I have two Lap Master 15 lapping and polishing machines. I have used them for lapping and polishing flat surface metals. My question is if I can use them faceting gemstones and if they would be accurate.
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Is this just the flat lap? You could buy a second hand faceting arm to try with the lap.
It can be done, no problem. I say that from experience.If you can get a faceting tower or arm, mount that …important…securely to the lap master so the arm and lap are as one, you should be able to turn out some nice stones.
If you want to facet descent quality gemstone… get a faceting machine, preferably one with a mast. The standard settings for changing a vertical setting are ten subdivision of every degree from zero = Horizontal to 90-degree vertical. Thus, the faceting machine gives you 900 vertical setting. You will be cutting the girdle are at 90-degrees and the table of the gem at zero… and many cutting diagrams give 100 subdivision for every vertical degree of change. You will not be able to get this accuracy with the make-shift set up that you are describing. Once you get the right vertical angle you will have to rotate a “tier” of facet about a perpendicular axis and each click on the dial is about 3.75-degree (that is using the 96 division index wheel). YES… it can be done… but you will likely not achieve the accuracy to be able to following the directions printed out on all those cutting diagrams.
I do agree with you that faceting with a dedicated faceting machine is the best way to cut stones…as I do now with my UltraTech, turning out beautiful gems both with standard cuts and fantasy cuts using the UltraTech fantasy machine. Having said that, if a person isn’t sure that he/she may want to facet gems, there are ways, as I suggested, to get started for a much more affordable price instead of spending $5K on something one may or may not enjoy. Also, using the set up I made in the early days taught me some super valuable tricks, lessons and methods of turning out stones that came out with perfect meets and angles. Things like listening to the stone, how the lap is working, using sound, feel and eyes. Now if using home made faceting equipment you may not be able to turn out a stone that you are sending in for competition and certification, you can still produce a beautifully faceted gemstone. A faceted gem on home made equipment takes longer to cut because one is not relying on gauges, depth of cut and so on, but on visual and sound. And that gem can be a beautiful perfect cut with 10X mag.,which will last for generations and be enjoyed by many. So what if it won’t qualify for a CSM…
I first started with one of those chinese made jade faceting mast, LOL, mounted on a piece of beam with soft rubber pads under it to keep it in place and had to make a set up out of metal rods and bolts for adjustment for the girdle and table. I still have that set up, years later, on a shelf in my faceting room, just to remind me how I started! I also have some of the first gems I faceted, keeping them for the same reasons. But I did turn out some nice gems with that set up.(And some incredibly crappy ones!)
Now, over 3000 faceted gems later, working in my room filled with over $45K worth of equipment, I still look at that first machine, I made, (if you can call it that), and smile. 
Their use for faceting will depend on how accurately you can align a mast to the flat lap, it will need to be accurate to the point that you can touch a stone to all points across the lap without any variation in depth.
Beyond faceting, there are many lapidary arts where they can be used, cabochons would be a good one as there is no necessity for the accuracy demanded by faceting, plus you can get profiled polishing wheels to assist with the final form, they would be great for working boulder opal which calls for a lot of freeform work and financially rewarding results can be produced quickly with minimal technical skills.
Thanks for your input. I was thinking the alignment can be checked with a dial indicator. I was also concerned with a hysteresis due to the lap bearing. I was going to buy a lapping machine but just figured the lapmaster could work. It’s something to think about.
Also if I buy the laps and mast, it wouldn’t be that much more to buy the whole unit. I was just thinking that the Lapmaster 15 new cost $8000 to $10000 and work pretty good on flat metals. I’m going to check out the owner’s manual and see if there are any options other than buying 5 or 6 15" laps
I find a dial indicator provides a rough guide, I do final alignments with a smear of oil and a dop.