TAKAMAZ
RYU- GIVol .1 6

In the current edition of Takamaz Ryugi, we will introduce
the tools used in "Hard Turning Work" which is said to bea relatively difficult process, as well as a cutting method
is garnering attention.
Q

CBN ※Tools for Hard Turning※CBN/C= An acronym for Cubic, B= Boron, N= Nitride

Q

What are CBN tools?
The hardness is 4× that of quenched steel,andthe advent of CBN materials makes mass-production
cutting possible
Although these tools were first developed for use
in the cutting of heat-resistant alloys,they havecome to be used in hard turning work with Sumitomo
Electric Industries, Ltd having developed "CBN tools
for Quenched Steel Machining" where CBN is mixed witha ceramic binder and sintered.
The cutting tool hardness needed for stable cutting
is at least 3× that of the material cut.CBN has ahardness second only to diamond, and strong resistanceproperties even against cutting heat. Therefore it has
had great expectations since it was first developed as
the optimum material for machining quenched steel.
The Vickers hardness of quenched steel is about 720Hv.
Whereas carbide tools are about 2× this at 1500Hv,
CBN tools are about 4× this at 2800Hv, and maintainfull hardness in stable machining.
However, their use in mass production was put off due
to their extremely high price and low reliability due
to sudden breakage. In Japan, amelioration of these
problems has been underway since 25 years ago, with
the intent of propagation in mass production.
As a result, costs have today been successfully
lowered to about 1/10 of what they once were.
Regarding breakage, the toughness and the wear
resistance have been enhanced through the improvementof tip shapes and material blends. At the same time,
high-speed machining capitalizes on the "high temperaturestrength" that is a superior characteristic of
CBN tips, thereby improving efficiency, and dramaticallyextending their introduction in mass production.

TopicsWhat are the Pros and Cons?
Improved Cost Performance, Reliability, Machining
Precision, Suitability for a Variety of Applications
Cutting for machining straight parts such as cylinders and the likecan be performed relatively simple, while for compound shapes with R,
tapers, undercuts, and right angles, etc., total type grind stones
are used, and it is necessary to perform the machining with pressure
applied, generating restrictions, naturally. Moreover, formation into
total type grindstone shapes is difficult, and there are also difficulties
with uneven wear in parts. In hard turning machining, it is not only
possible to machine compound shapes through NC lathe programming, buteven parts machining of multiple items is possible simply by changing
the program. Although workers experienced in grind setup and tracing
are needed, hard turning machining using CBN tips is characterized by
the ease of obtaining the demanded precision, unaffected by the skillof the worker. Although the work surface roughness using CBN tools was
Rmax6.3μm 25 years ago, it has risen to Rmax1.6μm in recent years.
It has become possible to obtain high-precision machining while maintaining
stability. Because machining is performed using an NC lathe, there isalso an additional advantage in that machining facility costs can begreatly reduced in comparison to grinding equipment.
Although hard turning machining has such outstanding characteristics,
the occurrence of spiral patterns (fine thread patterns) (See Figure 1)
generated oil leaks from the seals, and is restricted in terms offinishing of parts associated with oil pressure or lubrication, andfuture improvement of this is desired.

Figure 1 （ Finished Surface Pattern )Spiral Pattern
Occurrence(Single Point Machining)(Skiving Machining)

Achieving seal surface machining, and making possible a great shortening of cycle time

Hard skiving machining
Hard skiving machining* refers to amachining method where a cylindrical
work surface of quenched steel isscraped off so that burrs are shaved
off through the up/down motion of
the tool (See Figure 2, 3). The greatest
advantage of this machining method isthat it can even deal with oilpressure/lubrication-related parts
because spiral patterns cannot bedifferentiated (See Figure 1).
In addition, regardless of the feedrate]
multiple of a single point tool,
it is possible to maintain a surface
roughness on the order of Rmax1.0μm,giving the prospect of a great shorteningof machining times and increased quality.

※Hard Skiving Machining/ Machining Method
Patent of Sumitomo Electric Industries, LtdT news

Divided Spiral
Pattern●C B N S t r a i g h t B l a d e

In terms of tool life, this can beextended to about 4× as the tool tip
is set upon the line (generally 20mmin length) like a deburring blade.
Moreover, because it is possible toachieve a machining surface with anunchanging grinding pattern, and since
it is possible to machine parts forseals, and dramatically shortens
machining times, etc., it overcomesthe conventional issues of hard turning
machining, and is garnering muchattention as a machining method promising
great results.
With progressing internationalizationand the trend towards small lots ofmultiple part types in Japan, as wellas the tail winds of "improved setup"
and the so-called environmental advantage
of "reduced waste oil processing"
due to dry machining, and future demandis expected to increase.

Figure 2 ( Tool for Hard Skiving Machining )●T o o l m o v e s f r o m t o p t o b o t t o mFigure 3 ( Skiving Machining State )