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Author Topic: Tangential arcs  (Read 2816 times)

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Offline rev

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Tangential arcs
« on: December 08, 2011, 03:54:58 AM »
Hello everyone,
Can anyone help please, I have mach3 driving a Homann MB-02V6 breakout board and gecko 203V's
When cutting tangential arcs or compound curves  the motor stops or stall between each arc and I
cant get the motors to move linearly.
Is there a setting I have missed?
Re: Tangential arcs
« Reply #1 on: December 08, 2011, 05:18:30 AM »
Are you saying that the motors stall completely? Or is it a hesitation?

If you are completely stalling, the first thing to try is lowering acceleration and/or speed in the motor tuning window. If you've already slowed down the feedrate without any difference then I'd start with acceleration. You may be trying to drive the motors beyond the system limitations, which is a combination of your motors torque and inductance, as well as the power supply. Higher voltage power supplies are better able to deal with motor inductance and the back EMF that they generate.

Offline rev

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Re: Tangential arcs
« Reply #2 on: December 08, 2011, 06:48:30 AM »
Its more like hesitation, it seem like it has to have a think about its next move.
When cutting a rectangle with two semi circles on either end it has no trouble transitioning from a straight line
to the semicircle and back to the straight line.
But when cutting a shape such as a airfoil where the curve is made up of many arcs it hesitates from one arc to the other.
The system is running at 35 volts regardless of motor running or not. 
Re: Tangential arcs
« Reply #3 on: December 09, 2011, 06:22:00 AM »
I also have and do experience this problem, although it hasn't presented a major problem so I haven't pursued it.

Can anyone who knows the inner workings of Mach3's trajectory planner weigh in on this?

Can others confirm whether they also have seen this issue?

In fact, I see hesitations before/after all of the canned cycles. For drilling, it is of no concern, but it does affect cut quality with arcs.

I usually let our production workers run the Mill and Lathe, but I do see it on the router when I happen to run it. I will check the mill for the problem and let you know. The lathe it can be hard to get time to play with - it's too busy, but I'll see if I can check it too.

Anyone with some insight? It would be very much appreciated!
« Last Edit: December 09, 2011, 06:25:21 AM by Sargon »

Offline rev

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Re: Tangential arcs
« Reply #4 on: December 14, 2011, 03:45:39 PM »
I found this post and has cured my problem
More technical explanation in "Using Mach3 Mill" users guide.
Page 10.1.5 Co-ordinated linear motion
Page 10.1.6 Feed Rate
Rev


A few folks on different boards have been having problems with jerky motion on their machines (including me). The machine acts like Constant Velocity is not turned on even though the little green border is lit around the CV button.

A poster on the Mach support board suggested I turn off CV (go to exact stop) on the general config page, and turn off the CV Feedrate button on the Settings Tab then add G64 (Turn on CV mode) in my gcode. I tried this and it worked!

After a bit of experimenting I figured out that all I really have to do is go and turn off the CV Feedrate button on the settings tab and everything is good. Only problem is that it comes on by default whenever I start mach.

I also noticed that choosing "save settings" from the config menu somehow turns CV Feedrate on. So if I change anything I have to remember to go turn it back off again.

Hope this helps if anyone is experiencing the jerky motion.
Re: Tangential arcs
« Reply #5 on: December 17, 2011, 06:40:04 AM »
Thanks for posting this here. I will try it tomorrow!

Offline stirling

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Re: Tangential arcs
« Reply #6 on: December 17, 2011, 11:48:52 AM »
As you know - CV is a compromise. With Exact Stop you get the exact path you programmed but with a stop at "segment" junctions. With CV you get Mach's attempt at a constant feedrate (never sure why it's called CV as technically a constant velocity can only ever be a straight line - any whatever - I digress ;D). If you're happy with the "out of the box" CV for any particular job - then cool - it works very well. However there are times when you might want to tweak the way CV behaves.

So, when Mach needs to do (say) a corner, with CV it will cut (round) that corner as much as IT likes to keep as close to the commanded feedrate as it can. BUT - suppose you want to tighten that "rounding" a bit. There are a couple of ways you can do it and ONE is to set the CV Feedrate button ON. Then it effectively says to Mach - don't worry too much about keeping the feedrate up - I'm prepared to let you drop it to - whatever's in the CV Feedrate DRO. If you go with the default value in the DRO i.e. 1 you're saying to Mach - you can drop the feedrate to 1unit/min if you like. Which is about the same as turning OFF CV - i.e. just about exact stop.

So, instead of turning CV Feedrate OFF - which says "go as fast as you can round corners etc." you could set (say) 75% of your commanded feedrate in there. Now Mach will allow the blended feedrate to drop to 75% of the commanded feedrate if it needs to. It'll tighten your corners a tad but at the expense of dropping the feedrate a bit more than it would have done.

Hope that makes sense.

BTW - not sure why it's turning itself ON when you set it off - but it doesn't do that here. Checkout the <CVFeedOn> tag value in your XML after you've turned it off and closed Mach. It should be 0. If it is and when you restart Mach it's back on again then something's turning it on again - a macro maybe? guessing...

Ian

PS - I'm going to move this to general - FAQ's is not really the place for this.
« Last Edit: December 17, 2011, 12:41:37 PM by stirling »