MIcroMill CNC: Z Axis

A project in process

(aren't they all?)

Since I don't need another project, I decided to immediately start one.  I wanted (and actually will have a use for) the MIcromill when it's CNC'd.  So what I decided to do was read that entire part of the  Yahoo  Micromill CNC archives that I had,  I got a part number of the gas strut, and a part number for the bearing assemblies.  Time to do it all.  I also ordered the bearing blocks for the X and Y axis so I could replace them when I modified them.
This is part one of a 3 or so part series, covering the Z axis, then X and Y.  It will be posted in pieces as I start to work on it. 
The first thing I needed to do was to install the gas strut for the Z axis.  The part was gotten from Mc-Master Carr and is a 25 pound thrust extension with xxxx travel and an M8 thread on each end.  Time to make your own mount here.  I was rather surprised at the size of the strut, but as it turns out, it's the extension that determines the size.  Next, it's up to me to make sense of it.
First, I find out that the base is not square to the bottom of the column.  That makes it imperative that I mount the base of the spring differently.  I can't use the little red base.

 

This gives you an idea of how high that base will need to be, and how long the strut is, in relationship to the whole micromill.  The strut is at full extension.
I will mount the bottom of the strut on a simple aluminum plate, 1/4 inch thick.  I've drilled holes and will mount the plate under the column bolts.
The plate as mounted.  I decided to mount the strut so that the base is out a bit, and clears the column by a bit. 
I've actually mounted the strut on the plate here.  Closeup is next. 
The placement is rather arbitrary, but seems to work so far.  I could have picked the back, but the spring would have gotten in the way of the screw.  Mounting it on the front would have put it in the way of the work, and on the left side would have put it in the way of the gib adjustments.

 

I decided to use a piece of thick angle bracket.  There are several reasons.  One is that I had it.  Second is that the additional metal will give me strength in two dimensions, both at right angles.  The third is that it was just long enough as is.  Mounting both the spring and the plate, I have the headstock Z axis clamp removed and I will be mounting the support angle on the side.  I'll be using a 1/4-20 thread cap head screw.  No countersinking needed and the allen heads are hard to strip.
This is how to tap.  You use a tapping adaptor which is spring loaded, which fits in the end of the hand tap.  If you don't move the table in X or Y, you're automatically aligned.  Very worth while addition.
Details of the bottom of the strut support.  You can't remove the angle with the strut attached, but that's not a big deal.  I had designs on that aluminum column for motor mounts, but I think I won't go there.

 

I've put a scrap piece of 1/2 inch aluminum at the top of the column.  With maximum extension, it's about 3/8 of an inch inside the maximum extension of the spring, so that the spring will never hit the stop.  I will be doing metal finishing later.  This is just for fit, not finish.
The side view.  The screws are 10-32 because I had them.  They're about 1 inch long or so, and the strut is tapped between them.  I could put one on the 90 degree part, but decided against it for the moment.  Be sure to round the inside corner, since the inside corner in aluminum extrusion is rounded, not square.
The column partway down.  If you do this and want manual operation, pick another way, since the hand wheel will get in the way of the angle.  It'll also be awkward to use.
This is the lowest extension of the head, which is not bad.  However, if you get the chance, you might want to mount the block differently.  This is not absolutely the best way to mount that spring, but it will work.

The bottom of the Z axis screw has a washer that rubs against the bearing block, the top has another washer.  Lots of friction.

 

This is the lowest extent of the head.  That's about a 1 inch parallel for an example.  This low extent is fine.  The quill can always be lowered if needed.  The maximum extent is the original extent of the head, so no big problem there. 

I had thought that the gas spring was way too big, but in fact, it was not.

 

Top view at the top extension.  The head will not go up any further.  You can see the clearance between the strut and the head.

I decided to try to build another Z bearing block.  I was going to use the standard skateboard bearings.  Those were cheap, so all I'd be out would be a small piece of aluminum. 

