View Full Version : Z-axis huge slack / beginning a conversion

26-02-2012, 08:05 PM

Please is it at all feasible to convert this sort of a machine to CNC:

- I bought this King Midas (TFY-515) mill a year ago, with some tools.
- It is a lovely machine, cuts nice and smooth

Machine looks like this (though this is not mine):

Problem is a huge slack in Z-axis.
- One can either use a knob, turning clockwise / anticlockwise (there is a lot of slack, when changing direction, eg. first move down, then move upwards, slack is in changing direction)
- OR: alternatively use a lever aside. There is not much slack there. (but movement of lever pulley is only some 1/3 of a full turn, so guess one could not attach a motor to it...)

Attached a couple of photos of my mill. (this is the one which is my own :) )

Please suggest, where can I go from here?
- Source another mill?
- something else ____ ?

John S
26-02-2012, 10:29 PM
In a case like this the easiest way would be to clamp a plate around the quill at the bottom and use a ballscrew or similar to push / pull this plate and bracket. The existing pinion would be discarded.

26-02-2012, 10:51 PM
I would have a go at replacing that pinion first made up of some MOD gear and rod or just another one machined?

27-02-2012, 12:51 AM
Hi. I have a Myford/Rodney mill mounted on a very nice base. This is always a problem with rack and pinion z axis. My Myford drives the rack thought a worm and wheel which is hinged so it can be diss engage. I tighten up the vertical and always approach in from the same direction (very normal on a lead screws). The hand wheels marked in thou, I reckon about 5 accuracy but its a challenge. But it drives the rack via a worm to gear everything down.


Robin Hewitt
27-02-2012, 11:06 AM
I agree with John S. If you leave any springs or gears in the Z drive you will come to regret them.

Best to make the plate detatchable from the screw. That way you can still use the sensitive drill handle and you don't have to hammer against the screw when you dislodge taper tooling.

27-02-2012, 08:36 PM
Thanks, like this advice, this is what I wanted to hear. For a while thought this is it, need another mill, but now there is some light.

Had another look at the mill today.

1) Z-axis (the original question)
- Do you think it might be best to fit the ball -screw as in picture?
-- (and yes make attachment -plate detachable from screw)
- Or perhaps try to hide the screw inside the "tower"?
- Or attachment to the quill ?

2) The Z-axis screw is possible to disable. (picture)
- perhaps one could leave it there, then?
- or would you take that out altogether?

3) Noticed, actually there is slack in X/Y -axes,too.
- So while we are at it, maybe retrofit ball screws, there too?
- How to choose correct ball-screws?
-- Length: existing screws + a bit more?
-- Diameter: similar to existing screws? (does it matter if they are not quite the same?)
-- Where to go from here? Take the table apart, to measure existing screw? Make a mockup / sketch?
- View from underneath of my mill (X/Y -table)

4) X/Y -table drawing / plan of parts to change?

Would appreciate any comments (no need to answer all questions). Sorry for a long post . Thanks!

28-02-2012, 10:03 AM
A friend of mine converted his vicroy and here is what he did:

1.left original screw in the lower knee Z and a powered this (backlash is less of an issue here)
2.left the spindle Z as manual so at to always keep this up and maintain as much rigidity as possible during milling ops (still had the option to lower if needed)
3.replaced screws in the X an Y planes with ball screw

29-02-2012, 09:22 PM
I didn't realize Z-axis backlash is a smaller issue than X/Y -backlash. But it starts to make sense. Thanks for comments!

Right now, I will try to figure out exactly what to do, by studying other builds.

Perhaps will then come up with a "shopping list", and can start, and hopefully finish, too, my own CNC conversion thread.

This CNC -thing is such a fascinating, quite addictive topic :smile: Admire you folks a lot.

Robin Hewitt
01-03-2012, 10:22 AM
This CNC -thing is such a fascinating, quite addictive topic :smile: Admire you folks a lot.

The question is, are you going to become anal about backlash and go for true zero? I know I did. Also retaining the handles and dials on X and Y was a pain but I am ever so glad I did :heehee:

A man has to ask himself, would I be happy with my circles 15um out due to backlash? It is very nice when digi calipers occasionally measure the finished job as something.00mm

I'd suggest starting with a crude CNC conversion, get it cutting, identify the problem areas. Then do the pukka job with the benefits of CNC and make it beautiful.

03-03-2012, 08:16 PM
Think you got it spot on, Robin, crude conversion first = thanks I like that.

I am a cnc -virgin, and perhaps a big concern is, will there ever be a running machine.

My primary aim at the moment, is to get a shopping list ready.
- For that purpose, have done now two (2) things

1) Take measurements of the mill (King Midas)
- take photos too.
- Here are a couple of greasy pics, from dismantling the X/Y -table (From left to right: 1) overview; 2) X-axis dismantled; 3) X-axis leadscrew is 260mm, think it could be a bit longer; 4) x-axis one end; 5) Leadscrews are 18.34 mm thick; 6) Y-axis leadscrew at 175mm, think it could be longer)
5415 5416 5417 5418 5419 5420

2) Make a sketch of the Z-axis ballscrew -conversion
- think how the motor would be mounted
- how long ballscrew one might want
- how the thing would work
- This sketch is from side of the mill. (used inkscape to create the original graphics)
- Green is the body of the mill
- Blue bits are those that I'd try to buy ready made
- Red bits move up/down as "one unit", with the quill (and ballnut)


I am a bit concerned about the (horizontal) distance between the ballscrew, and the quill.
- the King Midas -milling machine is a bit difficult shape, like a mushroom.
-- Thought about using pulleys/belts, to bring ballscrew horizontally closer to the quill. But then, if possible I would like to avoid any added complexity, because of my non-existing experience with these beauties.
-- Another concern I have is the length of the Z-ballscrew (somewhere around 370mm), and no support in middle.

Robin Hewitt
03-03-2012, 10:50 PM
Thought about using pulleys/belts, to bring ballscrew horizontally closer to the quill.

Close as you can, even if it means taking an angle grinder to the mill head. Nothing wrong with timing belts and pulleys, much preferable to microstepping and you don't have to align the motor to the screw. Avoid small pulleys, you want lots of teeth engaged on the belt.

Are you not using taper tooling? I don't see where you plan to release the quill from the screw when you hammer the taper out. The two point quill nut mounting looks really good.

04-03-2012, 09:26 PM
Thanks Robin, yes need "release the quill from screw". Will keep that on the list :) Need to still think ways to bring the ballscrew closer to the quill.

This has been a very useful thread, helped a lot. Thank you!

Hope to continue as a build-log here...