Machsupport Forum
Mach Discussion => General Mach Discussion => Topic started by: ftec on July 27, 2008, 07:39:54 AM
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Hello folks,
I have been calculating motor size for a fairly heavy ganrty using Emerson servo sizing program. Quite understandably the size of the motor depends heavily on how quickly it needs to accelerate the mass to full speed. From another discussion group I got a piece of advice to use an acceleration time of 0.1 - 0.2 s to full speed (10m/min). With the acceleration time of 0.1s I get fairly decent motor sizes, appr. 0.37HP or 275W with a suitable rotor inertia to load mass inertia ratio.
My question is: is Mach able to handle this max acceleration, I mean can/will it optimize the routing path so that this max acceleration will never be exceeded? This qustion may be ridicilous but as I'm just starting with DIY CNC I hope you can bear with me.
Thanks,
Risto
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Yes, when you set up your Mach for your machine, you set the acceleration and max velocity for each motor in the motor tuning screen. The acceleration is always what you set, and the max velocity is your rapid speed.
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Yes, when you set up your Mach for your machine, you set the acceleration and max velocity for each motor in the motor tuning screen. The acceleration is always what you set, and the max velocity is your rapid speed.
Great, thanks Gerry! I have not yet purchased Mach as the machine is still on the drawing board, maybe it's now time to get to know it better.
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I keep saying I will get shot for this - but you do not need to buy Mach 3 .
The free downlaod version will get you 95 % of features of the purchased one - certanly enough to get you going and try - for nothing.
I am still using the free downlaod version. The only major difference between the two is that the free version is limited to 500 lines of code - and I have never come up against that barrier.
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tried any threading yet on the lathe Jim ;)
Hood
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I keep saying I will get shot for this - but you do not need to buy Mach 3 .
The free downlaod version will get you 95 % of features of the purchased one - certanly enough to get you going and try - for nothing.
I am still using the free downlaod version. The only major difference between the two is that the free version is limited to 500 lines of code - and I have never come up against that barrier.
OK, maybe so but I think the guys need to be paid for the nice job. Besides I will be using Solidworks models and VisualMill for CAM so paying the licence fee for Mach is a decent thing to do for the vital core program.
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Nothing wrong with paying for it, but download it and start learning how it works. I too also use the demo, but I don't actually have a machine to run. (5 year work in progress) ;)
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OK, maybe so but I think the guys need to be paid for the nice job.
That's what they are looking for ftec. Trying the free demo is there for just that, trying. Once anyone with good sense tries it they will gladly pay for a licenses out of gratitude for such a great product at such an insanely low price. Anyone doing less should be embarrassed.
Brett
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What a small price to pay for such a dynamic learning tool. LCam too !
RC
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I coiuldn't agree with you fellas anymore, for the price of Mach3, it's an act of thievery to not buy it :D
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O.K. - I'm sorry - ??? :-[ :-[ :-* :'(
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Nah, Jim, I don't think our comments were meant that way, I think once you do finally run out of your demo amount, you'll be happy to contribute to this software, no?
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We are getting a bit off the post here - Yes - I think Mach 3 is the best thing since sliced bread ( to coin a phrase). It let me convert a lathe/mill into a CNC machine for a very small price. It is, as far as I can see, complete in evey way, and opened up a whole new approach to my machining. I was getting a lot of my stuff done professionally - in steel and in plastic - and I am capable of doing this at home now (if my machine is good enough). At least I can understand what the CNC guys are on about.
I haven't run up against a limit with the download version - and being from Yorkshire is like being a Scotsman with no arms, we don't like spending money if it is not necessary.
Now another short coming is threading - I think - and I want to have a go at that sometime - at the moment I have a good range of taps and dies - .
Yes - you are right - when the shortcomings start limiting my work I will gladly pay my money for what I consider to be a superb product.
In a post reply to Ftec -
Yes - Mach 3 can be set to any acceleration - and the ones you are quoting are at the bottom end of the scale.
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In a post reply to Ftec -
Yes - Mach 3 can be set to any acceleration - and the ones you are quoting are at the bottom end of the scale.
I see. I have no installed Mach yet as I need to get another computer for it so I havent been able to see what it says. So what do you think, does a low accel value have a negative impact? (The gantry weights actually appr. 95 kg. With the accel time of 0.1s from 0 to top speed 166.7mm/s the 300W servo is running in it's countinuos range. It can deliver a 900W peak if necessary).
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So what do you think, does a low accel value have a negative impact?
