Saturday, February 15, 2014

The Z-setter

Along with all of the fun metal working toys I have I also have a CNC Router that I built from scratch.  Last fall (15 months or so ago) I managed to make a stupid mistake and broke it.  I destroyed part of the Z axis, burned out the router and managed to do something to the electronics.  In the end it turned out that the issue was a broken wire in one of the motor connections.

I fixed the physically broken parts and then realized that the electronics weren't working.  Two weeks ago the electronics on the CNC made it back to the top of my list and I purchases a G540 4 axis controller and break out box.  I bought it from  A damn good company.

As I was going through their website I saw a Z axis position setter for only $90.

So what does a Z axis setter do?  Well when you are using CNC you need to know where your material is.  Normally X and Y axis aren't critical because you just want to make sure you are cutting inside the material.  If it is critical then you can do standard edge finding but it doesn't happen very often. 

The more critical position is the Z axis.  If you miss the surface by a few 1/1000s you could end up cutting to deeply or you could cut to high and miss features.  Worse, it is often the case that you will change tools in the middle of working a complete CNC program.  If the tool you put in is not have exactly the same stick out as the last tool or you don't know exactly how much it sticks out your 2nd (or third passes) could cause problems.

If you are doing CNC work you often do a roughing pass.  As long as you don't turn off the machine your X and Y axis will stay exactly the same.  When you change tools for the finishing pass unless you get the tip of the tool exactly right then the roughing and finishing passes might not match up.  Leaving a poor result.

There are lots of ways to set the Z axis.  You can use the paper squeeze where you put a paper under the bit, lower it till it is "stuck" then slowly raise it till it is barely free.  You know know your bit is exactly the thickness of the paper above the surface.  You can use a dowel pin that is exactly 0.500 thick.  You lower the tool until you can no longer roll the pin under the tool, then you raise the bit until you can just barely roll the pin under the tool.  Then you know the tool is exactly 0.500 inches above the surface.

There are other methods but they all are slow and have the potential of error.

In the best of all worlds you would want to change the tool then just press "figure it out" and have it "work."

That's where a Z setter comes into play.  A Z setter acts like a Normally Open (NO) switch.  There is a sensor plate which is on one side of the circuit.  The tool is attached to the other side of the circuit.  When the tool touches the sensor plate the circuit closes and the computer senses that the tool is exactly sensor plate away from the surface of the work.

You could just use a flat plate but that plate has no give to it, so when the tool touches down it puts lots of stress on the CNC system unless the tool is moving very slowly.  I originally tried a piece of printed circuit board but it was too light and flexed upwards causing still other problems.

This Z setter is designed to protect the tool as well as be heavy enough so that it doesn't flex or move when placed under the tool.

The design is a 2in diameter cup with a spring loaded plunger.  The entire cup is one side of the circuit.  When it is time to set the Z axis the setter is placed under the tool and then a CNC program is run.  The program does a rapid lower.  It doesn't matter if the tool bit hits "hard" as the AL is softer then the tool bit and the plunger will give.  Once the program detects that it has touched the plunger is backs out rapidly until the tool is no longer in contact with the plunger.  At that point the program lowers the tool at a much lower rate of speed but it only has to go a short distance so this doesn't take long.  When the tool touches down we know exactly where the tool is relative to some surface.  (top of the work or the bottom board or something else.

I plan to make a second z-setter.  This one will be placed in a fixed position on the router table.  This will give us an absolute Z location.

To start using the program we would use the movable Z setter to determine a point above work surface.  (We don't have to use the movable Z setter for the bottom of the work because that can be determined from the absolute location from the fixed Z setter)

With two Z locations, one absolute and one relative we can calculate an offset and remember that offset.  When we change tools we can recalculate the offset from the absolute Z and not have to measure to the work surface again.

Enough about what a Z setter is, let's get into making it.

I started with a 2.250 round of AL that was 12 inches long
I set this up in the 3 jaw chuck and then added a steady rest way out at the end.  My son Michael then faced it off and turned a section about 0.375 down to 1.500 +0.000 -0.005.  This was parted off to create the plunger/plug.  When parted off the part ended up about 0.250 thick with a very smooth top space.  I was pretty happy as we hit 1.499 for the diameter.

Next the work piece was faced and a section a little longer than 1.000 was turned down to 2.000 +/- 0.005.  I hit this at 2.000 which was very nice.  Now that I had the diameter of the cup and retaining ring established it was time to start boring.  I started with a center drill  then I drilled with my largest drill 0.500.  I drilled 0.750+ a little more deep.  I do have a 5/8 and 3/4 in drills but they are MT2 and the tail stock is an MT3.  I've not yet gotten the adapter sleeve.

After drilling I then set the micrometer stop and started boring.  The goal was 1.375 +/- 0.20 diameter.    This is the interior diameter of the retaining ring.  Once the bore reached diameter a slice 0.063 was parted off.  Since one side was faced it was "perfectly" smooth. This face will be against the top of the cup and plunger which will bring the plunger into alignment with the top of the cup.
Next the cup was again faced and the boring was continued.  The goal was 1.500 +0.005 -0.000.  When I hit the 1.500 mark I test fitted the plunger and decided it was a little to tight so I took another 0.001 off the diameter.  The final size of the bore was 1.504 which gave a loose slide fit to the plunger.

