times, a problem with the bed is the real cause of troubles on older machines. Since this is an easy area to check, let's start here.
Your lathe bed (ideally, made of cast iron) must be rigid, straight, and level from end to end. Any problems with the bed will translate to a misalignment between centers when the tailstock is locked down. This, in turn, causes problems with processes like boring accurately from the tailstock.
You can check a cast-iron bed visually for cracks, and sighting down the ways usually tells you if they're straight. Sometimes the beds of older machines will be worn down in one spot from constant repetition of a production process. Use a steel straightedge to check for suspected twists or low spots.
If the bed is worn or twisted, you have three options: You can learn to live with the problem, you can have the bed milled flat by a machine shop, or you can build up the damaged ways with a product called Multifil 426 Bearing Tape.
Multifil 426 Bearing Tape (available from Bearings, Inc.; see "Sources," page 39), is a sliding bearing material that comcs in varying thicknesses from 0.015 in. to 0.125 in. and in standard widths of 12 in. and 24 in. It's used by machine-tool rebuilders to restore machine ways and gibs. You glue the bearing tape on with epoxy, and if necessary, the surface of the tape can then be machined, ground or hand-scraped to the tolerances you need.
Don't forget to chcck the underside of the lathe bed. Cracks in the cast "webs" that give the bed strength can cause flexing under load. And take a look at the underpart of the bed, against which the tailstock locks down. Excessive wear here can cause a loose tail-stock. If the tailstock is loose, you can adjust it by tightening a nut.
If you do find cracks in a cast-iron bed, they can be welded, but this is a job for a qualified machine shop. Welding cast iron can be tricky (and expensive).
Steel-bed lathes—whether pipe or angle—can be straightened. It's a good idea to replace the offending section with a new, straight piece. Some manufacturers of lathes with steel-pipe beds suggest filling the pipes with concrete to add rigidity and minimize vibration.
The headstock (the powered end of the lathe) basically consists of a spinning shaft connected to a drive system. However, there's a lot going on here, especially on newer, more complex lathes.
The Bearings and Shaft—First there are the bearings. The bearings of a lathe arc the heart of the machine. If the bear
Taking the play out. On lathes with tapered bearings, spindle-shaft play can often be eliminated by tightening the load nut slightly.
Wiggle the shaft. Play in the shaft is a sure sign of worn bearings. To check for play, grab the inboard and outboard ends and wiggle. If you detect play, or a '"clunk," there's a problem.
ings are worn, the lathe will produce eccentric (oval) turnings instead of round ones. Lids won't fit, tenons will have play, and turning will be frustrating rather than enjoyable.
Play in the shaft is a sure sign of worn bearings. A quick way to chcck a shaft for play is to grab the inboard and outboard ends and wiggle. (Sec photo above.) If you detect play, or a "clunk," there is a problem. For a more scientific approach, you can measure runout by placing a dial indicator on a smooth portion of the spindle (not the threads). Runout of around 0.10 in. or greater is more than you want.
Modern lathes usually have tapered roller bearings that feature a load nut or screw for adjusting the load on the bearings. In many cases you can eliminate spindle-shaft play by tightening this load nut slightly (see photo, left), but don't over-tighten! If this bearing adjustment doesn't get rid of the play, you may have to replace the bearings.
Older lathes, with oil-impregnated bronze or babbit-metal bearings, aren't so adaptable. Spindle play on these machines generally means worn bearings and a trip to the machinist for (expensive) replacement.
If you need to replace oil-bronze or babbit bearings, also be sure to have your machinist check the shaft for true. A bent or out-of-round shaft will "eat" new bearings quickly. If the shaft isn't true, you will need to have it re-machined before replacing the bearings. If the shaft is too far out, it will have to be replaced.
To sum up, play of any sort on the
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