For Marc,
No, this is not normal behavior. Under the right conditions you should get a very smooth continuously un-interrupted cut at a constant velocity. However, the effects you describe are not surprising and happen to many others on all types and brands of CNC controllers when either not set up properly or the physical motors are under sized for the weight, load, gearing and inertia that the table can handle.
Honestly we have not read through all of the chats on the message board, but we do have some solid advice and explanations on this topic.
First the explanation. The better the problems are understood the better you can decide upon one of these solutions.
When stepper motors do not have enough torque (power) or are geared incorrectly for the application they are notorious for losing torque during acceleration while taking off from a dead stop, decelerating too fast to a stop, travel at the upper reaches of the RPM range for that motor or making an abrupt change in travel direction. When this happens, the motors may kick out, stall, ignore steps or freeze up.
You may be losing steps by accelerating too fast or rapiding too fast. The faster a stepper motor moves, the whimpier the stepper motors becomes thus losing torque. Although they have good torque at slow speeds. The drawback to an open loop system is that you will never know this unless you can humanly visually notice it.
The key solutions are presented in order of consideration for effectiveness, difficulty and cost.
(1) Change to closed looped servo motors.
(2) Possibly keep the stepper drives but change the stepper motors to a larger size.
(3) Use gear reducers. There are many types to select from. In-line ones that attach to the face of the motor or side mounted gear boxes are most common. Each time you reduce the gearing ratio by a factor of 2 you will increase the torque the motor has, thus doubling its power. The trade off is that each time you do this you also reduce the RPM and travel speed by half. It is common practice for companies to use , 1.5:1, 2:1, 5:1 pitches on their ball screws or install gear reducers in factors 2,5,10 or 20 to 1 for example.
(4) Use SmartPath to make automatic on-the-fly intelligent decisions about when to decel and when not to and then only apply the amount required per cutting scenario, material type, physical mechanics and up coming look ahead geometry in the cut path.
(5) Set the SLOWDOWN and NEXTMOVE parameters to automatically calculate the correct distance to commence a reduced feedrate before reaching each target position on the basis of percentage and distance of the original feedrate. This feature in part is the missing ramp up/down your speaking of. SmartPath does this same thing but with much more intelligence and versatility.
(6) Keeping in mind that steppers have the most torque at their low RPM ranges. Some solutions are in this area are:
(6a) Use lower the feedrates on G01, G02 and G03.
(6b) Open the position error allowance using POSERROR which keeps the motors from kicking out so easy.
(6c) Lower the maximum rapid speed using RAPIDSPEED to keep from running the stepper in the high RPM ranges.
(6d) Open the TOLERANCE setting up slightly. This will allow each target position reached to be satisfied more quickly and move on to the next. Don't worry about position accuracy, this system will always strive to make position and always output the exact number of steps. TOLERANCE only confirms your in-position by doing a check within the user defined tolerance range. If in-position tolerance must be confirmed on each move within a certain small value then an alternative exist by using the logic command BLEND.
(6e) Set a minus BLEND factor. This settings is in milliseconds. When the value is negative it will have a similar effect as opening tolerance, except for the unit are time based not distance based.
(6f) Insert DECELSTOP commands in your GCODE.FIL file or use G11 codes in your G code program only where necessary to automatically ramp up and down.
(6g) Lower the acceleration rate and deceleration rate using ACCEL and DECEL settings to make softer starts and stops or ramp ups and ramp downs.
Keep in mind that the focus of what your trying to accomplish is to reduce the factors that make stepper motors lose torque. The only way to approach this is to start with conservative values and make repeated tests each time slightly increasing or decreasing the values until you feel that the settings reflect adequate acceleration and deceleration for the motor size, table weight, load, gearing and inertia that your machine can handle.
For more detailed information read the Question & Answer Solutions numbers Q127, Q176 and Q195 in the CNC Professional manual. Also information for existing users can find SmartPath, SLOWDOWN, NEXTMOVE, POSERROR, RAPIDSPPED, BLEND, TOLERANCE, ACCEL and DECEL parameters in both the printed and electronic manuals.
Tech Support
CamSoft Corp.
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