Saturday, June 4, 2016

The Long Story of Z

When we last talked about the Z axis for my G0704 CNC Conversion, I had just completed boring the big hole to clear the ballnut housing.  Then I got a virus, took a week off to visit the granddaughter, and, well, next thing you know, it was June.

As I said in that piece, the next task was to drill and tap six 1/4-20 holes to match the ballnut. I barfed it up. The holes were far enough off that I could get only one screw in the ballnut at a time.
Now what? I've never relocated holes before, but it really is a fundamental skill we should all have. I know in commercial shops they may drill it out larger and put in a Heli-Coil, or use a welder to fill it, but I don't have a TIG welder and wouldn't know how to use it if I could borrow one.  So acting on the advice of a frequent visitor here, I got a quarter-inch diameter aluminum rod, turned it down enough to thread and made 1/4-20 aluminum screws to plug the holes with.  Used red Loctite to hold them in place, sawed them off with a slitting saw on my Sherline, then milled them just a little proud of the surface (like .002" high) with the G0704. This picture is with it still on the Sherline, before I milled them down.
I think my error was in setting my zero on the Sherline before I drilled, because after flipping the part, finding zeroes again, and putting a small peck at each hole with a center drill, the marks all appeared to be in the right place.

My adviser also suggested that since I have the master pattern (the ballnut that it has to mount), why not make a tool to fit in the holes that will better center a drill and mark the mount that way? So I turned down some 1/2" aluminum rod to fit the holes in the ballnut, then drilled them on the lathe for a 1/16" drill bit. I made four and put them in the four corner screw holes in the ballnut, and clamped the ballnut to the mount with C clamps. Drilled the holes with a 1/16 bit. Broke the first bit and while muttering obscenities and looking for the next size up to continue, found that "someone" had bought and stashed three 1/16 bits. No more excitement because I remembered to use just a tiny dab of tapping fluid. Once the four were drilled, I moved two of my tools to the center holes, re-clamped, and drilled them.

Moved it to the G0704 and mounted my drill chuck. Carefully centered the bit six times, using a 7/64 bit, then six more times using the bit that came in my old Craftsman drill and tap set for 1/4-20. While drilling the larger holes, I could feel that sometimes the drill advanced way too easily. Turned out that two of the threaded aluminum filler screws had backed out. So I put them back with more Locktite. The result: perfect fit.
The little tools are in the lower right.  These have a half inch head to grab with a turned down section that fits in the ballnut with a tight fit, and are center-drilled 1/16".

This part also has two holes in its base, perpendicular to these screws.  Those are drilled and tapped 5/16-18.  I won't be able to tell how well they're done until I have the mill apart to modify. 

So that's where I am now.  X, Y and Z ballnut mounts are complete.  I'm down to four parts to make: the X and Y motor mounts, the Y axis spacer and the X-axis end cap. The end cap has a lot of machining that needs to be done, and I think I'll do that last. I spent a lot of time making this part, though, so I hope I pick up some speed.  My first mention of it was May 1.  Even subtracting the week vacation, nine or ten days out of the shop, I spent close to four weeks working on this part.  I don't think any machine shop would hire me with time like that.  Not that I care.



  1. You're having fun, and learning new skills as you go. That, to me, makes it worthwhile to do what you're doing.

    In my prime wrench-turning days, I could have had the EGR system on my Supra taken apart, cleaned out, and reassembled in maybe 10 hours.

    It took me over three weeks!

    Keep up the good work!

  2. Impressive! Keep at it, you have a very nice hobby there. And don't forget ... the learning curve. First time through is the longest especially if you don't have all the right tools.

    1. For sure about the learning curve. This Z-axis piece has been one massive learning curve. Never used a boring head, and never cut chamfers like those big ones. I'm pretty sure it's the biggest single piece of metal I've worked on.