3. Preparations for thread cutting by dies and taps

Prior to thread cutting by dies and taps, position in readiness and in proper order all required tools and auxiliary means, allowing so quickest possible access times.

In this connection, pay attention to the following rules:

- Check that all tools function properly; do not use defect tools.

- Working means to be used must not be stacked one above the other.

- Place all measuring and testing tools at the places provided therefore.

- Lay aside tools only when properly cleaned.

- Select all necessary auxiliary means in line with the work assignment and position solely on the provided supports.

Which parameters must be followed when checking the functionality of the tools?
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The setting-up of the lathe is mainly made according to the following points:

- Chucking the workpieces for thread cutting

· Extending soft chuck jaws as, normally speaking, the lathe parts to be processed have already been preworked. Extending the chuck jaws ensure the true running of the parts and the workpiece surfaces are not impaired.

· Checking the proper dimensional preparation of the thread diameter and deburring.

Figure 12. Deburring the workpiece

1 minor diameter

The thread diameter roust be turned about 1/10 smaller than the thread pitch as, whilst cutting, the tool somewhat compresses the material, pressing it outwards. Consequently, the external diameter increases and the die may tear out the thread turns.

Figure 13. Compression during thread cutting (1/10 greater pitch)

1 minor diameter of the bore, 2 thread minor diameter, 3 tap, 4 compression, 5 workpiece

Example:

Thread M 16
- thread increase 2 mm
1/10 of 2 mm a 0.2 mm
the thread diameter is turned to 15.8 mm

A thread M 30 is to be cut on a shaft. What must be the nominal diameter of the turn?
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If the diameter is too small the thread cannot be fully cut. Sound deburring ensures a faultless starting taper. Precise true running of the workpiece is the precondition for a uniform starting taper of the thread turns.

Why must the nominal or minor diameters when thread cutting be 1/10 less or greater than the corresponding increase?
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- Clamping the tools for thread cutting

· The die is fixed to a die capsule and then, together with the capsule, accommodated in a die holder (or directly in the die holder).

Figure 14. Die capsule

1 clamping nut, 2 expanding nut, 3 clamping nut, 4 holding nut, 5 die, 6 die capsule, 7 holding nut

Figure 15. Die holder with capsule

1 die, 2 die capsule, 3 die holder

· The die holder inclusive of the capsule and die must be thoroughly cleaned so that the front surfaces are positioned properly and in a distinctly angular manner.

· The expanding screw roust mesh into the slot of the die. The expanding screw opens the die whilst the adjusting screw closes it (see Fig. 12).

· Use an unhardened thread bolt to set the die.

· A support for the die holder is chucked in the tool holder.

· The support for the die or tap wrench holder must be securely chucked and selected according to the thread length.

· The tailstock quill is brought into working position (Fig. 14) whereby special attention must be given to quill cleanliness.

· When thread cutting by a tap, the tap wrench is positioned on the tap square.

· When thread cutting by a tap ensure correct centring and tailstock middle-positioning.

- Set the cutting values.

Generally speaking, low cutting speeds are selected because of the substantial cutting operation (entering, plug tap and plug third tap in one work sequence), coupled with complicated chip removal. The selected cutting speed depends on the workpiece material and the thread size. When using a die on steel workpieces the cutting speed is 3 - 4 m min^-1 and 4 - 15 m min^-1 when employing a tap.

Which rotational speed shall be selected where a thread M 24 x 100 is required for 100 bolts given an St. 60 material?

Formula:

Calculation:
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Explain your reasons for selecting the speed?
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Why is only a low speed chosen when thread cutting by a die and tap?
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