(1) The worm wheel, made of bronze (in a previous life it was part of the prop shaft from a naval launch). The final diameter and number of teeth were calculated and then slots milled across the circumference for the tap to engage during the hobbing operation. Initially there is a fair amount of argument between the cutter and the work (because the circumference of the work, at this stage, divided by the number of teeth required does not match with the pitch of the cutter) but this subsides as the diameter is reduced until harmony is reached with the circumference divided by the number of teeth exactly matching the cutter pitch.


(2) A standard 4 flute tap used for the hobbing operation. Three flute taps are not really suitable because the gap between the cutting edges is almost as great as the width of the wheel and tooth engagement would not be as good. (professional hobbing cutters have quite small gaps between the cutting edges).


(3) The hobbing process - the tap rotates slowly in the mill whilst the gear is free to rotate. This operation requires conviction (a bit like the knurling process on a lathe) and the cutter has to be fed into the work quite quickly to start with.


(4) The nearly finished worm wheel. The flange has still to be machined down to give the correct mesh when it is assembled tight up against the inner race of the front bearing.


(5) The finished worm shaft. Great care has been taken here to get the surface finish of the thread as high as possible.


(6) This is how the worm and worm wheel will mesh to produce the 60:1 reduction gearbox that forms the basis for the 4th Axis.

A 4th (rotary) Axis is a useful addition to any CNC machine and a small, light duty, unit which would have reasonable accuracy can be quite easily constructed with readily obtainable materials and simple tools. My 4th Axis is basically a 60:1 reduction gearbox with a stepper motor fitted to the input shaft and a chuck fitted to the output shaft - pretty simple stuff really. The most difficult part of the construction was always going to be the worm and worm wheel so these were tackled first and if the results were successful the rest would follow. As it turned out the finished worm wheel could almost be passed off as professionally made (but not quite). The casing has been constructed using 20mm alloy plate, keyed and bolted together, and makes for an extremely rigid assembly. A mating ‘centre’ is still to be made and this will support the other end of longer items which are to be machined using the rotary axis.

As far as projects go, I am extremely pleased with the end result here - it was not too difficult to construct and anyone with basic machine skills would be able to do the same.





(7) The housing (made from aluminium) is quite substantial and is constructed in such a way that adjustment of mesh between the worm and worm wheel can be set by the use of shims. I have no doubt that initially there will be a degree of wear or ‘bedding in’ between the rotating components and this adjustment will be needed.


(8) This is the front bearing. All the bearing pockets were milled using the ‘circular pocket’ wizard which is included with Mach3. The front bearing is more substantial but the other three are all standard skate board bearings.


(9) CNC pocket milling for the front bearing. A mill (such as Bob) with zero backlash ballscrews is essential if the pockets are to be truly round. An additional pocket, 4mm smaller in diameter and 1mm deep was milled (after this picture was taken but just visible in picture 7) to provide side clearance for the bearing.


(10) The holes in the end plates were spotted through from the, CNC drilled, top and base plates then drilled and tapped manually.


(11) This is how it will look when it fits together. The base plate has still to be cut to size and the mounting holes drilled.


(12) The output shaft. There is still some more machining to be done on this part so that the chuck can be fitted.






(13) The parts count is beginning to grow.


(14) This is the base plate, it is not finished yet, it has just been cut off the end of an anodized sheet and the holes CNC drilled. The holes are countersunk on the underside and this was done manually on a drill press.


(15) The top plate, again CNC drilled. The positional accuracy of CNC drilled holes is well worth the extra setting up time involved.


(16) The chuck I am using is a nifty little Sherline 3 jaw (perhaps not quite the same degree of accuracy as I would get by using a 4 jaw but I think it will be more than adequate for my purposes). The output shaft has now been screwcut and a small 45 degree bevel machined to accept the chuck. Also two flats have been milled on the flange so that a spanner can be used when fitting and removing the chuck.


(17) A graduated scale 0 to 360 degrees which will be used as a visual indication of the axis rotational position. This has been engraved into ‘Traffolite’ engraving laminate and CNC profile cut.


(18) Vertical slots or keyways have been milled into each of the mating surfaces of the case components and keys made so that the parts slot together whilst still allowing for the mesh of the gears to be adjusted.

Many materials, especially plastics, can give off toxic vapours when they are heated or burned. Never inhale the fumes and never work in an enclosed or confined space unless adequate ventilation is provided.








(19) The position of the 4 keys can just be seen in-between each of the mating surfaces. Additionally 4 holes have been drilled and tapped through the end plates so that 4 bolts can be used to hold the whole assembly together.


(20) A collar has been fitted to the centre of the scale with the id bored so it is an interference fit onto an ‘O’ ring placed on the output shaft. This provides just enough friction so the scale rotates with the shaft but still allows it to be rotated manually for zeroing if required.


