Notes on Workshop Techniques


INDEX

THE LATHE

6. Drilling and Reaming

6.1 Drilling

Drilling a concentric and parallel hole in a workpiece is a very common requirement and, as always, success depends on good lathe alignment and correctly sharpened tools. Most often this is achieved with a drill mounted in a drill chuck installed in the tailstock socket with the work held in the 3-jaw or 4-jaw chuck. Drill chucks of good make are very accurate, and the average runout (TIR) of 5 new Jacobs No.34B 1/2" chucks I have is around 0.0005". Although these chucks will hold very small drills it's better to use a smaller capacity chuck with drills less than about 3/32" diameter. It's very easy when inserting small drills into 1/2" chucks to get them mis-aligned, and when tightened the drill gets bent rendering it useless. For this reason I have 1/4" chucks mounted on 2MT arbours, and even a tiny Jacobs No.0 1/8" capacity chuck for the smallest drills. An alternative to small chucks for holding miniature drills is to use a pin-chuck, a small collet chuck which is held in a larger chuck, the only problem with this arrangement is a lack of sensitivity which can lead to broken drills. In addition to the keyed chucks listed above I have two 1/2" capacity keyless types (Jacobs and Validus) which are just as accurate and quicker to use. However, these chucks, whilst fine for drilling, are of less value when holding taps as reverse torque loosens their grip. It should be remembered that drills are not designed to produce the most accurate diameter hole, if this is critical then drilling undersize is normally followed by a boring operation to correct any eccentricity and finally reaming to exact size.

1/2" capacity keyless and keyed drill chucks, and a 1/8" capacity Jacobs No.0 chuck.

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In order to drill a hole which is concentric, straight, and is not over-size, requires control of a number of factors. Firstly, the tailstock needs to be aligned correctly with the headstock, and the method of achieving this has been described elsewhere under tailstock setup . Secondly, and perhaps more importantly, the drill bit needs to be accurately sharpened such that metal removal is done evenly by both cutting edges of the drill. Several modifications of the drill face have been described which claim to improve performance compared to the standard shape. These include narrowing the web or chisel edge in the centre, which is easy to do with the corner of a square-cornered grindstone, and an added benefit is that the pressure required to drive the drill through the work is greatly reduced. Also, grinding the lips with 4 or more facets reduces the tendency for the drill to wander off-line, however, this procedure requires a tool and cutter grinder. These modifications are of benefit but by no means essential to achieve accurate drilling. The best judge of your own setup is whether or not a new drill out of the box (and of good quality manufacture) is able to produce an accurate, parallel and concentric hole - if it doesn't you have a problem with your machine alignment. Other intermittent problems are most likely the result of incorrectly sharpened drill bits. It's important to sharpen drills using a good jig (not all grinding jigs produce good drill points) which will correctly grind the angles and will leave the leading edges exactly the same length and with equal back relief for each cutting edge. The jig I have leaves something to be desired in that it needs sympathetic use to get a good result, there seems a bit too much flex in the mounting for my liking and to get the correct amount of back-relief takes very careful placement of the drill in the jig. Some folk claim to get good results with 'off-hand' grinding, where the drill bit is sharpened with no more guidance than hand and eye to gauge angles and length of cutting edges. This is all very well, and I admit to using this method myself for very small and very large drills, but where substantial metal needs to be removed to restore the cutting edge, or an accurate hole is required, I always use the jig.

Drill sharpening jig.

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The drill angle normally used for steel is 59 degrees (118 inc.). A larger angle may be used where a lot of sheet metal drilling is done, as this reduces the tendency to grab and tear the hole as the drill exits the other side. When drilling brass the normal twist drill flute design will produce a cutting edge which is too aggressive for the material, and the drill will be pulled into the work - particularly when following a pilot hole with a larger drill which reduces the resistance for infeed. Again, if a lot of work in brass is envisaged then the situation can be improved by stoning a small flat on the cutting edges. However, this will leave the drill unsuitable for use on steel so keep such modified drills for brass only.

The Martek - a small integrated drill sharpener, in this case driven from the lathe overhead.

