Z axis "jittering" and a couple other ??'s

[nbman]

Sorry, can't think of a better way to describe it..

Basically I've pretty much always cut thinner sheet metal, hobby stuff, so cut quality has never really been a big issue, but recently I've been doing some heavier materials, 1/4"+, and the small issues are really showing up in the cut quality so I'm wanting to try to improve it.

The jittering happens when the torch adjusts for THC movement(i think)..Z axis constantly bounces up/down very rapidly as it cuts, not a lot in terms of movement, maybe 1/2 or 3/4 turn of the screw, but it does it probably 5-6 times/second, which causes wavy edge to the cut.

My switch offset is .5200", which is set in my POST. When I reference the Z axis, sometimes the DRO reads <-.5200>, and other times it reads 0.000. At first I thought it was because sometimes I referenced it with no file loaded, since then I make sure to do it with the file loaded, but it's still random.

What function does voltage have on the THC? I find for the most part I have to run at speeds a little slower than the book says for the thickness of materials, otherwise it won't cut through..the other possibility is that it's not at the correct cut height..visually, it looks high..so maybe I need to adjust the voltage setting on the THC to lower it rather than slowing it down...Question is, what is the correlation between voltage and height, raising the voltage raise the head and lower the voltage to lower it, or vice versa?

I've never really completely understood exactly where Z-0 is..is it top of table, or top of material?

Thanks

[acourtjester]

The THC causing the torch move up/down rapidly could be you have the voltage sensing to sensitive, try increasing the voltage range it senses, this will allow the arc voltage to swing a little more before it reacts. The set THC voltage is to have it maintain the torch height to stay at that voltage, voltage up move torch down, voltage down move torch Up.
Switch off set of 0.520" seems high did you mean 0.052". This is for the metal surface to get to a pierce height and you should have Mach zeroing out the Z DRO and then correct for the switch offset and zero the Z DRO again and then move to the pierce height before torch on.

[cutting$burning]

[]Hi nbman,
Not to take anything away from what acourtjester said but I was having the same problem a while back. I searched on this forum and others to find the post that explained it best and corrected the problem for me but I could not find it again. Basically you just need to slow down the height control as it is reacting to fast and therefore the z axis is jumping up and down. I posted a picture of the Mach 3 screen that I use with ess smooth stepper, this may or may not be the same screen you use. I drew a half-assed arrow at the number that needs to be changed. As you can see my setting is at 9 if the number on yours is higher then try lowering it to around 9 or 10. I may even lower mine to 8 as I noticed some rough or quarter looking edges on some 3/16" mild steel I cut yesterday. I hope this helps you out as I know how frustrating this is when you know you can make a near perfect cut but for some reason it doesn't. Also as far as I know the Z axis zero is top of material for plasma cutting.D
attachment=0]mach 3 screen shot.jpg[/attachment

[nbman]

Switch off set of 0.520" seems high did you mean 0.052". This is for the metal surface to get to a pierce height and you should have Mach zeroing out the Z DRO and then correct for the switch offset and zero the Z DRO again and then move to the pierce height before torch on.

My table is home built, so that's just how my Z switch worked out. When I reference Z, it will travel down until it sets the Z limit switch..from the point it hits the switch, until the point its back up to zero, is .5200" My normal sequence is

Load material
Load G Code
Reference x,y, which sets it to my table zero position, and zeroes both DROs
Reference Z (torch tip to material) - Here, on the occasions where the DRO reads 0.000 after reference, I manually raise it +.5200 to reach zero (top of material) and then zero the DRO. If it reads <-.5200> after the reference, I leave it alone.
Cut

Thanks

[acourtjester]

Home built not a problem I have built a few too, it sounds like you need a different post processor.
This operation can be automatic so you position the torch at the start of a cut and hit run
Attached is part of an older G-code this shows where I set the table to start a cut.

top line move torch up 1"
move to start of operation
home Z until home switch trips (G28.1 Z0.02)
when Z home trips (g92 Z0.0 auto set of Z dro to 0000)
move torch up for switch offset (goo Z0.030)
set Z dro to 0.0" reset Z to zero after moving for switch offset
now move to torch pierce height (g00 Z0.090")
M3 light torch start pierce

G-code.jpg (18.04 KiB) Viewed 2209 times

G-code.jpg (18.04 KiB) Viewed 2209 times

[nbman]

My THC rate in Mach on the settings screen is set to -30..? I can't change it, every time i do, it reverts back automatically. From what I've read elsewhere, that could be the problem?? How is that value changed?

