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.)
John
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