First thing is that the old bearing block is used as a pattern.  A transfer punch is used to match the screw location.  It is not symmetrical in the piece of aluminum I used.  I'm using the tap guide as a spud.
I drilled a 1/2 inch hole completely though.  This clears the center raceway of the bearing. 

I first made cuts and then did not quite realize that I was going to cut into the vise to make the hole for the bearing.  Either use a larger piece of aluminum (.75 is not enough) or use two pieces of sacrificial plate to start with.

I bored this about 1.25 inches deep.  It really had to be that deep.

This is the boring head tool set for the right size.  You sneak up on it with about 0.040 or so cuts. 

 

This is the first part done.  There are several things here.  One is that the bearings extend into the column.  The second is that I did not properly align the top bearing and bottom bearing, so that the shaft, which is about 8mm, did not fit properly.  I ended up turning down the shaft, which was not needed, and should not really be done.  I did wobble the top hole, which allows the bearing to move a little.  Next time, I will be more careful. 

The screw length requires that the two bearings be this close.  I want to use the existing screw thread to preload the bearings.

Roughly, this is where you want it.  The bottom of the bearing is exactly where the old bearing block was.  The right most hole is easy, since there's a lot of space there.  The left most hole is tricky, since it has to clear all the bearings and not hit the side of the column.
This is the bearing block before holes were drilled to mount it.
A side view.  The thing that irritates me is that the bearings are a very nice fit, with very little slop.  Too bad that one of them is wrong.
You can see that I turned down the shaft a bit.  Don't do this, wobble the hole or remake the bearing block if you must.  The existing washer is left on the screw.

Behind the screw is the U shaped slot that i cut to clear the bearings.  I used a flat mill, then use a ball mill to complete the job.  I ended up wobbling it a bit, making parallel passes down the slot.  The middle was made a tiny bit deeper.

Note also that the right most hole is new, and the left most hole is almost in the existing hole.  If I have to, I can always bore it out and put a nut in place to hold the old bearing block.

A good view of the need for the bearing slot.

 

A back view  You can see how the mounting hole on the left is between the bearings, and just about as far left as you can get.  Any further left and you are in the column wall.
Bottom view, bored, you can see how the hole just misses the outer diameter of the bearing.
The top bearing assembly, with the hole widened a bit.  The bottom is the only part maintaining radial alignment, and the top is acting as a thrust bearing only.
I put three bearings in to even out the spacing.  A washer and knob nut is sufficient.  This ought to align the bearing axis to the shaft.  The right most screw is loose,  and the left most is not drilled and tapped yet.  Use a transfer punch.  You might be able to see an alignment line that marks the edge of the bearings to the edge of the screw hole.
Removing the extra bearings gives a better idea of what's up.  Note that I have bought a new screw to replace the one that I messed up.
Now to make the adaptor.  I decided to use a belt drive, since I had one (all electronics).  The adaptor will screw down on the existing shaft, preloading the bearings.  The thread is 6 mm 1.0.  I want the bore to clear the shaft, and the top of the adaptor has to be machined to match the existing plastic pulley.

This is the plan I worked to.

