2719Re: [mill_drill] Re: Tramming Spec?
- Jun 4, 2002Hi Dan;
I'm not familiar with the JET JMD18, but I think this is a Rong Fu mill
drill with a round column? If so, here goes:
There are four (at least!) critical planes and or centerlines involved
in mill/drills. They are:
The plane of the Y axis motion
The plane of the X axis motion
The centerline of the spindle during Z axis motion
the centerline of the column
The plane of the X and Y axis motions should be parallel, short of hand
scraping and fitting you tend to trust the manufacturer to have gotten
these pretty close to right. Tramming can compensate for a bit of error
here by the maker, depending on just where the error occurs and how far
off the original work is.
The centerline of the Z axis and the centerline of the column should be
parallel, this is also something that crosses into the serious side of
machine tool rebuilding to correct if too far off in the case of
mill/drills, as you don't have the articulated head and ram combination
found on large mills.
What we're really after is to get the centerline of the spindle to be
truly perpendicular to the plane of the table. If off, cutters that
should cut a flat plane cut coves or steps, and drills poke holes that
are tilted with respect to the reference surface of the workpiece.
To get to this point, you mount an indicator via some assortment of rods
or indicator holders to the spindle, preferably using a collet so you
are referenced to the R8 spindle bore centerline. You then lower the
quill to the point you can sweep the indicator across the surface of the
table, checking for a consistent reading at all points on the circle.
The circle should be as large as fits on the table, you can go bigger
but then you add in the variable of having to do a table move to sweep
Here's a simplified example, assuming a table depth of 10 inches or so:
You sweep a 9 inch circle, and find the indicator reads 0 at 3 o'clock,
9 at 12 and 6 o'clock, and 18 at 9 o'clock. Your column is leaning
slightly to the left, but is fine front to back. The plane that is
perpendicular to the table plane is tilted, if you use a 4 inch fly
cutter and indicate the machined surface, you'll find it to be a shallow
cove several thou deeper in the center than the edges, rather than a
flat surface, assuming X axis motion only while cutting.
Now make the situation 0 at 12 o'clock, 6 at 3 and 9 o'clock, and 12 at
6 o'clock. A fly cutter will cut flat, but will create a sawtooth
surface (assuming X axis motion only again) on subsequent passes, as the
plane of rotation is tilted forwards.
In reality, you'll probably find a combination of the two above
situations, with the column leaning towards a corner. This adds fun, as
a shim in any corner effects the reading in ways you may not expect. It
also means you have to shim at least three corners, as you don't want
any corner floating in the air, or the casting will deform or crack as
you tighten down the bolt at the unsupported corner. If it wasn't fun,
we wouldn't do it, right?
To fix this, the bondo fillet at the base of the casting that holds the
column to the base of the machine is scraped off cleanly, the 4 bolts
that secure the column to the base are loosened, and shims are inserted
as needed to shift the column vertical axis in the direction desired.
Remember to raise the quill a tad before loosening things, crashing an
indicator is really considered to be poor form :-).
The final point you aim for is naturally zero variation of the needle
through 360 degrees of rotation, how hard you chase this goal is a
function of the sort of work you do. If you mill small engine heads
with a 4 inch flycutter with two overlapping passes, a 2 thou ridge down
the middle of the work is no good! While this isn't the "correct"
tooling approach for this job, we do often have to do overlapping passes
in our work while generating a flat surface, as most of us can't afford
the expense or space of machines that can swing a 10 or 12 inch inserted
cutter head. If you normally use a 1/2 inch end mill to face off 2 or
three inch work, and lap to final flatness, a tenth or two variation in
cut depth across the surface may not matter to you.
I chase zero variation pretty hard as a result of the work I do, and
have just added a surface grinder to the shop to let me generate really
flat surfaces. How far you have to chase that last thou is a function
of need and patience. Some folks prefer a thou high at 9 o'clock,
feeling this gives a bit of clearance for large cutters at the backside
of the cut as you move the work from left to right. I don't happen to
set things up this way, but some folks whose opinions I respect like to
do this. Just proves there's usually more than one way to do things
well in this hobby/business!
Hopefully this rambling isn't too long winded and answers your
><snipped my old random wanderings>
> Hi Stan,
> I just purchased a used Jet JMD18 mill drill and would like to know
> more about tramming. Where to I install shims and where do I put the
> dial indicator? Sorry but I'm really new to this machining thing so
> any info would be great! Any other tips on what to buy for it as far
> as tooling? The machine is 6 years old and came with a Shooting Star
> two x,y, dro and a enco power table feed. Looks like it had lite use.
> I can also purchase a Jet rotary table and index table for another
> $200 from the guy I bought the mill/drill from, should I get them
> Thanks, Dan
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