Internal skinpass developed by TGS PIPE, a South Korean ERW carbon steel tube manufacturer, is the technology that makes As-Welded tube can assure DOM level dimensional accuracy. Users can take advantage of cost reduction through TGS PIPE's Internal Skinpass.

1. Technology Overview
■ Name of Technology : Internal skinpass technology for pipes to achieve a precise tolerance in internal diameters
■ Background and Purpose
Of all steel tubes used for machine structure usage, ones that have other components pushed inside them must have
dimensional accuracy for internal diameters; otherwise the inserted component would not be properly fixated. This will lead to
collision between the component and the inner side of the steel tube, affecting durability. Thus, to manufacture steel tubes for
such purpose, a precise dimensional tolerance of +/-0.1mm or lower must be guaranteed for the internal diameter. Despite this,
during the ERW manufacturing process, making a cylindrical shape by applying pressure on both sides of a slitted coil using a
roll, the internal diameter deviates depending on the coil's thickness. Such deviation is caused by a lack of control process
which uses a tool contacting with the internal wall of the pipe. Thus an additional step such as cold drawing or tube expansion
by press is required after the pipe manufacturing in order to achieve exacting levels of dimensional precision. These additional
steps increase the price of the final product.
This technology can replace costly procedures like cold drawing or tube expansion by press for pipes that have components
inserted and thus require precise dimensional tolerances. The purpose of this invention is therefore to cut manufacturing cost.
■ Details
The presented technology is designed to achieve dimensional tolerances of the internal diameter of a tube without cutting it.
The process consists of the following steps:
1. Fixate the pipe to a skid
2. Insert a supporting rod inside the pipe.
- Both the rod and the entire pipe must be equal in length.
- Using a chain conveyer, force in the supporting rod from the back-end to the front-end of the pipe
3. Insert a plug at the front-end of the pipe and merge it with the supporting rod
4. From the back-end of the pipe, pull the supporting rod that is joined to the plug.
- When the plug and the pipe’s internal surface come in contact, dimensional tolerances are achieved.
Diagram 1

2. ■ How It Differs from the Existing Technology
○ How it differs from Tube-Expansion by Press
- The existing technology of tube-expansion by press first cuts the pipe into short pieces, and then fixates each piece and
use the press to push plug into them from top to bottom to expand the internal diameter. (Refer to Diagram 2)
- The repetition of pipe-piece fixation, pressing, and relocation of the pipe pieces is time consuming.
- Moreover, if the pipe’s internal diameter is shorter than the front-end of the plug, pushing plug from top to bottom using
high-pressure press damages the product and therefore more likely to cause defects.
- However, the proposed technology works with a uncut pipe, expanding the internal diameter in one pull.
Thus it eliminates the need for the pipes to be moved, reducing time consumption for manufacturing.
- Furthermore, supporting rod and the plug are combined at the front end of tube and the plug and the internal surface
come in contact, which therefore eliminates concerns for defects.
○ How it Differs from Cold Drawing
- The existing Cold Drawing technology simultaneously carries out contraction of the outer diameter using dies while
enlarging internal diameter using a plug. Thus, dimensional tolerances of the outer and inner diameters are corrected at
the same time (Refer to Diagram 3)
- The dimensions are fixed by moving the pipes that are attached to dies and a plug. However, to move the pipes the end
part needs to be contracted and be attached to a chuck (a process called Pointing), which leads to an occurrence of a
loss when the end part is cut off/discarded after manufacturing.
- However, the proposed technology only fixes the dimension of the internal diameter and omits the use of dies. And the
plug attached to the supporting rod is pulled out while the tube is still fixated. So, aforementioned loss caused by
pointing process is avoided.
- The absence of a dies lessens stress on plug, minimizing damages on the inner surface due to plug's deterioration or
friction heat.
Diagram 2
Diagram 3

