What are Linear Motors in Electric Discharge Machines for?
Don't we have conventional ball-screws?
But the EDM process is slow.
What are the advantages of linear motors compared to traditional ball-screws,
if speed in EDM process is not necessary?
Why ultra-fast linear motors in a slow EDM process?
Ball screw drive
with belt reducer
of a Swiss EDM.
The two main parts of a Sodick linear motor (LM):
block of electromagnetic (EM) coils
and a panel of permanent rare earth NeFeB magnets.
Speed solves other problems.
For the EDM process, it is not the speed itself that is important, but the accuracy and response speed of the drives.
More precisely, the kinematic accuracy multiplied by the responsiveness or what can called dynamic accuracy.
And here planar linear drives are beyond any competition!
Ball screw drives are a constant dynamic, the delay from the moment led to the start of movement. The EI machine with a ball screw practically does not work with the optimal clearance, the processing modes are approximated, the loss of speed and quality is increased. All this is exacerbated if there is a belt or gear reducer between the electric motor and the ball screw, as in many non-Sodic machines.
For the EDM process, it is not the speed that is important, but the dynamic accuracy
What is required from the drive of the EDM first of all? Let's figure it out:
The EDM process is non-contact, spark discharges occur in the inter-electrode gap. The characteristics of the discharges and the entire process depend to a very large extent on the size (width) of this gap. A wider gap – the discharges are weak, more wide- they disappear altogether. The gap is smaller – instability, arc, burns, etc. It’s like an automobile spark plug (experienced motorists should know), but it’s an order of magnitude more complicated, because the process itself is much more complicated! So, the dynamic maintenance of the gap value – and the gap in the EDM process is constantly changing – this is the main task of the drive in the EDM.
Ideally, for a high-quality and productive EDM process, the drive should correct the gap tens of times per second, positioning the electrode with an accuracy of microns or even more precisely.
Can conventional drives with ball screws do this with that accuracy, if even in the best of them the gap (and, accordingly, backlash!) is at least 4 microns? And if in a drive with a ball screw, to reduce the cost, there is also a belt or gear reducer?
The EDM process in many cases is a sequence of micro-movements.
In die-sinking EDMing, micro-movementss are required for the so-called orbital oscillations,
and often for electrode relaxations.
In Wire-Cut EDMing, tracing and cutting any complex curvilinear contour is a chain of micro-movements.
Can ball-screw drives correctly excercise micro-movements
of 1…2 microns (or sub-microns), if the gaps and, accordingly,
the backlashes in them are several times larger?
Compare and decide for yourself what is better for electric discharge machines:
Ball screw drives are sofisticated structures with a complex chain of multi-stage conversion of energy into rotational motion and then rotational motion into linear motion – with backlashes, a large dead zone and uneven coarse feeds. There is a long delay from the command pulse to the start of movement at the start and each reverse. And if in such a drive there are also reductors (belt or gear), then the delays grows almost into idle time-outs:
of magnetic fields
rotation of the motor rotor
(functioning of a belt
or gear reducer, if any)
ball-screw backlash play
A long distance from a command to execution!
Ball screw drives are a constant dynamic, the delay from the moment led to the start of movement. A ball-screw drive EDM practically does perform with optimal spark gap, the machining conditions are approximated with invariable losses of machining speed and quality. All this is exacerbated if there is a belt or gear reducer between the electric motor and the ball screw, as in many non-Sodic machines.
Linear drives with flat (planar) linear motors are an extremely simple construction with non-contact transmission of force, a direct drive without any kinematic chain for converting energy into motion and rotational motion into linear motion, without backlash, dead zone and uneven bumpy feeds. In fact, the moving part of a linear motor is also a mover. All that happens when processing each motion is:
of magnetic fields
And it’s all!
Barely moments from a command to execution!
Sodick Linear Motors
correct the gap up to
1000 times per sec
with feedback response
in linear EDMs of up to 1 µsec
and linear scales resolution of
10 nanometers or 5 nm).
The backlashes “crawl out” at each start, reverse and stop of movement. A reminder: backlash in ball screw drives exacerbate elastic deformations, thermal deformations, kinematic errors of drive parts – losses from friction, twisting of the ball screw screw. You may object that, they say, this is nonsense, but it is precisely this nonsense, which is called microns, that we catch with our EDMs! The efficiency of ball screws, although higher than that of other mechanisms for converting rotational motion into translational motion, reaches 90% at best. But it’s not 100%!
Linear drives are direct drives free from all ball screw defects. In linear drives, multi-stage conversion of energy into motion is excluded, any factors for the occurrence of backlash and uneven feeds are excluded. Sodick linear actuators are capable of adjusting the gap up to 1000 times per second with a feedback of 10 nanometers. The result: optimal clearance at virtually any point in the EDM process, consistently optimal operating conditions, stable maximum removal, high EDM performance and surface quality!
Why stand still?
Ball screw drive and a ball-screw construction.
Sodick flat linear motor in a Sodick EDM.