MAKING AND USING A TEST BAR
FOR TESTING AND ALIGNING THE LATHE
Imagine having just purchased your first or maybe even your second lathe and after lugging everything into your garage, or as with the majority of us, down the kitchen steps into our basement shops, you sit there, sweating, gazing happily at all that new pretty tooling. You spend all that time recuperating as you read and re-read your new lathe's instruction book and marveling at your brand new tool and accessories. Some how you have managed not to botch anything up during the transportation and installation and you are very pleased at how easily and smooth everything went together. Now you can hardly wait to create some chips with it. Even if you just merely reduce a piece of bar stock to nothing. You just have to get some cutting under your belt with your new machine tool. Being the careful worker you are, you carefully chuck a piece of mild bar deep in your three jaw chuck, so as to not have more than 1/2" of stock protruding beyond the chuck jaws. You perform a beautiful facing cut across the end and marvel at how great a cut it makes. Now you proceed to center drill the end with a #2 center drill taking precautions to not let the straight portion of the bit enter the cut. Now you repeat the procedure on the opposite end of the bar so it can be mounted between centers and you can then take a turning cut along its length know well it is the most accurate way you know to turn an accurate cylindrical workpiece. You remove the headstock chuck and insert a dead center into the spindle's tapered bore. You have made doubly sure that the inner surfaces of the spindle and the MT dead center are perfectly clean and free of any stray chips or dirt. You gingerly place the bar stock between the centers in the spindle and the tail stock and secure it by tightening the tail stock center and locking it. The ram of the tail stock is extended only about 1/2" from its bore for the highest possible rigidity. A driving dog linked to a face plate and clamped to the work will power it. You are now ready for that first exciting cut. The cutting tool you have selected for the job is of the correct grind profile and has been freshly sharpened and honed for the premier cut. You have set up the carriage auto advance to the finest feed rate to insure the most even of cuts across the work. Now is time to begin that long anticipated event, so you begin the cut, making sure to maintain a sufficient amount of cutting oil on the surface of the bar as the cut proceeds. As the tool tip begins to get too close to the driving dog, you stop the carriage and turn off the lathe. The surface looks beautiful, smooth and even. You see no ridges of grooves, only very closely spaced machining marks. You are ecstatic with the results and then someone suggests that you check the diameter at several spots along the length of the freshly turned surface. You whip out that expensive micrometer and with a smug grin on your face you begin to take some measurements. First one is taken close to the driving dog. Then one near the center. Your throat begins to feel dry and your heart begins to sink as if it was made of lead. Your face is no longer grinning. You cannot comprehend what has gone wrong. You now get off the floor and finally confirm that you have just turned the finest example of a tapered cylinder immediately after you have taken that last measurement of the tail end of the bar. Your wife is sensing that there is going to be an eruption that will dwarf the Mt. Saint Helens event. She wisely leaves the house to go play Bridge with her friends. You can't seem to be able to deal with this and can't wait until daylight the next day to call that BLANK-BLANK tool company and begin to kick some much needed ethics down their throats. First thing next morning you have that service rep on the phone with your hands around his imaginary throat. You are told to calm down as there is nothing really wrong with the lathe itself, it simply needs to be aligned. Your still spitting fire and brimstone and barely allow the poor service rep to get a word edgewise, but eventually you do calm down long enough to listen to his good advice. He confidently explains that you are simply suffering from a slight tailstock center lateral misalignment and it shouldn't take more that 10 minutes to get everything lined up perfectly.
Magically, the smile has all of a sudden returned to your face and you almost begin to feel like you can once again get near that lathe. You seem to recall that you sort of read about this very same problem in one of your metalworking magazines or was it in a book?
Their solution to the problem called for the fabrication of a test bar which you could then use to align the tail stock to the spindle to within a fraction of a thousand of an inch. You couldn't ask for better results that those.
