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close this bookCentring, Drilling Counterboring and Countersinking, Reaming and Tapping on the Drilling Machines - Course: Techniques for Machining of Material. Trainees' Handbook of Lessons
View the document(introduction...)
View the document1. Purpose of centring, drilling, counterboring and countersinking, reaming and tapping
View the document2. Kinds, construction and applications of drilling machines
View the document3. Kinds and construction of centring, drilling, counterboring and countersinking, reaming and tapping tools
View the document4. Clamping possibilities for workpieces and tools
View the document5. Preparation of work
View the document6. Centring, drilling and counterboring or countersinking
View the document7. Reaming and tapping

5. Preparation of work

The prophylactic health and fire protection and labour safety are an essential requirement for carrying out this technique. Therefore, the following is to be observed:

- Keep your workplace always clean and in order.

- Use only such working tools which are in a good condition.

- Use appropriate protection for your body (safety glasses, protective gloves).

- For cleaning, checking and clamping place the machine out of operation. Do not make safety devices ineffective.

- Use appropriate aids for chip removal (brush, broom, etc.).

- In case of defects which affect the operational reliability of the machine do not start the machine and eliminate the defects.

Prepare the work in such a manner that all necessary working tools and objects of work can immediately be used without losing any time. This includes above all:

- Receive the work order, instructions and workpieces to be machined. Check the workpieces for completeness and accuracy to size, i.e. check the premachining quality.

- Study the technical drawings (working drawing) and data sheets

- Lay out and check the necessary working tools (tools, measuring and testing means, clamping equipment) according to the instructions for work or your own decision.

- Determine cutting values.

- Fix sequence of operations.

Choice of cutting values

When determining the cutting values, pay attention to the following hints:

As a rule, the tools for centring, drilling, counterboring and countersinking, reaming and tapping consist of tool steel or high-speed steel. The choice of cutting values will depend on this.

The required speed (n) is calculated by rearranging

The machining recommendation for v (cutting speed) is as follows:


25 m/min.


5 m/min.


18 m/min.

tapping (boring)

3 m/min.

counterboring or

15 m/min.


The feed can be effected both mechanically (by using the feed gear) and manually. When tapping, consider the thread lead. Choose such a feed that the surface finish required is guaranteed. Always choose clockwise rotation as direction of rotation of the tools. Maintain this clockwise rotation also for returning the tools (with the exception of tapping).

A bore with a diameter of 20 mm is to be machined into a work-piece by means of a twist drill. What speed is to be chosen?

n = _________________________ r.p.m.

Setting of the machine places high demands on the knowledge and skills of the operator. This includes:

- Fix the workpiece and tool in a certain position to each other
- Set the cutting values accordingly.
- Lay out further necessary working tools and auxiliary equipment.

In view of these factors the techniques are carried out in the following sequence and show a degree of technological unity:

1. centring or punch marking
2. drilling
3. counterboring or countersinking
4. reaming and tapping.

In order to reduce the wear of the tool cutting edges, it is necessary in most cases to use a coolant or lubricant. In this connection the following should be considered:

- Cast iron is always machined without fluid.
- For machining aluminium it is advisable to add spirit (pay attention to danger of fire).
- For other metallic materials use an emulsion of water and oil (drilling emulsion).
- For reaming and tapping use cutting oil.

In what technological sequence of operations is a tapped hole made?