The following points highlight the eight special types of milling machines used in industries. The types are: 1. Rotary Table Milling Machine 2. Drum Milling Machine 3. Profile Milling Machine 4. Tracer Milling Machine 5. Duplicating Milling Machine 6. Planetary Milling Machine 7. Multi Spindle Copying Duplicating and Profile Milling Machine and a Few Others.
Type # 1. Rotary Table Milling Machine:
It is an adaptation to the vertical milling machine. It consists of two vertical spindles, each equipped with a facing mill. Cylinder heads are roughed at one station and finish milled as they pass the second station. The operation is continuous and there is an ample time for the operator to load and unload the machine during the milling operation. The machine is fast in operation but is limited to milling of the flat surfaces.
Type # 2. Drum Milling Machine:
Drum milling machines are used for production work only. This type of machine has a vertical central drum which rotates on a horizontal axis much like a Ferry’s wheel. The cutters are of face mill type and usually both roughening and finishing cutters similar to those of a rotary-table miller are employed.
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In operation, the drum- fixture rotates slowly, carrying the work against the rotating cutters. Usually there are four cutter spindles. The operation is continuous, since machined parts are removed and new ones added after the work has completed its cycle. Automobile cylinder heads and small shafts are milled on this machine.
Type # 3. Profile Milling Machine:
In appearance, it resembles the vertical spindle machine. It has one to four cutter spindles. The cutter is a small diameter shank type end mill. Its movement is controlled, either by hand or automatically, by the path of a stylus or tracer which has the same diameter and shape as the cutter. In this machine the operation is performed in two dimensions. A good commercial finish and a tolerance of within 0.1 mm can be expected from this machine.
Type # 4. Tracer Milling Machine:
This machine is best suited for very complicated parts like embossing and forging dies and moulds, and for die sinking and contour milling. The cutting tool is traversed across the work by means of a tracer mechanism (Refer Fig. 16.14).
The tracer contacts the surface or contour of a master which may be a simple geometric shape in one or three dimensions, and the active cutting spindles are guided in a course determined by the shape of the master. Machines to perform such a job can be either vertical or horizontal, and with one or more spindles (Refer Fig. 16.15) for multi spindle tracer milling machine).
Type # 5. Duplicating Milling Machine:
It is also called by the name of originator, Keller machine, die sinker or automatic tracer controlled miller. In this machine, milling operating can be performed in third dimension also. The template used in this machine must be replica in three dimensions of the work to be performed.
Typical work performed includes the making of forging dies, steel molds for glass, plastic and certain metals, auto body dies, ship propellers and air-craft connecting rods.
Fig. 16.16, 16.17 and 16.18 shows three more types of milling machines.
Type # 6. Planetary Milling Machine:
It is a unique machine in the sense that the work is held stationary while the revolving cutter or cutters move in a planetary path to finish a circular surface on the work, either internally or externally. Many of the operations would be lathe operations, if the nature of workpiece permits. The cutter may be plain, form or thread cutter and may work on either the inside or the outside of the work or inside and outside simultaneously.
This machine is particularly adapted to work which is heavy, difficult, delicate to hold or work too unbalanced to be rotated itself. Other applications of this machine are milling of internal and external threads on all kinds of tapered surfaces, bearing surfaces, rear axle end holes, aero plane crank cases, shell and bomb ends.
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These machines are available in either horizontal or vertical spindle models and may be tooled for magazine loading if the type of work permits.
Fig. 16.20 shows a milling machine for milling cheek contours of crankshaft assemblies.
Type # 7. Multi Spindle Copying Duplicating and Profile Milling Machine (Fig. 16.21):
The copying machine functions efficiently in many fields of application:
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2- dimensional—engraving, drilling, contouring, intricate profile milling, slotting, bevelling, etc.
3- dimensional—molds, stamps, hobs, dies of all kinds, electrodes, medallions, artistic creations, etc.
Many times, the 1 : 1 reproduction capacity will eliminate the necessity of making an expensive master. An existing mold, a broken tool or stamp or a casting of a piece can serve as model to trace from.
The manual control of the machine not only permits faster machining than with automated equipment, but also a better copying of very fine detail and surface finish.
The movements of the machine are extremely sensitive and frictionless, so that maximum precision and true reproduction can be accomplished. Bench work is reduced to a minimum and even eliminated quite often.
The copying machine can also be used for reductions from 2 : 1 to 6 : 1 by mounting a horizontal adjustable pantograph unit and an auxiliary copy holder table.
