Boring tool holders.
Boring tool holders.
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Boring in the lathe falls into two catagories; where the work is turned whilst held in the chuck or faceplate with the tool mounted on the carriage, and secondly, where the work is mounted on the boring table (or vertical slide) and the tool - a boring head or bar - is mounted on the lathe spindle. The latter option is normally reserved for work which is too large to be easily be turned on the lathe, or where secondary operations are to be carried out.
Boring provides a method to machine accurate holes which are truly parallel and of a size for which there is no reamer available. Boring tools for operations on work mounted on the lathe spindle have been referred to earlier, and it's worth repeating that my preferred setup consists of a boring bar with inserted bit mounted in a turret on the rear toolpost. The reason is simply that this position offers the most rigid mounting and the tool can be adjusted for the minimum overhang necessary to complete the job. Small holes are dealt with by one-piece boring tools held in the same turret.
As always, the accuracy of the bore is dependent upon the accuracy of the machine - in this case particular attention should be paid to wear of the lathe bed, aligment of headstock spindle, and correct adjustment of the carriage gib strips. If the lathe can turn parallel then it will be able to bore a parallel hole, if it can't it won't (one caveat being that work mounted on the boring table and bored with a between-centres boring bar will always produce a parallel bore - it might not be in the right place or parallel with a datum surface of the work but the bore will be true).
The shape of the boring bit should offer adequate front clearance otherwise the tool will rub and form a bell-mouth hole. Side and back clearances are similar to a knife tool and are shown in the accompanying diagram.
There should be a slight negative approach angle to the tool bit so that facing of the bottom of a blind hole is possible. It is important that large positive approach angles, and large-radius round-nosed bits be avoided, as these tool shapes tend to deflect the tool away from the work - again leading to bell-mouth holes.
It will be found that with a boring tool set up to cut on the nearside of the bore that another fine cut will be taken as the tool is withdrawn from the bore. If there is any amount of flex in the tool re-entering and withdrawing the tool again will produce yet more fine cuts. For this reason it is tricky to measure the progress of the boring operation and interpretation of the readings of the micrometer collar requires some care. The best way of boring to a dead size is to first make up a plug gauge, accurately turned beforehand to the correct diameter. The gauge should incorporate various steps in diameter by which progress may be assessed. A typical gauge for a 1" diameter hole would consist of 3 steps, the first 0.01" undersize, the second 0.002" undersize, and the third exactly 1.000" diameter. Callipers would be used to assess the hole size until the point where the first step will enter the bore, the experienced machinist will then be able to gauge how much more needs to come off to reach the 0.002" step - the less experienced would do well to take 0.001" cuts until the sencond step enters the bore - at which point you will know there is less than 0.001" cut to come off. Fine shavings can then be taken until the desired fit on the 1.000" portion is achieved.
For work mounted on the faceplate, or in the 4-jaw chuck, some method of centring the position of the intended bore is required. If a center punch mark is provided then a wobbler can be used to align this with the lathe axis. If the datum point is a previously machined hole a wobbler can still be used if the pointer is provided with a ball end. For larger bores it will be necessary to use a DTI against the inner surface of the bore.
A typical job in model engineering is to bore a cylinder casting. A special attachment is available for this job called a Keats Angle Plate which is clamped to the faceplate. The Keats incorporates a deep 'V' so that the bore will be true with the cylindrical outer shape of the casting, and there is a clamp to hold it firmly in place during maching operations. If the casting is for a slide-valve engine only one bore will be required, if it is for a piston valve engine then a second smaller bore will be required offset by a specified amount and parallel to the main bore. This secondary operation is accomplished by slackening the clamps holding the Keats to the faceplate and sliding the whole thing across to the desired position. The bores must come out parallel in this situation. It is normal to temporarily plug the bores beforehand in order to mark out the exact position of the centres and to provide centre punch marks as datum points. To bore the hole (which will usually be cored out), whether it be cast iron or gunmetal, it will be necessary to machine away the rough inner surface of the casting. This plays havoc with HSS tool bits so it is as well to use a carbide bit initially until sound metal is reached. Out of preference I would then exchange the carbide bit for one of HSS as I find a better finish can be achieved. The casting is usually mounted in such a way that sufficient is protruding beyond the front edge of the Keats that a facing cut can be taken at the same setting thus ensuring it is square with the bore (this end is usually the one that mounts the piston rod cylinder cover - the other end being less critical).