I have the thing internally threaded, the top cut to length, and I've left the 3/4 inch rod at the normal thickness, but taken off the surface oxide layer.  I've made all the left hand direction cuts, and will change lathe tools to even up the shoulder.  I need to remove the metal up to the right hand edge of the middle oxide layer.
Cut, more or less polished, and ready to cut off. 
Cut off with the bandsaw, I will turn the little piece off. 
I've already threaded the middle for a 1/4-20 screw.  That will hold the pulley on with a washer.  However, the gear has a D slot with a keyway.  It's molded plastic, and does not have enough to profitably make a setscrew.  However, I can make this a splined shaft.
A milled slot will cut half of the keyway.
Duplicating the same cut on the other side of the keyway gives me the D/keyway combination.  It turned out to be a trifle too large, so I put it back on the lathe and just turned down a thousandth or so.  Once it fit, there it was.
The pulley as a side view.  This is an XL style pulley that I bought as a set from All Electronics.
The adaptor will screw down onto the shaft and bearings, preloading them.  You then tighten the setscrew and that keeps the adaptor locked to the shaft.  Tapped the same way as before.
This is the Z axis bearing assembly without the pulley.  Note that the bearing block is firmly bolted to the column, and extends beyond the column itself.  I might trim it to the right.
Pulley mounted on the top.  The screw heads will get in the way, so I have to countersink one of them a bit, or get a smaller screw head, perhaps pan head or flathead if needed.
I know that I will have to have a flat on the stepper motor shaft.  You do this because the setscrew raises a small ridge on the shaft.  If you make the shaft a pretty much exact fit, then the adaptor will jam on the shaft.  Putting the flat on the shaft moves the dimple out of the way and the adaptor does not freeze on the shaft.  A single pass with an end mill is enough.  The depth is about 0.020 at the max.
I've turned the adaptor from 0.75 aluminum.  I bored through by 8mm, which is the diameter of the shaft on the pulley above.  I cut the shaft short, and removed the bearing.  Then I reamed out the adaptor to accommodate the stepper shaft.

The pulley shaft is straight knurled, so it was either bore out the pulley, or turn down the stepper shaft, or knurl the shaft, or try to make the pulley a setscrew.  I could also have made an adaptor that cut out the center and just had a bore in it.  Things you think of after the fact.

Because of the various shaft lengths, this is where the motor goes.  I could have bolted the bottom of the motor to the bearing bracket, but that seemed inelegant somehow.  Again, adaptors and a bit of different thinking could have resulted in a design that is lower to the motor and the bearings.
I bored out the motor mounting holes to 1/4 inch for this.  Since the machined part of the motor plate is the part that is perpendicular to the shaft, that is the important part.
A piece of angle bracket on the side is sufficient to mount the motor.  I put in a few washers to space the bracket from the motor end.  I also had to notch the bracket to clear the adaptor.  You'll see that later.
Right now, a single screw is holding this in place.  In case you wondered, I had to countersink the one cap head screw that this bracket is covering up.
I'm testing stuff here.  The bracket is a bit tilted, and that means that I will (of course) need that extra screw to mount the bracket properly.

The little blue box is the stepper motor tester.  Since it's unipolar only, and the motor is a 6 lead motor, I can use it to test this out.  Since a bipolar driver will be better, not at low end torque, but at higher speeds, I can see how the design works.  As is, the tester does 1 and 10 steps/second.  The power supply is giving me 12 volts for the tester, and the stepper is being run at 4.6 volts or so and 1.55 amps (if the meter manages to follow it) for two coils. 

The motor is a bit under driven, but is a good test. 

Here's the second screw holding the bracket in place.  You can see the notch for the adaptor as well.
Looking at the side, you can see the washers, notch for the shaft, shaft flattening, and details of all the brackets.
As mounted, and pretty much as done, but without the connectors and the home and limit sensors.  They're next.
First, the system is running at 171 halfsteps/second.  The ratio of the belt drive is 25:41 (reduction).  The leadscrew is 20 TPI, and the stepper is 200 steps/rev. 

So we're getting  170/200 rev/sec which is .855 RPS which is 51.3 RPM (way slow), and that's 31.3 RPM delivered to the shaft. 

That works out to 1.56 IPM. 

So that means that I'll have to get up to about 1700 PPS drive half step to get about 15 IPM, which is not too bad.

 

I took a 1 inch by 1/4 inch bar and mounted it parallel to the column.  It will be spaced out with small spacers from the column.  More on that in a bit. 
A little down the path.  The bar has the three sensors mounted on it.  The upper one is the limit switch, the bottom one here is the home switch.  You can see that they are slightly adjustable. 

The 4-40 tap broke in the headstock mounting block.  I thought that was really special. 

So I moved the brass bar over a bit, and tapped for 6-32.

