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Mini Mill Gearbox

I have been using my mini mill (Amadeal XJ-300) for around 2 years and all was fine. Then today 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 methodicially whilst trying not to get worked up and do not apply too much pressure to any joints.

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.


The black gear is broken into more than two peices.

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.


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.


One solution is to fit a belt drive system from littlemachineshop.com.

Just had feedback from the team at LittleMachineShop and they don′t believe the belt system will fit. Reading other comments online and it looks like it is possible, but only with a lot of modifications.

The alternative is metal gears, again from littlemachineshop.com. I have a pair on order....

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 gear on the left is the metal gear and on the right the original.


The head of the mill has been dismantled into all of it′s parts.....this got me thinking about the bearings on the main spindle which are 62mm OD, 30mm ID and 16mm thick and are quite rough ball races. I found a close match with the SKF 30206 J2/Q on size, but these are taper roller bearings and are around 33 for a pair including delivery from Bearingboys.


Another image of the bearing showing the two parts.

Note: the original lower spindle bearing comes apart very easy and the balls spill out and go free - take care if you intend reassembling with the original bearings.


The milling machine is now all back together and the metal gears are quieter than the original plastic gears. Will let you know how I get on with this in the long run.

The taper roller bearings make a huge difference to the shaft, it runs smoother and have not measured it yet, but I can see that the chuck is running much truer than it did. Taper rollers are worth the effort of upgrading even if you do nothing else.

I have been using the milling machine for a year since replacing the taper roller bearings and this is a must upgrade. There is just no play in the spindle and it runs absolutely true.

See also: Precision Tool Vice, Tools, Vice - Soba.