@ Cutting$Burning, your screenshot is too small, I can't see it....

[SeanP]

I don't see a way of entering a minus amount to thc rate.
If you enter the number then press enter it should keep that figure.
I have seen that jittering you talk about, usually in thicker cuts like over 10mm, I find 29% for those works well, sometimes on thinner material less than 1mm I might go to 31%.
30% seems to work for everything else on mine.

[cutting$burning]

Hi nbman,
Sorry for the pic size, going to try this again. I just highlight the number with the mouse and then type in the number I want and hit enter and it's good untill I change it again. D

[acourtjester]

A side note I use a program to modify images with shapes and text called HiJaak® PhoTags, I think both eBay and amazon have it.
To get a screen shot Windows has "Snipping Tool" under the accessories menu.

[tcaudle]

My THC rate in Mach on the settings screen is set to -30..? I can't change it, every time i do, it reverts back automatically. From what I've read elsewhere, that could be the problem?? How is that value changed?

@ Cutting$Burning, your screenshot is too small, I can't see it....

The value cannot be negative. Its a % of the Z max velocity so 0 is a low as you can go! It will just use 30 for the value. 30 is a little too high for most Z's .

The functions of DRO's (input boxes) are defined by code behind the button and the way the numbers and types are part of the parameters. You set all of that in the MACH screen builder. Its sounds like the DRO is not functional and just shows the value stored in the XML file. It would ignore the negative sign so you are at 30% and that can be a bit "hot" for some Z''s and lead to instability. The Screen Set has all of the buttons and DRO codes and the values are "hidden" in the Screen Set.

[nbman]

Well, there's definately a negative sign in front of the number, and when i change it, it remains...

I'm running a Bladerunner a few years old, with Sheetcam TNG and Mach3 .067. What is the recommended POST to use for this setup?

Thanks

[BTA Plasma]

Type a number into the box and press enter on your keyboard. On some older Mach's the plug in made the number a negative number. Doesnt matter...it still works well.

[tcaudle]

You need to upgrade the MACH support files . If the unit is more than 3 years old (before 4/15/14 ) you need to use the Master Installer Legacy. Backup your MACH folder BEFORE you do the upgrade. Also pick just ONE from the menu to install. It will update screens and drivers and give you a new ICON (profile) you will need to copy the motor tuning from the older profile.

What POST you use is largely dependent on which model DTHC you have.

[nbman]

You need to upgrade the MACH support files . If the unit is more than 3 years old (before 4/15/14 ) you need to use the Master Installer Legacy. Backup your MACH folder BEFORE you do the upgrade. Also pick just ONE from the menu to install. It will update screens and drivers and give you a new ICON (profile) you will need to copy the motor tuning from the older profile.

What POST you use is largely dependent on which model DTHC you have.

I will run the Master Installer Legacy.

Stupid question, but how do I know what DTHC I have? Its a Bladerunner purchased from you in 2011.

Thanks

[tcaudle]

Its labeled next to the jack where the cable plugs in. There have been three models. DTHC, DTHCII and DTHCIV . If it has the 9 pin computer cable as the plug to the THC sensor card then its the DTHC.

[nbman]

Its labeled next to the jack where the cable plugs in. There have been three models. DTHC, DTHCII and DTHCIV . If it has the 9 pin computer cable as the plug to the THC sensor card then its the DTHC.

OK,its DTHCII