Overloading a Chester Champion V20 Mill

I am about to have my Champion V20 Mill back from repair at Chester tomorrow. I was told by Chester that the control panel blew out, possibly due to 'overload', and that the Champion is 'only a hobby machine'.

Would any Champion (or equivalent) users comment on this in particular do you let your mill cool down at regular intervals or look for signs of overload ? Also is there any other way to avoid this e/g better cooling of the control panel ? Have any of you replaced the Chester controls by third party system or replaced both the motor and the controls for this reason ?

As an aside Chester have been very good about repairing it under warranty, but not having the mill for a month is a nuisance and this is the second time in three months it has gone back for repair (previously the power cable to the motor had frayed inside the Mill (!). Before that I had an intermittent loose wire. I think I may have the Monday morning machine...

Peter

What do they class as an overload?

What was the exact nature of the fault on the controller board 'It blew out' is a pretty glib statement

They state a max cutter size of 20mm but do they give any indication of max depth of cut, feed rates etc?

Do they give a figure for MTBF?

Do they state a max running time, load limits etc etc?

If you've had to have this repaired 3 times in a short time frame I would be persuing the 'Not fit for purpose' route and request a replacement.

I was looking through a German model eng mag the other day and they had a similar mill where the owned had milled some cooling slots around the bottom of the motor cover and fitted what looked like a computer cooling fan to the top.

What you have to watch with the variable speed machines is running them for a long time on low speed as the fan is fixed to the motor it does not run at full speed all the time unlike a belt or geared head machine so things run hotter.

J

All good questions Coalburner - unfortunately not questions I can answer. Nothing like that is stated in their documentation hence my questions about practical experiences of owners.

Jason yes the cooling fan is the sort of thing I was thinking off, and it's interesting someone else has done that already. My guess is that the control board 'blows out' if the power transistors run too hot, and they are in a closed metal box so just extended running of the machine will heat them up (they are not mounted directly to the control box), almost regardless of load. Thank you also for the advice about not running at low motor speeds. I am milling aluminium and have kept the gearing set to 'High'. I will try it on 'LOW' gearing with the motor running at max refs, and a big mill.

Peter,

For your information , in the very good Newton Tesla variable speed package I have on my lathe it states quite clearly that the motor must not be run for long periods below 800 rpm , obviously to avoid over-heating (as per Jasons comment above ) which is why I still use the change belts.

Also, not familiar with your machine , so would ask what types of protection are there on the machine in order to try and prevent the overheating , and since this would appear to be a regular problem , I wonder if someone does a mod whereby a thermal protection device may easily be fitted to the board ? I have in mind as a precident the improved American made circuit boards for the mini lathes.

Trevor

Hi Peter,

This has been discussed previously on this forum, I don't know if you have found it. Johan Crous had a post about how he mounted a fan on top of his Mini-Mill motor. It worked well for him.

Thor

I think there is some confusion here.

Mounting an extra fan on the motor helps with problems related to the motor overheating at low revs.

As I understand it Peter's problem is not with the motor but with the controller board. If this fails due to overheating or extended use it is badly designed and as someone else said, "not fit for purpose". Yes it may be possible to improve it with a heatsink or a fan in the controller box but that shouldn't be necessary.

Russell.

Russell,

Yes they are separate issues, and I was aware that some people have had problems with overheating motors, just not overheating control boards. It is difficult to say if they are fit for purpose or not as the purpose is not specified in detail by the supplier other than it being in a 'hobby machine' category, and I just dont know what that means in practice.

So I'm still interested if people let their mills cool down at regular intervals or watch for some signs of stress like hot quils, control box or motors - anyone ?

They advertise it as a "hobby size machine". Any machine tool should be able to run at maximum load indefinitely. It is very straightforward, when designing a controller, to build in thermal and overload protection.

Having said that I always run my mill in the lowest gear suitable so that the speed controller doesn't have to reduce the speed too much.

Russel can't agree with you there I previously had an Emco Unimat 3 which is definately a hobby machine and it clearly stated something along the lines of 10mins run, 5mins cooling etc.

Peter can you give us some idea of what you are actually doing regarding size and depth of cut, material and how long the motor is running.

I generally find that you spend as long if not longer setting up a piece, changing tooling, etc than the machining opperation so that setup time is sufficient to allow things to rest.

J

I was swapping my gas bottle the other day and was looking at some big 3 phase MiG welders.

Lincoln and Esab amongst them.

The Esab 350 amp welder was rated at 350 amps 40% duty cycle, this means 4 minutes in 10

100% duty cycle was down to 215 amps.

The Lincoln was even worse at 30%.

These are serious bit of kit, well into 4 figures and certainly not hobby machines but still have limitations.

Lets face it these machines are built to a price - full stop.

If they wasn't they wouldn't sell.

So the answer is for the importers to get on to the manufacturers and tell them because you have built to a price you have cut corners and it's not acceptable.

So One Hung Low disapears into the R&D department and comes up with a mini mill equipped with a 1HP 3 phase motors and quality VFD

Unfortunately the price has now more than doubled, the head is that heavy it drops on it's own and the extra weight causes to to tilt and not mill square.

So they go back and redisign the frame so it fully supports the weight.

