Hi Guys: I think there's a real opportunity for sharing some knowledge here.
I'm a complete novice and figured I'd use my little $50 surface plate to check some measurements with a height gage, or scratch a few layout lines prior to putting in punch marks and drilling holes. Wishful thinking on my part, as I don't have a real height gage yet.
This is the typical way of using a surface plate. You can do work of near identical quality with nothing more than a mill table and a caliper. If you watch some videos of people on YouTube, you will very commonly see them using a dial caliper for scribing lines. Purists will balk at such mis-use of a caliper because it will eventually ruin the tools ability to measure accurately. I use a cheap dial caliper for this purpose myself, and it is the same caliper I keep at my mill or lathe for making rough size checks. It has fallen to the floor countless times, and no long is trustworthy for measurements under 0.003". The old school method that is probably the most accurate method for scribing lines is to use a flat bottomed scriber on top of a set of gauge blocks on a surface plate. In this era, scribed lines are mostly a sanity check to make sure we are not milling past the line, and we use a DRO or the handwheels for actual positioning of the cutter.
BTW, center punching by hand has it's own set of problems. It is almost impossible to hold and strike the punch vertically, and you end up with a dimple around the punch mark that is taller on one side than the other. This little dimple will flex a small drill and cause the hole to start in the wrong place, drill off at an angle, and often break the drill. There are ways around it, but do not assume that center punching will always give you good results, usually you will get results just as good by using a spotting drill and hitting scribed lines by eye.
When I think of lapping, I know one uses various compounds/pastes, but the only thing I've done is set fine emery cloth face up to clean up flat faces (maybe just "precision sanding"?). If using "paste" directly on an iron plate, how does one properly clean off the paste if you needed different grades of paste? How do you prevent/minimize wear on the plate? Also, aside from having the surface reground or hand scraped, how would an iron plate be "fixed"?
Properly, lapping is done as you describe with lapping compound on a lapping plate. Lapping compound should not be used on a surface plate, and never should be allowed near a cast iron surface plate. The compound embeds in the iron and can only be removed by surfacing the plate. Lapping plates have groove cut in them and wear over time. Dressing the plate is part of maintenance, and done by grinding/scraping. If you have three plates of equal size, they can be lapped to each other until all three plates are flat. Many of us use small granite surface plates with sand paper as "lapping" plates, and call it lapping even though properly it is just flat sanding.
As to "setting tools", I'm guessing this refers to using gage blocks for setting precise gaps from the table or some other part on a mill. I don't have a working mill yet, but the only way I've seen anyone set heights closely was to slip a piece of thin paper between a part and the tool to grab the paper and zero a digital readout. If someone were doing really precision work, do you actually set each tool from the bed of the mill to hold tolerances? I'd like to understand the process, but best to use "little words", as I'm quite the novice.
I hope you folks are better prepared for Christmas than I am.
Time to get wrapping.
TomKro
Short answer - it depends...
Generally, the way we work is to make a test cut and measure, then adjust. For instance, to mill a part exactly 1.000" thick, I would mill it slightly over sized, measure, then adjust the cutter down to reach the final dimension. This is much more reliable that trying to set a reference from the table top, and takes into account things like expansion from heat. I was milling a piece of aluminum the other day. I milled it to near finished height and took a break for the night. When I returned the next day, I found the cutter was slightly too high to scratch the work even though I had left it all set up from the night before. When my mill head cooled down, the quill actually got shorter by just enough to raise the tool away from the work. The work itself might have contracted a bit over niht as well, but it was very cool to the touch when I quit for the night.
I use gauge blocks constantly, far more than I ever expected to when I purchased them. I find them invaluable for setting stops and for making quick go/no-go measurements.
I am in agreement with wrecks on this topic. gauge blocks are far more useful and valuable than surface plates, and a surface plate and height gauge are wasted with out blocks to set the gauge. I do suggest getting a cheap class B 9"x12" plate for "lapping" and the like. If you wait for an Enco 20% off and free shipping coupon, it will run you about $35.00 to your door. If you find you are using it a lot for measurement, then get a bigger one. If you are not using it much you have not lost out very much cash, and it will eventually find some other use. No need to buy a height gage, just get blocks and grind a 1/4" HSS tool blank into a scribe. You won't be able to fit a height gauge on a small plate easily anyway. Get economy blocks to start with, you can always verify the set up with a micrometer and if you end up to a lot of very precise work, they can be put to use for machine set up and a nicer set purchased for metrology. They should be considered consumable tools in any case.
I found I was using my plate a great deal, so I purchased an 18"x24" Class B plate. My work is just fine enough that I could probably use a class A plate, but it is not worth the added cost, and I doubt even my nice Taiwan machines would hold those tolerances anyway.
. Another Craigslist find.