Re: Tramming a mill - update
New guy and lurker here. How about using a spherical washer at each bolt? See www.mcmaster.com/#washers/=3wh5xh These would self align in any direction regardless of the shim thickness needed. You would need to install the washers before the initial tramming to compensate for variations in washer thickness. Just my 2 cents worth.
--- In email@example.com, "Rick Sparber" <rgsparber@...> wrote:
> As I think about tramming, a few things come to mind:
> * I put my DTI on a support and swing it in a circle around the center of
> rotation of the spindle. The diameter is around 7" because my table is 8"
> wide and I want to hit the center of the surface between T slots.
> * I take readings at the front, back, left, and right sides
> * however, my bolts are in a rectangular pattern
> * I can't recall who pointed this out, but I now understand that if I put a
> flat plate on the mill table with marks at the same spacing at the bolts, I
> would directly read the thickness of each shim if I moved the DTI to each of
> these marks.
> I would not want to use a plate because, (1) I don't have one, and (2) it
> may not sit flat and therefore would add more error. I also don't want to do
> it because I can do the same thing with math and get less error. I believe I
> can take my 4 readings plus the bolt spacing and calculate what the shims
> must be. In the past I have taken the front to back error and used it to set
> my back two shims. Then I have taken my left to right error and set my two
> right side shims. This was never exact and maybe that is because I didn't
> use enough math. Stay tuned for more on this one.
> Furthermore, I got thinking about how the shims contact the mill base top
> surface and the column's bottom surface. I assume these surfaces are flat.
> Say I add a pair of shims near the back 2 bolts. I then am forcing this
> mating surfaces apart and will have contact only at the front edge and at
> the shims. That is not good support. So what about making a shim with a self
> adjusting top face?
> Picture a 1/2" thick block of CRS. Milled into the top with a round nose end
> mill is a groove. Then take a piece of 1/2" diameter round stock and cut it
> down so you get a D shape on end. The flat on the round stock contacts the
> column casting and the force is distributed to the block via the groove. I
> see a lot more support with this arrangement. Shims could then be put under
> the block. As a side effect, I will get a small riser for my mill. If this
> still makes sense tomorrow, I will draw it up and maybe make it. I might
> even be able to combine this design with the double wedge idea so no shims
> are needed.
> What say, all?
- I just realized that my column bottom to base test was done with only one
bolt tight on the back side. My back left bolt is not installed. So maybe
with both bolts in place the flexure would be 0.5 thou rather than 1 thou.
It doesn't change the conclusion, just the magnitude.