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CNC router shudder and jerky / jitter doing 3D toolpaths
« on: February 04, 2016, 05:35:46 PM »
Hey everyone.

I have some toolpaths where I cut out a guitar neck. While doing any straight cuts, my CNC router runs great. When I get into doing the heel or volute carve on the guitar neck, the CNC is going through A LOT of little code to make the moves and it can start to shudder / jitter pretty bad. It shakes the whole CNC.

I am running those paths at 50 IPM but Mach 3 only says it makes it to 10-15 IPM for all those lines.

I don't know anything about CV mode but could it be that? Or would it be my post processor for Mastercam X4?


Any help would be greatly appreciated.
Thanks.
Re: CNC router shudder and jerky / jitter doing 3D toolpaths
« Reply #1 on: February 04, 2016, 05:53:31 PM »
Can you post your XML file

You will need to call it something unique like Alexander James guitars... Or it won't upload.

Can you paste a bit of the code when this is happening to have a look at?

What is your setup... Parallel port, motion controller (which one), are your drives microstepping, at what resolution?

Give as much info as you can... No one ever says you have provided too much info
Rob

Albert Einstein ― “If you can't explain it to a six year old, you don't understand it yourself.”
Re: CNC router shudder and jerky / jitter doing 3D toolpaths
« Reply #2 on: February 04, 2016, 05:57:30 PM »
Make sure you are set to constant velocity and not exact stop, see pic for CV setting.

Exact stop mode will make the machine stop at the end of each line and give you what you are describing.

Re: CNC router shudder and jerky / jitter doing 3D toolpaths
« Reply #3 on: February 04, 2016, 06:00:36 PM »
...you can also prefix your gcode with g64 in your post processor...
http://machmotion.com/cnc-info/g-code.html/
Rob

Albert Einstein ― “If you can't explain it to a six year old, you don't understand it yourself.”
Re: CNC router shudder and jerky / jitter doing 3D toolpaths
« Reply #4 on: February 04, 2016, 06:12:00 PM »
I am using an Ethernet Smooth Stepper.
My drives are microstepping at 1/13 (I think that is how you write it), or 4000 pulse/rev

I have uploaded my XML.
Here is a sample of the guitar neck volute toolpath that I get a lot of shuddering on:

N100 G17 G20 G90 G40 G80 G64 G49 G0 M05
N102 G8 P1
N104 G90 M5 Z0
N106 G52 X0. Y0. Z0.
N108 G43 H5 Z.25
N110 T5 M6
N112 G0 G90 G54 X7.3898 Y9.0113
N114 S13000 M3
N116 Z.1
N118 G1 Z-1. F50.
N120 X7.4171 Y9.0341 F100.
N122 X7.4443 Y9.0571
N124 X7.4713 Y9.0801
N126 X7.4981 Y9.1031
N128 X7.5249 Y9.1261
N130 X7.5515 Y9.1489
N132 X7.5781 Y9.1715
N134 X7.6047 Y9.1939
N136 X7.6312 Y9.2159
N138 X7.6577 Y9.2375
N140 X7.6842 Y9.2586
N142 X7.7108 Y9.2793
N144 X7.7373 Y9.2993
N146 X7.7638 Y9.3188
N148 X7.7903 Y9.3375
N150 X7.8168 Y9.3556
N152 X7.8434 Y9.3729
N154 X7.8699 Y9.3894
N156 X7.8964 Y9.405
N158 X7.9228 Y9.4198
N160 X7.9492 Y9.4336
N162 X7.9756 Y9.4465
N164 X8.0019 Y9.4585
N166 X8.0282 Y9.4696
N168 X8.0544 Y9.4797
N170 X8.0806 Y9.4888
N172 X8.1068 Y9.4971
N174 X8.133 Y9.5044
N176 X8.1593 Y9.5108
N178 X8.1858 Y9.5162
N180 X8.2125 Y9.5209
N182 X8.2396 Y9.5246
N184 X8.2671 Y9.5274
N186 X8.2953 Y9.5294
N188 X8.3241 Y9.5304
N190 X8.3313
N192 X8.3387 Y9.5303
N194 X8.3463 Y9.5302
N196 X8.3542 Y9.5301
N198 X8.3624 Y9.5299
N200 X8.3707 Y9.5297
N202 X8.3793 Y9.5294
N204 X8.3881 Y9.529
N206 X8.397 Y9.5286
N208 X8.4062 Y9.5282
N210 X8.4156 Y9.5278
N212 X8.4252 Y9.5272
N214 X8.435 Y9.5267
N216 X8.4449 Y9.5261
N218 X8.4551 Y9.5255
N220 X8.4654 Y9.5249
N222 X8.4758 Y9.5243
N224 X8.4865 Y9.5236
N226 X8.4973 Y9.523
N228 X8.5082 Y9.5223
N230 X8.5193 Y9.5216

Offline RICH

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Re: CNC router shudder and jerky / jitter doing 3D toolpaths
« Reply #5 on: February 04, 2016, 07:56:49 PM »
Are you using steppers or servos?

