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Author Topic: When is a circle not a circle? When it's 4 arcs!  (Read 8009 times)

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Offline stirling

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Re: When is a circle not a circle? When it's 4 arcs!
« Reply #20 on: April 22, 2014, 06:19:03 AM »
you want your accel as high as possible. However it will be limited both by your motor power and jerk. From what you say it would seem you have rigidity problems if an accel of 1200 is causing wobble.

However, concentrating on the difference between your 4 arcs and a single arc try your accel at 1200 and slow your feed down - 9m/min is pretty fast if you a) have rigidity issues and b) your accel is not so good.

Using a single parallel port you have 5 inputs of which your e-stop should be one. Pin 2 is an output, as are 7 and 8. Look at the bottom of the input/output dialogs - it tells you the valid input and output pins.

accel and decel are the same in Mach.
Re: When is a circle not a circle? When it's 4 arcs!
« Reply #21 on: April 22, 2014, 04:04:59 PM »
Success! I increased the acceleration to 1100 and now have much improved circles. There is still a wee bit of a flat area at one point which is 0.3mm smaller overall. I think I can live with that for the time being, with a bit more testing I will see if it is consistent. Now that I think about it I was getting the wobble when the 90deg corners were being cut in tiny radii so I might be able to increase the acceleration further.

Changed the e-stop pin to 11.

There is still a wee stutter in certain places which seems a bit strange. I was cutting large smooth arcs today and at the transition point where the arc is split there is a a slight hesitation and then it carries on. Is it seeing that there is a join in the arc and thinking about it? Is that a characteristic of stepper motors? The look ahead is set to 200 lines as recommended by mach handbook.

I realise that I may be barking up the wrong tree here but would a smooth stepper improve the performance?

Offline stirling

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Re: When is a circle not a circle? When it's 4 arcs!
« Reply #22 on: April 23, 2014, 07:15:40 AM »
Sounds good. So is it now cutting the same for a 4 arc circle as a 1 arc circle?

Is it seeing that there is a join in the arc and thinking about it?

It shouldn't be if CV is doing what it's meant to. Just to re-state: If CV is working properly there should be absolutely no reason for it to cut 4 tangentially meeting arcs any different to a single arc. i.e. there is absolutely no need for a speed change OR for any rounding at the "joins".

Is that a characteristic of stepper motors?

Not sure what you're meaning. Is what a characteristic of stepper motors?

The look ahead is set to 200 lines as recommended by mach handbook.

Hmmmmm. Where does it say this? My default was 20 and my manual says it shouldn't need changing and indeed I never have.

Lookahead tells the planner the maximum number of sequential feedrate moves it should consider at any given time. The maximum number of sequential feedrate moves in the whole code you posted was only 20. I think you can see where I'm going...

I realise that I may be barking up the wrong tree here but would a smooth stepper improve the performance?
With Mach I've learned never to say never. But if we're still concentrating on why a 4 arc circle cuts differently to a 1 arc circle, then personally I can't see how it could make a difference.

Anyway - things seem to be improving so that's good.

Cheers

Ian
Re: When is a circle not a circle? When it's 4 arcs!
« Reply #23 on: April 23, 2014, 05:55:23 PM »
Yes, it is now cutting the same for a 4 arc circle as a 1 arc circle.

I can see your point about the lookahead and it was set to 20 steps by default but again, possibly I imagined reading it or someone recommended to change it to 200. I did think it a bit odd when trying to run a circle program that had at best 20 lines so setting it to 200 seemed a bit pointless. Perhaps I should trust my own thinking but at the time I thought it may have been better suited to when I actually got round to running bigger programs. Anyway as a newcomer to this every day is a learning experience. I will set it back to 20. I also learned to never try and open the config when cutting something on the machine. It did not like that.

With respect to the CV distance value is that lines of code or is it steps in a direction? Is it worthwhile turning that on? What is it's purpose? When you say that if CV is working properly... how do I know that it is not? Is there a shape (don't say circles) that I could cut that would show it is doing what it should?

The stuttering effect I sometimes see when it comes to a transition point in a curve is what I was referring to when asking if it is a characteristic of the stepper motors. Setting the lookahead back to 20 may cure this I suspect.

I think I may be able to reduce the feedrate of the cutter a bit but I am using twin fluted cutters and I think that is about the right feed speed for them. When increasing the acceleration at what point will I know that it is not actually having any physical effect due to the trade off between velocity and acceleration? I have been watching the feedrate DRO as it has been cutting and on curves and anything that is not a straight line then obviously it drops a fair bit but once it gets to a straight line it accelerates well up to max feedrate. I am bearing in mind you have never seen this machine... however ... what is an ideal acceleration value to aim for? Or am I not actually looking for a value just the performance of cut?

I apologise in advance for continuing to ask foolish questions!

Offline stirling

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Re: When is a circle not a circle? When it's 4 arcs!
« Reply #24 on: April 24, 2014, 09:40:41 AM »
CV distance is in your machine units. It just says - don't blend until you're at least this close to the join. Sounds great - nice tight (small radius) corners. Problem is - you don't get anything for nothing. The smaller you make this the more the axis HAS to slow down and of course the name of the CV game was NOT to have to slow down.

TBH - my view is really that if you have to start tuning CV you're already in trouble.

Velocity/Accel - always a trade off. Get as much of both as you can. Great velocity with crap accel is no good. Neither is ace accel and crap velocity. If you can't get a good balance that does the job - it's hand in pocket time again.

Ian
Re: When is a circle not a circle? When it's 4 arcs!
« Reply #25 on: April 24, 2014, 03:55:28 PM »
Thanks Ian.

I will spend some time playing about with the acceleration and velocity. I hope to not have to use the additional CV settings.

I bought this machine to make furniture components with but not in huge quantities. I think it will do pretty much what I am looking for but if I had to spend another couple of hundred quid I would, providing it was a benefit. I am talking about a smooth stepper rather than upgrading motors or drives. But then which smooth stepper to buy.... If I thought it would be a straightforward installation then I would probably just get one. Buying a smooth stepper and finding it causes more problems would not be ideal. Being self employed and trying to make money during the day and finding a few hours here and there to test the CNC is proving tricky. Although to be fair I did not expect it to be straight forward.