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CLOSE THIS BOOKOperations on Wood Turning Lathes - Course: Mechanical woodworking techniques. Trainees' handbook of lessons (Institut fr Berufliche Entwicklung, 19 p.)
6. Turning Technology
VIEW THE DOCUMENT6.1. Wood Selection
VIEW THE DOCUMENT6.2. Turning Long Trunks
VIEW THE DOCUMENT6.3. Turning Cross Pieces
VIEW THE DOCUMENT6.4. Grinding Surfaces

Operations on Wood Turning Lathes - Course: Mechanical woodworking techniques. Trainees' handbook of lessons (Institut fr Berufliche Entwicklung, 19 p.)

6. Turning Technology

6.1. Wood Selection

When selecting wood for turning operations the following requirements vis-a-vis material must be heeded:

- According to its purpose and subsequent further processing the wood shall possess sufficient stability and the necessary grain and colour standards.

- Only select properly dried wood.

- The workpiece must not have any fissures. Otherwise it may splinter during processing - danger of accidents!

- Long trunk workpieces with knots and knot ends may break during processing.

- Ensure no alien bodies (nails, screws,...) are in the wood.

Straight, round woods (branches or thin trunk pieces) can be used for long trunk turning operations. Such woods, however, require special care. Fissures arise at the grain ends if drying ensues too quickly. Such tearing can be prevented if:

- the grain ends are coated with cold glue,
- the bark around the trunk is notched,
- no too thick round wood is selected,
- the wood dries slowly and not in the sun.


Figure 19 - Round piece of wood prepared for drying

Thick trunks are split once, thrice or four times and the grain ends protected from too rapid drying. This prevents to a marked degree the emergence of drying fissures.


Figure 20 - Splitting trunks

(1) split simply
(2) split threefold
(3) split fourfold

The partial sections, after drying, are set up as blanks for turning operations. This ensues through splitting, sawing or planing.

Blanks of considerable diameter can also be yielded by sticking together thin board pieces with joints. These joints should, if possible, not be visible. Heed the wood structure of the various partial pieces (Figure 21). Blanks for various operations can be simply yielded by ripping up thick sawn timber (Figure 22).


Figure 21 - Blanks made of board pieces


Figure 22 - Blanks which arise by splitting up thick sawn timber

Much care is required when marking workpiece centre points. Workpieces which have not been centrically chucked may easily, because of the unevenly influencing centrifugal forces, detach themselves from the machine. - Danger of accidents!

Furthermore, material consumption increases given inaccurate marking and chucking.

How can dry fissures be prevented at the grain ends of raw woods?
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Why should a workpiece be chucked centrically in the machine?
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6.2. Turning Long Trunks

Round turning

After workpiece marking and punching the plugging chisel (trifurcate) is lightly struck with a hammer and chucked with the tailstock centre under medium pressure. Thereby the driving tongue of the chucking device shall engage the diagonal lines of the grain end wood faces. This reduces the danger of splitting and, unimpeded by the tailstock, the hammer can be directed to the top, right-hand comer of the workpiece.

The suppport strip should be positioned close to the workpiece without, however, restricting the movement of the turning workpiece. The height of the support strip is determined by the necessary cutting rake for the various wood types.


Figure 23 - Arrangement of wood during long trunk turning

(1) very hard wood, (2) hard wood, (3) soft wood

When turning, select the most favourable cutting speed for the wood being processed. This ensues by selecting the proper spindle rotational speed.

Table 2: Spindle rotational figures related to tool diameter

Workpiece diameter in mm

Revolutions per min-1


soft wood

hard wood

to 50

2500

2000

to 80

2000

1400

to 200

700

700

Prevent bigger workpieces from splitting by applying suitable chucking means.

Initially turning is undertaken using the roughing tool. The tube is thereby placed firmly on the support strip at roughly right-angles to the rotational axis of the workpiece. The right hand seizes the grip whilst the left hand directs it to the strip. If turning is undertaken carefully the tube with the hollow side is somewhat inclined in direction of feed. The feed can be reciprocal depending on the length of the support strip, respectively the workpiece length.