Problem is now it even dearer and looks like, feels like and costs the same as a Bridgeport but Joe Blogs can't get this in his spare bedroom.

What he really wants is a mini mill for small jobs that he can afford to by with his pocket money and taking a few empties back.

Said if before and will keep on saying it.

There is a BIG difference between FIT FOR PURPOSE and FIT FOR PURSE

If you want to mill something big then get a big mill. An industrial milling machine will cut as you wish and I would say that the only limiting factor on such a mill is the tool itself. Plus of cousre a far greater cost of machine and tooling.

Talking of a 20mm cutter, doesnt mean ploughing into a block of steel at 2mm depth full width and hope not to stall the machine with a small DC driven motor. Stalling the machine will either blow a line fuse or as as happened burn out the power diodes on the control board.

having used multi cutter face mills of 250mm diam. trueing up blocks of Alu, no high speed and a slow traverse speed to get a good finish on HE30. The same machine with a 10mm 4 flute Rippa cutter certainly removed metal quickly. This on a Schaublin 13.

The amateur/model making machine has limited capacity to carry out heavy work due to several factors, rigidity, and lack of drive power with attendant vibration from the drive train. My X3 just about gets through with a 1.5mm slitting saw hand fed without stalling. but again with a small dia carbide cutter performs well. Here I have to work within the capacity of the machine power and ability to cope with the work load.

Dont expect too much from these bench machines or you will be disappointed. At the very least work within their capabilities, it will take longer but you will be well satisfied by the quality of finish and accuracy.

Clive

John yes that is one of my questions: what is a practical duty cyle for one of these mills ?

Jason I mill Aluminium only, and I take care not to take deep cuts where the mill appears to strain. Inevitably this means that I have to go over it several times and that means prolonged running times.. The machine did not stall at any time.

Clive with a 1 HP motor and specified max drilling and end milling of 20 mm and table of 700 X 180 this is hardly a toy. I understand the need to work within it's limits, my challange is to understand what those limits are (ie what is the duty cycle) and what the warning signs are for impending overload. I have emailed Chester today to get answers to those questions. Having said that I am keeping my eyes open for a good second hand grown up mill. I did think of trading up with Chester but felt I probably shouldn't dig myself a deeper hole.

Yup. There are limitations, but they don't blow up when they heat up. I have had cheap welders and I have a professional welder. The difference is very clear on the duty cycle and ease of use, but even the cheapest piece of crap buzz box will just shut off and after cooling it it will work.

Moreover all these milling machines comes up with a table that will tell what duty cycle they perform.

Electronics that overheats and dies from normal use is just faulty. Plain and simple. Bad design or poor manufacturing. Thermal cutout (on motor and control board) has very little impact on price and practically no impact on weight. A PTC on the motor weight few grams.

I do agree for the rest of the post regarding rating and duty cycle. Any improvement will cost money.

But serously....it is very sily to require a hobbyist to have an ESP qualities of estimating what these machines will do and what not before giving up on ghosts.

Pekka

Fan mod to my WM14 here http://www.model-engineer.co.uk/forums/postings.asp?th=85711

Edited By Dodweld on 16/08/2013 18:02:05

Thank you all gentlemen, based on your feed-back I am going to first measure the temperatures inside the motor enclosure and control box over time, then see if passive venitlation (vents!) keeps them down sufficiently (I am guessing below 50 C) and add thermal cut outs to both for added protection.

What is a good cut out temperature ? 50 C ?

40 C is a good limit for std cheap components. Every 10C rise halves the lives of electronic components. military spec components have higher ratings but don't expect them in budget priced machinery.

Steve

I would say that 55 C should be O.K. in the control box. We often have temperatures over 35 C here and 40 C would be far too low. Most integrated circuits (usually the most sensitive parts) are specified for use in ambient temperatures up to 70 C. The components' case temperatures will be much higher.

As Steve said the MTBF doubles for each 10 C rise in junction temperature but is usually specified for 175 C.

Motor temperature is more difficult. The limiting factor is the temperature of the windings and their insulation. There are several classes of insulation for use at different temperatures up to 175 C. However motors are usually specified at maximum power for maximum ambient (air) temperature of 40 C and should give a life of 20,000 hours at that again halving for every 10 C rise.

So you should try to keep the air in the box below 40 C. If the temperature is more than that you should go a bit easier or let it rest occasionally (with the motor running at full speed with no load if it has a fan driven by the motor).

Russell.

I should add that the case temperature of power devices on heatsinks shouldn't be allowed to exceed 70 C if they are operating near their maximum rating.

Russell.

Not scientific but I use to let my motor get hand hot then let it cool, since fitting the fan and thermometer it runs to a similar "hand hot" temperature of 50'C so I,ve set myself a limit of 40'C. I work until my thermometer nears 40'C, finish my cut then allow to cool, the fan only takes a few minutes to reduce the temperature back down to 30'C but this is only when taking cuts which are really too big. Just milled the spokes on a brass flywheel, 1/4 inch slot drill, slow feed, and the temperature never rose above 35'C the ambient temperature in the workshop at 21'C

The fan only cools the motor, the only thing I did to the control board was use some heats sink paste to ensure a good thermal connection to the case.

It has been a worth while modification and something I could recommend doing

See my photos on my album

Edited By Dodweld on 17/08/2013 16:52:55