For the functionality of a component, beside to the design and the material, mainly the quality of the manufacturing in respect of the adherence of necessary tolerances is of importance. The production cost of a component rise disproportionately the narrower the tolerance range is chosen. Therefore, these tolerances must always be aspired only as accurate as necessary. In addition to the predetermined tolerance for dimensions and surfaces, the used manufacturing process has a tolerance range too. This tolerance range depends on the chosen manufacturing process and on how well this process is controllable.

Ways of tube manufacturing

Since the most controlled manufacturing process always has a tolerance greater than zero, there are three ways to manufacture a tube as an example from a certain material under economical manufacturing conditions:

  1. The dimensions of the raw material used are chosen such that with the maximum manufacturing
  2. tolerance, the actual wall thickness falls not below the minimum required one
    1. a) Advantage: no rejects
    2. b) Disadvantage: higher material costs
  3. The dimensions of the raw material used are chosen smaller than in point 1 so that not all pipes have the
  4. required wall thickness
    1. a) Advantage: lower material costs
    2. b) Disadvantage: generation of rejects
  5. The dimensions of the raw material used are chosen smaller than in point 1. The wall thickness is continuously monitored during the manufacturing process and it is acted before the wall thickness falls below the permissible wall value.
    1. a) Advantage: lower material costs, no generation of rejects
    2. b) Disadvantage: very costly measurement required

In-situ measuring

BHDT GmbH is constantly striving to optimize the production quality and the economics of production. Our success is in part thanks to modern machinery, highly trained staff and the best possible control of the manufacturing process, the prediction and elimination of failure sources before they can cause a bad part. As an example, BHDT GmbH uses an especially developed in-situ measurement equipment for the determination of the wall thickness during the deep-hole drilling process by continuously measuring wall thickness at several points.

In this way, there are no rejects in the maximum 16m long pipes due to too low wall thicknesses. The equipment is also used to minimize the eccentricity between the inside and outside pipe diameter. The advantages of in-situ measurement in deep-hole drilling are:

  1. Large product quality because the operator can selectively influence the direction of drilling
    1. a) very small eccentricity of the bore
    2. b) no rejects due to falling below the minimum required wall thickness
  2. Drilling has not to be stopped for manual thickness measurements
    1. a) Improvement of surface quality since there are no steps due to stopping
    2. b) Reduction of drilling time
    3. c) Extension of drill-head lifetime

Increase of accuracy

The challenge in the development of the equipment was that the thickness of a pipe has to be measured while rotating and that the quality of the used ultrasonic measurement method is highly dependent on the speed of sound in the material. To increase the accuracy of this measurement method extensive sound velocity measurements were performed. Thus, the dependence of the sound speed of the chemical composition, the microstructure, the texture and the heat treatment condition could be determined. In addition, the measured sound velocities were correlated with the hardness of the materials. In this project, both high-strength quenched and tempered steels as well as various types of corrosion-resistant materials have been investigated. The results of this study were presented at the „World Conference on Non Destructive Testing“ 2012 in Durban, South Africa.

In Situ Process Control

The left-hand picture shows the ultrasonic measuring head on the bottom of a rotating pipe in the deep-hole drilling process. In the right picture, the orientation and size of the maximum and minimum eccentricity and wall thickness in the form of user interface are exemplary to see.