Work Head Unit:
The cutter spindle and stylus holder are mounted on a common plane, called ‘T-bar slide’. This eliminates errors which may occur when spindle and stylus are fixed on separate machine components. The stylus movements are directly transmitted to the cutting tool which, therefore, reproduces the traced model size for size (1:1). A built-in balancing device is provided for load compensation of the copying unit and for adjusting the weight when the machine is used with multiple spindles.
Pantograph:
When working at 1 : 1, the rigid pantograph serves to reduce the machining effort for the operator (leverage), and to control the movements of the cutter with utmost sensitivity.
One can also use a horizontal pantograph unit which is adjustable for reduction ratios between 2 : 1 and 6 : 1. This reduction pantograph is used in conjunction with an auxiliary copy holder table, and the multiple spindle model is advisable for increased production.
Tracing Stylus:
The stylus holder has 3-directional micrometer movements, permitting a precise adjustment of the relative location of work piece and model and of the depth of cut. The tracing point is clamped in a hardened precision spring collet. Collets are available for shank diameters from 1 to 8 mm. in increments of 0.5 mm.
The work piece and model are clamped on one common table with XYZ lead screws. No locating errors can occur once the setup is made. Regardless of T-bar slide or table
movements, the location of all components relative to each other remains constant. Therefore, the entire table travels plus the T-bar slide movements constitute the total 1: 1 work range of the machine.
Milling spindles:
The spindle and stylus holder brackets are rigidly fixed on the T-bar slide which is equipped with a T-slot. Therefore, the location of either bracket can be varied so that the center distance between cutter and stylus may be changed. The same adjustments are possible when using several spindles simultaneously.
Each spindle has a vertical adjustment with a total travel of 2 mm to provide for easy and precise alignment travel of 2 mm to provide for easy and precise alignment of the cutter, especially after re-sharpening.
As the spindle holder brackets are easily interchangeable, the machine can be equipped with a variety of spindles and motors to suit individual work requirements and to use a wide range of tools.
Line by Line Milling:
A line milling attachment (pick- feed) is provided on the cross movement of the table (0.05 to 0.5 mm per step), for roughing work and heavy chip removal. Because of this systematic tracing method, there is no risk of omitting any portion or detail of the pattern traced, and the finishing time via manual freewheeling control is substantially reduced.
Locking Devices:
Depth Stops-2 depth stops-adjustable between 0 and the maximum vertical travel of the work head- are standard and can easily be retracted to permit changing of cutting tools without disturbing the work setup.
Longitudinal and Transverse Stops:
These facilitate work where limited travel of the work head is desirable, such as machining miniature parts, milling on the bottom of a cavity, etc.
With these stops, each work head travel can be locked independently for maximum efficiency, time saving and preservation of tool life.
Spindle Locking Arm:
This device immobilizes the work head unit, and the 3 table travels can be used in the fashion of a standard milling machine.
For economical production runs, the machine may be equipped with multiple spindles which can become a very important factor in
3D: multiple or sectional molds or mold inserts, EDM electrodes for roughing, semi-finishing and finishing to be cut simultaneously from a single master in one setup
2D: multiple engraving of push buttons, stamps, trademarks, identification of production parts, bevelling, etc.
Symmetrical Milling Attachment:
Many times, right and left hand molds, stamps, dies or other cavities are required, especially in the footwear industry, for hardware, toys, jewellery, etc.
In using this special attachment, a normal piece as well as an inverted replica can be copied from a given model. From this existing model, the mirror image master is milled into easily machineable material. When the relief or cavity is very shallow, it is even possible to copy directly into steel.
Scriptamatic:
This attachment permits the fully automatic engraving and copying of 2 and 3 dimensional round stamps, dies, medals, coins, badges, coat-of-arms, etc., on a pantograph or copy milling machine.
It controls the movements of the machine’s tracing stylus which, from the center point of the model works continuously to the exterior circle in spiral motion. The distance between the tracing spirals can be adjusted in order to provide for superior quality surface finish. For extremely fine detail, the final finishing cut can be taken by controlling the tracing stylus manually, without disturbing the location of the model or template.
It is further possible to install a terminal switch so that the machine will stop automatically when tracing is completed.
With the automated set-up, one operator can handle the production of several machines simultaneously and, therefore, considerably reduce the labour cost for each work piece.
Type # 8. End Facing and Centering Machine:
