I Hear the Call of the Saw
It is time for me to get back to doing some cutting. So this month will be another article covering some of the general aspects of using a diamond saw and the finishing of cuts on a diamond lap. We have been here before so some is old stuff . Maybe there will also be something new.
Perhaps the first considerations before making any cut will be the size of the blade required and the placement of the cut. It is never a good idea to cut a meteorite with a blade that is too small. For example we had a meteorite that the business bought from another dealer. Little chunks had been cut off it every where and was a mess. The previous owner had hacked it up and the grooves of old cuts were overlapping and running everywhere. It is very difficult to make a cut with a small diameter blade and then turn the stone over and make another cut getting them to meet. It can be done but there will always be a slight difference in the angles of the cuts and a lot of lapping will be required to clean up the faces of the cut. Just obtaining a larger blade and making the one good cut is always better.
If the meteorite is to be only cut open to examine the interior, then a small slice off one area or edge May be enough. Or a clean up cut on a fractured surface May be just the thing. In which case the planning decision is rather elementary. On the other hand if the meteorite is larger and is being cut in half to split between two partners or to make book end type portions of equal size. Then the decision can be complex. The shape of the meteorites is usually irregular and judging the placement of the blade difficult. In my experience getting exactly equal portions on odd shaped large meteorites is hard to do.
If the meteorite is to be cut completely up into slices the thin blade trim saw May not be large enough in diameter to make some of the cuts. A larger thicker blade May have to be used on one or more of the cuts. Thus reducing the size of the pieces to that which the trim saw can accommodate. This will increase the waste some, but in the end May save a lot of time and make achieving nice parallel slices easier. The thick blade will produce nice flat surfaces that can be used to align the piece for the following series of cuts.
After a scheme for cutting has been arrived at it is time to position the stone in the saw. The meteorite must be held securely so that it does not shift during the cut. If it does it will certainly spoil the cut and necessitate the removal of a lot of waste to bring the two sides of the cut into flatness. It May also damage the thin blade or even cause the saw to stall if the blade is pinched hard enough. If the cut is with a thick blade the meteorite could also crack apart. So make sure the stone will not move in the vise. Clamping the stone May not be so much a matter of how hard the stone is squeezed in the vise as how you design the clamp itself. I have many times made custom piece of wood with curved faces to insert in the jaws of the vise to hold a stone. Sometimes you just can not clamp one down real hard without risk of crushing it. The need to be creative in making holders come in to play in those situations.
The very first few seconds of the cut are the most important. With a thin blade it is possible for it to wander a little at first across the surface of the stone before it bites in. Especially on the rounded contours of a smaller stone. This will not only leave a mark on the surface but also May begin the cut in the wrong spot. This could eventually cause the blade to pinch up or have to bend. Go very slow at first watching to see that the blade stays where it is supposed to be until it has bit into the stone. Then you can stop feeding by hand and begin to use a slow power feed rate. Or if no power feed is available you can hang a weight on for a gravity feed. On the saw I am using currently which I made I have a variable stepper motor drive that will allow for anything from a half step to fast continuous turning. I usually opt for slow and steady letting the sound of the saw guide me in setting the feed rate.
It is important to always be present during cutting and be ready to pull the meteorite back off the blade if something unexpected should happen. I realize that it is tempting to set it up and begin the cutting and go do something else. Some cuts can take a long time and most are uneventful. But, stay near the machine. That is my advice. Listen to the sound of the cutting. It is the first warning that something is wrong. If the meteorite cracks and falls apart the piece May not fall away from the blade but the sound of the cut will instantly change.
The previous paragraphs would give the impression that I make all cuts using the meteorite clamped in the saw’s vise. That is not true. I make a lot of cuts hand holding the slice or meteorite. But, these hand held cuts are single cuts from the meteorite or dividing cuts on slices. If you want nice parallel slices from a large chunk it must be clamped one time and the series of cuts taken. It is difficult to reclamp the piece exactly parallel in all axis to the last cut. You will often have to make a cut that produces a slightly wedged slice as the first one of the next series. This wedged slice can either be ground parallel on the lap or sold at a discounted price. All slices should be as free of wedge as possible to meet the standard that most collectors expect.
It is quite normal for the meteorite to break free just at the end of the cut. This is especially true if the chunk of meteorite which is being sliced off is heavy. Supporting the piece by hand and vastly reducing the feed rate through the blade can minimize the size of the teat that is produced when the break off ultimately occurs. What you do not want is for the piece to break off and take a lot of material causing a cavity in the surface of one side of the cut. There is a lot of waste generated in smoothing down the entire slice to the depth of the cavity.
After the cutting is complete there is always the need to smooth off the lines from the diamond blade. I have done this with aluminum oxide sand paper hundreds of times. But, for the last few years all the slices have been put on a diamond lap. This is fast and produces a better job. There is no rounding off of the edges and the finish is perfectly uniform. I will lay out the slices and work through them on one of the disks. Then change the lapping disk for the next finer grit and work through all the sides of all the slices. Continuing the process till all are finished to the extent that I want them. Some will continue on to polishing on a leather disk. Some slices will be finished at 600 or 1200 grit. It depends on the meteorite. If all of its characteristics disappear in polishing then it serves little purpose to simply polish for polishing sake. It is what the meteorite has to show that is important not the finish I put on it. I use 99.9 isopropyl alcohol for the coolant on the lap and have never had any problems with fire but I am sure that it is a concern that one must keep in mind. It is always possible to create a spark with a stone on a diamond lap. I always make sure the lap is fully wet before placing the slice on it to grind. And I never allow the lap to dry out while grinding.
Cerium oxide is the only material I am using now for polishing. I have try and used many other polishes. But, cerium oxide is a great universal polishing agent. It is also relatively clean to use and does not stain the slice or the outside of the meteorite as rouge will. It is not stubborn to clean off as some of the new white alumina based polishes are.
Nothing replaces experience in lapidary work and this could be a long series of articles taking months to cover every aspect of the process. I have cut nearly every kind of stone there is. From expensive material for jewelry, to rocks I picked up in the desert. Some so hard they would barely cut like ruby zoisite. Others like selenite are so soft they cut like butter. All have often been in my saws. I have just a few words of advise. Take a moment to look at and plain the cut. Even if it is a single simple cut. Check to make sure the stone is truly sound and has no hidden fractures. Practice a lot on other kinds of rock so you are truly confident of what your saw and you can do. That will make cutting that R chondrite, Howardite or Eucrite a lot less stressful. Remember some meteorites are better left uncut. Buying an additional slice May be preferable to cutting one from a beautiful fully crusted individual. Save your chips and fragments. You will think of something cool to do with them eventually, we all have. Remember you are cutting material that is limited in a way nothing else is and it is not replaceable. We are custodians only of our meteorites. Hopefully they will survive us. What we have done to them should enhance their value and their ability to be appreciated.
Scale of the Month
I am adding a new little feature to my articles for a while. I guess as you can see I'll be calling it the "Scale of the Month". It will be pictures and a short amount of text about one of the precision scales from my collection. So here is the first offering. It is the scale I am currently using for all really small measurements. The graduations on the finest range are in .01 mg. Mettler was ever refining the 160 gram series of scales and this is one really good one. I had one a year or so ago that was originally in use by NASA. It had one additional range of sensitivity. However, I sold that scale and for some reason never liked using it.