Very early in my apprenticeship, I was given my first real lathe job. It was a fairly close tolerance part, about 6" in diameter and 2' long. There was also a 32 rms finish requirement on the O.D. The only machine that was available was an old lathe that had been run hard for many years. I had the part completely roughed in and was ready to take a .010 finish pass. Everything was set, so I cranked up the RPM and let her rip. About 1/2" into the cut, the machine started howling. It got louder and louder. Finally the noise could be heard around the block. My boss who was standing close by walked up to me and hollered in my ear "Kenner, What The HELL Are You Doing?" With a deadpan look on my face, I hollered back "I'm Going For The Finish"..... He hit the floor. That was many years ago. Now let's talk about how to get an old lathe to make a proper cut.

Over time, all machines that get used become worn to one degree or another. On a lathe, the primary components that wear out and will give you the most trouble holding a tolerance are spindle bearings, the ways on the bed, saddle, and cross slide. Also the leadscrews and nuts. These items are difficult to repair. There are many other moving parts on a lathe that can wear out or break. If possible, they should be fixed. Let's start with getting a lathe to cut straight and hold a tolerance. The three main culprits keeping a lathe from cutting straight are wear in the machine ways, tailstock being off center, and a twist in the lathe bed and/or the machine is not level. If you need instructions on centering the tailstock, please email me at eztram@aol.com. Leveling a lathe is easy enough, taking the twist out of a lathe bed is quite another story. If you have a large lathe, you can take the twist out by bolting it to the floor and making adjustments with the leveling screws. Let's assume that you aren't going to regrind the lathe bed and scrape the ways. So we will get started with what we have. First we need to find out how bad things really are. Chuck up a piece of material that is close to the length of the part you are going to make. Touch off your tool on the O.D. of the material and dial up .010 - .020 depth of cut (.020 - .040 on the diameter) and take a straight cut along the z axis. Stop the machine and begin measuring the O.D. with a micrometer. With a magic marker, identify each .0005 change in diameter from end to end. Spin the chuck by hand and lay out these lines around the diameter. Let's say your part was big at the start of the cut and got smaller by .0040 at the end of the cut. This is usually how a worn out lathe acts (part of this condition is also due to tool deflection and deflection of the part being turned). You should have 8 parallel magic marker lines all the way around the part. If you don't have a digital readout on your x axis, you need to rig up a dial indicator on your cross slide. I use a mighty mag base with a Brown & Sharpe dial indicator with a 2 1/4" dial, .0005 increments and .030 per rev. Model number 7207. It looks like this.

                                   

All set, now let's take another cut. Touch off your tool, dial up the same depth of cut as the first cut. Set your indicator at zero. After you set zero on your indicator, dial up 1/2 turn on your cross slide handwheel in the same direction. Now reverse direction on the handwheel and bring the cross slide back so the indicator is reading zero once again. This will take the backlash out of the leadscrew and nut. (You may need to snug up the gib in the cross slide before you start). Begin the cut. As the tool approaches the first magic marker line, gently begin tapping on the hand wheel so that the needle on the dial indicator moves out gradually .00025" (.0005 on diamter) Continue this process as you approach each line until you reach the last line on the part. Now stop the spindle and measure the part. It should be fairly straight. If you practice this procedure, in a short period of time you will get very good at making a straight cut and holding a tight tolerance. The next problem to overcome is bad spindle bearings which will show up as a poor finish when taking a shallow DOC. Make sure you have a good setup ie; correct speed, feed, proper tooling, sharp tool, rigid setup, etc. The solution is to take a heavier DOC, this will load up the spindle bearings. But this creates problems with holding a tolerance and turning a straight diameter. I want you to practice the procedure you just learned only now you will take heavier Depth Of Cut's until you are comfortable with a DOC that will give you a good finish. Let's say you have determined that you need a minimum of .040 DOC (.080 Dia.) to achieve a decent finish. Now here is the trick. Rough the part in leaving it .100 oversize, touch off the tool and dial up .080 on the diameter, set your dial indicator at zero. We only want to take about 1/4" length of cut, (just enough to measure the O.D.) so with one hand on the z axis feed wheel and the other hand on the feed engagement lever, start to cut. When you have taken a cut long enough to measure the O.D., disengage the feed and quickly move the carriage back so the tool is off the part (do not move the x-axis). You measure the part and it is .0210 oversize. Dial up .0105 on your indicator. If needed, take up the back lash to make a straight cut (see above, remembering to tighten up the gib on the cross slide) and then make the cut. Again, this takes lots of practice. When you become proficient at this procedure, turning straight diameters to a close tolerance with an excellent finish will become almost second nature to you.

One final note, sometimes I will saw cut the part 3/8 to 1/2" longer than finish size. This will allow for facing off the part of the diameter that is used for measuring finish size.

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