The test bar is nothing more than a length of bar stock of at least 8-10" long ( depending on your lathe's capacity ) and of a diameter large enough to insure that it is stiff enough to avoid any sideways deflection while you are machining it. It must be pretty straight to begin with so a good piece of cold rolled steel or better yet, a length of large diameter drill rod would be best for the test bar. Drill rod is normally ground to very fine tolerances so it is perfect for this particular job. If the price of drill of that size worries you, just think that once you have finished this tool, you and maybe some of your closest machinist friends that may get to use it will treat it as a prized piece of high precision tooling.
The first step in the machining process calls for you to face and carefully center drill both ends. If your chuck can swallow a 1" wide bar, that's great. More than likely it won't, so you must support the ends with a steady rest. Adjust the bearings on the rest so the bar's end is running as close to zero runout as possible and proceed to face and center drill each end. You will want a pretty stout center hole for maximum support so use at least a #2 or #3 center drill for this. The following step is the most important step so do it as carefully as you possibly can. Remove the head stock chuck and insert a morse taper center in the spindle. Make sure that the inside spindle and the side surfaces of the center are clean. Install a catch plate, mount the work piece between the head and tail centers and tighten the tail stock ram so it is nice and snug but not overly tight. You do not want to deflect anything by using excessive force here. Install the driving dog and connect it to the catch plate. Using a freshly sharpened tool that cuts to the left, take a turning cut beginning at the tail stock end of the bar, taking off only about .010" of material. Use the power advance and finest feed possible for this. Proceed until you have machined about a 1/2" to 3/4" length of the end of the bar. Use a good cutting oil to insure a very fine, clean finish. Take one last finishing cut of no more than a few thousands and proceed to congratulate yourself as you have just finished making one of the most important lathe aligning tools you will ever own.
The alignment process is actually a simple one that once mastered, will take the operator no more than a few minutes to perform whenever needed. The objective here is to determine and measure if the tail stock ram axis coincides with the spindle axis. The relationship between the two points will not become important until you attempt to turn something between centers. You will need a small dial indicator either on a magnetic base sitting on the cross slide surface or held in the tool holder in leiu of a cutting tool. Either way you decide to mount the indicator, the plunger must be aligned so it is as close to center height as possible and operating horizontally. Now you can mount the test bar between centers so the machined end is toward the spindle. Using the cross slide, bring the indicator plunger to bear against the machined surface of the test bar so the needle is displaying a solid reading at a convenient, easily visible region of the dial. Re-zero the dial and adjust the cross slide if needed to bring the needle to exactly zero reading. Without moving or displacing the cross slide in any way, remove the test bar and move the carriage toward the tail stock so the indicator plunger is near the tail center point. Be doubly sure that you do not move the cross dial setting. Better yet, it should be locked during this operation. Re-insert the test bar between centers but this time, reversing it end for end so the same machined end is toward the tail stock just as it was when you first machined it. With the bar in place so the indicator plunger is bearing against the machined end, the reading should also be zero, indicating a perfect alignment condition or as in most cases, it will show a difference of a few to several thousands. This will depend on how bad the initial misalignment is or if you purposely displaced it to cut a taper. All that is left to do is to loosen the adjusting screws for the tail stock deflection adjustment and move the tail stock position out or in order to bring the indicator reading back to zero. Check the alignment once more and re-adjust if needed. In the beginning, when you are first learning the technique, it will probably take you several attempts to get everything back in line.
Once you have gotten everything perfectly aligned, the proof of the pudding will be whether you could turn a perfect cylinder with parallel sides. Remember way back when our friend's brand new lathe first arrived? Turn another piece of stock just like our friend did the first time and check the diameters again. These should be perfect or at worst, no more than half a thousands off. If you are off a couple of thousands out of parallel just adjust the tail stock slightly while continuing to take very light turning cuts and re-checking the diameters. Remember that if the tail end of the bar is narrower than the spindle end, you have to move the tail stock toward the back of the lathe bed or away for you. If it is larger, you move it toward the front or nearer to you. You only compare the tail end to the spindle end diameters, never the other way around. When you get it all perfect and you wish to purposely cut a taper, you just slap on that nice test bar and while taking a reading directly off it, just deflect the tail stock whatever amount of set over you need. Once you have finished machining the part, it's easy to set everything back in line with the test bar again. This will become such a simple task that you will no longer fear setting the tail stock at any other position from center.
