What is the causes for contaminated laps?
Ok, you are cutting and polishing happily away and then. Suddenly out of no where you are getting scratches across your facets. It is usually a really nasty series of scratches, at least for me. This is really a pain in the neck type of problem that every faceter will have happen to them sooner or later. Sorry, but it is just a matter of time if you facet enough.
There can be many causes, but the main cause of scratching is contamination on the lap(s). The contamination can be cause by quite a few things. But generally the list boils down to one of these three things. The stone you are polishing is brittle, and shedding chunks. This is not all that common but it does happen. Look at the leading or trailing edge of your stone and see if you can spot pits and comet trails. A chip at the leading edge of the facet with a larger to smaller scratch profile as the chip runs across the face of the facet is typical of this problem. This can be hard to tell for sure because often a contaminate on the lap will hit the girdle and then scratch the facet anyway.
Note: When a scratch shows up, especially a contaminate scratch, STOP IMMEDIATELY. Clean the lap and stone carefully, as well as the splash bowl and the rest of the machine. If you continue when a scratch of this nature shows up, you take the very serious risk of not only damaging your stone further, but even worse, you could imbed the contamination into your lap. Think about it. The chip is scratching your stone, so your stone which is probably a 7 hardness or more is running over the top of the chip (which is hard or it would not scratch the stone) on a lap that is a metal lap (typically tin or steel) which is only 3-5 in hardness. Where else unless you are really lucky is the chip going to go except for into the lap surface? You might get lucky and the chip will flush off the lap, but I seldom am that lucky.
Some times a chip comes off a stone, but the stone is not really brittle (maybe a flaw, bubble, some thing of that nature and one time deal). In this type of case, clean everything well and if you are lucky the problem will go away. You will not know if the stone is really brittle until your try several times and the problem keeps happening. If the scratching reoccurs and you have cleaned everything. For example you manage to polish a few more facets with no problem and then the scratching starts again and you can see a brittle chip. Well in my opinion start fresh with another stone, put the brittle stone in your reject pile. Life is too short. You can struggle through the polish, depending on how close you are to being done, but remember the odds are not with you and you could very easily destroy a good lap.
The lap is contaminated but not from the stone. In this case it is not the brittleness of the stone, it is some loose bort (diamond) or particles from other cutting that has gotten on your polish lap. This is really the most common. Stop right away and clean the laps and the machine (see Note above). If the chip reoccurs and is in one particular location on the lap, you maybe able to dig it out or avoid that area of the lap. If you cannot dig it out, well the lap is done, at least for polishing unless you can have it resurfaced or some thing. Some laps you can resurface and some you cannot.
The lap is contaminated because of age and wear and the plating is starting come off of the lap. This is pretty common, and if you think this is the case, replace the lap. As laps become older and get a lot of wear on them the platting will usually start to come off. This problem is common and typically noticeable on a 1200 or 3000 plated lap. Replace them and be sure and clean the machine and bowl completely. This problem often shows up at the polish stage, because some platting or diamonds has shredded off the lap and managed to get on the polishing lap. You can often see this problem as a slightly deeper scratch that shows up randomly on the 1200 lap facets. Of course as you polish the facets the slightly deeper random scratch will be easy to spot. If the random scratch is showing up a lot on the facet faces and the lap is well used or old, then it is time for a replacement.
Note: Laps are not that expensive and it really pays to just replace them if the problem starts to occur.
Other issues to consider about contamination, charging laps, and plated laps. Yes plated laps can cause a problem when they wear out, but it is not that big a deal usually, just watch and replace them. Plated laps are seldom a big issue and they are an easy fix. Plus in most cases they are cheaper than trying to charge a lap.
You do not think so? Well here is an example. Say you are wanting to charge a Batt lap ($135.00 or a regular tin lap $148.00 Crystalite). You need the lap $135.00 Batt, the diamond $11.00, extender fluid $9. Prices vary a little but say $150.00 just to start. A new 1200 steel lap is $78.00, if you need a coarse lap like a 260 steel it costs $96.00 new. So as you can see there is absolutely no money savings. If you consider toppers that cost around $30 each, there is no comparison. Toppers win hands dow, particularly for coarse laps. People say “Well I can just keep using a charged lap…” This is true, but remember that a charged lap will wear and groove badly over time. This is a flatness issue eventually, a new plated lap is always flatter. Yes, depending on the charged lap it may be resurfaced, but this is not cheap, time consuming and frankly not many machine shop people want to do it because the diamonds on the lap ruins tools. Plus there is cost involve to the machine shop and usually postage.
So there is no real advantage to charge most common laps, contrary to what some people seem to think. I do not especially like or recommend people charging their own cutting laps with loose diamond bort. The reason is that there is a very high probability of some kind of bort/diamond contamination with all that loose diamond being used and flung around the faceting machine bowl, work bench, and general working area.
Note: I am talking about cutting laps, not charging polishing laps with fine polish grit. Polishing laps seldom cause a scratching problem because the diamonds are well. polish sizes, not cutting sizes.
Can you charge your own cutting laps?
Of course, but remember you are increasing your odds of having a scratching problem in polishing stones exponentially if you do charge laps with loose bort/diamonds. Why? Think about it. You are putting loose diamonds that are coarse (when charging a cutting lap) 100, 260, 600, or even 1200 grit on the lap. The lap turns when cutting, especially at a higher speed for coarse cutting, and the diamonds are flung all over the splash bowl as well as the machine and any laps (or any thing) you my have out and not covered in your working area. My personal experience is that loose diamond bort will get into places you cannot imagine and pretty much every where. It is hard to believe just how good bort is at getting into and on to things that you do not want it near.
Remember that when loose bort charging laps the diamonds are not “plated” down to the lap, the diamonds are loose, yes they will embed, but not all of them and not right away. Sooner or later you will have loose diamonds coating almost all of the top surfaces of your machine. If you touch a surface, or your stone is not completely clean when you go to a polish lap. You just contaminated your polish lap.
Charging cutting laps creates lots of opportunity to have a contamination issue. Can people charge their own cutting laps? Yes. Is it a good idea? No, it is not a good idea, especially for a beginner. For some one with a lot of experience? I do not recommend bort charging laps for cutting laps, but it can be done. Why would people want to bort charge? Well in the old days it was often cheaper and some times the only way to get a particular grit cutting/polishing lap.
In short faceters often had to charge their own laps. Now a days plated laps and toppers are very good quality and cheap to get. It just does not make sense to bother charging cutting laps in my opinion. There are of course people that will not agree with me, but just remember what I have said the first time you really have a polishing and contamination issue. When you do have the problem, I think you will agree with me. It is best to keep any chance of loose bort contamination as close to a bare minimum as possible. That generally means plated laps, at least to me.