How To Buy A Vertical Boring Mill
One of the largest machine tools manufactured today is the vertical boring
mill (VBM). It is mainly used for turning, facing or boring large workpieces
that are shaped symmetrically. Because of the machine's design, circular cuts
can only be made as the work is rotated against the fixed tool(s). Applications
for a VBM would be machining large steam engine turbine casings, ring gear
blanks, locomotive tires, water turbine runners, flanges for large pipe and
machine tool tables. The rotary table houses the mechanisms that drive it
and sits on the machine's bed. Two vertical uprights are mounted to the back
of the diameter of the table, which bolsters a cross rail. The cross rail
can be maneuvered up and down on the support columns in order to accommodate
various workpieces. It also has one or two toolheads mounted on it as well
as each vertical upright. For smaller VBM's, there is usually one turret head
and one heavy boring bar that tilts at different angles. For larger VBM's,
the turret head and swivel bar are replaced by two heavy boring rams to supply
vertical feed of the tools.
Principal Parts
Bed - A circular, hollow unit that is the main casting of the machine. It
covers the spindle and the bevel gear and pinion which drives the table.
Table - a circular shaped casting mounted on top of the bed and is rotated
through gearing. It is the workholding part comparable to the lathe's head
stock and faceplate.
Housings - two vertical members which support and rise from opposite sides
of the base. The faces of the housings are finished in order to furnish bearing
surfaces for the cross rail.
Cross rail - a rectangular casting mounted on the housings to stay parallel
to the surface of the table. It's counterbalanced and can be raised or lowered
to accommodate various workpieces.
Saddles & Toolheads - normally VBM's have two saddle and toolhead assemblies
on the cross rail. They're maneuvered across to feed the cutting tools parallel
to the table. The motion of the tools creates plane surfaces.
Rams - Similar to the cross rail, the rams are counterbalanced to aid movement
in their bearings. The rams can be maneuvered vertically in their toolheads
to feed cutting tools at 90 degrees to the surface of the table. The motion
of tools generates cylinders.
Arch - connects the housings at the top and maintains rigidity to the structure
of the machine.
Selection
When selecting a machine to suit your production requirements, the user should
consider a VBM over a standard lathe for the following reasons:
(1) machining a workpiece that is too large or heavy to be handled on a lathe;
(2) turning, facing and boring operations on various workpiece sizes for maximum
machine flexibility;
(3) work which necessitating during setup;
(4) counterbalancing irregular or off-center work loads for high speed turning.
When the user has chosen a VBM best suited to his production needs, he should
also consider some available features. Table drive systems also play an important
part in meeting production requirements. There are three main types:
(1) Conventional geared drives-alternating current motors supply a range of
16, 20, or 24 discrete table speeds and maximum horsepower is delivered at
any speed. Usually, this system offers maximum flexibility at the lowest cost.
(2) Adjustable speed direct current motors with gearbox-these drives supply
an infinite amount of variable table speeds. It provides the user with great
speed flexibility, but the horsepower given will vary with the speed in each
range and will drop down rapidly at the low end of the speed ranges.
(3) Variable, voltage constant, horsepower direct current drives-produces
nearly a complete range of infinitely variable table speeds at maximum horsepower.
This electronic system is suggested if the user calls for specific speed selection.
Inspection
Non-power
(1) Look over the mainline casting, check for any breaks, cracks or welds
that could affect the rigidity of the machine.
(2) Make sure the condition of the cross rail bearing surfaces have no signs
of excessive wear, since it can affect precision movements of the tools.
(3) Make sure there is a proper amount of gib adjustment left for maintaining
proper alignment.
(4) Make sure there is proper adjustment remaining for the gibs, which are
fitted on the saddles of the cross rail for taking up wear.
(5) Look over the cross rail screws for wear with the use of a precision gauge
drawn across the screw thread. Also, check over the cross rail for sag, which
can show wear in the vertical screws or a structural flaw in the camber beam
spanning the rail.
(6) Look over the condition of the table. Look for hammer marks, gouges, rounded
corners on T-slots.
(7) Test for table top flatness through the use of a leveling device, check
it in several directions at various points in the surface.
(8) Examine the rams, look for excessive wear since it can affect accurate
mounting of the tools and therefore the machine in maintaining close tolerances.
(9) Look for any evidence of oil leakage. Check inside gear mechanisms and
the table drive mechanism for any evidence of oil.
Under-power
(1) When testing for table run-out, use a dial indicator. Generally caused
by worn or loose bearings.
(2) Listen carefully to all gear boxes while the machine is running. Listen
for any meshing, grinding, or grating sounds.
(3) Operate the machine through its entire cycle, make sure all speed selections,
electrical controls, and rapid traverse functions properly.
(4) Check over the tool travel in both directions for differences which are
excessive for your operation, seeing what will require precise work and what
will not.
This is one article in a series of How to Buy Metalworking
Equipment. Each article showcases and explains a particular type of metalworking
machine. They were originally published in the Metalworking Machinery Mailer
published by the Tade Publishing Group.
