Thread-turn-milling of internal threads - INDEX-Werke GmbH & Co ...

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Thread-turn-milling of internal threads - INDEX-Werke GmbH & Co ...

Thread-turn-milling

of internal threads


High quality and concentric internal threads are produced

on turning machines preferably by single point threading,

thread milling or thread whirling. To unite the advantages

of all these processes, INDEX-Werke developed the patented

thread-turn-milling process. Thread-turn-milling can

produce high precision threads with high surface quality in

the least production time and also with increased tool life.

Comparison of the different processes

With single point threading the thread is produced in several

passes, resulting in a long machining time. Single point threading

also produces long chips due to the tool being in cut over the

entire length of the thread. These can clog the thread bore and

lead to breakage of the threading tool. Furthermore, tool life is

unsatisfactory because the entire machining is done with only

one cutting edge. With standard thread milling processes, almost

the complete length of the thread is produced in one pass, which

results in a short machining time. Due to the short engagement

of the milling hob-teeth, only very short chips are produced.

The draw back is that these teeth are reproduced as short flats

on the surface of the thread. With thread whirling, the surface

of the thread is very good, but because of the speed difference

of the tool and the work-piece, it will take more time. Threadturn-milling

combines the advantages of each of these three

methods. The chips are similar to those of the thread milling.

The machining time is short, making the process most

competitive. The surface finish and the accuracy of work are

comparable to single point threading or thread whirling. Tool life

increases because the workload is equally split between the

work-piece and the tool. Additionally, the drive power is shared

between the tool drive and spindle drive.

Description of the method

With the thread-turn-milling process, the thread is produced in one

pass. The rotating milling cutter has several teeth on the

circumference and has a lead that is the same as the thread to be

produced. The direction of lead of the tool and the workpiece is

opposite, however. For the production of right hand threads, left

hand-milling hobs are used and vice versa. The work-piece and the

tool are revolving with different speeds and in opposite directions.

The feed rate or traverse rate is determined by the amount that

the ratio deviates from 1:1. The work-piece and the tool traverse

against each other with a relatively small feed (the feed does not

correspond to the lead of the thread). Feed rate, speed of the cutting

tool and speed of the work-piece are synchronized by the control

(electronic shaft). Two different methods can be applied; the

traversing method and the plunge method.

The Traversing method

Thread-turn-milling with the traversing method is done with a

tool parallel or slightly inclined to the work-piece axis. If the tool

driving attachment is inclined by twice the lead angle, the profiles

of the threads fit together exactly. This results in a high surface

quality, comparable to single point threading. If the work-piece

is parallel to the cutting tool, the milling cutter has to be equipped

with special form ground inserts, which show a profile-correction.

In both versions the milling

cutter is positioned on the

thread outside diameter and it

is in cut in one longitudinal

pass.

Traversing method – Tool axis Traversing method – Tool axis

inclined

parallel

Due to the large diameter of the tool and work-piece, the number

of cutting edges is increased and the feed is decreased. With a

relative radial movement during the longitudinal movement, taper

and/or several threads with different diameters can be produced.

The threads, however, must have the same lead.

The Plunge method

With the plunge method the cutting tool and the work-piece are

parallel. The complete length of the thread can be produced in


one pass. Therefore, the length

of the milling cutter has to be

at least as long as the thread.

The surface quality is not as

good as with the traversing

method, but significantly better

than with thread milling.

Plunge method

Cutting tools and machine equipment

The thread-lead of the cutting tool and the thread to be machined

has to be the same, but the direction of the lead must be

opposite, i.e. workpiece thread right-handed, tool left-handed

and vice versa. The diameters of the thread and the tool have to

be different. Depending on the method, there are different kinds

of milling cutters. For the traversing method, the tool is equipped

with only one set of cutting edges corresponding to the lead.

Depending on the diameter, it has 3 - 12 cutting edges and is

spindle has to be tilted. The exact positioning of the beginning

of the thread is controlled by the C-axis. An optional cuttingforce

monitoring unit protects the tools.

Thread-turn-milling – A summary:

Thread-turn-milling can be used for internal threads from a

diameter of 12mm (1/2”) and larger. The maximum lead,

depending on the diameter, is 3mm (1/8”). Thread-turn-milling

is an alternative to thread milling, single point threading or thread

whirling and is already a well-established and proven process,

for example for mass production of self-locking nuts. Here the

advantage of a process capable to produce cylindrical and taper

threads in one pass shows the benefits very clearly. Other

advantages are:

• Short machining times

• Short chips, especially suitable for materials developing

long chips such as low carbon and stainless steels

• Outstanding surface quality

Threads can be cut to full depth

Tools Traversing method and Plunge method

Bush with thread M34x2, produced by thread-turn-milling

made with cutting inserts of solid carbide. It looks similar to a

groove-milling cutter for internal circular milling. For the plunge

method, a tool with several sets of cutting edges is required.

Depending on the diameter, it has 6 - 12 cutting edges and is

usually made of solid carbide. The milling cutter is a special tool

(similar to a tap) with ground profile correction, because it cannot

be tilted due to the ratio of diameter to the length. The machine

control has to support the synchronization of two spindles and

one or two linear axes. Depending on the method used, the

• Long tool life because machining volume is split onto

several cutting edges

• Possibility of producing different diameters with the same tool

• Transition from cylindrical to tapered threads is possible

• Splitting of power and torque between tool drive and main

spindle drive

• Interruptions of threads can be bridged in rapid traverse

without loosing the thread lead

• Choice between traversing and plunge method


LY9002.9416-12.00 ME Printed in Germany Subject to change without prior notice

INDEX-Werke GmbH & Co. KG

Hahn & Tessky

Plochinger Straße 92

73730 Esslingen, Germany

Tel. +49 (711) 3191-0

www.index-werke.de

INDEX Corporation

829, Bridgeport Avenue

Shelton, 06484 CT, USA

Tel. +1 (203) 926-0323

www.index-usa.com

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