I suspect that if you are running the motor at much reduced speed, its cooling is insufficient – especially if worked hard.

A fault of these cheaper machines. You might also find that the motor has a lower output power than you think.

It may be worth, after a hard cut, just letting the motor run at full speed, off load, to give it a chance to cool down, before the next hard cut.

These are hobby machines, so will not withstand the continual five or six 8 hour days hard labour expected of an industrial machine.

The Cincinatti vertical mills in the Training School weighed about 2 tons, and had 5 hp motors, so a 3/16" (4 mm ) cut with an 11" / minute feed was within their capabilities..

Our 150 Kg, 3/4 hp machines are not in the same league, and so have to be treated differently.

A bit of high speed no lad running will allow the heat to be dissipated between bouts of hard (for a hobby machine ) work,

Howard

Ian, what motor speeds and cutters do you normally use?

How much work can be got out of a hobby mill before it overheats or strips gears? Good question! Two different problems:

• gears are more likely to strip under shock loads, as when a speeding cutter crashes into something solid. The motor could be running within specification when the crash happens, and the gear breaks to protect the drive train and to limit the heavy current pulse pulled by a stalled motor.
• Motors deliver more their rated power when the user overloads them. Plastic gears are unlikely to strip under a steady overload, but the motor will overheat. The gear isn't useful as a fuse and if the user persists, insulation, brushes and electronics are all likely to be damaged.

Therefore the operator should avoid crash-bang cutting, and back off when that type of cut is unavoidable – adjust RPM, depth-of-cut and feed-rate to minimise the risk of damage.

To get a feel for how heavily a particular motor is loaded by the operator, I suggest measuring the input with a cheap wattmeter (example of type here). Note how much power is consumed when idling, and how much during a normal-to-you cut. The motor is working to full capacity If the wattmeter registers motor rating + 10%. And the motor is overloading if the wattmeter registers more than that. Overload is a silent killer; the operator may not know anything is wrong until the motor fails outright, or the dreaded magic smoke appears!

Hobby motors are unlikely to be rated for continuous output at full-power. As a result they gradually overheat unless a busy operator pauses to let them cool down. Not a problem in my workshop because I rarely remove lots of metal in one go, and spend ages resetting work and thinking. However, when I do a lot of work on a large lump of difficult metal, I run the motor for 10 minutes, then let it cool for 10 minutes, and repeat the cycle until finished. If the motor is accessible, check how hot it is with the back of your hand: give it a rest if the case is uncomfortably hot. If an owner often finds his machine gets hot, he needs a bigger machine! If that's not possible, take more time.

There's a reason Industrial Machines cost 6 to 20 twenty times more than hobby kit. They're very substantially beefed-up to slog through production work. We have to remember hobby machines are comparatively delicate.

I don't know if the plastic gears in these machines are fuses or cost-cutting. Possibly neither! Another common reason for using plastic gears is they are much quieter. Only the designer and production engineer would know for sure, the rest of us are just guessing…

Dave

I don't think it helps motor life much by putting the motor in a metal box with very little ventilation. It may help cool the motor to remove the louvres from one side of the motor enclosure and fit a 120mm computer fan to draw air through and around the motor.

John

Best not generalise too much, my X3 just trips the electronic protection and simply needs a flick of the switch and it's up and running again. I think it senses current not temp as otherwise you would need to wait while it cools down

Oh and it's not enclosed either.

The remaining ones are. How many were used up doing the jobs they were bought for?

A 60-70 year old m/c will be worn out, much like a 60-70 year old human.

Tony

Hi Ian,

I have a similar mill, a Chinese "Optimum BF20LB" mill. I've repaired the motor on several occasions ! I made and replaced the gears with steel ones because the plastic ones stripped the teeth, and it was more cost effective to replace them. Actually the gears stripped whilst using a slitting saw on aluminium, the work got hot and grabbed the blade stripping the teeth off the gears.

I do have a 3 phase motor to replace the DC one and have made some parts to mount it, however a ducted cooling fan mounted on top of the existing motor seems to have much reduced the heat that is created. The major problem with the heating of the motor is simply that the only way that the armature can get rid of the heat is through the bearings. There are no field coils to conduct the heat into the motor body !

The brushes and brush holders suffer badly because of it. Also the push on tags don't make a good electrical connection so they get hot and the resistance increases causing them to get even hotter. I've thoroughly cleaned and soldered the tags to the brush holder terminals. This removes the major cause of brush holder failure due to heating.

The motor still gets very warm but nowhere near as much.

Edited By John Baron on 08/11/2021 19:50:36

I have a Champion V20 mill, same configuration as the WM 16 with the motor inside a poorly ventilated shroud, had overheating problems when driving the mill hard during warm summer weather, smelt very hot. Solved the problem by fitting two small computer fans into the front of the motor shroud, driven by a 12 volt led power supply, the fans come on whenever the mill is switched on for use. The two computer fans can shift a fair volume of air and since fitting there have been no overheating episodes. Dave W

There are no field coils to conduct the heat into the motor body !

Field windings add to motor heating, they don't sink it. Most modern industrial DC drives with field controllers have a "field economy" setting that reduces the field current when the motor is at standstill to help reduce heating from this source.

A separate through motor blower will be a big improvement over the built-in "stirrer" and should help to remove heat from the armature – need to allow the warm air an easy escape route, though. Forced air cooling is the way that industrial DC motors get cooled & the (mainly centrifugal) blowers can be very powerful. In most industrial environments, filtration of the cooling air is important & some motors have pressure or airflow sensors to indicate when the filters need changing or cleaning – inadequate airflow leads to burned out field and armature windings.

Just had a 15Kw DC motor rewound from a Correa CNC milling machine at work, though this intitially failed due to a shorted interpole winding, not through an overheating issue – £5500 + Vat & 6 weeks turnaround due to delays getting the correct section wire for the rewind. No real option but a repair this one, as replacements not available any more.

Nigel B.

It sounds like a cooling problem, not an instantaneous power issue.

May I add my vote to cooling fans. It does no good to any part of the motor to cook it.

You may find that it only needs a few more slots, fans are cheap, so they shouldn't be a problem.