road and workshop
Page 33
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by Handyman
Benchwise: lathe sense (3)
MANY TASKS in the course of vehicle parts reclamation call for precision drilling to ensure the required fit. This is particularly so at this time of MoT test preparation and when there is increasing downtime from spares delay and the need for a permanently open eye in search of cost reduction. Much of the work of test preparation, and for that matter any programmed work, literally means putting back what has been taken out in a period of service. The more accurate your clearances, fits and tolerances, the more you wind the clock back towards the state of newness. Thus a few "thou" may make all the difference between a "pass" and a "fail". Yet those same few "thou" in the form of excess clearance caused by rough and hurried machining or -drilling can bring on early and rapid wear which can mean something less than half the expected life.
Many drilling jobs can be handled far better in the lathe than on the drilling machine; a far better accuracy can be obtained. There is also something to be said for the revolving workpiece rather than the revolving drill. The benefit is, less scrap or swarf build-up in the cutting area, better cooling as a result of movement through the air, and also, the fact that the Morse taper is immune from the chatter often associated with worn three-jaw drill chucks.
However, while all the aids to accuracy are there to be had, the trainee or part-time turner can often spoil the job by rushing it, or through ignorance of certain essential features and know-how that are part and parcel of the "professional". Many working parts around a vehicle or trailer, such as suspension beams, arms, spring brackets and turnable mountings, wear heavily on one side. It is far better to build up an oval hole rather than attempt to bore out for an oversize pin or bush. It is also wise to return to standard wherever possible to avoid odd sizes.
Certain of the hints, and also the pitfalls about which the trainee should be made aware, concern the task of drilling in the lathe, particularly where it is important that the hole is finished dead to size. Almost every turner in his early days has experienced the shock of finding the finished hole well above size for no apparent reason. This is something that can happen where a twist drill has been sent through a workplace under excessive feed load, and it is all too easy to wind on the tail stock wheel in too eager a fashion. It may appear to the trainee that he is going great guns in producing a thick coiling length of swarf, but he may be doing the one thing wrong that will shake him later.
Two mistakes
As a rule the trainee will make two mistakes both of which should be demonstrated to him as wrong, as quickly as possible, otherwise he may spoil many jobs and begin to lose faith in himself. First, after a short time spent using the twist drill in fairly deep holes in steel, the trainee will discover for himself that when sharpening the drill he can improve its bite and cut by grinding back a little more steeply. This is .called "backing off", and allows metal to be lifted out like a plough. The man may well feel that he has scored, and will also find that with the new angle he can force the drill to the work a good deal harder, and to him, more effectively.
This overload will, however, cause the drill to dig so deeply that its flutes will expand. As a consequence the finished hole will be considerably larger than the drill diameter, and any expectation of an intended size can be forgotten. It is therefore most important for the trainee or part-time turner to stick to standard settings of twist drills until he has gained a fair amount of experience and not to experiment. The experienced turner on the other hand, quite often varies drill settings to suit different materials and thicknesses.
Where this does call for an alteration to the backing-off angle, a device used by many of the older lathe hands can be of value. This is simply a short length of tube with a perfectly squared end; this tube should be fractionally smaller in diameter than the drill to be gauged, permitting the drill point to be entered about half way, the clearance angle can now be clearly seen and the required adjustment determined.
It is true that a good tool man can prepare a drill for a wide range of tasks in the lathe, and his experience can prove invaluable at this time. But we are concerning ourselves with the somewhat lesser-experienced turner, and where it may be possible to enlighten him with a few more tips on the subject of drilling accurate holes.
Often during vehicle repair, time is the essence, yet more haste less accuracy all too often applies. So in seeking a precision job, particularly in the lathe, always find time to drill a pilot hole first, right through. This is not done just to give more bite and quicken the job as many imagine, it is in fact to ensure that the larger drill to follow runs true all the way. The greatest risk to accuracy without a pilot hole, is that the actual drill point may not be exactly in the centre. If this is the case the drill will take on eccentric motion ending with an oversize hole. Where a really precision fit is called for, this can be achieved by drilling up to within one drill size of the finished hole, then dull the outermost tip of each cutting edge of the proper drill with a fine stone to remove the chance of any chatter or grab and run it gently and slowly through the hole and out again with ample lubricant—a dead-to-size hole will result.