Overloading a Chester Champion V20 Mill

Hello, good after noon.

My first milling machine was an Optimum BF20 Vario which after about two years of use was sent to the distributor in Portugal to some light repair. There, someone dropped a heavy object on the table causing deep dents on it.
The dealer replaced it with a new one and some time after its electronic crashed. The distributor's technician came to my workshop and replaced the electronic board with a new one and at the end of the installation I did ask him questions about the load capabilities that the machine was capable of. And he promptly did a test that left me amazed.
He asked me a chunk of cold rolled steel and set an end mill 10mm in diameter in a collet.
Then he started a cutting with about 10mm in depth, manually feeding at high rate which took only a few seconds to both the metal and the cutter to be red hot. He continued to cut a few more seconds until the cutter has cut about 50mm in length and almost all the time the metal and the cutter were some variation of red hot.
At the end he told me something like this: if with this load it did not broke its not likely ever becomes damaged by overload. And, indeed, so far never broke ...

After that I concluded some machines are only similar at first sight...

Best regards
Dias Costa

Edited By dcosta on 17/08/2013 18:35:20

Gentlemen thank you for the additional advice on temperatures, I will measure and experiment and report back...

p.s. I really like the dodweld fan mod - nice and simple.

Update:

I emailed Chester to ask for Duty Cycle information for the Chester Champion V20 and this was their surprising response:

"duty cycle is not available for these machines as they are hobby rated and not for use in any kind of setting where the machine needs to run for extended periods of time."

I take this to mean that the only safe use of the Chester Champion V20 is to not use it at all.

I asked them what periods they can be run for but there has been no response to this.

Chester also said that "the capabilities of the machine are listed in the manual". So I checked the manual but there is no duty cycle information in the manual. Nor any information about thermal cut-off so I have to assume there is none.

I thought I would see what other Chester Machine Tools Mills are 'hobby rated'. On their model engineering section (One assumes these are hobby rated) it lists everything from the Micro Mill at £ 268.80 inc vat to the Model T turret Mill at £ 6156.00 inc VAT. Wow, that is a lot of money for something that has no defined duty cycle. And I thought that spending £ 816.00 on my Champion V20 would buy a machine that has a defined performance however limited.

OK so I have learned my (rather expensive) lesson - don't buy Chester Machine Tools (or any other machine tools!) without a documented duty cycle.

I got some temp gauges today and will be doing some measurements on motor and control box with and without load to try and figure out what the duty cycle of this thing is without mods. I am stuck between a rock and a hard place. If I modify the Mill I will void the warranty - If I dont modify it then it will most likely break down again!

Thank you Mike. I may be missing something but there does seem to be something very daft about a machine whose virtue is variable speed but where the slow speed cannot be used because the motor overheats and its fan is ineffective at that speed! Sounds like mechanical speed reduction is a superior technology to its successor of electronic vairable speed. I also seems odd that the motor does not have any cooling ribs as many do, and that it is encased in a barely ventilated metal box. Some may say it's a poor design. I guess none of this matters for a 'hobby machine' which apparently is not expected to be used. Anyways some temp measurements first....

Peter: No, you're not missing anything, but people do seem to be strangely seduced by the 'ease' of changing speed by twiddling a knob. Of course from a manufacturing point of view it is much cheaper to provide a simple electronic speed controller, especially for a DC motor, as opposed to a belt or gear reduction system.

I much prefer a gear or belt speed change, so you get full power at low speeds, not just full torque. There are two exceptions in my workshop. My CNC mill uses an inverter and induction motor, but even that has a belt change for high and low ranges, and my cylindrical grinder uses a variable speed DC motor for the workhead.

If it's any consolation the same sort of woes befall some of the variable speed commercial CNC mills with regards to available HP as opposed to marketing numbers.

Regards,

Andrew

Thank you Andrew, I have been thinking about variators, simple yet versatile, and if it works for a Bridgeport and scooters....

Peter: I have a varispeed head on my Bridgeport. It's a bit noisy at high speeds, but seems to work well. I'm happy with it, although I know some people don't like them, and prefer the earlier stepped belt heads.

Regards,

Andrew

I had always assumed that the reason industrial CNC machines used such seemingly huge motors and drives was so they could get usable torque at low speed without having any gearing. The CNC router at work has a 20kw spindle motor which presumably gives enough power at the lower speeds. I would be interested to know if this is the case.

Most motors intended for variable speed operation will generate their max torque up to "base speed" which is typically 1500rpm for motors designed for 50Hz. Beyond that, they are typically capable of constant power until the back emf approaches the available voltage, beyond which point you need "field weakened" operation.

In the constant power region, the torque falls off as the speed rises, given that power = torque x speed. The speed range for constant power is rather limited for fairly conventional machines. Once you start looking at wider ranges of constant power speed, you need more complex and expensive inverters and control schemes. For instance, for a hybrid electric vehicle, the engine speed may be 1000-6000rpm, so designing a motor and inverter for that application is more challenging than for a machine tool spindle motor that may have a more limited range of operating speeds.

To answer the original query, you can buy fairly standard industrial motors that have an independently powered fan on the back end. In this way you can actually operate the motor with high torques and low speeds without overheating the motor. And many industrial motors will have a temperature sensor which will allow overtemperature protection amongst other things.

My lathe and milling machines both have variable speed drives but I generally try to get the speed range roughly right with the belt or gearbox and then modulate the speed around that set point. Trying to operate the motor much above or below its base speed would result in significantly limited power. In both cases the original motor power rating is fairly well selected for the application (and assumes the presence of a gearbox) so unless you want to pussy foot around, this is the only sensible way to shift respectable metal.

Muzzer

Other factors that affect performance, cost, quality, life etc:

• Quality of steel used in laminations
• Thickness of steel laminations
• Active cross section area of steel used in magnetic path
• Winding window area
• Fill factor of copper windings in the winding window
• Quality (purity) of copper
• Geometric tolerance of parts
• Size of air gap (tolerance etc)
• Balancing of rotor
• Rotor construction (partic induction motor)
• Bearing quality and life
• Cooling effectiveness
• etc

A lot of things to get right!

Gentlemen thank you all for your contributions, I did some measurements today with interesting results which I will post in a new thread.