Mini Mill Gearbox

The Mini Mill gearbox or more accurately, repairing the Mini Mill gearbox after you break the plastic gears.

This is the Real Bull XJ9512 milling machine or for this particular case the Amadeal XJ12-300.

I’d been using my mini mill (Amadeal XJ-300) for around 2 years and all was fine. Then I pushed it a bit too hard and the spindle stopped rotating but the motor could be heard whizzing round.

I have read elsewhere that these milling machines have a tendency to gearbox failure…..

Before considering replacing the gears it is worth thinking about going straight to belt drive. A belt drive is so much better than gears and if you are worried about the cost you can quite easily convert to belt drive yourself as I found out. Read about the mini mill conversion to belt drive.

So the first job was to pull it apart and have a look.

The manual that is supplied with the machine is not a lot of use – you need to just work your way through slowly and methodically whilst trying not to get worked up and do not apply too much pressure to any joints. A machine breaking part way through a job is a stressful time and can turn the best of us into gorillas with a spanner….

You can see here the head broken down into the major parts and the offending gear that has split into two.

Inside the head you can see the larger of the gears on the main shaft and in front of this the intermediate shaft around which the black once belonged.

You can also see the lever that has a fork attached to it to move the gears between high and low ratio.

The black gear is broken into more than two pieces.

There is a suspect white tooth here as well, which if you look in the image above you can see a missing tooth.

When you put the pieces back together it is rather obvious that the gear would fail at some point – the material around the keyway is very thin.

The fork that goes around the gear and allows you to move the gear and change from high to low is quite rough and ready.

Again I imagine this does wear the gear.

The output gear on the motor is metal. However, the screw in the centre was rather lose and something that I didn′t think of checking when I bought it.

The motor drives this gear which is keyed onto the intermediate shaft.

Looking at other websites where people have had gearbox issues this gear has also failed.

The back of the head with the keyway and pinion drive. Another problem with this design is aligning the bearings on the fine height adjust axle as they span the two major head castings and although these have alignment pins there is quite a bit of play. I ended up using some thin material to shim the bearing blocks.

The shaft has a left handed thread on the top – note this as it can catch you out. Once again no mention of this being left handed in the manual – be careful and inspect the thread before you go any further.

And so rather than replacing the gears with plastic I decided on metal gears, from

After ordering these from the USA I then realised that I could have got them from Arc Euro Trade.

The gear on the left is the metal gear and on the right the original. The pair of gears arrived from the USA in just 4 days – that was with the minimum postage – a great service.

The milling machine all went back together easily and the metal gears were quieter than the original plastic gears. But then I decided to go further and make a belt drive system….





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