Mini Mill Belt Drive

This is all about making a mini mill belt drive for the gear driven mini milling machine. A great upgrade that will result in a quieter and smoother milling machine.

These mini milling machines (eg Amadeal XJ12-300) 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. 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. However, 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 resulting 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.

Optibelt

One of the first things was to find a suitable belt. 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-6×335-Li Cogged Classical 6mm Wide 4mm Deep V-Belt (VB-6×335-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. All in all this added quite a few hours to the time to make the whole project.

The main plate you can see here is 8mm thick aluminium. The hole was machined using the fly-cutter and gradually extending the length of the tool.

A trial fit of the main plate onto the top of the mill. As you can see the aluminium plate is countersunk with the intention to use countersunk caphead bolts so that they are completely flush and do not protrude above the surface of the plate.

Once again I need to use the mill to make some of the parts, hence 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.

This image shows clearly the left-hand threaded locking nuts on the top of the main shaft, I will come back to this later when I show you how I made and attached the main pulley.

Balanced here is the motor mounting plate. As you saw in the main image at the top of this post this plate is fixed with 3 bolts, one of which the plate rotates about.

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. Again, in the final assembly I’ve used countersunk caphead bolts so that I could minimise the distances and ensure the belt could not catch on anything.

The drive belt and the part machined motor pulley. As you can see, the parts are laying on one of my rough sketches of the parts I needed to make with a few rough dimensions.  I really did design this as a bit of a trial and error, the best bit is it worked first time.

The outer diameters of the drive pulleys are: 34 and 30mm

Once again the drive belt: Optibelt VB-6×335-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.

I’m not sure this is the best thing to machine on a lathe as you end up applying a lot of pressure to the saddle. I did all of this machining on my Warco WM240B, a very solid lathe. Next time I would buy a keyway cutting tool kit.

The outer diameters of the pulleys on the spindle are 70 and 76mm.

The optibelt-VB sits nicely in the pulley.

There is a good amount of grip between the belt and so I don’t think this needs a tensioning wheel. The main function of a tensioning wheel would be to increase the distance the belt runs around the small pulley.

The great advantage of the belt system is that if needed you could back off the tensioning of the belt and allow it to slip a small amount.

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.

Before starting this modification I felt daunted by the amount of machining required to make the parts and the size of them. However, 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 disassemble it every time I get a jam.

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

Belt Drive Ratios

I have tried the mill on both high and low ratios and it works very nicely, a joy to use. The ratios are roughly: 66/30 = 2.2:1 for the higher speed and 72/26 = 2.8:1 for the low speed.

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

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

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

The outer dimensions of the pulleys are shown in the image below. The pulley on the right is attached to the motor and has pulley diameters: 34 and 30mm

The pulley on the left is attached to the main shaft and has pulley diameters: 70 and 76mm

The difference in pulley sizes is quite small. The resultant ratios are given by taking the ratio of the diameters at half the belt depth. The belt is 4mm deep and so if we take 4mm from the diameter of each pulley we get the correct working diameter.

• (70-4)/(34-4) = 66/30 = 2.2:1
• (76-4)/(30-4) = 72/26 = 2.8:1

I must admit that if doing this again I would stretch the larger diameter pulley to 80mm and increase the larger diameter on the drive motor pulley to 38mm. This would result in a wider ratio spread, but my milling machine works very well.