Re-grinding only one V-way of a lathe bed

MichiganAmateur

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This is my first post, hopefully this is the right forum.

I am rebuilding a new-to-me South Bend 9C, and the front V-way is visibly worn near the headstock (a fingernail catches on it), but the rear V-ways show minimal wear.

To quantify the wear, I did the following test of my own creation (hopefully it is sensible): I slid the carriage and tailstock together so that they are touching. I mounted a dial indicator on the carriage at approximately a tool cutting height, so that I can measure the relative distance to the side of the tailstock. I then slide the carriage and tailstock together along the bed and see how much the dial indicator changes, which gives a measure of how much a cutting tool and the tailstock are out-of-parallel.

When I do this test, the indicator shows a relative motion of approximately 8 mils, with most of the deviation near the headstock, as expected based on the front V-way having visible wear in this region.

From reading forums, I see occasional discussions about regrinding the ways, and the usual conclusion being that the cost is simply not justifiable for an older machine.

Rather than regrinding everything, would it be a bad idea to simply regrind the front V-way to be parallel to the tailstock V-way? My thought is that the tailstock V-way is in much better condition (based on look and feel), so grinding just the front V-way would help align the carriage to this reference, and bring the machine into better alignment.

I know that this would result in the carriage sitting ever-so-slightly low in the front, but it is already sitting slightly low due to the bed wear near the headstock, so if the grinding does not exceed this wear depth, I am not seeing any harm being done.

Before I do anything foolish, I wanted to see if any of the more experienced people may have any thoughts about grinding just the front V-way. Thanks for any suggestions or cautions!
 

Richard King 2

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Years ago when I was a kid...maybe 60 years ago my Dad bought a lathe for our work shop in the garage at the cabin. The V was was terrible. He rigged up the compound on the bottom slide of the tail-stock and he had me pushing the sled up and down the TS ways that weren't worn to bad and with a tool holder and bit, I planned it. Them after it was close he had me file it, then we hand scraped it . It worked and I remember it was a pain and dirty as heck with that cast iron dust. It worked. I have scraped one side that was scored on grinders when we had an emergency repair. It worked, so it is possible. Would I do it on a rebuild with time, no. But if your short of time and $ go for it. I dis some math..8 mm is .350 inches. You must mean something else. what is it in inches? Bedtime for me......
 

MichiganAmateur

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Thanks Richard for the helpful info. The relative out-of-parallel motion is 8 mils (0.008 inches), not 8 mm, in case that makes more sense.
 

Richard King 2

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be sure you have the bed level aligned before anything. I would also relive the bottom of the TS to so it doesn't rock
 

MyLilMule

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Thanks Richard for the helpful info. The relative out-of-parallel motion is 8 mils (0.008 inches), not 8 mm, in case that makes more sense.
That would be 8 thousandths, or "8 thou" in machinist-speak ;). Took me a minute to pick up on some of this.

1.000" = 1 inch
0.100" = 100 thou (thousandth of an inch)
0.010" = 10 thou
0.001" = 1 thou

And this is fun:

.0001" = 1 tenth
.00001" = 10 millionths (.000010")
 

MyLilMule

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Incidentally, 8 thousandths is not bad if you can learn to compensate for it.

Another way to check wear in the bed is to attach a dial indicator holder on the carriage and then set the dial on the TOP of the V-way. Zero the indicator on the tail stock end, and then run it down to the head stock. You'll notice it drop if there is wear in the ways.

My 1941 South Bend has .015" of vertical drop. It still makes great parts.
 

MichiganAmateur

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Hi MyLilMule, thanks for the info about units.

I tried your test on all the V-ways, and I'm measuring a carriage drop of 0.006" against both the rear V-ways, and 0.012" against the front V-way.

I'm guessing that this asymmetrical drop causes the carriage to rotate slightly as it moves along the bed, moving the tool bit in and out, which I'm picking up with the test I noted in my original post. I currently compensate by shimming the lathe's feet to twist the bed until I get equal diameters on a two-collar test with ~6" of a test bar extending out of the chuck. This means I can cut consistent diameters near the headstock, but the carriage and tailstock are then out of alignment over the rest of the bed length. (I think that's a correct statement, but I'm still a novice learning about machining.)

I'm trying to figure out if there may be a partial remedy (only grinding the front V-way) which could provide 90% of the benefit of a full bed grinding, but at a lower time or cost expended. I contacted 3 professional machine shops today to see what they may say or quote for this job, I'm still waiting for their response after I provided them lathe bed dimensions.
 

tq60

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Tangent to a circle drawing...

Draw a circle with a compass

Large one is better.

Take a ruler resting on the center point and draw a line across the circle at the center point.

Now using that line as a reference mark a dot a quarter inch above at both ends then connect them.

Next do same but mark a dot 0.008 below and connect them.

Now look where they cross the circle.

The first one is tool at center and at perfect target point.

The high one is just demonstration but notice it is not that great of difference.

The bottom one, we'll you really could not draw that one thus the top one for demonstration.

The 0.008 drop does not make that much actual difference.

You can be aware of error and compensate for it.

One can change the angle of their cutter so it can be above center and make less of a difference.

Being AWARE is the critical part.

A good machinist can make good parts with a bad machine.

And at the "hobby" level the error here is less than operator error.

So learn from it.

Sneak up on final size.

And watch for the next lathe...

Sent from my SM-G781V using Tapatalk
 

MyLilMule

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Hi MyLilMule, thanks for the info about units.

I tried your test on all the V-ways, and I'm measuring a carriage drop of 0.006" against both the rear V-ways, and 0.012" against the front V-way.

I'm guessing that this asymmetrical drop causes the carriage to rotate slightly as it moves along the bed, moving the tool bit in and out, which I'm picking up with the test I noted in my original post. I currently compensate by shimming the lathe's feet to twist the bed until I get equal diameters on a two-collar test with ~6" of a test bar extending out of the chuck. This means I can cut consistent diameters near the headstock, but the carriage and tailstock are then out of alignment over the rest of the bed length. (I think that's a correct statement, but I'm still a novice learning about machining.)

I'm trying to figure out if there may be a partial remedy (only grinding the front V-way) which could provide 90% of the benefit of a full bed grinding, but at a lower time or cost expended. I contacted 3 professional machine shops today to see what they may say or quote for this job, I'm still waiting for their response after I provided them lathe bed dimensions.
That's not too bad, IMHO. The front will wear more because of tool pressure. My guess is that you also have wear in the saddle ways. If this is your first lathe, I'd be happy with it.

I suspect that when the quotes come back, you're going to have a sudden case of sticker shock. To have a bed way reground is very labor intensive. Only grinding part of it would be a waste if you're not going to regrind it all. Plus, you still have to deal with the saddle. And now that it's been ground, you have to compensate for the loss in material and scrape it flat.

Unless your lathe has deep sentimental value (it belonged to your grandfather perhaps) or has some significant historical significance (it was used aboard the USS Missouri during the war) then I think you will find it is not cost effective.
 
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