I hope you readers will appreciate the effort going into this blog. I’m not yet at jewelry with a CNC mill and will take a long while to get there. I’m treating my own journey into the unknown as a travelogue with notes and pictures.
Today I will fix a mistake I made previously… attempting to fly cut artwork off both sides of a silver ingot, before first considering whether the vise was level.
The less than pleasant results that obtained were:
1) My resulting faced ingot was much thinner on one edge than the other
2) I damaged the top surface of one of my vise jaws, making it useless as a reference surface.
Oops! Unindicated Vise!
For better work, what needs to be done to to measure how out-of-level the vise is with respect to the milling spindle. This is because an endmill needs to be normal (right angles to both the x and y axes) with respect to the plane of the vise, the more normal, the better.
I tackled the preliminaries first. If you care at all about getting your engravings or cuttings parallel to the long edges of the ingot, it makes a great deal of sense to use a precision square, such as the one I used below, to ensure that the vise is true (vise’s X and Y axes parallel to milling table X and Y axes).
Use a precision square to true vise with respect to mill table.
Next step is to determine how far out of normal the spindle is with respect to the vise. Using a dial indicator measuring down to ten thousandth of an inch divisions (.0001 inch) is the method of choice for many CNC mill operators.
Your basic dial indicator, an essential tool for this exercise.
Unfortunately, when I unpacked my workspace out of mothballs, I discovered I had lost my dovetail mount for the dial indicator. I had to make do with using some Crazy Glue (a cyanocrylate based adhesive) to bond a broken endmill along the dovetail mount line as a temporary fix. The bond is actually quite brittle and therefore needs to be handled very carefully.
A broken endmill and some cyanoacrylate-based glue for spindle mounting.
As you have probably guessed, I am going to mount the dial indicator into the spindle by means of inserting the shank of the affixed broken endmill into a standard 1/8 ER collet. Before this, I need to make a simple safety precaution. The spindle is operated by a simple switch next to the mill, we want to make certain that a stray cord or a stray movement does not cause the spindle with dial indicator to rotate at high speed, causing damage that we really would not wish to contemplate, either to the dial indicator or to the operator.
SAFETY FIRST! UNPLUG THE SPINDLE MOTOR BEFORE INDICATING!
The business end of the dial indicator is very sensitive, excessive pressure can harm the inner workings of the dial indicator. I don’t want to measure how out of true the tops of the vise jaws are, because that doesn’t really matter to me. What does matter is the alignment of the spindle with respect to where the bottom of a piece of work is going to make contact with the vise.
I can simulate this condition by mounting a gauge block in the vise. The gauge block is known to be fabricated to exacting measurements and angles, therefore the top surface of the gauge block will be parallel to the vise’s bottom surface.
Once everything is in place and the axes are homed, and with the dial indicator well above the block, I first command the X and Y axes to a corner of the gauge block, avoiding the holes. I will then command the Z axis to lower the dial indicator so that the business end makes contact with the surface of the gauge block. About the final hundredth of an inch toward meeting the surface, I recommend very short jogs (a thousandth, or .001 inch) downward on the Z axis (downward is + for my setup, because of how I mounted my Z axis home switch), until the needle on the dial indicator makes its first short jump. Next, back off by a single .001 inch jog, then resume downward by jogs of a ten thousandth (.0001 inch) each until the needle jumps to a convenient number on the dial, like a zero (0) that you can use as a reference point for out of true measurements. The diagram below illustrates what things should look like after accomplishing this:
How indicating works...
The idea now is to call that corner Z = 0. Command the X and Y axes to the other 3 corners of the gauge block, and take measurements of the indicator at those corners. This will give you the deviations of the Z axis on the other 3 corners relative to your first corner.
Measurements taken with dial indicator on the gauge block before correcting with shims.
For making jewelry to precision measurements, this is horribly out of level. I fixed this by using shims, thin pieces of metal calibrated to exact measurements, placed as needed to compensate for the inclines of the surface. A couple of years ago on a family trip, I purchased 4 sets of feeler gauges for half a dollar (0.50 USD) each. Normally a feeler gauge is a sheet of metal of calibrated thickness used to determine the size of a gap, but at that price I felt that the feeler gauges were expendable and could come in handy for shimming.
A surplus feeler gauge set provides excellent calibrated shims.
After shimming, I remeasure the surface using the dial indicator again, to determine whether the levelling had improved. I can apply the shim to an entire side of the vise, but sometimes it helps to apply the shim only to one corner. After several tries I got fairly close. Since my feeler gauges only went down to a 0.0015 inch thickness, I resorted to using a pair of shims along opposing sides to obtain a .0005 inch incline adjustment. As a final adjustment, I used a piece on aluminum foil as a shim for corner zero only.
(NOTE: the vise can also go out of level simply because of how tightly the vise is screwed onto the milling table. I advise for consistent results when using shims, to screw the vise down to were the screws no longer move upon a comfortable touch of the allen wrench. If I do that consistently I will get consistent dial indicator measurements that are dependent on the shims rather than on the degree of tightening. Also, never overtighten!)
Shim the vise with feeler gauges. Follow up with aluminum foil in a corner.
Professional crafters have suggested to me that a vise having level deviations below a thousandth of an inch is “good enough” for most amateur needs. After several attempts I can happily present the following final results for my own setup:
STET!!!
Which Was What We Wanted.
{ 1 comment… read it below or add one }
I’m glad to see you’re making chips!
For what it’s worth, my usual CNC practise is to hold the stock down onto the bed of the machine with little hold-down clamps, like this:
http://jarkman.co.uk/catalog/jewel/doctordonna2.jpg
That supports the silver well, and makes it possible to work on thin stock which isn’t strong enough to clamp sideways.
The disposable acrylic spacer allows me to cut the work out of the stock (with a few holding tabs left in, of course).
Doing it this way also saves worrying about alignment, as long as the bed itself is true.