm ABOVE the G54 0.

Try this:
0) remove bit from spindle
1) home machine
2) jog 5mm above work surface (40 mm down from the Z home position)
3) zero G54
4) jog up 30 mm (10mm down from the home position)
5) G38.2 Z-20 F25

• This reply was modified 5 years, 7 months ago by SteveD.

Have you read the previous comments in this thread? Reply #10844 in particular.

Regards,
Steve

Hi Reeve,
you are correct. It’s an absolute versus relative co-ordinate thing. The TinyG developers have decided to implement G38.2 in G53 absolute terms, ignoring any user offsets (G54-G59) and absolute/incremental modes. This is confusing because other commands like G0/G1 don’t work this way.

Working through an example, assume the WCS is G54 and mm (G21). The block “G0 Z-10” can mean one of two things. If you’re working in absolute mode (G90) then this means move the Z axis to position -10 in G54 space (an absolute move). If you’re in incremental mode (G91) then this means move the Z axis down by 10mm from its current position (a relative move).

G38.2 Z-10 F25 always means move the Z axis to position -10 in G53 space.

It doesn’t help that the GCODE from the touch plate widget and CNCJS are mixing useless G91’s into the sequence. Using your previous example:
; Z-Probe
G91
G38.2 Z-10 F50
G90
; Set the active WCS Z0
G10 L20 P1 Z19
; Retract from the touch plate
G91
G0 Z4
G90

This can be rewritten as:
; Z-probe
G38.2 Z-10 F50
; Set the active WCS Z0
G10 L2 P1 Z19
; Retract from the touch plate
G91 G0 Z4 G90

The first G91/G90 pair have no effect. G38.2 ignores the current absolute/incremental mode setting i.e. it is hardcoded to be G90. The current WCS is ignored and G53 is always used.

Cheers,
Steve

Yeah, the G38.2 command ALWAYS uses absolute G53 co-ordinates. It ignores G54-G59 offsets and G90/91.

When you’re at the top of the Z axis the G53 value is 0 so it goes down, i.e. seeks absolute -10. When you jog below the G53 -10 point, say at -20, it is going to seek up.

To get the Chilipeppr touch plate widget to work right try this:
1) put a thick sponge under the touch plate to prevent bit/plate damage
2) home machine
3) set G54 Z to zero i.e. clear out the offset
4) jog down close to your touch plate
5) G28.3 Z0
6) run the touch plate widget (be ready with the ! just in case)

I think I may have worked this out.

Try this:
1) Home your machine, do not jog once in home position
2) G38.2 Z-10 F25

Which way does it go?

What I neglected to mention in my previous post was that I was running with the edge firmware, i.e. 449.01. My suggested workaround applies to it only.

Taking the previous comment from cmcgrath on board I decided to revert the firmware to the current master 440.20 and repeat the exercise.

Immediately after programming the chip I noticed something different. My TinyG controller is version 8. It came preloaded with a firmware that reported itself as 440.20 and 0.97. However, every time it went through a hardware reset cycle it would flash one of the red LEDs as it booted (I forget which one). Once reset, the LED would stop flashing and remain on.

The newly programmed device also reports itself as 440.20 and 0.97. When reset the PWM LED turns on and stays on. It does not flash.

Is it possible that there were two versions of the firmware with the same build number but different code?

• This reply was modified 6 years, 9 months ago by SteveD. Reason: clarity

Try setting $ztm=0 and $ztm=115.

I was having the same problem and this seemed to be the thing that made G38.2 go the right way.