Another area where the tail stock could possibly be off line is the tail stock height. You may have the tail stock perfectly aligned but somehow you are still ending up with tapered cuts. This will more than likely be caused by a tail stock that's too high or low. If the work being held between centers is higher at one end than the other, the tool will cut at different levels along the work, instead of along the center line like it should, resulting in dimensional inaccuracies even though everything seems to be OK. While you still have the test bar mounted, just set the indicator so the stylus is running along the center line of the work from above. If the tail stock is a bit too high, the reading at the tail will be corresponding higher and vice-versa. The only thing you can do about that problem is to shim or very carefully lap the under surface of the tail stock assembly, removing the excess material until bar is held at the same height as at the head spindle. There is just one more quick check you can perform and that is to check for radial misalignment. That is whether the horizontal axis of the tail stock, i.e. the axis that the ram travels in or out of the tail stock bore relates to the lathe bed. It is supposed to be parallel. If it is not, then every time you attempt to drill a deep hole with the tail stock, the drill bit will be forced to the right or left. Obviously not a good situation. The least that can happen is a hole of differing diameters, getting wider the deeper you go since the twist bit will flex. The worst scenario is a bad jam, broken drill bit and thus, a ruined workpiece. Some tailstocks on expensive lathes may have a provision for radial alignment. If your does not, you can very carefully scrape the offending areas of the "V" slot on the underside of the base in an attempt to get it back to a parallel condition with the ram travel. Cheap lathes will suffer from movement of the ram in a lateral and possibly radial direction as well as you lock them in place. This occurs by a sloppy fit of the ram and the tail stock bore. Higher quality machines will not have that problem.
Since my lathe is of the "imported" variety, I am well aware of some of these problems so I have learned some preventative methods that I use while I work.
Say I want to machine a cylinder that is well over 4" long and it must not differ more than a fraction of a thousands along its total length. I will of course have mounted it between center and have just finished taking a preliminary cut along most of its length. With an indicator on a magnetic base and vertical rod, I set the indicator contact at center height and I take a reading along its length but from the rear of the work. If I take it along the front edge it will of course show a perfect reading as the front edge will always be cut parallel to the carriage travel. Taking the reading against the rear the back of the work will immediately show you any errors as a reading away from zero. Say I set up the indicator contact so it is taking a reading from the spindle end of the work and I then zero the indicator dial. As I slide the carriage toward the tail stock I begin to notice that the indicator is showing a gradual change of diameter resulting in a total of .002" too large. You then adjust the tail stock in the correct direction to correct it by the half the amount of the error. In my situation, since my tailstock ram suffers from a slight case of slop. I adjust the tension of the ram lock while watching the reading on the dial. Usually a level of lateral tension against the ram will be found that will nullify any small amount of error initially encountered. My particular machine's tail stock ram can be made to move about .0015" to the left and right of perfect centered position by adjusting the ram lock tension while watching the indicator needle. This is a situation you cannot detect or feel without taking a direct reading from the workpiece's side. Make one more light pass and check once more. The indicator will either show no deflection or a very minor movement. You must now determine just how much allowable error you or that particular part can live with. If the part is destined to become a decorative column, then you are really wasting valuable time splitting hairs. If on the other hand, you are making a piston and rod assembly or a crank shaft whose journals must be parallel to less a couple of tenths, then you should spend the time to tweak out as much accuracy as possible. Remember that you can actually produce more accurate work than the overall accuracy built into the lathe. You could use your equipment to actually manufacture a more accurate duplicate tool. That's the way machine tools have evolved from rather crude, not so accurate machines to very accurate ones. So the moral is that if you work very carefully and know how to determine when great accuracy is imperative and when it is not, you can have a much more enjoyable and productive time with your machine tools. And remember that it's not the tool, but the tool operator that determines the quality and accuracy of the work!