Tramming--Why Do We Care (So Much)
- After tramming my new HF mini in the x-axis, I started to contemplate
the error in the y-axis and all the trouble it's going to be to square
the mill in the y-direction (fore and aft). My table is flat, but
like many mini users, I found that my head is not square with the
table in the y-direction and it exhibits nearly 0.020" of error from
one end of y-axis table travel to the other end when measured at the
front and back of the table, i.e., the measurements are over a total
y-displacement of 7" or so (~4" of table travel + ~3" wide table).
If the table is flat, why do we care so much about these errors? All
the cutting is done under the spindle, head, and mill. Even though
the head may not be square to the table, all points on the table are
at the same height as they pass under the cutting mill or bit.
Therefore, the only error in cutting that results from a non-square
table/head is the error due to the error angle subtended across the
diameter of the cutting tool. My largest end mill is 1" so my 0.020"
inch error in 7 inches is only about 0.020" / 7 = 0.0029" across a 1"
end mill. In other words, with a 1" mill, one side of the end mill
cuts a bit less than 0.003" lower than the other side.
Well--maybe 0.003" is still too much, (it depends on the nature of the
milling operation) but it doesn't seem necessary to me to reduce the
error to 0.001" over the range of the table. If I can get it down to
0.007", I'll have 0.001" error across a 1" diameter end mill.
I'm interested in how others analyze the significance of the problem.
Shimming out the y-error is going to be a pain and, with only shims
to work with, It'll be difficult to get the y-axis anywhere near as
square as the x-axis.
I've read a bunch of postings on the subject here and thanks for any
insight to the significance of tramming error that more experienced
users can lend.
- --- In GrizHFMinimill@yahoogroups.com, "Ray Livingston" <jagboy@...>
>IMHO many possibly spend more time squaring up their little mills to
> I really have to wonder if the value of tramming isn't somewhat over-
.0005 as opposed to just getting on with the task of making parts to
within .005 of square. I've owned my shiny new red X2 for about 8
days now, I squared it up with my bad eyes (aided by a magnifying
glass), a couple machinist squares, a length of 3/8 drill rod held in
the EMH and my flashlight. I had to add a couple shims under the L to
make the light shine equal but other than that the heck with it! So
far I've made a cut off tool holder for my 7x12 and as well I faced
the bottom of it's tool post so 3/8 bits sit at center height since
mine was very roughly configured for 5/16 bits. I think the next thing
I'll make with it is some better gibs for the little lathe and maybe a
couple bits for my old Logan 10x24 so it will still feel loved and
cared for too. Soon as I find a chunk of aluminum near the right size
I'll make a fixture plate for it then maybe some stops and indicator
nubs for the X and Y and then some...ahhh the joy of watching the
metal chips fly with my cheap little Chinese mill :)