RICH
Re: CNC router shudder and jerky / jitter doing 3D toolpaths
« Reply #7 on: February 05, 2016, 05:28:16 PM »
Hello,

Sorry, I have only just got a chance to look at your XML file

Can I ask what leadscrews are you using and what microstep resolution are you running?

Do you REALLY need a step resolution of 0.049 thou per step?

I don't know your setup or how much or little you know, so please bear with me..... (not trying to be a smart A$%%^ or something).

Motor torque diminishes with increased microstepping.

Basically if your drives are set on full step resolution you will get your motors rated torque, however the torque diminishes via the following equation

T actual = Torque Rated x Sine of (90 / microstep setting)

Hence if you are running say 16microsteps.... you get about 9.8% of your motors rated torque...

The rule of thumb is generally microstepping up to about 8 is fine (~20% of the rated torque).

___________________________

I suspect as your drives are closed loop what is happening is your machine is loosing steps as it lacks the holding / stepping torque (because of a high microstep setting) and in turn your drives are trying to automatically correct and jumping

Note..... I've never used close loop drives so its just a guess (only you know your machine, and setup as to why it is configured as it is!)

______________________________

Question.... how are you controlling your router, spindle / mill?

You seem to have disabled M3/M4 controlling output1 on the ports & pins >>> spindle setup tab

And you are not using step & direct / PWM for the spindle but have a defined output 1

____________________________

Although its not your problem..... it may be worthwhile increasing your lookahead to 200 lines on the general configuration tab

Have a look in the FAQ's on the Warp9 site.... if you run out of data at any point this is likely to be your problem.... it comes up quite a bit on the SS forum on the Warp9 site.

Rob

Rob

Albert Einstein ― “If you can't explain it to a six year old, you don't understand it yourself.”

Offline Tweakie.CNC

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Re: CNC router shudder and jerky / jitter doing 3D toolpaths
« Reply #8 on: February 06, 2016, 02:48:24 AM »
Quote
Motor torque diminishes with increased microstepping.

Basically if your drives are set on full step resolution you will get your motors rated torque, however the torque diminishes via the following equation

T actual = Torque Rated x Sine of (90 / microstep setting)

Hence if you are running say 16microsteps.... you get about 9.8% of your motors rated torque...

Hi Rob,

Once, this info was correct but it is now many years out of date and still the myth continues to be perpetuated.

The manufacturer's of the various stepper driver chips (eg. Allegro/Toshiba/etc.) are well aware of the stepper motor characteristics and design their circuitry accordingly. Basically their chips are ‘smart’, using a form of back emf to proportionally increase the current through the energised windings to balance the available torque in relation to a micro-step position and shaft loading.
It’s a few years now since I carried out comparative angular deflection testing between a number of different manufacturer’s chips and I can confirm that the test stepper motor’s rotational shaft deflection, under a controlled load, (which is basically, holding torque) was within a knat’s whisker between whole step positions and micro-step positions.

Tweakie.
KEEP SAFE !
Re: CNC router shudder and jerky / jitter doing 3D toolpaths
« Reply #9 on: February 06, 2016, 03:11:10 AM »
Here is another way to look at my steps comment.....

Looking at your G-code, your x and y movements are in the range of 0.0001"   or 0.1 thou

Which to me is really impressive....

so if your step resolution is ~0.049, you are getting two steps for every commanded movement

Hence half'ing your microstep resolution will align your steps to your commanded motion. (and improving your stepper motor torque)

You also need to consider backlash (you cannot really eliminate it.... just reduce it to the point that it does not affect what you are doing..... in order for a ballscrew (nut or anything) to slide over a leadscrew, it requires tolerance to provide slip and movement.... hence you always have some small amount of backlash)

0.1 thou of movement is impressive, and I know nothing about what you're machining or your required surface finish or detailing, but what does 1 thou of cutting accuracy do for you (0.0254mm).... have you tried it, is it noticeable?   it will again 1/2 your microstep resolution and increase your available stepper motor torque.

I think at that sort of resolution of cut I would also be considering if there is any movement in spindle, guide rails etc

Only you know your machine and I am by no means an expert, only you know your machine and these are just things to consider.

___________________________________________________

(Tweakie, thanks very much for the guidance, I'll take note and stop myself perpetuating the myth..... I think it makes a little difference, I went from 50uStep resolution to 10uStep on my plasma and the torque improve dramatically (AM882H drives) .... I considered the step resolution of what I was asking the machine to really do..... with a blunt instrument (plasma cutter) and the finish I was getting and my backlash).



Rob

Albert Einstein ― “If you can't explain it to a six year old, you don't understand it yourself.”