Lost Steps

[SMA]

Hey Guys,

As I have discovered chasing down lost steps is very difficult.  Worked through mechanical issues and ended up cleaning up some minor binding issues etc.  I have quadruple checked mechanical stuff.  Even swapped Y and Z to eliminate a bad drive/transmission and issue still stands.

Z loses steps.  Measure in real world it adds up to 3mm.  This is over a variety of sizes of files.  In this case I am cutting surfboards and there are more lines of code for a board that is 6' than 7'.  No matter the size of the file there is a 3mm discrepancy at the home position from start of file to end of file.  

This would not normally be an issue if the discrepency was throughout the board but it all happens during the cross over portion of the file where one side of the board is complete and the tool moves to the other side.  The command is xo yo zo f1000.  Then the machine proceeds to cut other side of board.  Only in this case z does not come to 0 in real world instead it comes up about 2.5mm short.  This is an issue since one side of the board ends up thinner than the other by, you guessed it, 2.5mm.  This is not the end of the world but is irritating. 

I slowed the cross over F com to 100 and watched to see where z gets to and it comes up 2.5mm short xo yo zo.  Although the total mm off at home is 3mm but losing .5mm over one whole side is normal.  Just seems like it is a crossover issue.

I have slowed motors settings all the way to 50% and still get a 3mm discrepancy.  I am missing something?  Is it a settings issue.

There are two files to a surfboard.  A top and a bottom.  To make things even more confusing the issue only arises with the top file.  The bottom file only loses a mm max.  When cutting the bottom file Z comes up about a mm short of z=0.

I have included a spreadsheet that includes my settings and the different speeds that I have tried running the drives at.  I have what I call a z test file.  It just simply moves Z up and down so may times.  I measure position at beginning and again at end of file relative to a fixture that is fixed in place and get a .5mm diff everytime I run the file.  Two cycles of the file equals 1mm.  This is the case no matter the F com or Motor settings.

My issue is this.  How can there be the same 3mm discrepancy over different number of lines of codes if it is step loss?  How can I run at all these different speeds and motor settings and get the same .5mm for all speeds since step loss can usually be overcome by slowing machine.

sorry for the book

Stefan

[Tweakie.CNC]

Stefan,

Returning to the surfboard - when you move from one side of the board to the other (with Z raised clear of the work) does the Z axis 'tick' steps during this crossover move ?

Tweakie.

[SMA]

Hey Tweakie,

Thanks for the reply.  I am not sure what you mean by ticking.  These motors are microstepping sort of "humm" in my mind.  I will run the file again and see if I can make out a ticking noise.

S 

[SMA]

Well I had a look around for what this ticking might sound like.  It sounds like it is pretty obvious and so I would say no to ticking.  In my Gcode generator I can control the F command for the crossover.  When I run it low, like F200, all I can hear is the rail bearings quietly rolling away and the even humm of the motor.  As well I did swap Y and Z but it made no difference.  Hopefully I am answering the question?

Thanks for the reply
S

[Tweakie.CNC]

OK, At what point during the crossover move do you suspect that the 'extra steps or lost steps' are being generated ?
Perhaps if we know when they are occurring a solution may be easier to find.

Tweakie.

[SMA]

And this is the crux of the matter.  It is a long file at about 6000 lines of code.  As it cuts it is very difficult to make out where the steps may be lost.  I found a couple of "rough" spots where some binding was occurring but they have been rectified and now all axis move very smoothly.  I am almost 100% sure that the lost steps are not due to resistance.  I am not even sure that it is due to lost steps.

I thought someone might spot something in the motor settings or the file itself that I have not.  When you look at a problem to long you start to just stare and don't change perspective enough to look at it from a different angle.

What I have observed is that when Z comes up to "zero" on the crossover it does not get to "zero" but rather comes up 3mm short.  I can observe the cross over maneuver at F100 and watch as Z climbs up but comes short of the mark by 3mm and plunges back down to start the other side.  My mind is that the crossover is the issue.  There is nothing hard stopping it.

S

[SMA]

Maybe a soft limit.  Not familiar with them as I don't have any set up.  I know the parameters of the machine, can see it, and design well within its limits

I am pretty new, only had the machine a month, and so a bit green.  I learn fast though and I am willing to take the time to read.

Here is a screen shot anyway.

S

[Tweakie.CNC]

Sorry to keep plaguing you with questions.

What I have observed is that when Z comes up to "zero" on the crossover it does not get to "zero" but rather comes up 3mm short.

Is that 3mm short on the Mach screen Z Axis DRO  or 3mm short in physical distance when the DRO correctly reads 0.0000 ?

Tweakie.

[SMA]

The DRO reads 0 but I have a laser level and a bit of measuring tape that the laser level indicates on.  When I home the machine I note the postion of the laser on the tape and then as it crosses over I watch closely, with a slow setting like F100, and see that it does not reach the same level as the laser indicated at the beginning.  I have attached a photo of this rig.  Works well for seeing how many mm's out at the end of a file.  Y is usually off about .5 to 1mm.  Perfect in the surfboard world !

S

[SMA]

 Questions are good.  Make me think about all that I am doing.