King Midas mill conversion

[Robin Hewitt]

Good, only one small mistake. You only leave a gap on the left hand side, the tension end. On the right you want the 12mm nut to grip the angular bearing inner ring against the step in the screw. That gives you something to tighten the nut against so it stays where you put it. Don't want that right end nut free to unscrew itself.

The gap on the left only needs to be long enough to stop the 16mm part of the screw ever touching the bearing. If it does touch then the Belleville washers stop pulling on the screw.

My handle fitted inside the saddle, if it didn't I would have extended it. My X motor didn't fit so I extended that end a long way.

The extension can be as long as you need, it doesn't have to be wide.
A wide extension could get in the way of the motor. Allow room for the motor, you don't want the motor hitting the saddle or getting in the way when you want to turn the handle.
The extension has to take a big axial load from the Bellevilles.
The radial load is the motor torque.
The side loading is ordinary workshop bumps and knocks.
It has to hold the bearing square, otherwise the screw will flex when you turn it.
It is comforting if the extension carries any motor heat in to the bed, so the screw doesn't warm up.

[Jonathan]

What's the dimension I've marked with '???':



Pretty minor, but I changed the end bearing surface from 7mm to 9mm since the bearing is 10mm wide and it's best not to have the bearing resting on too much thread.

[Robin Hewitt]

What's the dimension I've marked with '???':

I'm confusing my milling machines, on this one I used full nuts rather than pairs of half nuts. I didn't want to stress a half nut by asking it for 500 lbf of tension.

10mm for the bearing, 10mm for the nut, say 1mm protruding, 21mm total.

Doesn't really matter how far the thread goes inside the bearing, the big axial load from the Belleville washers overwhelms all the other loadings.

I made all the bearings an easy sliding fit, inside and out. This made it very easy to assemble but it went rock solid the moment I tensioned it.

How do you plan to mount the motors? I suggest you cobble them on somehow with plates and studding then replace everything when you have CNC. Motor mounting is important now only because it determines the length of the screw.

Things to remember when designing motor mounting plates...

Putting the motor on so it faces away from the machine is best.
Putting the motor on facing the machine saves a couple of quid on the ballscrew length but makes everything else difficult.
Leave a tadge of motor movement to adjust the belt tension.
Cut the outline for a belt cover and it's mounting bolts, assume you have CNC for making the belt cover.
Somewhere to attach a flexy conduit over the motor wires saves bodging it later on.

[Jonathan]

10mm for the bearing, 10mm for the nut, say 1mm protruding, 21mm total.

My point was not knowing those dimensions means the length of ballscrew directly between the bearings is unknown, which is a critical dimension.

Doesn't really matter how far the thread goes inside the bearing, the big axial load from the Belleville washers overwhelms all the other loadings.

I guess so long as the mounting surfaces are accurate, but there's no reason to have the thread far in the bearing surface.

[diy-john]

changed the end bearing surface from 7mm to 9mm

That is a sweet picture Jonathan. Good point that 9mm.

10mm for the bearing, 10mm for the nut, say 1mm protruding, 21mm total.

Final versions of X, Y, Z -axis screw are like this (changed according to Robin's / Jonathan's comments)
- X-axis:::


- Y-axis:::


- Z-axis:::




Revised shopping list is here. A couple of notes about that
1) Power supply perhaps is slightly undersize. Thought to trial and see.
2) Need two (2) of the Nema 23 -motors WITHOUT the rear shaft
- Need one (1) --------------//----------------- WITH the rear shaft (Z-axis, will attach hand wheel there)
3) Items 12, 14 -- 19 not sure where to source from, have not found yet
4) Bearing housings / fittings ---> DIY ?

ITEM NO.	Item			QTY
1	Nema 23 stepper motor, 3Nm (425 Oz-in)	Zappautomation	http://www.zappautomation.co.uk/sy60...cPath=9_159_42	2
	Nema 23 stepper motor, 3Nm (425 Oz-in)	Zappautomation		1
2	PM752 Microstepping Driver	Zappautomation	http://www.zappautomation.co.uk/pm75...ml?cPath=9_3_4	3
3	Power supply / PS705 - 75V DC at 0A & 68V DC at 5A	Zappautomation	http://www.zappautomation.co.uk/sps705-p-587.html	1
4	X-axis R 1605 ballscrew 853 mm (not machined)	Zappautomation	http://www.zappautomation.co.uk/r160...crew-p-93.html	1
5	Y-axis R- 1605 ballscrew 508 mm (not machined)	Zappautomation	http://www.zappautomation.co.uk/r160...crew-p-93.html	1
6	Z-axis R 1605 ballscrew 259 mm (not machined)	Zappautomation	http://www.zappautomation.co.uk/r160...crew-p-93.html	1
7	Ball nut R1605T3-FDID	Zappautomation	http://www.zappautomation.co.uk/prod...roducts_id=574	3
8	Ball screws machining	Jonathan		3
9	Pulleys, larger around 70mm diameter, 21T5/44-0	Zappautomation	http://www.zappautomation.co.uk/sync...Path=14_72_153	3
10	Pulleys, smaller around 35mm diameter, 21T5/22-2	Zappautomation	http://www.zappautomation.co.uk/sync...Path=14_72_153	3
11	Belts, 3 for the pulleys (10mm)	Zappautomation	Ask Gary	3
12	Angular Ball Bearings 7201B -type	Local bearing shop	http://www.vxb.com/page/bearings/PROD/Kit1085	6
13	Belleville washer	Robin		6
14	E-stop switch	TBD (to be decided)
15	Limit switches	TBD
16	Connectors for stepper motors	TBD
17	4-Core CY, 1.5mm^2 cable	TBD
18	Sleeves for handwheels	TBD		2
19	Handwheels	TBD		3

Think this should be pretty good now. (until someone points out inconsistencies )