Boring a casting using a between-centers boring bar.
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Where the cylinder casting (or any other casting for that matter) is too large to be mounted in this way it is usual to clamp it down onto the boring table with packing to raise it to the required center height. I should have mentioned that it is good practice to machine one face flat (usually the port face for a cylinder casting) to act both as a datum surface and to enable clamping to the boring table. Again, leave enough of the casting overhanging the edge of the boring table so that the end can be fly-cut square to the bore at the same setting. In this case boring can proceed using a between-centres boring bar, the bar being of sufficient length to allow the casting to pass right over the cutter bit from one end to the other. The bit is usually set in at an angle such that a micrometer can be used across the point of the bit and the underside of the bar, simple math will enable the cutting diameter to be measured. It is difficult to provide for precise adjustment over any appreciable range of cutting size, the best method perhaps being a cutter bit backed up by fine-pitch screw which pushes the bit forwards (i.e., outwards). The only drawback with this method is that if a secondary operation is required, such as another bore for the piston valve, the casting will have to un-clamped, raised on more packing, and re-aligned - with the consequent danger of misaligment.
Small boring head.
One option not mentioned so far is the use of a small boring head mounted in the spindle socket and used in place of the between-centres boring bar. The advantage this offers is that the head is usually provided with a direct reading micrometer adjustment so that it's easier to estimate the size of the bore being produced, and over a greater range. One drawback is that it can potentially produce a tapered or bell-mouthed bore just as normal boring can. I have one of these tools and I make more use of it in the vertical miller where it makes a fine poor-man's jig borer. I don't think I would use it for deep bores though (if there was an option to use the between-centre bar), and not for something as critical as a cylinder casting. One sophistication present in some heads is the ability to face work as well as bore holes, the Dore-Westbury is a design that spings to mind. Here, the head is mounted in the lathe and a star wheel on the end of the head's feed screw impacts a lug mounted on the lathe bed. This action turns the feed screw one notch each revolution and so the cutter bit scribes an ever increasing radius. I've never had a job where I've needed to do this but it sounds fun!
In both examples above the boring operations should use fine self-act to give the best finish. I think of the two methods, if I were making a cylinder for a slide-valve engine I would use the between-centres boring bar, if the cylinder were for a piston valve engine I would use the Keats angle plate (if I had the option that is).
Where taper bores are required then naturally the tool will need to be mounted on the top-slide in the normal way, so it is necessay to provide a tool holder which will (preferably) hold the same tooling that fits the rear toolpost turret. This can be as simple as a V-block and standard toolpost clamp, or a drilled and split bar which will do the same job but can be mounted in the 4-tool turret. The method of producing a MT socket (for example) is essentially the same as for making the MT shank. The only difficulty is in initially setting the top-slide accurately enough to the required angle, and some information on this aspect is given in the construction notes for a simple attachment to aid Morse Taper Turning .
To bore very long small bores (such as rifle bores) is an art in itself, and probably is not within the normal remit of the model engineer (if there is one!). Anyway, special equipment will be called for, not least of which are long hollow drill bits and high pressure suds equipment. The drill bit is a simple affair, more akin to the D-bit or spade drill than the normal twist drill. Chip clearance is by forced lubrication down the centre of the drill to the cutting edge, and up the outside and out carrying the chips with it. If I had to produce a concentric shorter bore (say, 3/8" diameter and 6" long) I would probably drill and ream a bar of metal which was of larger diameter than required, then mount between centres and turn the outside down.
(c) Chris Heapy 1996.
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