The sensors are not wired yet.

A side view of the senor and the interrupter.  The left/right (towards the column, away from the column) can be controlled by adjusting the number and thickness of the spacers.  The entire bar can be removed with two screws.  This was one of the better ideas. 
The head at approximately the upper limit (actually, about zero on the up/down scale. 

 

I actually expect it to trigger a bit lower than this.

   
   
   
   
Analysis:

So far, it works, without much of a problem.  I will have to see how it works with better (or different) drivers. 

Things not to do: 

1) do not machine the shaft at all.  I think I will eventually replace that shaft.  I may or may not remake the bearing block, but it doesn't really need to be replaced now.  Main problem is that the pulley assembly and the shaft will wobble.  I may have bent the shaft a bit when I was working with it, or it may have just have been bent naturally.  I also had a loose fit on the top bearing, and that was within the bearing assembly itself, shaft to bearing hub.  This allowed slop. 

2) It's possible to move the strut mounting up a bit, that will work.  Another option is to move it down a trifle.  The main problem is that one of the screws is in the way of either the Z axis handle or the drive.

3) Probably better to make the belt drive lower and closer to the motor body and to the bearing block. 

 

Plusses:

1) I like the way the sensors are mounted.

2) the belt drive (All Electronics part number CGB-33).  The gears need to be mounted differently, but are useful as is.  The adaptors are relatively simple lathe work.  The ratio is 41 teeth on the large gear, and 25 teeth on the smaller one. 

3) I had enough sensors for the entire mill (I think).  All Electronics does not have any more of them.  They can be replaced by something generic.

 

 

AFTERWORD:

I did a few more things to clean up the implementation, but not the design.  The first thing I did was to order another z axis screw from Little Machine Shop.  Since this did not have the unthreaded part narrowed down, it would fit the bearings much better.  However, I found at least one  thing that had to be done.  Because the shaft had been cut down in the original design, the upper bearing was off center, and could be assembled that way.  This was not a Good Thing . 

I have a centering indicator, so I put the bearing block back in the vise.  I added the little side plates as best as I could, and then  left the bottom bearing in.  I indicated from that bottom bearing's inner race the diameter.  That centered the shaft to the bottom bearing, the inside of which had to clear the 1/2 inch hole that I had bored.  This way, the concentricity of the half inch hole didn't matter as long as it cleared the shaft.  With the boring head set to the proper outer diameter for the bearing, I was able to make a pass down the block, and clean up the hole.  It was probably about .010 or so off. 

An 8 mm rod would run down the two bearings without a problem, so the two bearings were concentric.

I found that the washers that I had used were suitable for 1/4 inch stuff, not 8 mm stuff (the shaft diameter).  Adding a few 8 mm ID washers made up for the (slight, if at all) manufacturing tolerance, and the small amount that I had probably removed from the inner thickness of the bearing block.  Note to self: put locktite on the outer race of the bearing to help hold it in, but probably not needed.

With this addition, the bearing block was reattached, and the very slightly modified leadscrew was put in.  You'd think I hadn't learned anything, but I did modify the leadscrew a bit.  What I did was to drill and tap the end for a 1/4 - 20 thread, and put in a cap head screw.  Since the screw head diameter was bigger than the thread itself, I turned it down.  This is to give me a way of grabbing onto the end of the screw without messing up the threads.  Locktite can be used if needed.  When you assemble the top part, it screws down on the bearings and preloads them.  How well that will work is another matter.  However, they are cheap bearings.  If I absolutely have to, I can put in thrust bearings and either remake the block or just add spacers as needed.

Overall impression is that there's much less wobble, and the screw did not move from side to side.  It seems to be easier (by hand feel, nothing else) to turn the Z axis screw.

Overall last comment so far:  Do Not Turn Down The Lead Screw.  Second comment, you really have to watch the concentricity in the two bearings.  I'll remember that on the X and Y axis.

 

On to the X and Y axis.