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The Vario option, available on all DB series lathes is an electronic variable speed unit allowing stepless control of the motor speed between the set
minimum and maximum speeds. The motors on these lathes may be specified as single- or three-phase units on the mechanical-transmission models whilst the Vario system employs a three-phase motor but which is fed from the Vario
controller, the input to which is single-phase domestic (220/240 volt) supply. The motors used in the DB range are totally-enclosed, fan-cooled (TEFC) types to prevent the ingress of dust and moisture and are of substantial
power output in relation to the task to be undertaken. Control of the motor is effected through a no-volt-release (NVR) switch which ensures that the lathe will not re-start after a power supply interruption, until the unit
has been manually reset. This is an important safety feature that no workshop machinery should lack. The Vario system has an integral reverse facility whilst a forward/reverse switch is standard on the fixed-speed motor
variants (types "m" and "d") of the DB1000 and DB1200 models. Dependent upon model, the vario system may be integral with the headstock (DB801), or mounted on the inside face of the left-hand pedestal of the lathe structure
(DB1000Vario, DB1200Vario).In all models a poly-vee-belt drive system is used, offering the advantages of a smooth, quiet, resilient drive which is compact, virtually vibration-free but capable of high torque transmission.
Many misunderstand the use of variable-speed drives and may question why these lathes still employ a mechanical speed change system in addition to the Vario unit. The truth is that however much marketing departments
may opine otherwise (!) the basic and immutable laws of mechanics apply just as powerfully today as when Newton first wrote them down. Very basically, whatever torque is produced by the motor fitted to a lathe, that torque is
multiplied (i.e. increased) by interposing a mechanical drive between the motor and the spindle. When we are turning, the force we apply through the tool is resisted, and the workpiece kept turning, by the torque of the
motor. (NB: NOT
the power of the motor - this is simply a measure of the amount of torque applied - the work done - in a certain time). Torque and power are related, but only by bringing the rotational speed into the equation - for the technically minded, in imperial units:
Torque (lbs. ft.) = (HorsePower x 5250)/Speed in revs per minute. The truth is that an electric motor is an imperfect device, like most mechanical devices, and as the operating speed is reduced, the
torque output by the motor (torque being the turning-moment or rotational force) drops dramatically. Hence, the torque available to drive the workpiece drops proportionately and suddenly the user can find himself able to stop
the lathe "with a little finger". Modern sophisticated electronics go some way to resolve this problem but there is an economic limit to the energy that can be provided to drive the motor before the motor literally
fries. What actually happens in a variable speed drive system is that the operator - you - sets a value that represents the speed at which you wish
to turn: the speed controller then outputs electrical energy to the motor and measures the resulting speed (it does this measurement many times a second). If the measured speed is too high, the power output is reduced: if too low, the power delivered is increased until the set speed is achieved. At some point a limit is reached where the energy being delivered to the motor is the maximum that can be allowed before "frying" commences (!) and this defines the limit of torque availability from the motor.
What is really needed is a way of multiplying the torque available to drive the workpiece: it is this that is achieved by coupling the motor to the spindle through the step-pulley drive transmission. Quite
simply, torque is increased so that the risk of stalling is obviated. Each step of the pulley system affords the user a wide range of speed variability, significantly more than is likely to be required for any one job, but
that speed range can be optimised for any particular job. Direct drive systems, however sophisticated, cannot by decree of the laws of nature (mechanics) ever be as effective in the delivery of torque as can a "greared"
drive. Remember, in conjunction with all of this that the larger the workpiece diameter, the slower the spindle needs to be turning in order that the peripheral speed of the workpiece is optimised for best
turning. The optimum for hand turning is reckoned to be 25 ft/sec: that means that for a 3" diameter workpiece, a speed of 1910 rev/min is desireable, whereas when turning a big bowl at 15" diameter, you need to be able to
reduce the speed to 382 rev/min. It is more important to be apply to supply high torque at low speeds, in the world of woodturning, than at high speeds - we are applying lots of cutting load when turning that big bowl,
working seven inches away from the spindle axis, but almost none when creating the three-inch spindle and working just over an inch away from the axis. For a simple, but fairly true comparison, consider your car: using
a "direct drive variable speed system" is akin to driving your car with an accelerator, but without a gearbox (no, not even an automatic gearbox!). Using the WivaMac preferred system combining belt drive with various pulleys
plus electronic speed variation is like driving a car with an accelerator and a gearbox. Which would you prefer? If you're not sure, step outside and take your car for a drive firstly using the full gearbox, then using
only top gear. Now which do you think makes for better performance? Isn't that what you wanted - performance? Speed changing is quick and simple, utilising only one lever to lock the belt and tension it. The
step pulley in use at any time can be easily checked through the transparent door in the side of the headstock housing and moving the belt from pulley to pulley, when required, could hardly be simpler. All in all a very
simple but very efficient and effective drive system, entirely appropriate to a hard-working lathe. |
