Calculate maximum mass of an workpiece on a vertical CNC turning machine 2

[reggie92]

Greetings fellow colleagues,

I work with an EMAG VSC 500 vertical turning CNC machine. Currently, I am offered a task to machine a 125 kg workpiece, which is quite a challenge. How can I precisely calucalte the maximum mass of an object, as it is not mentioned in the specifications of the machine? What I do have are torque and power characteristics of the machine.

I think I should do it this way: Calculate the angular momentum of the workpiece using equations from Dynamics. After that, I can use the angular momentum to calculate the necessary torque at a certain speed rate. I will get some torque value. I can add the torque necessary to achieve the machining process (I can measure it on my machine using Sinumerik 840 D Servo/Trace Option).

But that doesn't include the force used for clamping the workpiece, which makes it useless in my calculations, which kinda doesn't seem logical to me. Is my caluclation OK and what should I add to it? If not, please give me some advice how to make it better and more precise? I am very thankful for every advice.

Thank you in advance,

Nermin

 

[JNieman]

Any forces used in workholding shouldn't have any effect on the capacity of the machine. The only time you worry about workholding clamping forces is to ensure they are adequately compensating for any cutting forces in any vector. Whether you have a pie jaw chuck clamped or loose, the mass is the same.

 

[3DDave]

The angular momentum might affect the ability of the motor to get the part up to speed, but asymmetric mass distribution would be a greater problem. Once the part is up to speed, there is no torque associated with maintaining that speed.

The answer is to contact the machine maker. You can't do what you want without a full structural model of the entire machine.

 

[BrianE22]

CNC machines generally give you an error message or stop the process if the error between the commanded point and the actual point is too large. They'll also trip on over-current (too much load). If you are only doing one piece I'd suggest entering your desired feed rates and see if that works. If the machine doesn't like it then slow down the feeds.

 

[reggie92]

Thank you all very much for your answers.

I am currently machining workpieces with diameters around 250 mm and mass around 5-6 kg. The angular momentum I calculated for them is very small. I measured almost no torque on the machine when starting to spin the workpiece to 2000 rpm (which I limited as the maximum value). This new one with the 125 kg mass and 520 mm diameter is quite a challenge for me and my team. We will start it around june if we conclude that we can machine it.

I found out through experience that by a certain relatively low value the workpiece gets bumped out of the clamping tool and flies away (which is logical, but very, very dangerous). So I know that only the cutting forces can bump the workpiece out. I wonder if the mass of the workpiece has some influence on that. Currently, I am in the phase of finding out if such a workpiece (made out of gray cast iron) can stand still in the clamp at any cutting speed (it is OK if I get like 50-100 rpm, time is not the matter). So I wonder what other factors (except the cutting force) influence the fact that the workpiece won't bump out of the clamping tool, and won't vibrate a lot? Mass, angular momentu, moment of inertia, asymetric mass distribution etc... ?

Thank you once again.

 

[JNieman]

Worse comes to worse, a safe test is to chuck it up. Assure the part is secured adequately. Command the spindle to 5 RPM and watch the spindle load monitor as you do. That will tell you a lot. Then you can try it at 50 RPM. Or whatever sounds good for testing.

Asymmetric mass will be a big problem if it's not already mostly radially symmetric. However, if you're not so fortunate, and you're having to bore something in a weird casting shape that's nodular or something... slower speeds and possibly counterweights in the fixturing to simply offset mass.

Whether or not the piece will work in the 'clamp' is dependent upon how you choose to hold the piece. Given that it's a small-ish VTL I was assuming it had a multi-jaw chuck at least. If it's a t-slotted table and you use an array of different clamps, that would be different. Not impossible. But then you have to look at other factor.

Resisting cutting forces is nothing terribly special compared to your 5-6 kg parts. The principles are all the same.

 

[hendersdc]

I think you may have a problem. Your machine has a maximum work piece diameter of 440mm (according to their website Link) and you are trying to machine a 520mm diameter part. You may have some wiggle room since they list the swing diameter as 520mm but you are definitely pushing the size capabilities. Can you test fit a part before accepting the job?

 

[reggie92]

yes, and this is why i need to do a calculation to test if it is possible to move those limits up a little bit. i hate to do that, but i see no other choice right now. a colleague of mine measured the whole workspace of the machine and it can fit somehow. i have a chuck with 3 jaws.

the part is symmetric, but since it is a casting, i believe assymetry to some point is guaranteed. the only question is in what rate will it affect the machine.

thank you all for your advice, i am very grateful. if you have anything to add up, i would be thankful to listen to your advice/experience/way of calculation. i will certainly send you my results if i accept this job.

 

[SwinnyGG]

Typically part sizes for those machines are based on the physical envelope available and have nothing to do with part mass.

The machine cares very little how much the part weighs. From the CNC machine designer's point of view, if they give you a part envelope, they are going to have to assume that at some point in the machine's life, it's going to have a part on the table or spindle that is a solid bar at or near the size of the full envelope.

You're basically only limited by accel and decel rates based on how much current the motor can handle, and the machine has built in alarms for that.

We're talking about a machine with 70 kW and 750 Nm on tap.