BORING

BETWEEN CENTERS

 

Holes are bored in order to fine tune as well as true up an out of round condition in an existing bore in order to accept another component as a press, sliding or free running fit. A reamed bore is always more desirable over a plain drilled hole as just about all drill bits will produce a slightly oblong and oversize hole. When the holes are small, I will almost always opt to finish them by reaming to their desired final diameter. They are a bit more trouble to produce but you can always be assured of perfect fitting holes especially when the diameter of the part that will go into the hole is known. Most people will choose a bored hole if they are going to be larger than 1/2" as reamers in the larger sizes tend to be rather expensive as well as too long. After drilling a hole about 1/16" undersize, you can bring the hole to the correct size by boring with a tool post held boring bar. The requirements for perfect un-tapered bores are perfect axial alignment between the head stock and spindle as well as lathe bed and carriage travel. If any of these four alignment requirements are not met, the resulting hole will not be perfectly straight with parallel walls along its complete length. Even if all seems to be perfect, you still have to contend with tool flex. The longer the boring bar shank, the more tool flex you will experience and thus, that more likely you will end up with tapered bores. Imagine a piston attempting to slide up and down a tapered cylinder bore. Enough said about that! You will know if your lathe is suffering from that condition if you turn a 5-6" long rod unsupported by the tail stock center, taking a very light cut along its whole length to an overall smaller diameter and after measuring it along its length, you notice that the diameter gradually decreases or increases along the freshly machined length. Measure the amount of the discrepancy between the widest and the narrowest part and divide by two because any error will always be magnified by a factor of two whenever one turns a round object, then divide by the number of inches of length you turned. The resulting number in thousands will indicate the amount of axial misalignment between the headstock and the lathe bed ways per inch of travel. Lathes like the SHERLINE that feature a swiveling headstock used for tapering, will be easy to bring back to proper condition. Others may prove more difficult to fix. On the TAIG, the headstock is held in a sliding dovetail and it is said to be accurate to .0004" maximum error ( along the 10" bed, or .000,04" per inch ). So long as very deep holes are not being bored, the slight amount of error will be almost immeasurable with most common calipers or micrometers, and will not affect the vast majority of work. If a bore of more than one inch in depth and less than .0002" error in concentricity and parallelism is required as in the fit of a ring less piston into a cylinder, or the bore of a pump housing, you would be better off boring between centers. A between centers boring bar is nothing more than a steel rod about 1" shorter than the maximum between centers turning capacity of your lathe with both ends center drilled to accept the spindle dead center and tail stock dead or live center point. Around the middle of the rod is a centered transverse hole that goes all the way through to house the cutter. A set screw hole is at right angles and in line with the cutting tool bit hole. Before any drilling takes place, you securely clamp the workpiece on the cross slide so the direction in which the hole will be drilled runs parallel to the walls of the cylinder block. The work will most likely have to be jacked up with the proper combination of spacers and shims under it to place the central axis of the future hole in line with the spindle center. Chuck and center a short pointed rod so its point can be used to indicate the spindle center. Raise the work on top of the spacers and add shims as needed to align the center punch hole mark on the work with the point of the rod in the chuck. Move the carriage back away from the spindle. Remove the chuck and replace it with a small faceplate you know to be true, Use a small square to align the straight sides of the work square with the faceplate. I have a small faceplate without a central bore so I can just align the front face of the workpiece against the faceplate front surface to instantly align and square it to the lathe central axis. Clamp the work down with strap clamps or any other type of hold down system you might have handy while you are pressing it against the faceplate. Re-check with the square and adjust if needed. Set your boring cutter to provide the correct diameter as it swings in a circle. Do it this way. Micrometer measure across the tip of the tool and the opposite side of the rod to get a reading that represents the amount the tool protrudes beyond plus the diameter of the rod. Subtract the radius of the rod and multiply by two to arrive at the total diameter or swing the cutter will transcribe on a single rotation. How difficult is it to set the cutter at the right setting? It actually is pretty simple, if you figure out before hand what the correct cutter extension plus boring bar diameter should be by reversing the steps in the equation. Set the cutter extension well beyond what would be needed and slightly tighten the set screw so it holds the cutter but allows it to slide in an out with some resistance. Set a micrometer to measure between the bar and the cutter and slowly close the micrometer, slowly pushing the cutter inward until the correct pre-determined setting is reached on the micrometer. Tighten the set screw to secure the cutter in that position. Now is time to begin the drilling and boring operation so put the pre-set boring bar aside and get ready to center drill and begin to drill out the workpiece. The drill bits are held in a headstock drill chuck and the work mounted on the carriage cross slide is moved into the rotating bit. Gradually enlarge the hole until the bore is about .010" to .020" short of the correct bore diameter. Remove the drill bit and move the carriage to the center of the lathe bed. Slip one end of the boring bar through the right end of the workpiece bore and insert it to the headstock center and attach it to a drive dog and catch plate making sure that the cutter is oriented so it will cut as it rotates counterclockwise. You could orient it backwards by mistake so be careful. Bring up the tail stock center point to support the opposite end of the boring bar. If a dead center is being used, make sure to lubricate it well. With the lathe running at a fairly good speed, feed the workpiece toward the tail end of the lathe and through the whirling cutter to effectively bore out the last few thousands from the inside as it slowly passes through the workpiece. Do not touch the cross slide dial as you are performing the boring operation. In fact lock the gib screws to insure that it will not move laterally during the boring run. Remember that you had originally centered the work laterally by lining up the hole prick punch mark with the head stock center point. After the work has passed through the cutter, it should be returned, again passing through the cutter to clean out any possible irregularities. Use a very slow and deliberate feed of the carriage to insure the finest finish possible. It should be such that only fine lapping compound would improve it. The workpiece that is to be fitted into this " perfect " bore is turned to a running fit of .0003" or less total difference between the two diameters to insure a good leak proof seal. To achieve this level of fine tool advance you need to cock your compound slide about 4-5o to the left of the lathe center line so as the tool is advanced with the compound, it moves toward the work at a very shallow angle. Because of the slight angle that tool is being feed to the work, for every .001" of actual compound slide feed, the effective distance that the tool point travels into the workpiece will be more in the order of about .0001" or approximately 1:10 ratio. You still need to check the fit by actually attempting to slide the two parts into each other. If the part is just a tiny bit snug meaning that it won't fall through by gravity alone, It can be brought down to perfect fit by lapping it as it rotates with 400 to 600 grit black silicon carbide paper coated with light oil or simply water. Thoroughly wipe off all of the grit and slop that is produce during the lapping procedure. Fit the parts and enjoy the great feeling you will surely experience in a job well done.