3. ■ Why does it keep cost competitiveness over cold-drawing process?
❍ The proposed technology enlarges and corrects internal side with just a small reduction. So, it doesn't need heat
treatment for stress relief. Furthermore, since drawing process is executed while tubes are fixated, material loss caused by
swaging (pointing) process is not generated.
<<Comparison of Process Flow>>
❍ However, because of its simplicity in process composition, it has limitations compared with cold-drawing like below.
① Tubes' outer side tolerance is dependent on as-rolled tubes' quality level.
② Coverable size range is also dependent on tube mills' roll possession.
③ If the tubes are too thin or inner diameter is too small, tubes rotate during drawing process because dies are not used
❍ As a result, applicable size range for the technology is restricted to the scope below in current status.
① Tubes' I/D (Inner Diameter) > 30.0mm
② Tubes' wall thickness > 2.3mm

4. ■ Application of the Technology
The proposed technology is invented for a purpose to fix the dimensions of internal diameters of pipes in a cost-efficient way.
Therefore it focuses more on the dimensional tolerances on the inner diameter than the outer.
The technology is expected to be applied to components (that have parts inserted while having parts welded to the outer part).
○ Link & Arm of Car Suspension Parts
- Collar steel tube inserted in a module to weld to a bushing  Outer surface welded to other components in the module
allows relaxed tolerances, however, tight tolerances are required for the internal surface as the bushing must be forced inside.
○ Outer Tube of a Bushing
- As weight lightening prevails in car manufacturing, usage of thin-wall tubes is expanding. However, as corrosion can affect
durability, vendors are considering plating them.
- But since the unit price of cold-drawing products is high, the cost of plating for enhanced durability offsets the cost
saved by weight lightening, which results in higher manufacturing cost.
- To make the products competitive in an equal or lower cost vendors consider applying the proposed technology to a
bushing's outer Tube.

5. ○ iRing to Connect Shock Absorber and Arm Component
- To mount a shock absorber, iRing is used to attach it with an arm. The outer part of iRing does not require such precision
as it is welded to a cylindrical part, but high precision is required for the internal part since a bushing will be inserted.
■ Quality Level
○ Measuring Method Overview
- Products: STKM13B 45.95 x 37.85
- Measuring Method:
① Cut 6,000mm-long products into three parts (front-end, middle and back-end), and then measure the outer and inner
diameters
② When measuring outer and internal diameters, measure every 30 degrees clockwise, totaling to six times
③ Analyze the process capability from the data collected, and apply a tolerance range of CPk > 1.0 to each internal and
outer diameter
○ Result: Sufficient process capability is found within +/-0.1mm tolerance range in both internal and outer diameters
- Inner Diameter: Process capability of CPk >1.0 is found within a tolerance range of +/-0.09mm even when a bead part is
included
- Outer Diameter: Process capability of CPk >1.0 is found within a tolerance range of +/-0.05mm
<<Analysis of Process Capability: Internal Diameter>>
① Outer Diamter
② Inner Diameter

