Machsupport Forum

Mach Discussion => General Mach Discussion => Topic started by: secretspy711 on December 26, 2018, 02:16:56 AM

Title: Is this a "look ahead" problem?
Post by: secretspy711 on December 26, 2018, 02:16:56 AM
I'm getting this weird corner on a part I machined today in polycarbonate.  It's supposed to be round, just like the adjacent corner and in the model, but it's like the machine started turning the corner too early and then created this sharp-ish corner instead of a nice round.  The opposite corner did the same thing.  Also the radius on the lower section looks too large.

Could this be due to the # of look-ahead lines being too low, or the motor acceleration values being too low?

Backlash compensation is turned OFF.

Images here: (
Title: Re: Is this a "look ahead" problem?
Post by: TPS on December 26, 2018, 04:22:20 AM
are you running Constant Velocity or Exact Stop mode?
Title: Re: Is this a "look ahead" problem?
Post by: joeaverage on December 26, 2018, 04:35:31 AM
it sounds more like C(onstant)V(elocity) mode. In CV mode Mach attempts to keep the machine cutting at a constant speed an to do so it
'cheats' a bit by pre-empting corners and so on.  In most circumstances such cheats are perfectly acceptable but in some instances result in poor
toolpath following and poor parts as a result.

You might try ExactStop mode. This will cause Mach to follow the toolpath exactly but more 'stopping/starting/jerking around'. Worth trying to see
what sort of difference it makes.

The setting ig made on Config/General Config page per the attached. You may have to restart Mach to see the effects.

CV mode is a way to (try) overcome finite acceleration of your axes. If you had infinite acceleration then ExactStop and ConstantVelocity modes become the same.
Thus it is a good idea to tune your machine for the maximum reliable (no loss of steps) acceleration your steppers can manage.

Title: Re: Is this a "look ahead" problem?
Post by: robertspark on December 26, 2018, 08:12:56 AM
What does you gcode look like?

Is it a load of linear moves (G01)
Or is it a mixture of linear and arc motion G01 + G02/G03
Constant velocity tries to attempt to maintain a constant velocity but there are limitations as to how constant velocity can actually be maintained.....  Think of it this way...

You're running down a road flat out. Can you change direction 90 degrees still running flat out..... Exactly 90 degrees!   If your flat out is walking probably, if your flat out is Usain Bolt probably not.

Ok option a..... Move slower so you can turn 90 degrees exactly..... Option b slow down so you can turn exactly 90 degrees.

Mach3 is not very good at defining what Constant Velocity actually is .....  what is the relationship of how slow is slow enough?  How much in advance of the junction or corner does it slow down?  Is it different for G01>>G01 or G01>G02/3 or G02/3 >> G02/3 or G02/3 >>G03/2 or G02/3 >> G01 motion?

Try to use the motion planner effectively by using linear motion where the move is linear and arc motion where the motion is an arc.  Use roughing cuts and finishing cuts.  Consider using exact stop with finishing cuts if precision is required.  That way the unknowns / undefined elements of the motion planner and constant velocity are removed or minimised.

Jerk or jolt is something slightly different and is to do with trapezoidal acceleration which all lower cost motion planners (mach3/mach4 etc) seem to do and is related to your machine setup of drive mechanism, microsteps etc.
Title: Re: Is this a "look ahead" problem?
Post by: secretspy711 on December 26, 2018, 10:47:22 AM
Thanks for the replies!

I am indeed running Constant Velocity right now.  I'll first try increasing the acceleration values a bit.  I've never tried Exact Stop before, but I'll give it a shot as well.
Title: Re: Is this a "look ahead" problem?
Post by: robertspark on December 26, 2018, 11:39:36 AM
I have not tested this myself but I have been told by a motion controller manufacturer (a few times) that has mach3+4 plugins for their motion controller that mach3 has a known issue that it can occasionally under certain circumstances exceed the defined acceleration setting by 6x during some motion planning to do with constant velocity sonbe careful increasing acceleration too much