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Mini Mill Belt Drive

These milling machines with the back gears suffer from gearbox failures and mine did some time ago - read about the experience of changing the gears for metal ones. Even after changing the gears I was left with a milling machine that was still quite noisy, but more importantly with a drive that was quite lumpy due to the coarse nature of the gears. The result is this feels like it is hammering the cutting bits.

Therefore I decided it was time to change the system over to a belt drive.

I looked on the Little Machine Tools website and they sell a belt conversion kit, but after emailing them and doing some basic measurements I realised this just would not fit without a lot of modifications.

One good thing about this milling machine is the fact that there are two sets of gear reductions reulting in a drive direction reversal and then a second reversal that brings the main shaft back to rotating in the same direction as the motor. A belt would keep the drive shaft rotational direction the same.

One of the first things was to look for a suitable belt as it would appear that lots of people have trouble getting a belt small enough.


I found this belt on simplybearings.co.uk the description of it is: Optibelt VB-6x335-Li Cogged Classical 6mm Wide 4mm Deep V-Belt (VB-6x335-LI-OPTI)


The problem I had with this project immediately was that I needed the mill working in order that I could make a lot of the parts. So I had to take the motor and housing off, make the measurements I needed and then reassemble the mill.

The main plate you can see here is 8mm thick aluminium.


A trial fit of the main plate onto the top of the mill.

Once again as I need to use the mill to make some of the parts I cannot remove the intermediate gear without a lot of work.


Not very clear here, but the hole in the plate is 80mm diameter to clear the dust cover that fits above the bearing at the top of the mill.


Balanced here is the motor mounting plate.

At this point I′m still not sure whether to fix the motor and then use an idler pulley to tighten the belt or move the plate to adjust the tension.

The advantage of the idler pulley is that it will allow me to increase the contact between the small pulley on the motor and the belt.


I centre punch the motor plate and then used a pair of compasses to scribe the 74mm diameter location for the 5mm bolts.


At this point I must admit that I did some measurements of the motor, main pulley and the drive belt to work out where they would go and just thought it would be better if the motor moved back 15mm on the plate.


The 40mm diameter hole was bored on the lathe using a 4 jaw chuck.

Again some dismantling of the mill head to check the motor fits the plate. This also allowed me to make some measurements for the motor pulley.


The drive belt and the part machined motor pulley.

Once again the drive belt: Optibelt VB-6x335-Li Cogged Classical 6mm Wide 4mm Deep V-Belt


Looking at the angles on the pulley and the drive belt, I think it worked out quite well.

The belt makes a good contact and just testing it by hand I can feel that the grip is very high.

I was concerned about getting the angles on the pulley exactly to the specification, but having made the pulley, felt the alignment with the belt and the grip and having since used it in anger it really was easy.


I turned the larger pulley to shape and then located it in the 3-jaw chuck by the main boss so that I could turn the grooves and bore it in one go.

Then I turned the pulley around in the 3-jaw chuck (image to the left) and cut the 5mm wide keyway.


The cutter was made from a piece of 6mm square tool steel that I ground down to 5mm wide and then ground the cutting profile on the end.

Make sure that the workpiece is hard against the back of the chuck as the forces are quite high.


This image gives a nice clear picture of the profile of the cutting tool. I had to make a lot of small cuts and must admit that my arms were aching by the end of this.

I did not lock the chuck to stop it rotating, just took the cuts easy at first and once the tool had made a reasonable groove there were no problems.


The optibelt-VB sits nicely in the pulley.


The adjustment screws for the belt tensioning can be seen very clearly. This system works very well and the time to change between the 2 ratios is just a couple of minutes.

I did not refit the plastic covers over the motor, I may place a shield over the top to reduce the dust going into the motor, but not sure even this is necessary.

The spacers are 27mm long and were drilled and tapped M6. This then allows countersunk M6 bolts to be used to fix the bottom plate and keep the bolt heads flush. I just tightened the countersunk bolts as much as I could and left them - I did think of using thread lock, but must admit that I ended up thinking that is a job for later once I am completely happy.


The completed mill.

The noise levels are now much lower than either the metal or plastic gears and the rotation is very smooth.

I have tried the mill on both high and low ratios and it works very nicely, a joy to use.

The only small items that need to be done are a belt guard and a lever to lock the rotation of the shaft so that tightening the main nut onto the belt drive is easier and something to react the forces when tightening the collets.


You can see that the overall height is very compact. 27mm between the upper and lower aluminium plates.

The 6mm belt is more than man enough for the power and torque levels, will let you know how long the belts last.


Before starting this modification I felt daunted by the amount of machining required to make the parts and the size of them, but this has been a good project and has come together rather well. This is really worth doing as I will not have to remove the head and dissassemble it every time I get a jam on the head.

See also: Amadeal XJ-300, Tools.