This is Bob, a bench top CNC milling machine manufactured by Boxford UK (circa 1990) for use in education. It is driven by 200 step per revolution stepper motors and has 5mm pitch ballscrews for all three axis - the body of the machine is of stout, cast iron, construction with adjustable gibs on all three slides. The spindle uses quick change chucks and is belt driven by a variable speed DC motor. It is, I think, quite an impressive machine for its size.

 

I have removed it from its cabinet and discarded all the original electronics except for the stepper motors, Lenze spindle, speed controller and choke which will all be reused.

This 3 axis stepper motor controller seems ideal for the job - it can handle a peak current of 2.5 Amps per phase at 24 Volts and can be set for eighth step micro stepping and is just right for the existing stepper motors. It also incorporates the charge pump safety circuit (see the Mach3 pdf help file for further details of this) and a spindle on/off control relay. All I/O lines to the LPT port are nicely buffered with opto isolators with Schmitt trigger circuits for all inputs.

 

The board was marketed on ebay by MDfly who appear to have done just that - they come and go as it suits them. This always seems to happen to me - no sooner I buy a stepper motor controller and the suppliers leg it. (I did not want a warranty or any after sales service anyway - so there).

 

Nonetheless it is a very good board and does work extremely well. It was supplied with instructions, connection diagrams and Mach2 setting up procedures on CD. (if you would like a copy of this, it can be downloaded from here MDfly.zip). Not visible in this photo, there is a large heatsink fixed to the back which does get warm during use so forced air cooling will be necessary.

 

The new control box enclosure.

 

Spindle speed is set manually from a potentiometer on the front panel until such time as I can build a suitable circuit to enable program control of this function. Spindle on / off is fully under Mach3 control but at the first chance or risk of an accident I will revert back to manual control. I have fitted a mains driven fan to the top of the enclosure and this seems to circulate the air nicely. The little switched mode power supply hardly looks man enough for the job but so far it has performed perfectly.

This is the general schematic diagram and detail of connections to the speed controller. Fuses are incorporated within the incoming mains supply plug, the input and output within the p.s.u. and within the Lenze speed controller - no other fuses have been fitted.

 

It is possible that fuses in series with each of the stepper motor coils could prevent damage to the controller in the unlikely event of a short within a motor but it is also possible that the sudden open circuit of a fuse blowing may also cause just as much damage to the controller. Who knows ?.

 

At the moment I cannot foresee any requirement for me to need to run the spindle motor in an anticlockwise direction. As far as software control is concerned both the M3 and M4 commands will energise the spindle motor for clockwise rotation and of course M5 will switch it off.

 

I have fitted a small terminal block to the backplate of each of the stepper motors and used screened cable to connect each of them to the controller card. The screens for these cables are only connected to ground at the control box end. The power supply negative has been connected to the ground terminal on the control box and all other grounds or earths are connected to this one, same terminal thereby avoiding ground loops. The LPT parallel port cable screen must not be connected at the control box end - this is connected at the computer end only. The control box casing and the body of the machine itself is, for safety reasons, connected to the incoming mains protective earth.

 

As yet no limit switches have been fitted to the axis but they will also be connected with screened cable, the screen of which will also be connected to the common ground point.

 

WARNING - Mains voltages can be lethal and if you are in any doubt about making any electrical connections you should consult a qualified electrician.

Some of the original tooling and quick change sidelock tool holders - the facing cutter on the right looks like it takes no prisoners.

This mini Turner’s Cube is the first item I have made with Bob. As a turning exercise I suspect this would require a good deal of skill to complete but with a CNC milling machine very little skill, except that of compiling the GCode program, is needed. Two GCode programs were used here, one to cut the circular pockets into each of the six sides using a 4mm slot drill and the second, using a 10mm dovetail cutter, transcribing a circular path to free the centre cube.

The adjustable stop fitted to the side of the vice enables accurate work re-positioning.

In industry, machines are fitted with all sorts of guards and safety interlocks which have been put there for the safety of the operator. This is not the case with most home operated machinery therefore extra vigilance is always required.

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Bob the mill has now been mounted on its base together with it’s control box. Removable chip shields, which can be mounded on the edges of the base as required, are the next thing to be made.

Removing this machine from it’s original cabinet has reduced it’s footprint considerably and enabled it to fit into my small workshop. The down side is that all the safety features of the cabinet, which were designed to protect the operator, have also been removed.

 

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