Tuning Up A Lathe

Why Tune Up Your Lathe?

Machine tools are meant to be adjusted and dialed in for maximum performance, and periodically one should go back and revisit the machie to make sure its operating as well as it can. In some cases, actual rebuilding may be called for such as replacement of spindle bearings or rescraping of ways. In other cases, less intrusive procedures can improve performance. This page captures my notes on how to do a lathe tune up. So far, it's just research. However, at some point I want to do the tune up so that I can do some precision work such as spindle work.

My Lathemaster 9x30 lathe has been pretty good to me so far. Right out of the box I've been able to pretty easily machine things to a thousandth (0.001"). Now that is not an especially noteworthy achievement by most standards, but it is good enough to accomplish quite a lot of HSM projects. For my next magic trick, I'd like to be able to work down to a "tenth" (0.0001") as needed. After I've done sufficient research on the tune up process and find time in my crazy schedule (too many projects!), I'll have a go at this and see how I do.

Level the Machine

blah, blah, blah

Reduce Vibration Through the 3 B's: Belts, Bearings, and Balancing

For a while now I have been ignoring a slight vibration in my lathe, hoping it would go away. Stupid, I know, but I will be the first to admit I am lazy!

Finally tracked it down. I happened to be fiddling with setting up my 4-jaw chuck and spinning the spindle by hand when I felt a little "pop" in the motion. I thought, "Oh boy Virginia, this is the big one, my spindle bearings have gone!" So I opened up the gear area and fiddled around until I got the pop to repeat itself. Suddenly it all became clear. My timing belt was not tensioned quite properly and it would occassionally pop a tooth out fo the timing belt pulley.

SWEET RELIEF!

I suspect the motor shifted a bit. What I need to do is work out a proper tensioning arrangement. I either need to mill some slots on the motor mount (I didn't own a mill when I made the original DC treadmill motor mod 2 years ago!), or install some kind of an idler. It's funny because the vibration would disappear if I revved up the spindle rpm, and then I would back down to the desired rpm after the vibe was gone. I looked at the belt and it didn't seem worn, but its a wonder it didn't wear through.

What has all of this to do with finishes and Fenner belts? Well, I was reading another thread about a guy putting a real 3 phase motor and VFD on his mini-lathe and it jogged loose some memories of other threads and thoughts. First, I have read and been told in more than one place that a lathe running 3 phase will simply leave a smoother finish than one running single phase. Evidently you can see the pulsing of the motor on fewer phases in the finish. Hmmm, says I. Now another fella on this VFD thread opines as how he things timing belts give a rougher finish than v-belts. Hmmm again, says I, and I suddenly recall reading that running the belt a tad bit loose is good for surface finish. I think that's an old SouthBend trick or something. Now the timing belt can't exactly be loosened, or it starts to skip a step every now and again, like mine is doing.

So, methinks I ought to consider swapping out the timing belt for something else. I don't know if this will improve my finish or not, but I have heard that the best belts for reducing vibration are Fenner belts. That link is just one source, I've heard it in a lot of places. It sure is tempting to look into a Fenner (also called a "link" belt) that would fit the lathe, swap out my pulleys, maybe consider a little different ratio, and get a proper tensioning arrangement. OTOH, one fellow has reviewed every belt known to mankind and feels the vibration advantages of link belts are marginal compared to cogged v- belts.

Having a chuck out of balance is another potential source of vibration. I remember reading a tail on the Internet of a fellow who'd bought a new Jet lathe. Worked great below about 650 rpm, but vibrated "like a whirlpool washer with an unbalanced load" at higher spindle speeds. It was finally traced to a chuck that was out of balance. I'm not sure how to check the chuck balance, or how to fix it if I have a problem, but it bears thinking about. It can't be that hard. I'd start out with a static balancing rig that holds the chuck between two centers, stick on some weights until it balanced, and then carefully try drilling out (probably the backplate rather than my expensive chuck!) opposite the weights until I got it balanced.

As to bearings, if they're vibrating that's likely bad, but read below about setting preload before ye surrender all hope.

Spindle Runout and Bearing Preload

Take a feelergauge 0,01mm and place it between the roller and the outer racer Tighten up the bearings for 0,01 play that way Then check the play every 90 degrees If there is some difference you have a problem Also check rotating the spindle Difference in this way means you have a bent shaft

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Set bearing preload with an infrared thermometer. Your bearings should reach about 140 degrees F under load and stay there. If they get hotter, you have too much preload. If they run cooler, you have too little.

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I would check the spindle for play before going any further. To do this I put a long bar in a chuck mounted on the spindle and set a dial indicator up as close to the front bearing on the spindle as possible. Then I lift the bar and push it down while watching the dial indicator, any play will show up on the dial indicator.
If it shows zero play then I would look at the chuck to see if the jaws are bell-mouthed and not gripping the work piece properly. It could be something as simple as that which can be checked by moving the indicator to the work near the chuck jaws and doing the same test as with the bearing clearance check.

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A taper roller bearing should not need a lot of preload, just enough to allow you to feel some drag with oil as a lubricant (clean all the grease out for this test). Then put a tool in the post upside down and take a few cuts with the lathe running backwards. if the bearings are the cause of your problem this test should either cure it or at least diminish it substantially, if you are still getting the same chatter the cause lies elsewhere. Usually the slides are at fault.

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Fitting Your Lathe Chuck

• Mount the backplate sans chuck on the lathe and face it making sure the final cuts are light enough there is no distortion.
• Always store chucks with jaws downwards to keep swarf from entering the mechanism.