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Monday, March 17, 2014

CNC mill conversion - part 13

The end of major construction on the Z axis, not!

Now that the main static structure of the Z axis has been machined, it is time to concentrate on fabricating (there goes that word again) those parts that will deliver the stepper motor impulses to the mill’s head, and make it go up and down. 

“On the menu for tonight are a couple of flanges, with a side of pulley modified to perfection, on a bed of flat bearings and races, topped with ball-screw and ball-nut. For dessert, may I suggest two very dissimilar adapters, and a funky looking bottom plate?”

Time to put the 2.5” (6.4 cm) 2024-T3 aluminum bar to good use.

Raw material

Aluminum slug

I first turned down the shoulder, this would have to slip inside the center hole of the 2.3” (5.8 cm) ball bearing I “pocketed” last time.

Turning down the shoulder

Then, I used a series of drill bits increasing in size to create a safe passage for the ball-screw.

Adding a pass-through hole for the ball-screw

What I ended up with is the blank for the top flange.

Looking more the part

The next two operations were done on the mill, using G-code I typed in by hand to locate the four X and Y coordinates.

Drilling holes for the pulley to come

Counter-boring the bolt holes to clear the 2.3" ball bearing

Using the same G-code on the mill once again, I substituted the pulley for the flange, and drilled and tapped 4 matching holes.

Customizing the pulley I bought online

Tapping the holes that will connect to the top flange

Putting flange and pulley together, I drilled and tapped 8 smaller holes that will attach to the still-to-be-machined bottom flange.

Pulley and top flange mated. Flange drilled and tapped to meet with the bottom flange.

Moving back to the lathe, I enlarged the center hole through the pulley to match the one previously opened in the flange.

More customization

Back on the mill again, I tapped the pulley to accept the ball-nut, and drilled/tapped two set-screw holes on the side.

"Tap-zilla" at work (15/16"-16)

Set-screw holes drilled and tapped

Time to concentrate on the bottom flange now.

Raw piece sawed off the aluminum bar

Facing the slug

Drilling the attachment holes to the top flange

While I had the flange on the mill, I decided to surface it down to the proper thickness using CNC, but I accidentally took a huge 0.300” cut instead of a 0.030", and "crashed" the mill.

Monster cut!

Well, that’s all part of the long and steep CNC learning curve. After modifying the code, I finished the surfacing operation.

Weird but smooth finish

In retrospect, I should have done this operation (and the next one as well) on the lathe, but I was itching to test the CNC abilities of the mill. It all worked out well in the end, but the surface finish could have turned out better.

One design change that I made, was to machine the flanges out of aluminum instead of steel. I did this for two reasons, 1) I already had the aluminum in hand, and 2): steel is more difficult to machine with my small machines.

Since I still needed to have steel surfaces in contact with the needle bearing for wear resistance, I bought two steel races to sandwich the bearing with. Unfortunately, I had already cut the top flange to the original length, and I did not want to start back at square one, so I machined a recess in the bottom flange for one of the steel races to fit in.

I did so by using a Wizard in the Controller software (Mach3), but I tested the code first on foam, then on wood, to "massage" the code before I cut the actual part.

Testing the code on soft foam

More testing on wood after code adjustments

Satisfied with the results, I gave it a go on the flange.

Recess machined on upside down bottom flange

Steel race fitted in recess

Next came another trip to the lathe, in order to reduce the diameter of the bottom flange.

Reducing the diameter of the bottom flange

That's weird, and cool!

Finally, the time came to test fit the assembly.

Parts count

Ready to close

All parts took their rightful place without issues.

The next problem I encountered, was a slight rubbing of the belt on the motor stand-offs.

I didn't see this one coming!

To correct for this situation, I turned the front two stand-offs down on the lathe.

Front stand-offs reworked on the lathe

No rubbing here!

All clear here!

My love/hate relation with the ball-screw was briefly put to the test again, having  to face/drill/tap one end of the hardened steel screw. Because this was to be the last time for this project, I suffered silently as the hard ball-screw tried to destroy my mini-lathe.

Drilling hardened steel is not for the weak.

But once through to the softer steel on the inside, all is well again.

Like a dentist fixing a cavity.

The plate that attaches to the head of the mill, and to the ball-screw, was nothing special.

Surfacing, boring...

Drilling and counter-boring, boring...

Counter-boring and drilling, boring...

VoilĂ , our transmission is ready.

All the components of the Z axis could finally be assembled for the first time.

Z axis modification completed, and ready to be mounted on the mill.

Bottom view of the lengthy mod

Unfortunately, as fine as this assembly appears to be, there was a serious problem afoot, more like a fatal flaw, waiting to threaten everything I had been working on until now.

Once I finally tested the assembly on the bench, I was horrified to see that, while the ball-screw did in-fact move up and down, it did so in a very wild, almost violent way. 

The ball-screw behaved like an alligator in the midst of a death-roll, and I was too shocked and embarrassed to record it on video.

Well, let’s leave it at that for today. 

Next time I will go trough the steps I took to understand, and correct this problem. It was definitely not a quick and easy fix, but I eventually worked the problem out of my Z axis. 

Stay tuned...

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