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A recent aquisition is a Martek drill sharpener (illustrated above), bought more out of curiosity than the expectation that it would work particularly well. In fact, it does a reasonable job on drills up to 1/4" or so. As you can see from the illustration, the sharpener comes without any motor, though a proprietary power source is available at considerable price. It is expected that most users would utilise their hand power drill to drive the sharpener and a bracket is supplied for this purpose. Not liking the noise of a power drill over extended periods I simply fitted a pulley to the drive shaft and screwed it to the bench such that I got a power take-off from the lathe overhead drive belt. Very small drills are not catered for and the minimum practicable size that can be sharpened is around 1/8". It is supposed to sharpen drills up to 1/2" diameter, and tipped drills can be tackled by exchanging the standard grindstone (only some 1-1/2" diameter) for a small green-grit wheel. Grinding is a bit slow, and to put a new point on a broken drill takes quite some time. I would hate to have to put a new point on a 1/2" drill! Nevertheless, at least it manages to put sufficient back relief on the cutting edges, something that some jigs fail to do sometimes. The grind stones are very susceptible to grooving and need to be dressed regularly, this is catered for in the design. I am in the process of designing a new type of jig specifically for sharpening small drills below 1/8" diameter and this should appear in the projects pages shortly when complete.

For lathe work, more often than not, the depth of hole required does not justify using a normal length 'Jobber' drill, and a 'stub' drill will suffice. The advantage is that the shorter drill is stiffer and there is far less tendency for it to wander off-line. I have a partial set of stub drills and these get a lot of use. Where deep holes are required normal jobber drills can be used followed by 'long' or even 'extra long' series drills. It is also possible to extend normal drills by adding an extension piece onto the end, it's a simply matter to turn down the shank for a short length (the shank being left soft) and soldering on an extension rod.

Sharpening large MT shank drills is difficult as the shank often prevents mounting the drill on the jig, which is often designed only to handle parallel shank drills. On my jig I have to remove the jig's tail support and arrange for other means to control depth of cut (usually a small toolmakers clamp). Tool and cutter grinders are a better option for sharpening such drills.

All things being equal, provided the drill is started truly it will, in all likelihood, proceed without wandering off-centre. However, the lathe and drill bit are rarely so perfect so it helps for critical or repetitive work to give the drill some support in the form of a hardened bush. This is not always easy to arrange but one method is to use a piece of mild steel section held in the machine vice on the vertical slide and drill, bore and ream it to a size that accepts hardened bushes. This will naturally leave the hole at centre height, and the drill mounted in the tailstock can then be passed through the bush. A centre drill (Slocombe) is often used to provide a true-running hole in the workpiece to locate the end of the drill. The short and stiff centre drill will not flex and so the hole it produces is usually dead centre. In fact, the 60 degree angle of the centre drill is not an ideal one for starting the drill, and a 118 degree included angle would be better (I doubt whether these are still available though). The other option, and probably the best, is to centre, drill undersize, then bore out a short hole fractionally undersize (by a few thou) for the final size drill, thus giving the drill the best possible start. A quick way of doing this is to use a slot drill mounted in the tailstock chuck, cutting on it's end face it produces a true hole to start the drill - but you need a slot drill of the right size of course.

For holes of 3/8" or larger it is normal to drill a pilot hole right through first with a smaller drill (about 1/4" diameter). This serves two functions: the first is to relieve the pressure required to operate a large diameter drill, the chisel edge offers considerable resistance and the thicker web of large drills makes drilling hard work. The pilot drill should be a little larger than the width of the larger drill's chisel edge. Secondly, a larger drill will faithfully follow the pilot hole, so if this can be drilled truly then the large drill will also drill truly (provided it is sharpened correctly). In addition, with a pilot hole the larger drill will produce a hole that is closer to it's nominal size. A good plan is to drill the pilot hole, skim it with a boring tool to get it dead true, and follow this with the larger drill.

Getting lubrication down to the cutting edge in long drilling operations is difficult, and I find the best way is to use a plastic syringe and long wide-bore needle to squirt the cutting oil down to where it's needed. It's useless dabbing the drill with lube occasionally each time it's removed to clear chips, it just rubs off as the drill is pushed back down the hole. Brass, cast gunmetal, and cast iron are usually drilled dry, but steels need lubrication. Phosphor bronze (drawn rod) is nasty stuff to drill, the surface can work-harden if constant pressure is not applied, and frequent clearing of chips is essential to prevent the drill seizing in the hole. I use Trefolex cutting lube (which discolours the work a little) and a slow speed for this job, and also for non- free cutting stainless steels.

Accurate control of depth, where this is necessary for blind holes (a common requirement when making up small locomotive valves and backhead fittings etc.,) can be achieved either by using small bushes which lock onto the drill with a setscrew (convenient for a large number of similar parts), or some form of direct indicating depth measurement should be incorporated into the tailstock. Even quite sophisticated lathes seem to leave this out - I don't know why considering how important it is. The Myford Super 7 has a rather crude scale etched onto the tailstock barrel which reads in 1/8" divisions and is singularly useless. A modification described by Geo. H. Thomas gets around this by providing an accurate zeroing micrometer dial which can be read down to 0.001" depth. This is a very worthwhile modification, and one of the first I did on my new lathe.

