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close this bookThreaded Joints - Course: Techniques of Fitting and Assembling Component Parts to Produce Simple Units. Trainees' Handbook of Lessons
View the document(introduction...)
View the documentPreliminary Remarks
View the documentHints on Labour Safety
View the document1. Purpose of Threaded Joints
View the document2. Selected Types of Bolts and Screws
View the document3. Selected Types of Nuts
View the document4. Selected Types of Locking Devices for Bolts and Screws
View the document5. Selected Types of Washers
View the document6. Types of Threaded Joints
View the document7. Stresses in Threaded Joint
View the document8. Tools
View the document9. The Technological Steps of Making Threaded Joints
View the document10. Undoing Threaded Joints

9. The Technological Steps of Making Threaded Joints

The preparations for direct threaded joints differ from those for indirect joints.

9.1. Direct Joints

Direct joints can be found mainly in parts or assemblies of machines. Most threads are cut mechanically and the component parts are simply joined by screwing. Make sure that the parts which are to be joined by screwing coincide in their nominal thread diameters, pitch and sense.

Example:

Drill chuck on a machine spindle in a hand drill.
Closing caps on containers.

Pipes may be directly joined by screwing as well. Most connecting parts (pipe bells) and fittings (angles, bends) are manufactured industrially and only an external thread needs to be cut on the pipe.

Task:

An unthreaded 1-inch (25-mm) steel pipe is to be screwed into an elbow fitting heaving internal thread. A short external thread is to be cut on the pipe.


Figure 33. Pipe and elbow fitting

Sequence of operation:

1. Prepare the die stock

- Mount the cutting dies with pipe thread R 1-inch in the sequence of operations 1 to 4.

- Set the fine adjustment for the entering tap.

- Open the pilot.

2. Chuck the pipe, apply cutting fluid to the deburred end of the pipe.

3. Place the die stock, pilot end first, on the pipe end, and adjust the pilot.

4. Make an entering cut and rough cut over 19 mm length by turning evenly in clockwise direction. Then loosen the clamping screw and break the burr with a short jerk to the right.


Figure 34. Cutting a pipe thread

5. Open the die stock and remove it. (Do not turn it to remove it).

6. Set the fine adjustment for re-threading.

7. Place the die stock on the pipe and adjust the pilot.

8. Re-thread, then loosen the clamping screw and deburr the pipe.

9. Open the stock and remove it from the pipe (Do not turn it to remove it).

10. Clean the thread and apply some grease to it.

11. Screw the pipe into the elbow fitting until it stops. Use a pipe wrench.


Figure 35. Making a joint by screwing

Note:

- Where tight joints are to be made of pipes, apply a packing of hemp tightly, starting from the front end of the pipe backwards. Apply in right-hand direction if the thread is right-hand. Then apply acid-free grease and screw into the internal thread, first by hand, then with a pipe wrench.

- Where the pipe joint is to be made as a part of a permanent pipe installation, use a pipe with long thread to ensure that the screwed joint can be loosened even after a long time. Screw a pipe bell over the full length of the thread. Use short thread on the pipe and press it flush on the long thread. Then turn back the pipe bell and join both pipes without twisting either of them.

9.2. Indirect Joints

Most threaded joints for fastening purposes are indirect joints. When making the joint, make sure that the nominal diameters, pitches and senses of rotation as well the material of bolts, screws and nuts coincide.

There are two main ways of making a threaded joint the indirect way.

1. Bolt - component parts - nut
2. Bolt - component parts - component part with female thread

Task:

Two steel parts are to be joined firmly by a hexagon head bolt and a nut. The bolted joint will be exposed to dynamic stress, hence it must be locked.


Figure 36. Joint made with a bolt and a nut, locked with a spring ring

Sequence of operations:

1. Mark out the steel parts and punch a mark for the bore hole centres.

2. Set up the steel parts for drilling. Drill holes together, where possible. (The bore hole diameter should be slightly bigger than the thread diameter).

3. Deburr both ends of the bore hole with a spotfacer.

4. Apply some grease to the bolt. Insert the bolt into the bore hole and place a locking element (such as a spring ring) onto the end projecting from the hole.

5. Screw on a nut for a few turns by hand, then tighten with an open ended wrench.

Note:

- Use washers where the surface of the work is unclean or smoothen the surface.

- Where the bore hole is provided when the part is supplied, use a flat or pilot-type countersink to level the face which bears the bolt head.

Task:

Three steel parts are to be joined firmly by a hexagon head bolt. The receiving thread is to be in the last of the three component parts. The bolted joint will be exposed to dynamic stress.


Figure 37. Joint made with a bolt and a component part, locked with a spring ring

Sequence of operations:

1. Mark out all steel parts, punch a mark for the bore hole centre.

2. Set up the steel parts for drilling, drill the holes together, if possible. The bore hole diameter should be of the same dimension as the minor diameter of the thread.

· Use the following formula to calculate the drill diameter:

D= N-S

where:

D - minor diameter of the thread/hole diameter
N - nominal diameter
S - pitch

· For blind holes, consider the length of thread engagement and the run-out depth of the thread tap. The tap hole must be made deeper by that dimension. Formula:

TB = TG + TA

where:

TB - hole depth
TG - depth of thread
TA - depth of run-out

Where no values can be taken from handy tables, calculate approximate values using the following formula:

TA = 0.7 · D

· Read the speed of the drill from the table or calculate it with the following formula:


where:

n - speed (rpm)
V - cutting speed (m/min)

3. Take the component parts apart and work them separately. Face both sides of the hole with a 60° included angle countersink. The hole to be faced is that in the last component part in the joint. The sink diameter is to be the same as the thread diameter.

Then cut the thread.

DS = N

4. Bore an oversized hole in the two other component parts. The dimension of the oversize depends on the nominal diameter of the bore:

Nominal diameter

M3

M4

M5

M6

M8

M10

M12

M16

M20

Through hole diameter

3.4

4.5

5.5

6.6

9

11

14

18

22

5. Put the component parts are together, push the locking element, such as a spring ring, onto the threaded bolt and apply some grease to the threaded portion. Srew in by hand and then tighten with an open ended wrench.

Five details of making a tapped hole for a screw.
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Rules for assembly:

- Where several component parts are screwed together without a nut, the receiving thread must only be in the last part as seen from the head of the screw. All in between parts have through holes.

- Heads of bolts and screws, when they are not to rise above the surface of the workpiece, must be mounted flush. Cheese head screws are countersunk with a piloted counterbore. Hat-headed screws are countersunk with a 90° included angle countersink.

- Locking devices are always assembled at the side with the highest torque.

- Joints which consist of a bolt or screw and a nut always have the highest torque on the nut. Hence, the head of the bolt or screw is held and the nut is tightend.

- Always assemble a locking component at the nut end in joints which consist of a bolt or screw and a nut.

- Where several nuts are tightened on a component part (for example, the lid of a container), always start from the centre and proceed outwardly, crosswise.


Figure 38. Sequence of operations in tightening a screwed joint

Where several component parts are to be joined by a screw, which part must have a receiving thread?
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How will you proceed in tightening a joint which consists of a bolt and a nut?
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Where should the locking device be placed in a bolt-and-nut joint?
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How will you proceed in tightening several screws or bolts in a lid of a container?
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