Everybody is making eggs. Of course, it must be Easter time! So my four year old walks in the house with his newly acquired egg, excited and proud. It is a really pretty egg, but as I contemplate it I can't help but to wonder...
What if I could make a metal egg? Well as it turns out, we can!
In this Instructable I detail the steps I took to carve an egg on my CNC milling machine by using the 4th axis attachment.
Step 1: Drawing the Egg
Believe it or not, this has to be one of the hardest parts I have ever machined in my life! Most of the stuff that I do starts, continues and ends with a collection of rectangles, triangles and circles and as it turns out, and egg is neither of those. Heck, it is not even an oval!
After looking at many (MANY!!!) eggs and trying to draw it,even more times with splines and curves, I was able to set on a profile I could use. I drew the profile on Corel Draw because it is easy for me to do so, but this could have been also done directly on the CAD tool. And I used Corel Draw, but pretty much any vector based drawing application would have worked as well.
Notice I didn't draw the whole egg, but more like half of it. The reason is this is just a profile I will use to perform a turning operation (called "Revolve" on most CAD tools). If we use the line at the bottom of the profile as our axis, we end up with a rotated version which happens to be our egg. The result is a solid we can mill out of pretty much any material we can think of. Except egg yolk. That would be tough!
One last note. You can see a prong coming from the egg's bottom side. This is basically a support structure I added so that there is something to hold the egg throughout the different machining operations. We can cut this afterwards, or leave as is. I added this structure support in my Inventor sketch, but this could have been done in Corel Draw as well. Is up to you, really!
Step 2: Preparing the Material and Machine
To carve this egg out of solid aluminum we will use the 4th axis attachment which is basically a rotary table in the vertical position. Once added, it becomes axis A.
The 4th axis table accepts many attachments which help in securing the work. In this case I will be using a 3 jaw chuck. Other options would be a 4 jaw chuck, a collet chuck, etc.
Once the 4th axis is in place, we need to make sure it is square with the rest of the machine. In essence, we want the A axis to be parallel to the X axis and perpendicular to the Y and Z axis. To validate this, we run an indicator along our stock material. If the indicator needle remains motionless throughout the motion in the X axis, then the part is square with the table and machine.
Two things to keep in mind:
1. There is no way to use the tailstock on this operation because we will machine the part from the top side end. As a result, we can't have too much stuck pocking out of the chuck.
2. The stock material can't be too short, or the spindle will crash with the rotary table.
Clearly, how much stock material is used must be measured carefully. I recommend taking many measurements before running the program in order to ensure a crashing accident doesn't take place. You really don't want one of these when employing such an expensive accessory.
Step 3: The Coordinate System
This is probably the most boring of all the steps but if you want a pretty and symmetric egg, it might be the most important. The machine really doesn't know where the material is. It knows where the egg has to be, but if we don't align the stock properly, we won't get our egg. I cannot imagine what it is that we will get, but something tells me it will be quite SCRAMBLED...
I actually made a few recoverable mistakes while cutting this part, so rest assured there are many ways to do this wrong, and pretty much only one way to do it right, so here we go:
First notice that to carve this egg we need this solid to rotate in its X axis. Any offset and the egg is FRIED! As a result, we need to find this center carefully. With a CNC machine, however, this is piece of cake.
For finding the center in Y, we will touch off on both sides with the same edge finder. Touch off on one side, zero the Y axis on the DRO, touch off the other side, and then divide by two on the DRO. What you get is the Y offset from the center.
Finding the center on the Z axis, however, cannot be done by this method because we do not have access to both sides. We can touch off on the top side, but not on the bottom side. Hence, what I did (and it took me three tries to get it right because of silly mistakes) is to touch off at the top (with the actual cutter), but instead of zeroing out the Z axis, I typed the rod diameter divided by two (e.g. the rod radius).
Just for documenting purposes, my mistakes were:
1. I zeroed the Z axis instead of writing the actual offset. The result was (LUCKILY!!!) that the operation was performed in open air.
2. Once I calculated the rod radius to be 1", I accidentally typed 0.1" as my offset in Z. Again, the operation was done in floating air.
3. Once I typed 1.000" as my offset in Z, everything was a go! Notice this was only the case because my rod was a perfect 2.000". Had it been something like 2.010", my offset should have been 1.005" Yes, 5 thousands of an inch do make a difference!!!
Finding the edge in X is the easiest. Simply touch off with an edge finder on the right side of the rod and adjust for offsets on the DRO as you typically would do.
Be aware that once you have programmed your coordinate system you will not want to do something as silly as typing a "G0 X0 Y0 Z0" command at the MDI because that coordinate is now inside of the aluminum rod! You will crash that tool so badly...
Step 4: Carving the Egg
There is really not too much to say here. After all, once you have setup everything all we are left to do is press the START PROGRAM button. At this point all we can do is watch and hope for the best. About three hours later, I had an egg shaped piece of aluminum on my hands.
Notice the result is not a mirror finish piece. Could this be done with a CNC Mill? In theory yes, but I would have had to employ progressively smaller ball end mills and progressively smaller step cuts, and even then there would have been some cutter marks.
The egg on this project was cut on two steps. One roughing with a 0.5" end mill and then a finishing pass with a 0.25" ball end mill. With these two steps it took hours to cut! If I had added more steps to try and approach a mirror finish, it would have easily taken a good portion of the day to obtain said results.
A much more efficient way would be to file the egg on a lathe and then use sanding paper to remove the burrs. Do want to point out that any filing and sanding on the lathe has to be done with uttermost care and respect towards safety. This is probably one of the most dangerous operations you can perform on this industrial piece of equipment. So if you don't want this project to turn into a batch of DEVILED eggs, be sure to follow safety guidelines.
Once you have removed most of the grooves and cutter lines, you can go into the buffing wheel. That is how I obtained my near mirror finish results. In theory I could have had polished this piece even more, but I am happy with the results as they are. After all, it is not like I am going to give this to my wife for her to apply makeup with.
Step 5: Last Thoughts
Carving this egg with the 4th axis on my milling machine was a lot of fun. Do want to point out that as much as it was fun it wasn't efficient. It just takes too long to carve this "simple" shape.
If instead we had done this on the lathe, the output would have come out way quicker. Do note, however, that to follow this intricate profile would not have been a trivial endeavor.
The right way to have carved this egg the CNC way would have been to employ a CNC lathe. At the moment I do not have one, but perhaps in the future I will.
Now, had I chosen to carve beautiful motifs throughout the egg periphery, then the 4th axis would have been the perfect mechanism. Such a project, however, requires a level of artistry I haven't mastered. Hence I am leaving that aspect of this project to the virtuosos out there.
In the mean time, I wish you all a Happy Easter 2015!!!