6. Appendix 1. Outer Diameter Measurement
Appendix 2. Inner Diameter Measurement
*측정방법: 용접부 기준 30°씩 시계방향으로 12번을 측정
1 2 3 4 5 6 AVG MAX MIN DIFF MAX MIN DIFF
#1 45.950 45.960 45.960 45.920 45.940 45.960 45.95 45.96 45.92 0.04 45.96 45.92 0.04
#2 45.940 45.960 45.940 45.930 45.960 45.970 45.95 45.97 45.93 0.04 45.97 45.93 0.04
#3 45.940 45.960 45.940 45.940 45.930 45.950 45.94 45.96 45.93 0.03 45.96 45.93 0.03
#4 45.950 45.930 45.960 45.930 45.950 45.960 45.95 45.96 45.93 0.03 45.96 45.93 0.03
#5 45.940 45.960 45.940 45.940 45.930 45.970 45.95 45.97 45.93 0.04 45.97 45.93 0.04
#6 45.950 45.930 45.940 45.920 45.950 45.960 45.94 45.96 45.92 0.04 45.96 45.92 0.04
#1 45.950 45.970 45.960 45.930 45.940 45.950 45.95 45.97 45.93 0.04 45.97 45.93 0.04
#2 45.930 45.960 45.930 45.930 45.950 45.950 45.94 45.96 45.93 0.03 45.96 45.93 0.03
#3 45.940 45.950 45.930 45.920 45.960 45.950 45.94 45.96 45.92 0.04 45.96 45.92 0.04
#4 45.940 45.960 45.940 45.950 45.960 45.950 45.95 45.96 45.94 0.02 45.96 45.94 0.02
#5 45.940 45.960 45.960 45.950 45.960 45.950 45.95 45.96 45.94 0.02 45.96 45.94 0.02
#6 45.930 45.950 45.930 45.920 45.950 45.930 45.94 45.95 45.92 0.03 45.95 45.92 0.03
#1 45.960 45.970 45.960 45.930 45.960 45.950 45.96 45.97 45.93 0.04 45.97 45.93 0.04
#2 45.940 45.960 45.920 45.940 45.930 45.960 45.94 45.96 45.92 0.04 45.96 45.92 0.04
#3 45.950 45.960 45.950 45.930 45.930 45.950 45.95 45.96 45.93 0.03 45.96 45.93 0.03
#4 45.950 45.970 45.950 45.930 45.940 45.960 45.95 45.97 45.93 0.04 45.97 45.93 0.04
#5 45.940 45.960 45.950 45.930 45.930 45.960 45.95 45.96 45.93 0.03 45.96 45.93 0.03
#6 45.950 45.970 45.920 45.930 45.940 45.950 45.94 45.97 45.92 0.05 45.97 45.92 0.05
45.95
항목
선
단
중
단
후
단
비드부포함 비드부제외
외경 측정용 체크시트STKM13B
45.95 x 37.85
조관 , 내경인발
③
①
②
④
⑥
⑤
*측정방법: 용접부 기준 30°씩 시계방향으로 12번을 측정
1 2 3 4 5 6 AVG MAX MIN DIFF MAX MIN DIFF
#1 37.850 37.830 37.825 37.850 37.825 37.830 37.84 37.85 37.83 0.02 37.85 37.83 0.02
#2 37.875 37.830 37.835 37.830 37.835 37.820 37.84 37.88 37.82 0.05 37.84 37.82 0.02
#3 37.870 37.815 37.810 37.835 37.820 37.815 37.83 37.87 37.81 0.06 37.84 37.81 0.02
#4 37.885 37.830 37.805 37.820 37.830 37.815 37.83 37.89 37.81 0.08 37.83 37.81 0.02
#5 37.875 37.830 37.835 37.835 37.820 37.820 37.84 37.88 37.82 0.05 37.84 37.82 0.02
#6 37.885 37.815 37.810 37.830 37.835 37.815 37.83 37.89 37.81 0.07 37.84 37.81 0.02
#1 37.870 37.830 37.810 37.830 37.830 37.825 37.83 37.87 37.81 0.06 37.83 37.81 0.02
#2 37.870 37.830 37.815 37.820 37.835 37.837 37.83 37.87 37.82 0.05 37.84 37.82 0.02
#3 37.875 37.830 37.820 37.810 37.840 37.820 37.83 37.88 37.81 0.06 37.84 37.81 0.03
#4 37.870 37.830 37.810 37.838 37.800 37.850 37.83 37.87 37.80 0.07 37.85 37.80 0.05
#5 37.870 37.830 37.810 37.820 37.835 37.837 37.83 37.87 37.81 0.06 37.84 37.81 0.03
#6 37.870 37.815 37.810 37.830 37.840 37.820 37.83 37.87 37.81 0.06 37.84 37.81 0.03
#1 37.870 37.830 37.820 37.830 37.850 37.830 37.84 37.87 37.82 0.05 37.85 37.82 0.03
#2 37.875 37.830 37.825 37.822 37.830 37.810 37.83 37.88 37.81 0.06 37.83 37.81 0.02
#3 37.880 37.825 37.805 37.820 37.840 37.830 37.83 37.88 37.81 0.08 37.84 37.81 0.04
#4 37.880 37.820 37.850 37.805 37.840 37.825 37.84 37.88 37.81 0.08 37.85 37.81 0.05
#5 37.875 37.830 37.805 37.820 37.830 37.830 37.83 37.88 37.81 0.07 37.83 37.81 0.02
#6 37.880 37.825 37.820 37.805 37.840 37.830 37.83 37.88 37.81 0.08 37.84 37.81 0.04
37.83
항목
선
단
중
단
후
단
비드부포함 비드부제외
내경 측정용 체크시트STKM13B
45.95 x 37.85
조관 , 내경인발
③
①
②
④
⑥
⑤