Sunday, March 20, 2011

Mini Mill Upgrade. Adding a Larger Table

I just added an enlarged table to my Mini Mill, bought from Little Machine Shop (where else?).  When I acquired my Mini Mill, I soon discovered one of it's major shortcomings.  The small table had too much flexibility in the Y direction.  I found this to be more of a problem than the often mentioned column flexibility.  To give you an idea just how much larger this table, here are the two side by side.

Among other improvements, the feed screw is supported at both ends, while the old table's screw is only supported at the handwheel end.  In the upper left of the picture is the new table's end cap with brass bushing.  Perhaps most important, the saddle is much wider and has 4 gib adjusting screws, instead of only 2.  I believe that contributed to the lack of stiffness in the old design.
The whole assembly weighs 20 pounds more than the original, and it is noticeable.  The "mini" mill no longer feels so mini anymore.  It is far stiffer, and the 20 TPI feed screw feels more precise than the old 16 TPI one.  I highly recommend this mod, and if you are considering buying a mini mill, I would not buy one without this table.  The LMS High Torque Mill comes with this table and a powerful new motor.
Below.  My upgraded mill. The 4 inch vise no longer looks out of proportion, as it did on the smaller table.
Swapping the scales to the new table was a breeze, thanks to T nut slot on the front face.  The front T slot is much smaller than the standard sized slots on the table, so I made my own T nuts, with 8-32 threaded holes.   While the scales may appear to be inadequately protected,  they have held up well, and have performed flawlessly for over a year to date.
      All mini mill tables lack a decent thrust bearing on the Y axis, and the axis on my original table seized up one day.  I fixed that by adding a bronze bushing where the spalling took place.  I had to use a bushing, for there is not enough room there for a bearing.  I machined a recess in the shaft retainer for the bushing. It is a big improvement, but only works in the direction of the table traveling away from the operator.  
     Thrust loads in the other direction are carried by the handwheel, where it contacts the shaft retainer.  Again, it is simply steel rubbing against steel.  Here there is room for a real thrust bearing, and Enco sells some inexpensive, and very thin bearings.  The one I used is a 1/2" ID (12.7mm) bearing, but that is not critical, for the shaft retainer handles the radial load.  What is important is that the recess in the retainer is accurately aligned, for that is what keeps the bearing in alignment.

  Here is where a 4 jaw independent chuck and dial indicator are essential. Do not trust the outside diameter.  I found my shaft hole was slightly off center relative to the OD of the retainer.

  I am very pleased with the results.  I can now set the thrust bearing at zero clearance, which was not possible before.  That removed some of the backlash in the Y direction.  When moving the table towards the operator, it is nearly effortless, thanks to the roller bearing. More effort is required to move the table when the bronze bushing is taking the load, but it is a great improvement over the original setup.
Here is my solution to the lock nut problem. Rather than replace the lock nut pair with a nylon insert nut, I made a small wrench that fits inside the handwheel. This wrench wedges itself inside the handwheel, making it possible to tighten the outer nut without the inner nut moving.  It is very effective and never loosens.



      Here is something that was not possible with the original table:  A 4 inch vise, and a 4 inch rotary table with 5 inch chuck mounted on the table at the same time.
       I  moved my trusty Shumatech 350 to the lathe and installed a 550 on the mill.  The 550 remembers 10 workspaces, which means I can store separate zero values for the vise and rotary table.