Picture shows the modified tailstock handwheel. The small knurled screw in the centre of the wheel at the back operates the locking mechanism for the micrometer collar, an elegant design and plans are available in GHT's book (The Model Engineers Workshop Manual).

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An alternative to the micrometer collar arrangement is to attach a pointer to the tailstock barrel which reads off against an engraved scale (frequently a 6" rule) mounted on the tailstock body.

Drilling very small diameter holes requires special techniques. Firstly, drills within 60-80 SWG range require very fast rotational speeds, in all likelihood even the top speed of your lathe will not be fast enough (mine is about 2000 rpm). Secondly, these drills are very fragile and will break without any audible or tactile warning whatsoever. The speed restriction you can do little about unless you make a special high speed tailstock drilling attachment, the problem of breakage can be alleviated somewhat by ensuring the flutes are clear of chips by frequently withdrawing the drill. To aid in this, and to add some tactile 'feel' to the drill's progress you can use a home-made sensitive drilling attachment:

Sensitive drilling attachment.

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Using this attachment it's much faster to withdraw the drill to clear chips so you do it more often, and the reduced leverage provides the additional feel. It's particularly useful for tiny 1/8" center drills, the pilot of which is liable to break off just by looking at it. For all centre drills I have found that most breakages occur not whilst actually drilling, but when re-inserting after retracting to clear chips. It seems that one or two chips remaining in the pilot bore are fatal for causing a broken pilot. Just as well broken center drills make good boring tool bits!

It's comparitively much more difficult to sharpen these very small drills than drills of 1/8" and larger. The normal grinding jig will not be of any use but there are devices that can be used (the 'Wishbone' sharpening jig for example). With these drills I actually use off-hand grinding with an eye-loupe so I can see what's happening. I'm not always successful first time though. It's probably not a bad idea to simply buy a new drill if it gets broken or blunt, they are not that expensive unless you find you are getting through dozens of them.

Like everything in the world of machine tools, you tend to get what you pay for. I've bought cheap sets of drills before now at 'bargain' prices and been disappointed. Either the drills were soft or not sharpened correctly from new. You can always re-sharpen I guess, but a soft drill will blunt quickly and worse, the lands will wear so that the drill jams in the bore. There is good reason that American and European drills (and milling cutters for that matter) cost at least twice as much as these cheap imports, and that is the garaunteed quality. I thoroughly recommend buying top quality drills only and economise elsewhere.

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6.2 Reaming in the lathe

Machine reamers, 3/4" 2MT, 9mm 1MT, 3/8" straight shank.

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Two broad categories of commercial reamers are generally available to the model engineer, these are hand reamers and machine reamers. More often than not beginners buy hand reamers when they should be buying machine reamers. As the name suggests, for use in machine tools the latter are more suitable. The difference between the two is that machine reamers have a very short 'lead' and begin cutting to full diameter almost immediately, whereas hand reamers have a pronounced 'lead' or taper (like a taper tap) and will not cut to full diameter until a full 10-15% of the length of the reamer has entered the hole. Clearly, the former has advantages when reaming into a blind hole, or where space behind the work is limited. Machine reamers, especially the larger diameters, tend to have taper shanks for mounting in the mandrel of machine tools (not always, I have several parallel shank machine reamers), whereas hand reamers have plain shanks with a square end designed to be held in a wrench. Hand reamers are designed only to remove a few thou from the bore, and the tapered cutting edges have another unfortunate side effect in that shavings are trapped in the flutes and do not clear easily - which means that frequent removal is necessary to clear the chips. Machine reamers will remove a greater amount of metal and the lack of taper results in cutting occurring nearer the front edge of the tool, shavings are usually pushed forwards and do not pack the flutes to the same extent (clearance is still required though). If the shavings are not removed from the flutes regularly and they become packed the reamer will likely sieze in the bore and irreparable damage will be done to the work. Even if this doesn't happen, packed chips in the flutes will cause the reamer to cut oversize, off-line, or both.

It is necessary to use plenty of cutting oil for the reaming operation, I tend to flood both bore and tool with straight cutting oil (or Trefolex) beforehand, and each time the reamer is removed to clear shavings. Reaming should be done at a spindle speed that is about half that used for drilling, and the reamer pushed through the work fairly quickly. This will reduce the incidence of ridges forming in the bore. Where possible the whole of the working part of the reamer should enter the bore and then backed out at the same rate with the spindle still turning. Never reverse the direction of rotation with a reamer in the bore.

