King Midas mill conversion

[diy-john]

To put the screw in tension you just need one angular contact bearing at either end. The Bellevilles try to stretch the entire screw end to end.
I used one bearing at either end of my overlength X screw and it was fine and dandy.

Thanks again Robin

Please would this setup make the X-ballscrew dandy? (is this what you mean?)




Sorry if this obvious to some of you. I tried to figure it out, that
- if you now tighten the screws at each end of the X-axis screw ---> the nut(s) will push the Belleville washer ---> the Belleville washer will push the bearing ---> the bearing will push the fixed bearing mount. But because the bearing mount is fixed, then tightening the nut(s) will pull (try to stretch) the screw from either end.

Please correct. (is this getting better or worse? Tell me)

Thanks!

[Robin Hewitt]

Try this pic for the X screw in tension...

Right hand nut down tight.

Left hand nut tensions the screw.

Personally I'd put the pulley at the other end to reduce the overhang.

I should have drawn the handle fitting bigger than the nut. That way you can put the handle on and off without removing the nut.

Edit: I should also have put the Belleville washers on the other side of the pulley so the pulley grub screw doesn't have to slide. I can redraw it if you want.

[diy-john]

Cool Robin :) - Thanks!

That picture is worth a thousand words. Updated X-axis draft accordingly.
- added Belleville -washers on the other side of the pulley, like you say
- the sleeve, on top of which the handwheel comes is a nice feature.

Personally I'd put the pulley at the other end to reduce the overhang.

Now, have thought about this overhang. Might as well say now, what I have had in mind, but before reading further, make sure you sit tight and don't spill any coffee.. :lol:

The challenge is, that the X-axis movement is a mere 220mm, though the table is 500mm wide.

Thought of moving the
- handwheel
- pulley
- motor

way out of the end of the table, on the right hand side.






This "extension" would change X-axis movement from 220mm to around 400mm. Think there might be challenges ahead, with accuracy/slack, at the far end of the X-table movements. However, I have reasoned, that perhaps one could bring the slack down with careful adjustment. Just have been thinking, that there may be situations, I would rather live with a small slack at far end, than reposition the object on the table (because of lack of X-axis movement).

Of course people will wonder, why one didn't buy a larger mill in the first place. But what do you think, is this "extension" something one definitely should not do? Please comment :)
- extension would not be for attaching, or supporting pieces on table, but rather bring out the X-axis motor, pulley, bearing block.

[Jonathan]

On my mill the original handle / bearing arrangement already clears the casting that the bed runs on. The extra travel that 'gains' has proven invaluable on numerous occasions. As you say it's bound to not be as rigid as using the standard travel, but I've never noticed it.

You could use some decent size aluminium bar for the extension and use it to house the bearings. Convenient as you can do it all on a lathe.

Is it convenient to add another (e.g. 6201) bearing at the driven end of the screw? I'd prefer to have the screw supported with two bearings at one end as otherwise I suspect the significant radial force due to the tension of the timing belt will tend to bend the screw about the single angular contact bearing. Might be negligible in reality as clearly it works for Robin without ... ?

[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.

[diy-john]

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.

A tiny update to X-axis screw.
- Red texts are updates.

It is comforting if the extension carries any motor heat in to the bed, so the screw doesn't warm up.

Will take a note of that. Again really good suggestions. Thanks!

Project has been moving at a snail's pace, but that is okay, have already avoided many pitfalls.

[Jonathan]

£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:

http://www.bearingstation.co.uk/prod...HTD_Pulley_5mm

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:

http://www.tlc-direct.co.uk/Main_Index/Distribution_and_Switchgear_Index/Emergency_Stop_Buttons/index.html

One day I'll get round to buying one...

[Robin Hewitt]

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:

[diy-john]

Updates:

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).

£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?

Good point Jonathan. Thought about that, this time key criteria came to four (4) facets
- 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.

a toroidal transformer.

Erm, would love to try that. But my skills..not very confident, have set a soft limit of "no mains" electrical appliance -jobs for me. Think one can manage stepper motor -cabling, and drivers, because they are sort of behind the power supply (not directly mains)

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?

Have not yet figured out how that could be best done. Some sort of a quick-release system would be cool. But not sure about design, and whether it would introduce slack to the Z-axis
- Something like this maybe, to quick-release?

Really grateful for all comments and advice in this thread. They have been very helpful. Thanks!

[Gary]

You can remove them, but i would just leave them on the screw and screw the nut out of the way when doing the machining.

Updates:

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).



Good point Jonathan. Thought about that, this time key criteria came to four (4) facets
- 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.



Erm, would love to try that. But my skills..not very confident, have set a soft limit of "no mains" electrical appliance -jobs for me. Think one can manage stepper motor -cabling, and drivers, because they are sort of behind the power supply (not directly mains)


Have not yet figured out how that could be best done. Some sort of a quick-release system would be cool. But not sure about design, and whether it would introduce slack to the Z-axis
- Something like this maybe, to quick-release?

Really grateful for all comments and advice in this thread. They have been very helpful. Thanks!