On Jul 7 2013 10:21 PM, John Morris wrote: > On 07/06/2013 04:28 PM, Jon Elson wrote: >> Chris Morley wrote: >>> I am not an expert, just interested. I don't follow your reasoning. >>> Jerk limiting is about having the TP ask for movement that is >>> possible >>> for the machine to actually produce. >>> infinite jerk is impossible for a machine to produce movement for. >>> While we can ignore it in relatively slow and small machines, I can >>> not >>> see why you would want to turn it off in some cases. >>> >> G33 (lathe threading) assumes the spindle is mostly maintaining >> constant >> velocity. G33.1 (rigid tapping) assumes the spindle reverses fairly >> quickly >> at a certain point. The Z axis must follow the spindle quite >> closely, or >> it will break small taps and muck up the thread on larger ones. >>> It seems if you have the TP request infinite jerk, then you are >>> must realize >>> that your are asking the machine to NOT follow the TP command for a >>> small >>> instant. >>> I don't see how G33.1 is any different then other machine >>> movements. >>> >> Other than spindle synched moves, ALL axes are under TP command, and >> that should always keep them synched so they are all at the correct >> coordinated position. With a spindle-synched move, the tool must >> follow the spindle, which often is NOT a servo axis, and is just >> generally obeying a velocity command. When the G33.1 gets >> to the point of reversing the spindle, it can reverse fairly >> quickly, >> depending on the particular machine setup, and the Z BETTER >> keep up with however fast it reverses! Having any interpolation, >> jerk limiting, etc. between the spindle encoder and the Z axis would >> apply strain to the tap, and be very undesirable. > > Restating, to see if I understand: > > Conditions: Z axis needs to be slaved to the spindle speed; at the > same > time, Z axis jerk needs to be limited. > > Complications: Tapping operations have problems with friction and > grabbing, causing uncontrolled and rapid spindle deceleration. Also, > spindle acceleration and reversal control is not as precise as other > motion components. Nothing we can do about the uncontrollable! > > My naive impulse (don't be polite about shooting it down ;) : > Translate > maximum Z axis jerk into maximum spindle jerk, and then just do the > best > we can to keep spindle jerk within bounds, given whatever > complications. > (This can be generalized, for example when the spindle is at an > angle > and multiple axes must be slaved.)
While reading this I took a step back and thought of all the times I either wanted or needed jerk limiting -- they were all situations where the power to stage weight was high and I was trying to get it to move as fast as possible, or when I cared about any vibration in the stage (I'm not talking about being off by a few degrees, but by small numbers of arc seconds with positioning a LiDAR). I do not see tapping as being in the same ballpark. Then again, I have never used really high end forming taps... So, how much of an issue is jerk limitation when all the axes (+spindle) are moving at a small fraction of their max speed, and we have smoothed the start/stop/reversal of the spindle as best we can? Just asking. ------------------------------------------------------------------------------ This SF.net email is sponsored by Windows: Build for Windows Store. http://p.sf.net/sfu/windows-dev2dev _______________________________________________ Emc-developers mailing list Emc-developers@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/emc-developers
