King Midas mill conversion

[Robin Hewitt]

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.

[diy-john]

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
...
You could reduce it 5:2

Robin, with this rate, the mill will be cutting a couple of atoms per slice - maybe.

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.

No good, why spend more on 70V drivers then use them on only 36V?

Good Jonathan you say that, didn't even come to think about that. Updated into shopping list. So far only found some 50V PSU's, but will keep on looking, to add to the list.

	Shopping list colour codes:
	RED=NOT NEEDED, going DIY
	ORANGE=Maybe not needed (DIY?)
	GRAY=check quantity
	PALE BLUE=Ballscrews+ballnuts as units
	YELLOW=New since last list
ITEM NO.	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

http://www.youtube.com/watch?v=tasl9...eature=channel

Nice video Jonathan. Makes it easier to understand how the pulleys are setup.
- 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!

[diy-john]

Here is a new draft of the ballscrews. Lengths change a bit, from going to pulleys/belts instead of directly driving the screws
.

[Robin Hewitt]

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:

[diy-john]

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.

Belleville washers to compensate for thermal expansion, one thrust race at either end.

This is all new to me. Thanks will look for pics from google. Another thing I don't quite understand yet, is the construction of the "fixed mount" for the ballscrew. (still thinking whether to buy ready made, or DIY).

[Robin Hewitt]

Yes Robin, back to drawing board it is.

Wow! If you are okay to take advice you can make one hell of a machine. I got it right on my third set of screws when the penny finally dropped.

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...

[diy-john]

The XY screws appeared much too long until it was all together, have a pic...

Robin, that is one sweet looking machine. One can tell, a lot of planning has gone into it. Like the kind of contrast with the big, sturdy, used Warco mill, and then the precise CNC -parts. Very cool.

King Midas conversion last couple of days has been measuring + and thinking how/where conversion related components will be. From all your comments, I have realized how distant my thoughts have been from this project. Feel one has been watching this project sort of from cloud -level, but now there is need for much closer look. Exciting.

1) Y-axis ballscrew -related
1.1) Ballnut would be attached to "lower", "stationary" bed of the mill. (view from front of the mill)



1.2) Y-axis ballnut would be attached to "lower", "stationary" bed of the mill. (side -view, from right hand -side of the mill)
- white box, height is 55mm, width is 18mm (for reference only)


1.3) Overview / sketch of Y-axis ballscrew (side view, from LEFT -hand side of the mill)
- colours have no specific meaning, just different colours for different components
- length of ballscrew is now at 508mm. (note how it has grown, initially one was going to buy a 346mm screw...LOL)



1.4) A challenge in Y-axis ballscrew is a 35mm vertical space here
- thought initially there is 40mm space, since the "cave" bottom looked like level with "outside" (see picture attached)
- need to make DIY "slim" ballscrew floating end-mount. Haven't found a slim-enough ready made mount.
- SKF 7200 BEP bearing outer diameter, for a 16mm diameter screw, is 30mm
- Would mount screw to "moving upper deck".




I haven't got a lathe yet. Tool/workshop -wise I am in the wrong league. Looking to do premier league, but my skills/tools are only 5th division. But thanks to this forum and careful planning, maybe this game is possible..