Observations after a catastrophe and an annoying problem...

[Chet]

First off, I use 'lost foam' packed in green sand to make my aluminum castings.
At this point, it's pretty much routine, and the castings usually turn out well.
However to create the foam pattern, I use a homemade CNC router running Mach 3.
To cope with the problem of backlash, I've gone the route suggested by others, and use a nylon wing nut on my lead screws.
That being said, nylon wing nuts do have a limited life...
The limit of which is usually only discovered when the Z-axis gives out and plunges the unit deep into the pattern and CNC table.
Very annoying...
The reason I share this aggravating experience, is that on reflection, I think there may have been some clues that failure was imminent.
1st, unexpected black residue on the pattern several hours before the failure...
2nd, tiny variations on the finished surfaces several hours before the failure...
For those of you using a similar process, such observations may prove useful.
As to Part II of the subject line, the annoying problem...
I also encountered repeated Limit Switch triggering with my unit.
I readjusted the de-bounce interval, but Mach 3 kept stopping and telling me a limit switch had triggered.
There was absolutely no way any of the limit switches were triggered physically.
After much frustrating touble-shooting, it turns out one of my wires from the limit switches had developed an internal break...
I'd used solid wire when I built the unit because I had it on hand, but evidently the constant flexing of Y & Z axis wires caused the break.
Replaced the Limit Switch wires with stranded and am now hoping for a long useful life.
It's good to have everything running again...
At least for now... ; - )
Chet

[BClem]

Your backlash issue is not uncommon at all and is a major factor in either choosing the quality of a machine to start with or limiting the precision of the parts that are manufactured. The conversion of a milling machine to a CNC machine is a waste of time without due consideration to the lead screws and their backlash. Acme threads are available in increasing grades of precision but the nut is a 'wear out' part and is constantly adjusted for backlash. Ballscrews with preloaded nuts are the standard for CNC but even they have limitations and degrees of precision and usually their mounting will be a factor in backlash. Point being: your awareness to the limitations of a machine and the ability to compensate is a common thing that we just have to deal with...

Sounds like an interesting pursuit...

BClem

[Chet]

Acme threads and nuts certainly have a very useful purpose...
But, after pricing a single 12" 1/4-20 Acme rod and appropriate nut, I was financially challenged by the near $100 price! (that price also includes the collar that screws on the end of the nut...)
As an economically driven alternate, I think I'm going to prepare a 3/16 steel wing nut (similar in dimensions to the nylon wing nut even though the thread dimensions are different), drill and thread 1/4-20 hole through a 3/16 bolt, and screw that bolt in to the steel wing-nut and use that as my replacement for the nylon wing nut.
All of my force is downward on the Z-axis with the weight of Z-axis unit itself and the trim router, with little resistance offered by the styrofoam surface.
So basically my Z-axis is only working one side of the 1/4-20 thread...
However I am toying with adding a spring and a 2nd 1/4-20 steel wing nut which would 'float' on the rod and provide a constant tension on the modified wing-nut.
I'll post pictures when its done...

[BClem]

A counterweight for the headstock/quill is a good thought. If you use a spring it may need to be a flat wound spring such as used in the rope start rewind in a B&S or similar small engine. These will maintain a fairly constant tension over a distance maybe close to the Z axis movement. Having the weight of the head or quill on the z axis screw is not so good - as you know. All vertical NC mills have a counter-weighted quill - they are dangerous otherwise. The flat wound spring is rotated and tensioned to support the headstock and locked. Then the screw is not supporting that weight and you have free movement.

You may also make a nut with a means of tightening it axially on the screw to compensate for backlash such as a partially split nut. Belville springs are used as a preload for ball screw nuts and you may be able to fashion a means of doing that with the screw you are using.

BClem