Thread: King Midas mill conversion
The back-emf from stepper motors is significant - they're good little generators. The protection diodes in the stepper drivers will rectify the back-emf and if you turn it fast enough power up the driver which is clearly bad news. I found that below a certain speed they manually turn quite easily, but when that threshold is exceeded it gets a lot harder. Clearly it will vary significantly with the size of motor.
Easily solved - just unplug the motors.
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.
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 ?
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
Think this should be pretty good now. (until someone points out inconsistencies )
£300 on ballnuts! Are you sure you can't get the cheap RM1605 ballnuts from china to fit as they're $25 each from eBay. Surely you can at least use the cheaper ones for Z?
Pulleys - the tooth profile of HTD pulleys is much better suited to this application than T5 and similar price, so get pulleys and belts from here:
Most (if not all?) the 3Nm motors have dual shafts, so unless the rear shaft is going to get in the way you can just get 3 of the motors you linked to at Zapp.
I've got lots of 4-core CY, 1.5mm^2 cable for the motors...yours for £1.20 (I think, will check) per meter.
Making the handwheels can be the second project on the machine after cutting the motor mounts! I could make the piece that attaches the handwheel to the screw (18), wont cost much.
The power supply you've linked to is probably sufficient, but more power would be better. It's cheaper to make your own using a transformer like 88-3839 from Rapid Electronics. Just -earch on google for making a power supply with a toroidal transformer. There's not much to it - you just need the transformer above, bridge rectifier and capacitors. Let me know if that's not enough information and I'll explain better.
Have you considered in the Z-axis screw mount how to temporarily disengage it from the spindle when changing tools so that the force from hammering on the draw-bolt to release the tool taper isn't transferred to the relatively delicate ballscrew/ballnut?
Cheapest e-stop switch I can find is here:
One day I'll get round to buying one...
Last edited by Jonathan; 20-03-2012 at 09:00 PM.
Are these cheap chinese ballnuts doubles like the Zapp DF1605 at £44 ?
That would give about 10 microns backlash until it beds in, but with a bit of Belleville jiggery pokery you can get that down to around zero microns.
Transformers are okay until you get a mains surge and everything goes pop. Unlike back EMF from turning a motor, I have personal experience with this little snag :naughty:
1) Placed an order with Zappautomation, via the online-store.
1.1) Replaced pulleys / belts with HTD -type, as suggested by Jonathan
1.2) Plan is, screws would be going to Jonathan for machining.
- not sure if the ballnuts could be detached from the screws. They are preloaded. Will talk to Gary at Zapp(automation).
- Overseas shipping costs - located in Finland, buying from different places will hike costs up. Shipping costs add zero value to the conversion, so minimize them.
- Need for the smaller dimensions of the FDI -type -ballnuts.(X/Y-axis tight space)
- Quality ... don't know much about ballnuts, but did rollerskating / skateboarding in my youth.. what a difference bearings can make
Perhaps biggest factor I have is "risk aversion". Still must admit, not sure if one will ever complete this thread. It is a big project for me, and if there are many set backs, project gets longer and longer, you never know. Does this thread end up in classifieds -section? Need to stick to safe choices.
- Something like this maybe, to quick-release?
Really grateful for all comments and advice in this thread. They have been very helpful. Thanks!
21-03-2012 #50Visit Us: www.zappautomation.com
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