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Wednesday, February 18, 2015

CNC mill conversion - part 22

X travel limit switches

One of the items I should have addressed a long time ago, that was instead put off for a rainy day, was the installation of travel limit switches on the mill. 

Typical inexpensive snap action roller microswitches

All three axes need limit switches for reference and protection, but due to the diverse physical configuration of each axis, different mounting strategies are required. 

Limit switch operation

In this post I will discuss my development of the X axis travel limits.

Travel limit switches are needed on CNC machines for two main reasons. The first is to let the computer know where the table and head of the mill are in relation to the travel stops (machine coordinates). This is usually done at the beginning of the work day, or as needed, by giving the machine the “Reference all home” command, which slowly runs the head of the mill all the way up to the stop (Z=0), and the table to the furthest right and forward possible (X=0 & Y=0), where limit switches are tripped, the motors stopped, and the mill position recorded in the software. 

The second reason is to avoid damages to the machine by accidentally driving table, or head, all the way against the travel stops, which usually consist of hefty steel chunks. Positioning limit switches just inside the physical travel stops, causes the motors to be turned off before any hard hits are realized, and any damage done.

There are nearly infinite ways to attach limit switches to the bed of a mill, some are good, and some create more issues. 

What I wanted to achieve was a scenario where a slight bed over-travel (due to high speed) would not damage the switches, plus minimize the number of parts to be made, reduce the need for adjustments, utilize any already existing and unused tapped holes in the bed, and lastly reduce switches contamination from cutting operations.

The first prototype entailed a microswitch at the far left of the table, and a cam attached to two previously vacated screw holes near the center of the bed.

Left microswitch biased ramp (version #1)

This worked in principle, but the hole pattern didn’t match (my fault), and this device didn’t address right switch activation. It was however useful as a platform on which to develop the second and final version.

New cam mostly done

Version #2 of the cam is bidirectional

New cam bolted in two unused holes

Enough room still exists on either side to access the gib screws

Installation of the microswitches required drilling and tapping a few holes in the right places, so I didn’t bother taking any pictures, but I put together a short clip of the making of cam #1 and #2.

X axis travel limit development

I’d like to end by adding one last thought regarding the number of limit switches needed on a 3 axes CNC mill. 

While it might initially seem obvious that 2 switches per axis are necessary, this number can actually swell to 3 if you choose to have a dedicated homing switch, or shrink to 1 if you choose not to, and set up and employ the controller’s soft limits function. 

Setting this up in Mach 3 is actually a pretty easy thing to do, you just simply tell the controller how far you want it to allow the bed to travel in a certain direction, and the controller will slow down the speed of the motors as you approach this imaginary travel limit, and stop it as you reach it. 

This is a fine strategy to avoid having to mount ½ of the hardware, and it works well the majority of times, but I’ve had occasions when I turned the soft limits off to perform certain maintenance functions, then forgot to turn them back on, only to be really surprised at the next crash event.

For this reason, I will use a total of 6 limit switches on my mill, with 3 of them doubling as home reference switches.

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