Thread: King Midas mill conversion
Subject of conversion in this thread is a mill from 1980's.
There are some underlying issues that I would hope to address during the conversion:
- huge slack on Z-axis (covered here in detail: http://www.mycncuk.com/forums/showth...g-a-conversion )
- large slack on both X and Y -axis
Have read as much as possible, trying to learn from other machinists build-logs. As a result, here is a draft shopping list
- items in blue relate to axis/ballscrews.
PART NAME Length QTY 1 Nema 23 stepper motor, 3Nm (425 Oz-in) - 3 2 PM752 Microstepping Driver 3 3 Power supply 400W - 1 4 Coupling - 3 5 MBA motor mount - 3 6 X, Y, Z-Axis Ballscrew Fixed Support Block, FK-type
3 7 X-Axis 1605 / C7 ballscrew 581mm + ballnut 581mm (520mm usable thread) 1 8 Y-Axis 1605 / C7 ballscrew 346mm + ballnut 346 mm (285mm usable thread) 1 9 Z-Axis 1605 / C7 ballscrew 386mm + ballnut 386mm (325mm usable thread) 1 10 Ball nut housing (X,Y,Z-axis)
3 11 X and Y-Axis Ballscrew Floating Support Block, FF-type
2 12 Z-Axis Ballscrew Floating Support Block, BF-type
In view of the mill, please can anyone spot here
1) Something clearly wrong ? (something one would regret later?)
- eg. size of motors
2) USB vs. parallel port
- that is something I keep on thinking. Have read about arguments for / against.
- Kind of would like to go USB, but I have read parallel port is the right thing to do.
3) Cables and sundries
- worried about little bits missing from the list. (trying to buy all in one go, to save on shipping fees)
For now I have left out from shopping list "pulleys" and "belts". Hope could have this machine running some day, then later add belts/pulleys. Not sure how this conversion is going to turn out, will it run or not.
Motors and drivers are a good choice. What voltage is that 400W PSU?
I would substitute the couplings (4) for timing belts and pulleys. It makes it much easier to mount the motors as no faffing about getting them on concentric, reduces resonance and allows you to choose the ratio to get either better resolution or speed.
For (5),(6),(11),(12) make your own as you can use the milling machine to make them better than the commercial ones and much much cheaper.
USB / Parallel. If you use USB I doubt you'll regret it, but why not try the parallel port first as it costs virtually nothing to try?
For the stepper motor cables most people use 4-Core CY, 1.5mm^2 cable. Also add E-stop switch, limit switches and connectors for stepper motors.
Wattage 400 Watts
Input Voltage 240V AC
Output Voltage 36V DC adjustable +- 2.5V approximately
Output Current 11 Amps
Not sure if one wants more speed, at 5mm pitch. Thought of going lower pitch, even 2mm, but most of screws available seem to be 5mm.
- Perhaps my preference is machine / cutter longevity and accuracy.
Good an idea to make home-made mounts etc., though worry a bit about increased complexity.
The proposed set up looks good for a backlash of around .015 -> .025mm if you get it right. Plenty good enough to remake the machine after the screws bed in and .025 mm start to hack you off, which it probably will
A stepper does 400 half steps per rev. A 5:4 belt reduction on to a 5mm screw would give you 0.01 mm per half step, add one microstep for .005 mm and it becomes credible. You could reduce it 5:2 and avoid the springy microstep but that could seriously affect your rapid at 36 volts.
No good, why spend more on 70V drivers then use them on only 36V? The torque from a stepper motor is inversely proportional to speed and proportional to voltage, so if you get a 70V PSU you will get almost twice as much torque from the same motors. It's risking to operate right on the limit, so look for something just under 70V. One of the cheaper ways to do it is to use a toroidal transformer.
So far, added to shopping list some 48 teeth (around 75mm diameter) pulleys, and smaller 24 teeth (around 35mm diameter). Could make it into 5:2, guess it will be a bit more precise then.
Shopping list colour codes:
RED=NOT NEEDED, going DIY
ORANGE=Maybe not needed (DIY?)
PALE BLUE=Ballscrews+ballnuts as units
YELLOW=New since last list
PART NAME QTY 1 Nema 23 stepper motor, 3Nm (425 Oz-in) 3 2 PM752 Microstepping Driver 3 3 Power supply 50V – 68V, at least 400W 1 4 Coupling 3 5 MBA motor mount 3 5 Aluminium motor mount 3 6 X, Y, Z-Axis Ballscrew Fixed Support Block, FK-type 3 7 X-Axis 1605 / C7 ballscrew 581mm + ballnut 1 8 Y-Axis 1605 / C7 ballscrew 346mm + ballnut 1 9 Z-Axis 1605 / C7 ballscrew 386mm + ballnut 1 10 Ball nut housing (X,Y,Z-axis) 3 11 X and Y-Axis Ballscrew Floating Support Block, FF-type 2 12 Z-Axis Ballscrew Floating Support Block, BF-type 1 13 Pulley for 12mm width belts. AT5 -type. 24 teeth. Total width circa 22mm. Total diameter circa 37mm. For X, Y, Z -axis 3 14 Pulley for 12mm width belts. AT5 -type. 48 teeth. Total diameter circa 75mm, for X ,Y Z -axis 3 15 Belt 12 mm 12AT5 length circa 450mm, for X, Y-axis 2 16 Belt 12 mm 12AT5 length circa 330mm, for Z-axis 1 17 E-stop switch 1 18 Limit switches 6 19 Connectors for stepper motors 3 20 4-Core CY, 1.5mm^2 cable
- 6, 11, 12 ... not quite sure how involved a procedure it is to make them DIY? (and how big a saving can be made, and can one create those without CNC? )
Think the shopping list starts to look quite good (ready to go?). Really appreciate, your help has been great. Thank you!
A ball screw is best held in tension by Belleville washers to compensate for thermal expansion, one thrust race at either end.
If you lengthen the XY shafts at one end you can keep the handles.
Run the table to it's ful travel and ask yourself where the CNC parts are going to fit in the available space.
Experience tells me that being new to this you are very unlikely to believe this is necessary. You feel a compelling desire to see it cutting, don't want to go back to the drawing board now and spend extra money on machining screws. I only mention it so I can say, "Told you so" later on :naughty:
Yes Robin, back to drawing board it is. Really appreciate your advice, glad I didn't yet buy wrong length ballscrews.
Went to garage, moved X/Y -axis back and forth full travel. Need to lengthen the ballscrews, pulleys would now crash to the table.
Not only that, but realized that it might be possible to extend X/Y -axis travel a good couple of inches, with minor changes.
- Had not really thought, where one would fit the ballnut, and where screw would be mounted. (attach ballnut to upper part (the one sliding), or lower part vs. the other way around, that kind of things)
- Below is a sketch of the Y-axis movement (current vs. possibly future). Changes would be: 1) a longer ballscrew; 2) attach ballscrew to the table that does not move; 3) attach ballnut to the table that moves, at the centre.
Problem is, you need a CNC mill to make a pretty CNC mill which is why I was suggesting you cobble something together first. Hope you have a lathe, really do need a lathe.
I used pairs of Belleville washers to apply a preload of 500 lbf. One pair for each screw to hold the axial bearings in tension, one pair for each double ball nut.
That means there is no play in the system, apart from the column bending, so long as the load does not exceed 500 lbf. It was a lot of work but is quite delightful to use and all self adjusting.
The XY screws appeared much too long until it was all together, have a pic...
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