RatRig Stronghold Squaring end stops

Introduction

With two of these you can create hard mechanical reference points to reliably square up both Y-axis

Extra hardware:

• 2 * DIN 603 M6x45 carriage bolt

• 2 * M6 nuts with nylon locking ring

• 4 * any type of M4x20 bolt

Note: These are not designed to be sufficiently strong to work as actual hard mechanical stops during the homing. It relies on having sufficient back-travel prior to homing (GRBL parameter $27). I used 3 mm for this. During the calibration process I did accidentally run in to them without breaking, so if that's what you want, you could try but no guarantee it will not break.

How it works

1. If not already, home the machine

2. Power off the CNC (cut the power to the stepper motors)

3. Twist the Y axis spindles counter clockwise until both axis firmly hit the end stop

4. Power on the CNC

5. Home the machine

Calibration procedure

Obviously these end-stops don't magically make the Y-axis square w.r.t. the X-axis, this chapter goes into how I went about the calibration

Calibration reference

Unfortunately I don't know any way to do this without having a sizable reference. This chapter describes how I made my reference on the table saw using the 3-cut method (a condensed 5-cut method that is more material efficient). If possible use a solid material for this (solid wood / lumber-core / plastic). Ply wood can work (that is what I did) but requires more care in the next fase to stay on the "length wise grain". Definitely DO NOT USE MDF or particle board  

1. Cut a straight reference edge

2. Flip the board 90 degree and cut a first "square" edge

3. Flip the board over end (keeping the reference edge against the fence)

4. Cut a second "square" edge

5. Verify that the length of the reference edge and its apposing edge are the same length (to within the tolerance you want

Note: There can still be a little deviation (lets say < 200µm) on the length verification, will be able to check that in the next chapter.

Now we need to put the board parallel to the X-axis on the CNC-bed 

1. If you don't already have a reference edge make one.

   1. Put a thin strip (about 5 cm wide) on your CNC-bed, parallel to the X-axis and screw it down about every 30 - 40 cm 

   2. Plain mill it (via jogging is fine)

2. Put the board on the CNC with it's reference edge against the CNC-bed reference

3. Make the "opposing edge" parallel to the reference by milling it (via jogging is fine)

The actual calibration

1. Mount the two end stops on the machine

2. Dial in the "homing" Y-axis bolt so that it doesn't touch the motor plate

   

3.  Mount a dial gauge to the Z-axis 

   

   4. Dial in the "non-homing" Y-axis so that the bolt is already as close that what you think is going to be square

Repeat the following until you get a satisfactory result:

1. Do the actual "squaring" process from the "How it works" chapter

2. Go to the front of the machine (I have a program on my controller for this named #Y1, see appendix)

3. Jog the machine to the reference edge (Once there save it as the machining origin, to speed up the next cycles)

4. Run the dial gauge along the "square edge" of the board

5. Adjust the "non-homing" Y-axis bolt (see appendix)

Confirm calibration

Now you'll want to flip the board over end, so that the opposing / milled edge is against the bed's reference. Again run the dial over the "square" edge. Unless your reference board is perfect, what I doubt it will be, you will still see a deviation. If you divide that number by 2, that is to what tolerance you've squared up your machine over the length of the gauging distance.

For me this was 150 µm -> 75 µm square over 600 mm. This is sufficiently fine for whatever I am doing. 

If what you've accomplished doesn't meet your standard, you can use the knowledge that you know have about how "non-square" your reference is to continue the calibration process

Appendix

#Y1 program

This is for my machine configuration, adjust as needed and keep your hand near the E-break when testing it out.

G21 ; Ensure we are in Metric (mm)
G90 ; Absolute distance mode
G53 G0 Z0 ; Lift Z first to a safe machine height
G53 G0 X-696.95 Y-1277.44 ; Move to the target machine coordinates

"Non-homing" Y-axis bolt adjustment

It is not my goal do document this for all types of machine configuration. But this describes how I had to adjust the "non-homing" bolt on my machine. On my machine the "Non-homing" Y-axis is the right one.

If the dial gauge is moved from left to right, and the dial gauge is pushed in, turn the bolt out (counter clock-wise). 

Depending on the size of your "square edge" calculate the number of turns on an M6 bolt
(Distance_between_end_stops / gauged_travel) * (Gauged_delta / bolt_pitch). 

ex: If you have a 600 mm gauged_travel and measured a 250 µm delta. Having a 750 mm machine -> 1045 between end-stops and using an M6 bolt (1 mm pitch)
 (1045 / 600) *  ( 0.25 / 1 ) -> 0.5 turn