Monday, 4 May 2020

Z axis DRO scale mount - and revised home / limit switch concept

Background - DRO scale:
As noted, I delayed (re)fitting the Chinesium DRO scale from the Z axis, as it would have impeded access to the new ballscrew assembly. However, it's part of the final assembly and I'd like to be able to get a reasonably accurate measure of the absolute position. That will expose the accuracy / backlash as well as show up any missing steps or axis creep.

My original concept was implemented before I started the CNC conversion and picked up the quill position from the original feed trip mechanism. I then designed a steel ballnut yoke for the CNC conversion and was able to connect the DRO to that without difficulty.

The current arrangement for mounting the main body of the scale is simple and sound, so I plan to retain that. It picks up on a couple of flat, coplanar surfaces that are present on the std machine. From here it looks simple enough to make a bracket to connect the final ballnut yoke to the DRO scale slider.

Limit and home switches vs quill travel:
The other key element of this assembly is the arrangement for mounting the home and limit switches. I've made an effort to maximise the movement range of my revised Z axis assembly, as it's always an issue with turret / knee mills. I struggle with this on The Shiz as it is - and the BP has even less movement to begin with. The std BP manages 5" (aka 125mm) of movement and I'd like to preserve as much of that as possible, notwithstanding the unavoidable loss of movement due to the need for the limit and home switches to have a few mm of margin to work within.

The existing switch arrangement has resulted in lost movement - I can only get ~95mm, yet the mechanical parts are capable of closer to 120mm. That represents a loss of an inch or so - or 20% of the total. Not good. The reason for that is that there is no room for the switches to move before they clash with the head casting. They are maxed out here, with no possibility of further improvement.

So let's have another go, rather than bodge up the existing parts. And try not to get too carried away. I have a life to live and this doesn't need to be perfect.

New concept design:
I have the head assembly already set up in Fusion 360. So firstly, let's delete the old stuff and create a new bracket, keeping things nice and simple. This is little more than a strip of steel with a few holes in it:

The scale slider just needs a small spacer to connect up to the yoke:

I can mount a simple strip of 1" x 1/8" steel strip on the side of the DRO scale body using existing holes in the end caps:

Then design in place (within the Fusion 360 assembly) a slider component that will fit over the rail and lock in place with a set screw. Something like this:

The middle one will be the home switch. As it needs its own target, I need to mount it at a different height to prevent it triggering from the limit switch target. The (black) spacer should do the trick, while leaving the sliders otherwise identical.

This will be the final assembly. The switches can fit anywhere between the ends of the rail.

Fair enough. Enough talk - let's make the fucker.
The bracket is simple enough to knock up with the bandsaw, belt sander and hand drill:

With the various spacers flashed up, it all seems to fit together nicely. It even clears the 4 large bolts, although they have no obvious purpose on a CNC machine.

Doesn't look too silly:

It's not going to be quite as compact as the previous version but there again, the previous version didn't allow enough movement, so what do you expect?

Next - make up the sliders and the target piece for the proximity switches.

1 comment:

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