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.
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.
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.
(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.
(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.