FIELD OF THE INVENTION
The present invention is related to a method for machining used in ordinary
three jaw chuck lathe feed, more particularly for machining a slender hollow
5 cylindrical fixture used in thermomechanical fatigue test without including CNC
procedures.
BACKGROUND OF THE INVENTION
Thermomechanical Fatigue (hereafter referred as TMF) is an important point
10 that needs to be considered, when constructing turbine engines or gas turbines
with a cyclical thermal loading. TMF fatigue is a fatigue caused to materials
with cyclic mechanical strain and cyclic thermal exposure simultaneously. It
can also be considered as the overlay of a cyclical mechanical loading that
leads to fatigue of a material. The simple output of a TMF test is the number
15 of cycle it took to fail or develop a crack with reference to the number of
alternating or fluctuating thermal loads in combination with the fluctuating
mechanical load. The result or output depends largely on the specimen
accuracy. The test sample for such test is in compliance with ASTM standard
E2368, which is by default a hollow cylinder with a thickness of around 1.5
20 mm. This thickness being too small for accurate machining and a hollow sample
makes the process even more vulnerable for machining defects. Since there
are already samples listed for ASTM standards no deviations are allowed.
Hence for machining such standard material at a very narrow thickness
different approach is needed.
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OBJECT OF THE INVENTION
It is therefore primary object of the invention to solve aforementioned problem
by developing an insert to be used as a coupler during machining process of a
5 hollow cylindrical sample used for TMF test, in a manual lathe.
SUMMARY OF THE INVENTION
The present invention is therefore intended to solve one or more of the above
problems by providing a method for machining cylindrical samples used in TMF
10 tests.
According to aspect of the invention, an insert is fabricated which is used during
machine process of a standard hollow thermomechanical fatigue test sample.
The insert is used as an adapter/coupler to rigidly hold the semi-finished
sample and further machining is carried out so that the machining becomes
15 easy and less prone to machining defect. Without this insert the machining
becomes tough, inaccurate and there is high chance that the sample can get
punctured during machining. The machining becomes so complex as the
annular portion of the TMF sample is so thin. The insert helps in establishing a
tight fit with the semi-finished cylindrical sample and hence, making it work as
20 a solid cylinder. With this the machining of the sample become easy without
any fatigue test failure.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
The illustrated embodiments of the subject matter will be best understood by
25 reference to the drawings, wherein like parts are designated by like numerals
throughout. The following description is intended only by way of example, and
simply illustrates certain selected embodiments of apparatus that are
consistent with the subject matter as claimed herein.
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Figure 1 illustrates Thermomechanical test sample complying to ASTM E2368. Figure 2 illustrates the insert used during machining cylindrical samples.
DETAIL DESCRIPTION OF THE INVENTION
5 While the embodiments of the disclosure are subject to various modifications
and alternative forms, specific embodiment thereof have been shown by way
of example in the figures and will be described below. It should be understood,
however, that it is not intended to limit the disclosure to the particular forms
disclosed, but on the contrary, the disclosure is to cover all modifications,
10 equivalents, and alternative falling within the scope of the disclosure.
Figure 1 illustrates details of the sample which is used for conducting
thermomechanical testing. The sample as illustrated is round in cross section
area and hollow in nature. The reduced section (part R) is having an outer
15 diameter of 11 mm and inner diameter of 8 mm making the overall thickness
of the part 1.5 mm. since the thickness is very small there is high chance that
the sample might get damaged during machining. For the testing purpose,
three alternatives were devised to carry out the machining without causing
damage to the sample.
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Method 1
In order to machine the sample to the required dimension and accuracy, the
outer diameter is turned first for 15 mm diameter for required length of the
rod and then machining of the shoulder part (Part S) of the sample is achieved. 25 Once the outer diameter part is done, a hole has to be made through entire
size of the sample. But, practically this may lead to offset or eccentricity
through the hole when measured from either end of the sample. Hence, this
method cannot be preferred.
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Method 2
In this method, firstly the sample is machined to its outer diameter making it
a solid cylinder of diameter 15 mm then a hole is made in the solid cylinder for
an inner diameter of 8 mm diameter. Now a hollow cylinder of outer diameter
5 15mm, inner diameter of 8 mm, and thickness 1.5mm is formed. Then
machining of the outer part is done i.e. the reduced section of the TMF sample.
Machining such a narrow thickness hollow cylinder, there is high chance that
the machining tool can puncture the sample through its thickness. Hence, this
method is also not preferred.
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Method 3
In this method, firstly the sample to be tested is machined to its outer diameter
making it a solid cylinder of diameter 15 mm then a hole is made in the solid
cylinder for an inner diameter of 8 mm diameter. Now a hollow cylinder of
15 outer diameter 15mm, inner diameter of 8 mm, and thickness 1.5mm is formed
as earlier. Now the problem comes while machine the thinner part. Therefore,
in order to compensate an insert is prepared and coupled with the hollow
cylinder.
20 Figure 2 illustrates the insert employed for compensation of machining thinner
part of the sample. Needle stem (1) is made according to the inner diameter
of the TMF sample, so that a tight fit is established when the needle is inserted
in to the hollow cylinder. There is higher diameter end for this insert called as
shoulder stem which can fit to the chuck of the lathe machine. The advantage
25 is when the needle stem is inserted in to the hollow cylinder, it establishes a
tight fit and the hollow cylinder serves as good as a solid one. If the outer
profile is turned now, there is no chance of getting the sample damaged.
Hence, this method was preferred for machining of such hollow TMF samples.
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ADVANTAGES OF THE INVENTION
One of the advantages of the invention is that there are no chances of defects
due to machining of the sample at narrow thickness.
5 Another advantage with this method is that it maintains accuracy during the
machining of the sample as per the standard norms.
Yet another advantage is that this method is not material specific and can be
tested on any material type.
Still another advantage is that it is simple in construction and saves machining
10 timing.
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WE CLAIM:
1. A method for machining a slender hollow cylindrical samples used in
Thermomechanical Fatigue (TMF) tests, the method comprising steps of: - adopting an insert comprising of a needle stem (1) and a shoulder stem
(2); - establishing a tight fit in the hollow cylindrical sample by said insert, configured according to the inner dimension of the sample; wherein, said
cylindrical sample having cross section dimension in the range of 08mm to
11mm and a thickness of about 1.5mm.