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some questions about EDM machine

Jessie

Plastic
Joined
Oct 10, 2006
Location
uk
hello everyone,

I'd like to post some question concerning the mechanical structure of EDM machines:

1. How the speed of Z axis affect the machining performance of EDM machine? It's ok if the speed of Z axis is 1.5mm/sec?

2. what type of linear motor driven stages is applicable for EDM machine? Brush linear motor, brushless iron core linear motor, brush ironless core linear motor??

3. I am going to buy a spindle for a micro EDM machine, please recmommend spindle to me.
 
Jessie,
I can really only answer your first question as I am relatively new to Ram EDM.
Z axis speed effects machining in regards to the removal burnt material from the cutting zone. As the head retracts, if speeds are high enough, a mild vacuum will be created causing debris to follow the electrode out of the cutting zone. This is especially helpful in deep burns where side flushing is all but useless. Of course, through the electrode or through the part flushing will also aid in this type of burn. 1.5mm/sec should be ample.
There is also the obvious answer of "It takes less time for the head to retract and then return to work." This may sound a small attribute, but when you consider how many times the head will need to retract over the course of a long deep burn, those seconds really start to add up.
As far as the linear motor question goes, I believe Sodick is the only one using them. I could be very wrong about that however. Maybe you can look into what type of motor they are using. I can definitely see the advantages of a linear motor on the Z axis of a Ram EDM. That type of movement over a small section of the axis repeated over and over again just smells like heavy wear on any other type of drive system. I really don't see any benefits to using it on any other axis though. I guess if you're burning in X or Y direction... How often do you do that though?? Not very often here...
 
Hi, Jay, thank you for your reply.

Is it ok if the speed of Z axis is only 1.5mm/sec for a micro-EDM because I am going to build a micro-EDM machine?? I know that the speed of Z axis of some commerical micro-EDM is about 150mm/sec, but I have only got a stage with speed of 1.5mm/sec.
 
It will work, but you will have to use a fairly high retraction and lots of external flushing, needless to say it will be a SLOW machine.

Of course, since this is a "micro" EDM, you might not have much of a problem. How "micro" will you be going? If you are only going a few MM deep the speed might not be as critical, although flushing such a small area might be tough if it's a small diameter hole.

What type of work will you be doing on this?
 
Hey Mitstech, Are you really a Mitsubishi Technician?? If so, are you guys looking at linear motors? Do you see any value in them other than the Z axis on a Ram EDM?
 
First, you will need to define what your target capability for this machine will be. Micro EDM has many different facets, and the axis speed/servo reaction time requirements will be as varied as the applications. Your proposed axis speed translates to just over 3.5 inches per minute. Other machines doing similar applications will typically be capable of over 100 times this velocity. (near 400 IPM and higher is not unusual for these machines) Additionally, and more importantly, the physical reaction time of the machining axis is far more critical than full velocity speed. Linear motors, by the nature of their design, will typically take around 20 microseconds to react physically to servo input, while a properly applied ballscrew will typically react in the neighborhood of 3 to 5 microseconds. If you are producing fine details such as ribs or pockets, the 1.5mm/sec speed will not support that. Such fine details are difficult to flush, and will require much more frequent jumps at much higher speeds than general EDM work. If you intend to do fine hole machining, the axis speed requirement is lower than fine ribs and pockets, but mostly because this application is supported by a high RPM spindle and a very high pressure through-electrode dielectric supply. I would seriously question the possible effectiveness of such a limited axis capability in any micro EDM application. In this arena of machining, a $200K(USD) investment is on the low end of the spectrum, and there are a multitude of reasons why this is so. Good luck with your endeavor, however, and please let us know how things work out.
 
Hmmmm, I was unaware of the linear motors inability to react relatively quickly to servo input. Is this because there is no friction based contact and therefore the machine "floats" for a time while the input says move? Please explain more.
 
Jay Cee,

It is not actually an inability of the motor to react quickly. In actuality, both types of motors will begin to react within a microsecond or so 'with no load'. However, this is not reality with any machine tool, because you have the inertial load of the driven stage to contend with. You'll notice that I said "physical reaction time" in my previous post. I will attempt to analogize this with a very basic concept. Two identical cars with the same amount of horsepower apply throttle at the same moment. The "linear car" is in 3rd gear, while the "ballscrew car" is in 1st gear. The gearing analogy relates to the result of any given force being applied to the ballscrew as opposed to that same force being applied directly to the driven stage. Obviously, the mechanical advantage of the ballscrew produces a much higher linear force. While the linear will eventually build up enough speed to overtake the ballscrew, the ballscrew overcomes the inertial load much more quickly, hence reacting in far less time. In essence, the ballscrew car has moved a foot or so before the linear car has moved an inch. Reduced to units applicable to EDM, this first critical reaction might be a mere few microns, but directly comparable in scale. In the case of EDM, this "hole-shot" (as it would relate to drag-racing, not adult film) is far more important than the top speed. I also stated that this relates to a "properly applied" ballscrew. This would apply to systems in which the driven stage is coupled to a rigidly anchored ballscrew which is directly driven by the servo through a rigid (ie: bellows type) coupling, and not by a Gilmer pulley and timing belt type of system attached to a floating ballscrew as seen in nearly all models of a very popular brand of EDM. The reaction time of such a system is considerably slower than either of the other two types discussed here, and for obvious reasons. This is not intended as an attack on commonly (and popularly) utilized construction techniques, this is simple physics.
 
I am going to buy a spindle for a micro EDM machine, please recmommend spindle to me.

Which company produce spindle for micro EDM?

Many thanks.
 








 
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