The amount of metal left in a bore for reaming operations will vary with the diameter of the hole, larger diameters should have less depth of metal to remove than smaller diameters. If too small amount of metal is left (say 1 or 2 thou on a 1/2" bore) this can lead to chatter - especially with hand reamers, too large and you will have to clear the flutes very often. About 5 -10 thou would be about right. Small reamers (less than about 5/16") are far less sensitive to depth of cut but it's still wise to have the hole about 10 thou undersize.

For the most accurate jobs in the lathe it is preferable to use a machine reamer with a taper shank installed in the tailstock (of necessity the lathe must be accurately aligned). Holding a reamer in a drill chuck of dubious accuracy will likely lead to oversize and tapered bores. A better option is to mount a dead centre in the tailstock and locate this in the female centre at the rear of the reamer, and use the tailstock handwheel to push it through the work, a hand wrench or lathe dog attached to the shank of the reamer will stop it rotating as it enters the bore. An even better option is to use a Floating Head holder which will compensate for any misalignment between tailstock bore and lathe spindle.

Apart from commercial reamers there are the home-made variety called D-bits, so called because viewed from the business end they resemble a letter D (!). These are very simple tools yet are capable of great accuracy. However, do not expect to remove large quantities of material with a D-bit, treat it's capacity similar to any other reamer. One great advantage of the D-bit (apart from the fact that they are cheap to make) is that the cutting edge can be made square so as to form a flat end to the hole - very useful for forming the seatings of ball valves and such like. A D-bit can be made from a length of silver steel, a flat filed on one end leaving the width just over (about 5 thou) half the thickness of the parent bar. The front edge is then ground back at an angle to provide front clearance. In some designs (those for simple reaming where a flat seating is not required) the corner where cutting occurs is chamfered, with the opposite trailing corner given an even larger chamfer. In others the end is left dead flat - it depends upon the job in hand. Harden, temper to dark straw, and polish the bearing surface to prevent scratching the bore. Don't forget to use plenty of cutting oil - never try to use a D-bit dry in anything but cast iron or brass.

Home-produced D-bits made from silver steel.

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If you have dividing equipment it's easy enough to make special reamers (taper reamers for example) of a straight-flute design from silver steel. The straight flutes make them very prone to chatter though, so cutting should precede slowly with a firm contact maintained.

From new, commercial reamers have a tolerance of -0.0000" to +0.0005" on diameter, and most are towards the upper end. I have one 3/8" machine reamer which is a full 0.001" oversize which is surprising. It's as well to know the exact diameter your reamer cuts, this helps when machining interference or running fits for bores produced by that particular reamer.

The taper on the end of hand reamers is actually quite useful at times, for example, when producing holes for dowels. For a 1/8" diameter dowel I would not push the 1/8" reamer all the way though (or into) the hole, this would leave part dead to size and part undersized. The dowel would be a short length of 1/8" silver steel, very slightly tapered at the end with a fine Swiss file, which will then press into the part-reamed hole and be held accurately and very firmly.

I should perhaps make some mention of adjustable reamers. The most common are the design using 6 discrete blades mounted on an externally threaded shank, the blades are held in a cage and their seatings are tapered slots. Nuts at each end move the blades along the shank and the tapered seatings force the blades outwards or inwards. I have a couple of this type, and in a list of least-used tools these would come near the top. Their total range of adjustment is small, typically about 0.005". They are quite fragile and designed only to remove a small amount of material (1 or 2 thou). The straight blades make chatter a problem and the result is frequently a polygonal bore. The real problem is actually measuring the precise size the reamer is going to cut, and the only definitive way is to ream the hole first and measure the result - not the easiest of jobs unless you have a plug gauge to work by. The long lead caused by the adjusting nuts and shank means the reamer can only be used for through bores, you can't use them in blind-ended holes. Whenever I've needed a custom sized reamer I've always made one in D-bit form from sliver steel, less hassle and the size is right first time.

Store your collection of reamers in a drill stand so that the cutting edges cannot contact each other, and keep them oiled so rust doesn't cause pitting. I was once browsing around a market stall of used machine tools and dropped across a metal box full of reamers, the whole lot were just left bouncing around together and sad to say they were all ruined. When I pointed this out to the stall owner I just received a puzzled look. Some small nicks in the reamer's cutting edges can be repaired with judicious stoning and it's as well to keep an eye on your own set for the appearance of such damage.

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(c) Chris Heapy 1996.


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