--I've had one for years. They're good for roughing in but not
really precise enough to tram a head to the kind of accuracy I like.. Way to
test: try some flycutting with a big dia cutter and you'll see what I mean..
Well, if the top of the table has any dings, the disc
sits on top of them and may give a bad reading.
What I do is mill the top of a piece of stock with a relatively
small (3/8") end mill. I then indicate a ring of this with
a dial test indicator mounted to the spindle. This compares
the spindle with the XY "plane" of travel of the mill, not
the top of the table. The plane of XY travel is what is REALLY
important, and you can't be sure the table is parallel to that.
My ancient Bridgeport has wear on the ways, and so it does
not travel in a perfectly straight line, there is a slight
arc to the travel. It also has enough dings on the table
that it is iffy to indicate off that.
Kinda like that.
I use a small sheet of glass instead of a brake rotor though.
The indicator feeler doesn't chatter on dry glass like it does on dry metal.
(to use the coax indicator on real-actual-parts, I tend to squirt a
track of oil for the feeler to slide around in)
Yep. Mine's not that big though, it came out of a flatbed scanner that
didn't have Win7 drivers available.
Aquarium (tempered) glass is a little more breakage-resistant, if you
happen to ever get your mitts on a broken aquarium.
...Also to be honest, I have no idea what the typical +/- is of the
surface of ordinary window glass. I'm just assuming it's much less than
.001", since that's all I bother with worrying about anyway.
One can see and feel .001" variances on pieces of milled metal and
nothing on window glass looks or feels even close to that big.
I am gonna give it a try, .001" is plenty good for government work. If
i move the table X and Y with a indicator chucked in the spindle it should
tell me how flat the glass is and if it can be used for tramming.
Well, the tolerances on size are pretty wild, but with how glass is
made, I would have to say on a single sheet that the thickness/flatness
would be pretty good..
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