Milling Machine Problem

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tanweerzara Member Join Date: Feb 2009 Posts: 6	Milling Machine Problem 03/16/2011 8:23 AM
	Dear All Please help me to solve my problem, problem is in milling machine which has variable speed spindle , I was getting one problem of gear broken frequently , hence i have changed it into VFD driven spindle by eleminating the primary gear box which was used for speed change ,i have fix one gear at one point and now changing the speed by VFD but now the problem is spindle not taking load ,it stops after few minutes and current suddendly increased. Please help me to resolve .
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K_Fry Guru Join Date: Feb 2011 Location: Atlanta, GA Posts: 1053 Good Answers: 110	#1 Re: Milling Machine Problem 03/16/2011 10:44 AM
	It sounds like you have been overloading the machine, thus the broken gears. Reducing feed rates, tool sizes, etc would help, but perhaps you need a larger machine. To eliminate the motor overload problem, you would need to change the gear ratio, so that the torque delivered by the motor could be lower for a given output torque. With gearing, very low speed is naturally accompanied by very high output torque. With a VFD running at a low motor speed, high output torque means high amperage. To provide high torque at low speeds (after removing a stage of reduction gearing) the motor must be much larger to be able to operate at the increased load. Spindle bearing problems? Dull tooling? Is anything else (such as a feed) driven by the spindle motor, so that a bind in the feed causes overloading? But (assuming the machine is in good condition) the basic problem seems to be overloading -- gears do not break at anything close to normal intended loads. __________________ Think big. Drive small.
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Hendrik Guru Join Date: Jan 2007 Location: Centurion, South Africa Posts: 3921 Good Answers: 97	#2 Re: Milling Machine Problem 03/16/2011 12:03 PM
	Either reduce your feed rate or increase the number of passes. To really be of assistance you should give more detail. Type, dimension and state of cutter. Feed rate and plunge rate and type of acceleration control (ramp or s curve). Material spindle speed and kw and specks. depth of cut , climb cut or 2d or 3d etc. __________________ Never do today what you can put of until tomorrow - Student motto
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JRaef Guru Join Date: Sep 2006 Location: California, USA, where the Godless live next door to God. Posts: 4665 Good Answers: 804	#3 Re: Milling Machine Problem 03/16/2011 1:15 PM
	Just to explain what has happened, you made a classic mistake in applying a VFD to replace a gear reduction system. When you reduce speed with gears (or belts etc), there is a mechanical advantage in that as speed is reduced at the work shaft, torque is INCREASED by the same ratio. When you use a VFD to reduce speed, torque REMAINS THE SAME as it was at full speed. In many applications this is fine, but on something like a machine tool, the loss of work shaft torque can mean a loss of tool efficiency, So as was said by others you either have t reduce the feed rate or make multiple passes. 3rd option: calculate the shaft torque you had before and increase the motor (and VFD) size Sometimes you can get away with using a lower gear ratio on the remaining spindle drive and over speeding the motor and drive, so that you have that same torque all the time from the VFD. Nobody likes that idea though. But there are a lot of other pitfalls on that, something I don't recommend to be undertaken by amateurs. __________________ All I every really wanted to be, was... A LUMBERJACK!.
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