Got some numbers on the bed wear -- any comments?

^^^^^^ headstock, sorry

In order to tune the lathe, three tools are needed. The first is a precision level and the second is a spindle test bar and the third is a dial indicator. With these tools, all measurements and adjustments can be made. First, the level of the bed must be established. Not so much for the operation of the lathe in this instance, but to establish a measurement baseline. To do this correctly, be prepared to twist the bed. This sounds impossible, but the bed is actually quite plastic and it is easy to achieve. The bigger the lathe, the easier it is Remove everything from the bed. You can cheat and remove everything but the saddle if you want, but it is better to remove everything. if the lathe has an underlying bench or cabinet, you may want to unbolt the bed, jack it up and drill and tap two holes in the bed legs at the tail stock ends. Install two jacking bolts in these holes and remove any shims that may exist under these legs and rebolt leaving these new screws loose. Now level the bed horizontally at the headstock with the cabinet feet, then level the bed longitudinally with the cabinet feet, alternating back and forth to the best possible position. Don't worry about the horizontal plane at the tailstock, that's why we have the jacking screws. With the bed firmly bolted down measure the horizontal level incrementally from the headstock end to the tailstock end and record your readings. Now you have established the bed twist. Recheck the headstock side and be certain it is still level. Place the level across the tailstock end and with the jacking screws twist the bed to level, often checking that the headstock side isn't changing. If it does, adjust with the cabinet feet (the cabinet is twisting) Repeat this until the horizontal plane is the same at both ends. Then with feeler gages measure the gap between the feet and the cabinet, make a shim pack to the measured dimensions and rebolt the bed. Check for level again and repeat 'till square. Now by measuring the difference in level across the bed, you can establish the wear. (The level is calibrated) Once that is accomplished, reply with your readings and we will talk about that and perhaps the next episode, the headstock. Steve

"Ignoramus20788" wrote in message news:aamdnQKjUt1OiR3VnZ2dnUVZ snipped-for-privacy@giganews.com...

My first lathe had ~0.025" of bed wear and still could turn straight, which is the real test. Use it, make some parts and measure them, then decide if the lathe is good enough for the sort of work you do. I adapted to that worn lathe by doing critical work at the tailstock end where the bed and leadscrew were in good condition.

Since the carriage tracks the worn surfaces, the wear is probably worse than you measured. As you go toward the headstock with the indicator mounted on the carriage, the indicator is dropping in height as you head towards the headstock, nulling some of the reading.

That is quite good for a well-used lathe. I had about .013" on my Sheldon 15" lathe, and decided that was much worse than I could live with. Making estimates of the overhang and lever arm, I calculated that it would put a .016" taper or barrel shape in a 1" diameter part. That was totally unacceptable to me.

What you have to do is repeat the test with the indicator at the REAR way, riding on the reference surface of the tailstock way (assuming a classic American-pattern lathe). If the wear shows a roughly similar change, you are in great shape, as the wear is roughly symmetrical front and back. Also, gravity and the distortion caused by flame hardening the bed may have put a sag in the bed, and your measurement technique may be picking that up, too. (My Sheldon's 6-foot long bed had a .003" sag in the middle, probably caused by the stresses induced from the flame hardening.) The inverted-vee changes the force vectors causing more wear on the front way than the back, and the dribbling of chips increases wear on the front way, too. But, the critical thing is the relationship between the front and rear way. If the wear is relatively similar, the taper on parts will not be large. Assuming the front way has all the wear and the back is unworn, then you might get as much as .005" of taper on the diameter of a part. Measure the distance between the two ways, and measure the distance between a line front-to-back between the ways and the spindle centerline. On most lathes, the height of the spindle above the ways is very roughly similar to the spacing between the ways. You can work out the trig for your particular machine.

If the wear is similar front and back, then the tool will just move up and down, not in and out, and the taper will be vastly smaller.

Jon

On most "American pattern" lathes, there is a "reference surface" ground right at the top of the inverted vee, and another at the top of the tailstock vee in the rear. Nothing is supposed to ride on these surfaces, and they are ground at the same time as the rest of the bed's critical surfaces. You level and align the bed using these surfaces, for instance. I think Iggy was riding his indicator along these reference surfaces (actually only the front one). This should measure bed wear pretty well. it will miss sag in the middle of the bed due to gravity and stresses from flame hardening, but those are less important.

Jon

Mount the indicator on the tailstock, as the tailstock ways see the least amount of use on MOST lathes

Gunner