Drill Bit Depth Adjustment This invention concerns drill bits and the
control of the depth of drilling achieved thereby.
The drilling of holes to a predetermined depth in workplaces of various types presents problems which have been addressed hitherto in a number of ways. One such is to use the drill bit in a drill press provided with a depth gauge, and to effect drilling using the gauge. However, this is not possible with hand held drills, and depth gauges attached to the bodies of the drills are generally unsatisfactory because slight tilting of the drill and the drill bit therein can result in considerable inaccuracies in the depth of hole produced.
Other methods have been proposed, particularly for use with hand held drills, one being simply to adhere a piece of adhesive tape to the drill bit to make the depth to which drilling should be effected. A particular problem with this method is that the adhesive tape becomes progressively worn away as more holes are drilled, thereby leading to inaccuracies in drilling.
The above problem with using adhesive tape can be overcome using a collar secured to the drill bit, the collar having an axial - 2 hole slightly larger than the diameter of the drill bit and a radial grub screw which can be tightened on to the drill bit to secure the collar on the bit. Although such devices should in theory provide reliably accurate drilling, in practice the collars tend to slip on the drill bit, with the result that the set hole depth is lost. The reason for this is that the collar in general has to be set on the flight of the drill bit, which by its nature is not a smooth cylinder and so provides relatively poor support for the collar, and the radial grub screw has to be set on the flight of the drill bit resulting in uncertainty as to the position and security of the screw on the dill bit. An arrangement using such a collar is described in US5382120.
According to the present invention there is provided a depth setting device for a drill bit having an axially extending flight and associated groove, the device comprising a collar for mounting on the bit and a screw for securing the device on the bit, the device further including a radially inwardly extending projection for engaging the drill bit groove.
Devices in accordance with the present invention have been found to be more satisfactory than prior art simple collars with grub screws, the radially inwardly extending projection generally serving to minimise rotation of the collar about the screw used to secure the collar to the drill bit.
The radially inwardly extending projection preferably forms part of a tubular extension of the collar, for example a tab which forms part of the tubular extension or a crimped region of the tubular extension.
An embodiment of device in accordance with the present invention will now be described with reference to the accompanying drawings in which: Fig. 1 is a side view of the embodiment; 3 - Fig. 2 is a section on line I-I of Fig. 3; and Fig. 3 is a perspective view of the embodiment positioned on a drill bit.
The illustrated device 1 consists of a collar 2 with a central bore 3 having a diameter slightly larger than that of a drill bit 4 with which it is to be used, a grub screw 5 in a radial threaded bore in the collar 2 enabling the device to be secured to the drill bit 4. This is substantially identical to prior art methods of positioning drilling depth collars on drill bits.
The difference between the illustrated device 1 and collars used previously as depth setting devices for drill bits is a substantially tubular portion 6 which is secured to or forms an extension of the collar 2. Tabs 7 on the tubular portion 6 remote from the collar 2 extend inwardly towards the axis of the drill bit 4 so that when the device 1 is placed on the drill bit 4 they project into grooves 8 forming the flights of the drill bit 4. Alternatively, the tubular portion 6 can simply be crimped into the grooves 8 of the drill bit 4.
When the illustrated device 1 is used, it is threaded on to the drill bit 4 using the tabs 7 and the grooves 8 until the collar 2 is positioned at the desired distance along the length of the drill bit 4, and the grub screw 5 is then tightened down on to the drill bit 4. By suitably positioning the tabs 7 on the extension 6, the grub screw 5 can be made to engage the flight of the drill bit 4 rather than the grooves 8.
The tabs 7 project towards the axis of the drill bit 4 and thereby serve to resist pivoting movements of the collar 2 about the grub screw 5 on the drill bit 4 which with prior art collars 2 alone has been a cause of such collars becoming loose and losing their positioning on the drill bits. A similar effect can be achieved by crimping the tubular extension 6 into the grooves 8. - 4