(21) The stepper motor mounting will be adjustable in the vertical plane to compensate for subsequent gear mesh adjustment. Although this will only be a very small amount of adjustment it is necessary to take any lateral loading off the flexible coupling.


(22) The stepper mounting has now been completed and the stepper motor fitted ready for a trial run on a test rig.


(23) The mounting holes for the ‘T’ bolts have now been drilled in the base plate. Also two steel strips have been fitted to act as washers and spread the load of the mounting bolts when it is clamped to the mill table.


(24) This is the finished axis, all assembled and ready to rock’n’roll. Test results running the stepper at 15 Volts, 1 Amp proved that there is more than adequate torque here - in operation, on the mini mill, it will be run at 24 Volts 1.5 Amps.







Looking back on this project, if I were starting again, the order in which the parts were made would be slightly different (it is called learning from experience). As concentricity of the turned parts is essential they were all turned between centres and various mandrels and jigs had to be made to achieve this. If the output shaft had been made first then the worm wheel could have been hobbed in position on the shaft, this would have saved a lot of setting up time and work, as well as easily ensuring the concentricity of this part.

I CNC milled the bearing pockets and although it was easy to do and achieved a high degree of accuracy, because there was only four to do, it would have been a lot quicker to have set the parts in a 4 jaw chuck and bored the pockets on a lathe. CNC pocket milling is fun to watch but it is quite time consuming on a small mill with limitations on the depth of cut per pass.

The way in which the worm wheel has been hobbed will inevitably lead to some pitch variations around it’s circumference (perhaps dependant on the accuracy with which the original slots were milled) and this will determine the overall accuracy of the axis. Although reasonable enough accuracy for hobby use, this method of construction would almost certainly not produce an axis that would be accurate enough for a watchmaker or precision craftsman.

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Some notes on setting up a rotary axis for use with Mach.


For such purposes as cutting spur gears, machining splines or flats on round stock etc etc the position of the rotary axis is usually expressed in degrees. A GCode command such as G0 A10 will rotate the axis 10 degrees and so on. To set up the axis within Mach it is necessary to specify the ‘Steps Per’ (degree) and this is achieved by taking the steps of the stepper motor and multiplying by the micro stepping setting (if any) and then multiplying by the gear ratio then dividing the result by 360. In my case this is 200 (steps) x 8 (micro steps) x 60 (reduction ratio 60:1) / 360 (degrees) = 266.6666 steps per. Unimportant really but had I thought about this a bit more at the time I was cutting the worm gear for my rotary axis I would have made the reduction ratio 63:1, then I could have had a nice round figure for the steps per degree.


Angular mode is the most common and also the most useful way in which a rotary axis is used but there are alternatives….


When engraving on the surface of a cylinder, for example, it is convenient to have the rotary axis set for movement in mm but as this will vary, depending on the diameter of the work, there is a trick to getting it just right. One method for finding the ‘Steps Per’ is to calculate how many steps are necessary for the axis to complete one revolution and divide this figure by pi. In my case this is 200 (steps) x 8 (micro steps) x 60 (reduction ratio 60:1) / 3.142 (pi) = 30553.787 steps per to enter in Mach. Now this figure represents a linear movement of 1mm around the circumference of a 1mm diameter cylinder. In order that this can be used for any diameter of cylinder another calculation has to be performed and that is 1 divided by the diameter of the work (1/diameter). The result of this calculation is then entered into the rotary axis scale DRO within Mach. My version of Mach did not have an A axis scale DRO (like the X,Y &Z axis have) so this had to be added using Screen4 (which can be downloaded, free of charge, from the Artsoft website – For reference the A axis scale DRO is OEM Code 62 and it’s associated LED is OEM Code 44).

As an example, to engrave on the surface of an 80mm diameter cylinder it is 1 / 80 = 0.0125 so 0.0125 is entered in the A axis scale DRO now a GCode command of G0 A10 will rotate the axis so that the surface of the 80mm diameter work rotates 10mm. Using this method means that simple, conventional, engraving programs and existing GCode programs can be used for engraving onto curved surfaces such as tumblers, cups etc.

Although perhaps not quite as accurate as when the axis has been set up for angular movement in the first place - entering pi / 360 or 0.00873 into the A axis scale DRO will allow the axis to then operate in degrees ie. G0 A10 will rotate the axis 10 degrees.





(25) Very simple tailstock support made to use with the rotary axis when necessary.


(26) Typical mounting positions for the rotary axis and tailstock on the table of Tweakie (the additional mounting holes are for when it is fitted to Bob).


(27) Perhaps an unusual application ? - laser engraving an image into the calcium carbonate layer of a chicken’s egg.