Once the workpiece has been turned uniformly round it is smoothed by means of a planer which is held at some 65º to the rotational axis of the workpiece.


Figure 24 - Handling a turning tool

Position the planer higher up so that the cutter does not yet engage. Then the workpiece is slowly pulled back until chip removal takes place. The most favourable cutting angle is determined by trial and error depending of wood hardness. The tool shall cut rather than chisel. Cut chip removal yields a smooth top surface which requires little reworking. Turning by means of a planer requires a great deal of practise.

The turning process is over once the necessary size and required surface quality have been attained.


Figure 25 - Tool positioning as turning process commences

At which height, related to the rotational axis of the workpiece, must the support strip be set when turning soft wood?
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Form turning

This work technique presupposes prior round turning with a roughing tool. Frequently profiles are presented in Figure 26.


Figure 26 - Frequent profiles during form tuning

1 rod, 2 candle, 3 sharp rod, 4 chamfer, 5 round rod, 6 curve and chamfer

A templet of cardboard or a templet drawing is prepared. This aid is often held behind the workpiece during turning whereby the forms are compared. Prior to turning all profile limitation lines are carried over to the workpiece. The templet is positioned onto the support strip and the pencil point is held carefully to the rotating workpiece.

Turning is undertaken by means of the forming tool. The tool is held like a roughing tool. However, turning is only permissible from the greater to the smaller workpiece diameter section, thereby ensuring clean cutting faces.


Figure 27 - Profile templet

Cutting-off

A special cutting-off tool is required for this purpose. This tool is not part of the basic equipment of the wold turning lathe, however it can be easily made from a kitchen knife or a thin, flat file.

This operation serves to split up workpieces to precise lengths and also to separate small parts.

The support strip must be so set that the tool cutter is brought into play at the workpiece rotational axis level. Workpieces chucked inbetween the centres should not be completely cut off. Otherwise the workpiece may jam with the tool at the separation point and become damaged. This possibility denotes an additional accident danger with bigger workpieces.

Prior to cutting off the workpiece must be completely prosessed unless a special chuck, for instance a tongue clamp, is available for further working.

6.3. Turning Cross Pieces

Here the wood processing direction constantly changes. Mainly used are the roughing and finishing tools. The support strip is so set that the tool cutter engages at centre of rotation level. Initially the roughing tool is employed for preturning. The tool is moved from the workpiece centre to the edge and back again. Thereby the roughing tool with cavity is somewhat inclined to the respective direction of feed.

Next the surface is processed with the finishing tool until the required quality has been attained. Chamfers and curved sections cannot be processed using a finishing tool. Instead a tube must be used.

In the case of thin workpieces the narrow faces are processed from the wide face. Chip removal must ensue in the workpiece centre with minimal feed movement. Cutting conditions near the rotational axis also worsen due to cutting speed reductions towards the workpiece centrepoint. In the case of complicated turning operations in the centre section of the workpiece face, a slight increase in spindle rotational speed can improve cutting conditions. However, increasing the rotations speed is not possible with bigger workpieces for then too great centrifugal forces might cause the workpiece to splinter. This is extremely dangerous!

6.4. Grinding Surfaces

Grinding seeks to improve surface quality. According to wood variety grinding is undertaken with middle-fine to fine abrasive paper commonly used in wood processing work. The abrasive paper pieces are folded, bent of formed into thin strips depending on the profile size. Employing the prescribed processing rotational speed the workpiece is treated as for turning. When grinding the abrasive paper is always moved counter to the rotational movement of the workpiece. Consequently, no abrasive dust can collect between the abrasive agents. Nor are any burnt spots caused through too great frictional heat.

Always hold the abrasive paper so that it cannot wind itself around the rotating workpiece. - Danger of accidents!

Only grind the workpieces slightly. The workpiece shape must not be altered through grinding!

Why must the prescribed rotational speed not be exceeded given extreme workpiece diameters?
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