It can. If you cut parts in Constant Velocity mode, corner rounding may occur. The degree of rounding is related to acceleration. Faster accel = less rounding.
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So what do you think, does a low accel value have a negative impact?
It can. If you cut parts in Constant Velocity mode, corner rounding may occur. The degree of rounding is related to acceleration. Faster accel = less rounding.
Well, that was what I orginally was asking, if Mach can optimize the axis/routing path according to what the motor can deliver acceleration. So you mean it can't?
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The corner rounding when in CV mode is the only time that Mach will change the toolpath from what it's supposed to be. There are a few different CV settings that will let you customize how Mach behaves during CV mode. http://www.machsupport.com/docs/Mach3_CVSettings_v2.pdf
There is no "optimisation" of the toolpath based on acceleration. You tell Mach where to cut and how fast to cut, and it accelerates using the accel rate that you set. If the accel is too low to reach your intended velocity, then it won't reach it.
Your original question asked if mach would modify the toolpath so that the max accel would not be exceded. Mach ALWAYS accelerates at the rate you set, never more, and never less/ The toolpath has nothing to do with the acceleration.
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Your original question asked if mach would modify the toolpath so that the max accel would not be exceded. Mach ALWAYS accelerates at the rate you set, never more, and never less/ The toolpath has nothing to do with the acceleration.
Ok, my question was obviously inaccurately put. However, what I had in mind is that when the tool travels a certain curve, the path, and a change in it's shape occures, the velocities (in a 2 dimensional plane) dx/dt and/or dx/dt will need to change. Thus there will be changes in the time derivates of these velocity components ie. the accelerations. Now if Mach could make sure that neither of these accelerations exceed the given max value by changing the resultant velocity everything should be ok as far as I can see, even with a weak servo. The pdf file you gave only gives an overall picture, how can I get into detail what I can expect to happen in real life with an acceleration mentioned?
Thanks,
Risto
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After reading your example, I see that I mis spoke in my last post. Mach doesn't always accelerate at the set rate. It will not, however, ever exceed it. Which answers your question I think. :)
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Risto,
"how can I get into detail what I can expect to happen in real life with an acceleration mentioned?"
A practical approach would be to look at similar machines by vendors and individuals and see what they are using.
Make note of the sizes, weights, axis or drive chains, and speeds provided and also what they doing with the machines.
Use your computer program and compare its output to what others are using that works in real life.
I am not familar with servo's or the program you mentioned but have used other programs for steppers. The programs provide general paramters as input guides. So you can be conservative on those parameters. Now they usualy ask for additional loads you expect and only you can provide those. These values can certainly have an influence on sizing.
So play around with the computer program and see what it tells you. The sizing software may use some inertia guideline ,like a ratio of 10:1 of calculated to actual motor inertia, and often these are general guidelines / rules of thumb which they have found to be applicanle over time. It's just one parameter which provides for motor recomendation.
Like the story on computer use goes, "garbage in garbage out".
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Think I,m correct in saying, when going around a circle Mach uses the lesser of the either axis velocity for the movement. I know that in some other programs this dosn't happen. I also know that my faster computer will give me
quicker moves around a circle especially at the axis transition points.
Hope this helps
RICH
A computer program is a great tool for doing the calc's for a system and usualy provide for motor selction motor options.
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After reading your example, I see that I mis spoke in my last post. Mach doesn't always accelerate at the set rate. It will not, however, ever exceed it. Which answers your question I think. :)
OK, then I think I will be pretty safe for the time being. I'm quite familiar with calculations for pick 'n place robots but CNC is so much different with the constant interpolation. If necessary I can reduce the gear ratio to slow down the top speed as this is a hobby machine, not needing full scale production speeds. Thanks Gerry and Rich too.
PS. Obviously there was a typo in my previous post, should of course have read dx/dt and dy/dt.
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You dont need to gear to limit your top speed, whatever velocity you set in motor tuning will be the max you will ever get.
Hood
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If necessary I can reduce the gear ratio to slow down the top speed as this is a hobby machine, not needing full scale production speeds. Thanks Gerry and Rich too.
No need to gear anything. Rapid speeds are set in software. If your talking about slowing down cutting speeds, just change them in your code or use the Feedrate override to slow it down. There's also a separate rapid feed override.
Edit: Damn, a hair too slow. :)
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Uh guys, are you serious, I thought you knew the physics;-)
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There's a difference between slowing down because you want to and slowing down because you need to.