Bridges & jerky movements.

[Moondog]

Another question hopefully you can answer.

I use 3D bridges in my toolpaths to hold parts whilst cutting. If the 3D bridge is on a straight cut it works fine, however if the bridge is on a curve the movement becomes jerky etc..

Is there a setting in Mach that may eliminate this?...

Normal cutting of curves and circles are no problem, perfect.

Any help appreciated.

cheers

Frans

[DennisCNC]

You need to enable CV (constant velocity mode) and give it a try. Are you using Mach 3 or 2?

[Moondog]

Thanks Dennis

CV mode is already set.. any other ideas? anyone..

Using Mach2

[bgolash]

Hi
I can`t confirm this statement but I`ve heard something a while back.
Steppers can`t keep up to one another when the axes are moving
similtanously. I believe this is even more true when the speed is
high. I saw a video by Jeff Davies of homecnc using his servo machine
and didn`t notice any jerky movement. From what I saw the motion
was very smooth. I too have a stepper system and I see jerky motion
when two axes are moving at once. Maybe someone here can jump in
and clarify this condition.

Barry

[Kiwi]

I had the same problem with my Machine Centre with a Fagor 8055 controller.
The fix is to put G51 E0.0001 at the beginning of the program.
G51 is the Look-Ahead mode which enables the CNC to look ahead up to 50 lines, analyze the tool path and calculate the max feedrate with an max error of 0.0001mm.
Don't know if Mach has the equivalant feature.

[derekj308]

Hi Moondog
Are your pitches the same on X and Y? I have some vague memory about reading about issues when the pitches are different on X and Y. Something about circular interpolation.

Cheers
derekj308

[Moondog]

Hello Kiwi..

Had a look at the G codes and settings.. couldn't see an equivilent for Mach 2. maybe someone esle will know.

I have a feeling it may be a Mach 2 resolution issue?...

Bgolash....

Steppers can keep up with Servos etc.. and co-ordinate all axis together.... it is the drivers and controllers that govern how well it works.....


cheers

Frans

[Moondog]

Hi Moondog
Are your pitches the same on X and Y? I have some vague memory about reading about issues when the pitches are different on X and Y. Something about circular interpolation.

Cheers
derekj308

Derek... they aren't the same.. they very rarely are on most machines. Mach is supposed to compensate for drive/pitch differences as set up on Motor Tuning.... I'll have a look at circular interpolation (whatever that is).. is there a setting for it?

[derekj308]

Hi Moondog
I agree with you that Mach does compensate for different pitches via the setup but whether the pulses out are synchronised is another story. I believe (and I could be very wrong) that when you use circular interpolation (just by using a G02 or G03) that the contoller (Mach) may not synchronise the two axes very well for whatever reason resulting in a 'jerky' movement. I'd fire off an email to Art for his opinion on this. I can't for the life of me remember where I read it. It may even be in the Mach manual.

Cheers
derekj308

[Halfnutz]

If its just a matter of smoothing out those sections of code couldn't you just do a feed rate change to slow down for those sections?

[Moondog]

If its just a matter of smoothing out those sections of code couldn't you just do a feed rate change to slow down for those sections?

To do that would take large amount of code changes and involve numerous different toolpaths....

I still think it is a Mach2 resolution problem...

[Halfnutz]

I do it all the time with a simple F20 at the beginning of the problem and a F40 at the end of the block.

Just run the program, and wherever you hit a problem stop the program, open it in the editor, back up a line (or two) add a feed change to the first line of the block and one to the end of the block, and resume cutting at where you stopped. Once you run through the program once you have the problem fixed for good. If its a program you use more than a little, its not a big deal.

I use diferent feed rates throughout most of my programs. If Im at the beginning and doing the roughing with little or no Z movement and a simple raster scan I will crank the feed up to 70 or 80 IPM and then slow down to 30-40 for areas with lots of contouring and multi axis movements.

I geusse its a pain, but I only have to optimize the program once, and I can take a two hour program down to an hour sometimes. If its a design I like and plan on using over and over its well worth it.

I dont know exactly what your doing of course, maybe your talking about a six hour program with a tab every fifty lines, who knows, but its a something to consider.