Accurate Z-axis (Quill) Work on the Mill

[BobWarfield]

I'm working on a Turner's Cube, and being able to determine Z-axis precisely is imperative. In addition, I am still having a hard time achieving the level of accuracy I routinely get on my lathe over on my mill. On the lathe, getting it accurate to a thousandth is never a problem, and even if I am not paying much attention, things come within 3 or 4 thousandths. I haven't spent much time fiddling with tenths yet, but someday I will. On the mill, I seem to be doing good if I hit the high single digit thousandths, say accurate to 7 or 8 thousandths. I know its my technique, and its just not good enough!

I try to allocate time to experimentation aside from projects to improve my technique and learning, so tonight I resolved to experiment with z-axis accuracy, and along the way, dug out a purchase I had almost forgotten to test for the purpose. In the end, I tried four different methods. In each case I used my granite surface plate and digital height gage to determine how closely I had come. I thought I'd share these results in hopes someone else might learn something from them.



I find I use the granite surface plate and height gage so often I would hate to live without them!

To use the height gage, I zero it on the 1-2-3 block and then add 1" to the reading off the top of my aluminum cube. On the mill, I used each of 4 methods to find the top of the block, zeroed my quill DRO, dialed in a desired depth of cut with the fine quill adjustment, made the cut under power feed, and then checked how close I came to the expected result. Incidentally, the process of finding the top of the workpiece (or side on a lathe) is called a "touch off."

Here are the results of each z-axis setting method:

Touch off by feel: For my 1st Method, with the spindle stopped, clunk down the cutter onto the top of the workpiece. Zero the DRO and go from there. This produced a result with an error of 0.012". Not very good! The error was relatively repeatable, so one could add the fudge factor. In the end of the day, the cut was 0.012" deeper than desired.

Touch off by sound: For my 2nd try, I was gently lower the spindle under power and listen for when the cutter started to cut. This method proved slightly more accuate, and resulted in 0.0085" too deep a cut. Still not very good.

Touch off with paper: The traditional old school method involves holding a piece of cigarette paper (rumored to be exactly 0.001" thick) on the workpiece and gradually lowering the cutter until it starts to catch the paper. Add another 0.001" and you are at zero! Not having any cigarette papers, I used standard laser printer paper. I cut a 1/2" wide strip so I could hold onto one end from a safe distance, and waited for the cutter to grab. In my case, I got a grab at 0.010", not 0.001", but at least it was a nice round number and pretty repeatable. This seems really thick, so either I need different paper or maybe my facemill is just blowing so much air I need to consider that this thickness can change by cutter.

Z-axis Presetter: At this point, I thought I was done and would be using the good old paper trick. But I had a vague recollection that finally came back to me. Did you ever by a gadget that looked like a good idea, but before it ever arrived you started doubting it would work well, stuck it up on a shelf, and never actually tried it? I do that way too often.

In this case, thing I remembered was a Z-axis Presetter I bought off eBay around 8 months ago. Search for "Z Axis Presetter" on eBay, and you'll find the ubiquitous 800watt listed Chinese machine tool gadget. They look like this:



Yep, I bought it from the infamous eBay seller 800watt. I'm no schill for him, but I have not had a problem with him either. I have a dim recollection of some CNCZone guys recommending one of these to use with CNC machines, and I bought one after reading the post.

How does it work? Simple. There is a little knurled knob visible on the bottom left. It has a "test" and a "use" position. Set it to "test" and an internal standard swings into place so that if you press the anvil on top with your finger until you hit the stop, you'll have exactly 2" from top of anvil to bottom of gadget. You rotate the dial to zero in that position. Now reset the knob to "use", place it atop the workpiece, bring the cutter down until the needle registers, zero the needle, zero your DRO, and you should be exactly 2" above whatever the presetter is sitting on.

So, not expecting much, I plunked the presetter down atop my aluminum cube in my Kurt vise on the mill, cranked the head until the cutter almost touched. Locked the head and cranked the quill with the fine adjust until the needle zeroed, zeroed my DRO, removed the presetter, cranked down another 2" with the fine adjust, zeroed the DRO again, added 0.010" for a modest cut, ran the cube through under power feed, and hauled the block over to the surface plate to see what I had done.

The desired result was 2.396". I brought the height gage down to take a reading which was, drumroll please, 2.396"!!!!

Holy uncanny accuracy, Batman! The silly presetter actually worked, and it worked well, and even though the quill travelled 2", and I expected the worst, it came out deadly accurate!

Hooray for the gadget that actually worked. I guess I'll be using the silly thing more often. At $39, it was worth it!

Best,

BW

P.S. If anyone cares, the face mill I am using is a 3" Lovejoy 225, also purchased cheaply from Lovejoy on eBay (they use a different seller name though). These things work really well, even on my goofy "Wrong Fool" Asian mill.

[HuFlungDung]

Right on, Bob. I use one of those presetters for doing tool offsets all the time and really like it. If I am being really fussy, I will actually use it to check the height of each insert on the facemill to discover which, if any, is hanging lower than the rest. That further reduces the surprise factor on the Z thickness of face milled parts

[BobWarfield]

Originally Posted by HuFlungDung Right on, Bob. I use one of those presetters for doing tool offsets all the time and really like it. If I am being really fussy, I will actually use it to check the height of each insert on the facemill to discover which, if any, is hanging lower than the rest. That further reduces the surprise factor on the Z thickness of face milled parts

Ah ha!

That raises an interesting point, Hu. That Lovejoy face mill did a funny thing on one pass of many. I thought I'd broken an insert as it acted like it dug in slightly near a corner. I could find no trace of breakage. Assume I follow the procedure you describe, and there is a low hanging insert, how do I fix that? Just unscrew it and fiddle, or do you grind the insert slightly?

I still don't know what happened with the face mill. It did it one time out of like 30 or 40 passes I've made. The other thought was that maybe there was an inclusion in the aluminum, but maybe it was a low hanging insert.

Best,

BW

[One of Many]

You could also use the height gage to reference the part for DRO setting. Either by a known standard or off the seat the part is set on.

You can take a skim cut, then measure to get the reference with any other tool. Prefereably without removing the part, but eh.....if you have better options use'm.

The issue with presetters on weak machines is the reference is taken under no loads. The best result....final finish path should be a light cut.

Being concious of and minimizing things like material buildup on the cutter tips that can add to the actual cutting point, spring in the machine, setup or part itself, heat expansion and cutting tool pressure, are all detriments to precision. Recognizing what, where and when comes along after a few bad experience of not staying one step ahead.

Soon enough, even .001 looks big enough to throw a cow through!

DC