King Midas mill conversion

[Jonathan]

Lots of drawings on Gary's site:

http://www.zappautomation.co.uk/ball...2cf54c2f87cf0d

I can machine the ballscrew as you've drawn it for £15 plus postage.

[diy-john]

I can machine the ballscrew as you've drawn it for £15 plus postage.

Wow, cool Jonathan. Can't go wrong with that. Thanks!

www.bellevillesprings.com pn: D2812215

You are welcome to half a dozen, just shout.

Robin, yes please, need those.

suggest you put a thread and one bearing at either end.

Made another draft of X-axis screw. Please is this any good now?
- added a thread to the other end. And also a similar "structure, a fixed mount" for the ballscrew. So there are now two such mounts, one at either end of the screw.
- there are two (2) screws in picture below, but it is the same one single screw.
-- The other one just highlights, what components would fit, and where. Like hand-wheel, pulley, shims, Belleville washers.



Not sure about the dynamics, kind of figured out, that with this setup, the Belleville -washers + half nuts at either end of the screw ---> would tension the screw sort of around either end fixed mounts. Please correct if you think one has mis-understood the principle.

This is the link to the datasheet for the FDI and FDU type ballnuts and it is 34mm between the flats, so it is actually 6mm difference.
http://www.zappautomation.co.uk%2Fpdf%2FBallscrews%2FFDU-FDI.pdf

Gary, the 34mm height ballnut gets me off the hook. Thanks for pointing out these alternatives. Will speak to you about other components.


Thanks!

[Jonathan]

Robin - thank's for confirming that about the washers. I'll order some 'lighter' ones (http://www.leespring.com/uk_product_...ype=W&subType=)

Wow, cool Jonathan. Can't go wrong with that. Thanks!

:) Discuss details when the design's ready.

To put the screw in tension you don't want to have pairs of angular contact bearings at both ends, as that fixes both. You need a pair of angular contact bearings at one end, and a standard deep groove bearing plus Belleville washers and thrust bearing at the other end to provide contact force for the tension. If both ends were fixed rigidly with angular contact bearings the tension in the screw would depend on the relative spacing of the bearing blocks and the length of the screw between them, so there's no way to set it.

By 'standard washer' I assume you mean an accurately machined washer with parallel faces. You don't need a washer between the ballscrew end and angular contact bearing as the bearing is most accurate resting on the machined shoulder on the screw.

[Robin Hewitt]

That looks horrible. I'm obviously not explaining this at all well.

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. Adding more bearings just gives you an alignment nightmare. There is hardly any side loading on the screw to worry about, so don't worry. I used one bearing at either end of my overlength X screw and it was fine and dandy.

I used a pair of bearings at the handle end of my Y screw simply because I couldn't fit any bearings at all on the far end.

Not a problem for the X so I put one bearing at either end.

The whole she-bang should be sloppy, right up to the moment you tighten down on the Belleville's.

[diy-john]

You are all doing a good job explaining the principles.
- It was me who assumed X and Y -axis ballscrews are put together in similar way.
- Assumed too, that idea is to make it as sturdy/rigid as possible.
- There are some gaps in my knowledge, filling the gaps with assumptions doesn't always work

But it will be good. Will create another drawing of the ballscrews -setup.

Thanks again for comments!

[Robin Hewitt]

... and only one pair of Belleville washers, you don't want to balance the screw between two springs because that can move.

A few design tips.

I bolted the X screw pulley solid to the inner race. I sprung the sleeve that takes the handle at the other end. This reduced the bearing overhang I couldn't avoid on the Y.

Putting the handle on a sleeve means I can pop the handle off without detensioning the screw. Handy if I want to work on something that overhangs the end of the bed.

I mounted the X motor facing the unlikely way round and extended the screw to get clearance on it. This makes it a lot easier to mount, easier to change the belt and easy to pop a cover over to keep the crut and fingers out.

My X motor mount isn't actually bolted to the machine, it's free to turn. I was going to Loctite it in place but found the 500 lbf screw tension quite sufficient to hold it. I lean on it occasionally, if it ever moves I will know I have lost tension.

[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 ... ?