Drill bushes are classified as follows: 1. Fixed Bushes 2. Liner Bushes 3. Renewable Bushes 4. Slip-in Bushes 5. Screw or clamp Bushes 6. Special Bushes.
Type # 1. Fixed Bushes or Press-Fit Bushes:
These are used for limited production tooling where no change in bushing is called for. It should be light press fit. These may be headed or plain type. In headed type the head enables the bush to the used as a stop for the tool.
It also enables the length of the bush to be increased relative to the depth of the bush plate if the later has to be restricted. Sometimes the bottom of the bush may be required to conform to the profile of the component to prevent deflection of tool and it is possible with headed type bush. Repeated changes of press-fit bushings would affect their accuracy.
The advantages of plain fixed bush are low cost, it can be set closer, less likelihood of their interference with other parts of the drill press, can be used on all the six sides of a box type jig.
Disadvantage of plain type is that due to slip, it may blip in and that of headed type is that when box type of jig is used and several faces are to be used, difficulty is likely to be experienced in locating other faces as the projected head would cause interference.
Type # 2. Liner Bushes:
These act as hardened guide for both slip and renewable type bushes and sometime used as guide to tools. In this way the jig plate of soft metal could be used and after wear and tear only liner is to be replaced and not the whole of jig plate. The liner bush is either press fit in the jig plate or secured by means of screws.
Type # 3. Renewable Bushes:
These are used when bushings are changed many times during the life of jig. In these bushes, press fit type is not generally used but only light press fit type is used. In order that the bush may not come out with the tool, when it is withdrawn some sort of set screw for retaining the bush with jig plate is used. When the bush is to be replaced due to wear, retainer screw is removed and worn bush is taken out. As the operation of taking out the bush generally damages the bore of the bush plate, it is usual practice to use liner bush which has other advantages also.
ADVERTISEMENTS:
Slip-renewal bushes are used for performing more than one operation in same location, like drilling and reaming.
These bushings are used with a liner bushing and are held in place by the radial lock and the bushing head. When another size bushing is required for a hole, the first bushing is removed by turning it counter clockwise and lifting. The new bushing is installed by placing it in the hole with the radial lock aligned and turning it clockwise (Refer Fig. 28.56).
Where only one operation is performed in each hole, (but where several bushing are required during the life of the tool), fixed renewable bushings fitting into a liner are used. They are held by a mechanical clamp and take considerable longer to remove than slip-renewable bushings.
Type # 4. Slip in Type Bush:
Sometimes we require more drills to pass at one place e.g., we want to drill 5, 10, 15 mm holes in successive operations at the same position of the job, i.e. when any hole requires two or more tools to be used then it is necessary to use a slip bush for each tool except when the tool is guided by the liner bush.
The bore of each bush is made to suit a particular size of tool. In slip bushes arrangements are provided to stop the bush from rotating during machining process and also to avoid its following the tool when the latter is being used to clear swarf or when it is withdrawn at the end of the cut.
This is achieved by utilising a retaining screw which is fixed permanently in the bush plate. The slip bush is provided with a clearance slot with a radius slightly larger than that of the head of screw and is also provided with a step which, when the bush is rotated, will turn and lock under the flange of the screw. The lifting of the bush is prevented by the head of the screw.
Type # 5. Screw or Clamp Bushes [Fig. 28.59]:
In this type of bush, threaded portion is used for holding purposes and as threads can’t provide good location, the plain portion of the bush is utilised for that purpose.
Type # 6. Special Bush:
Suppose it may be required to drill two holes very near to each other and there may not be sufficient space to fit the bushes. In that case we provide a bush with more than one hole as shown in Fig. 28.60.
Oil groove bushings permit positive and complete lubrication for continuous high-speed operations.
ADVERTISEMENTS:
Important features for designing of jig bushes are:
(1) Bushes must be case-hardened for minimum wear.
(2) Loose or screwed in bushes should not be used where accuracy is most important.
(3) If threaded bushes are at all used, cylindrical guide surfaces must be provided. Bushing should be long enough to support and guide the tool properly.
(4) The bearing length for fixed bush should be to 2 times the diameter of bush. In case of slip bush, the length should be about 2-3 times the diameter of the drill.
(5) If the bushing is larger than the recommended length, the remainder of the length farthest from the work should be relieved as shown in Fig. 28.61.
(6) The wall thickness of the bushing should easily be able to withstand all the cutting forces and maintain tool accuracy.
(7) About one drill diameter clearance should be provided between the work and the bush for chip clearance etc. For small and accurate holes, and when drilling on inclined or curved surfaces, bushes should be brought down close to the work. If the bushing is unnecessarily very close to the work piece, chips would cause the bushing to wear rapidly. If the bushing is too far away, precision is lost.
(8) Holes designed to receive drill bushings must be made undersize and be perfectly round to allow the bushing to fit correctly. If the drill bushings are not properly installed, they could fall out during use, or bind and break the tool.