At this point it was time to part off the cup.  The cup was parted off at 0.800 to leave some room for facing and finishing to 0.750.  I had to make some soft jaws, actually brass shim stock to cover the jaws of the chuck.  I seated the piece against a pair of parallels then faced of the base.  Once the base was faced off I flipped the piece over, touched off, measured and cut to 0.750 +/- 0.000  I was able to hit this to the limits of my ability to measure (0.0001 on my micrometer)
Now that the major pieces are shaped it is time to drill and tap some holes. I wanted four screws in a standard bolt hole pattern 0.125 from the edge.  After doing a little research I decided on #6-32 as my screw size.  A #6-32 hole is produced by a #36 drill bit.

The fun comes from the fact that I don't have a fully functional DRO.  I only have Y axis capabilities on my DRO.  This of course complicates just about everything. 

To place my 4 holes in the bolt circle the right way to do it is to locate the center of the circle by sweeping the outside edge.  When this is done you keep track of which way the table was being moved when you found center.

From that point on you can move to the correct X,Y coordinates based on your dial settings as long as you are moving in the same direction as when you found center.

I was able to cheat a little bit.  Because I have one axis working with my DRO I'm able to locate the center value of Y.  At that point I move to just outside the cup in the X axis, move towards the cup until my edge finder twitches.  From there it is only a mater of turning the dials .225 in the same direction to end up where I need to go.  The extra 0.100 is to adjust for the size of my edge finder.

This gets me the "left" and "right" holes.  From there I just need to move in to 1.100 to be at the exact center of the cup.  A quick down stroke with the spindle and I can see that it is lined up exactly with the center of the cup as indicated by the drilled hole.

Using my Y axis DRO I'm quickly able to move to the correct locations for each hole and drill them.

In order to start tapping each hole vertically I'm actually doing three tool changes for each hole.  I start with the edge finder, touch off, move to final location, change to the drill bit, touch the top of the cup, set the stop to 5/8", drill to depth.  Switch to plug tap, take the mill out of gear, twist the drill chuck by hand to slowly tap the hole part of the way. 

I didn't go as deep as I could have because I was afraid I would not be able to feel when the tap hit bottom.  I'll have to test sometime when I'm not worried about destroying 4+ hours of love labor.

Once the four holes were drilled and the tapping started I took the cup out of the vise and started to hand tap the holes the rest of the way.   At which point I started cussing Grizzly.  The tap set I bought from them has taps that start at #4 and go up, but the tap wrench doesn't actually HOLD a #6 tap!

I ended up using the larger wrench and driving the square at the end of the tap.  This worked fine and I was very careful to feel when I hit bottom.  I had picked up a taper tap and a bottom tap as well so I finished out with the bottom tap.

Next I put the cup back in the vise on its side.  I used the edge finder to locate one side of the cup and then move to the center of the cup.   A quick edge find off the bottom and I moved 0.250 up from the bottom to drill and tap the edge hole.

This all seemed to work exactly as expected.

Now it was time to drill the clearance holes in the retaining ring/disk.  I carefully placed the ring in the vise, found the right place to drill, used the 9/64th drill with parallels holding up the sides.  The drill started cutting in with no problems.  A little tap magic did wonders.

Then as I went a little further the drill seemed to "stick" and the disk suddenly flexed and bent.  I repositioned the parallels for better support and finished drilling my holes.  Boy were they ugly.  The forth hole was the worse.  I barely started to drill when suddenly the ring popped out of the vise. 

I checked and there was a good mark so I went over to the drill press.  There I saw the drill cutting just fine and then again it seemed to stick.  I finished the hole with a little more pressure then cleaned up the ugliest holes I've drilled.

When I went back to get the drill bit I finally looked at the bit.  The cutting edges fine, but about 1/4 inch back from the cutting edge the bit suddenly got "larger" as if there was a second very bad cutting edge.  The bit is no in the garbage.

The end result was one hole that was misplaced so that I can't get a screw into it.  I'll go back and open all four holes up a little bit more (and clean them up).  This will allow all the holes to align.

Here is the Z-setter being put together:
The spring under the plunger might be a little two strong and a little to small but both can be easily adjusted later.

The retaining ring is then placed on top:

And here is the letherman screw driver acting like the cutting tool pressing down the plunger(sorry for the bad focus):
The South Bend 13"x5' that I used to turn the parts.  I'm hoping that someday it will be warm enough to enjoy cleaning it a bit more.  These chips are almost all from this project.:
Thats a DoubleBoost light pointing down at the three jaw and my Walmart special off to the back.

Here is the Bridgeport used for drilling the bolt hole circle:
Any rust you see on any of these machines came with the machines.  I've been cleaning and fixing as fast and as much as I can.  This vise was frozen when I got it.  I've cleaned it and it is now mostly in good shape.  The fixed face and bottom of the vise are in very good condition with the exception of a couple of nicks on the fixed face.

A special thanks to Marc L'Ecuyer for his rapid response to tell me how to do x,y bolt hole circles right and for some great teaching videos.

Doubleboost for his fun videos and methods.

Tom's Techniques for more great learning videos.

Oxtool, Abom79 and Kieth Fiener for production machinist videos.

And of course, Mr Pete (tubalcain) for the videos that got me started