Standard modern lathe misalignment

Howard E. said:
I've always used Rollie's dad's method-


"Level" is not really a requirement- it's '"twist" and "droop" that are the enemies. Lot's of lathes have been installed on ship decks that were very seldom near level for any extended amount of time.
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Yes!
I was just thinking about shimming the bed over the base to twist it with bolts!

Thank you
 
Howard E. said:
I've always used Rollie's dad's method-


"Level" is not really a requirement- it's '"twist" and "droop" that are the enemies. Lot's of lathes have been installed on ship decks that were very seldom near level for any extended amount of time.
Click to expand...
Well I give it a try and I am affraid that dad is wrong, or at least the method is not completely explained and some important details are missing, wich makes it harder to apply than it looks:

For this method to work, the starting point is the EXACT distance from the center of spindle to surface of bar so, yo would need a way to put your dial rod in the very exact center of spindle close to chuck, then pull and hold your dial rod enough distance to allow the installation of the bar in the chuck and then release the measuring rod. NOT THAT EASY. If you start taking measurements to a random distance from center you will never get the radius bar, which is, by the way, the heart of this method. That's what I did and I noticed soon that I was measuring only the chucking runout, wich is unsensitive to twist.

The bar does not need to be perfectly straight but it NEEDS to be perfectly round, since we are pretending to measure the radius bar. As an example, a bar with a slight high zone can put you far away from your target since you will wrongly consider this zone in your calculations.

I keep the idea of twist the bed over the table (for this kind of lathe) but i will do a twist then cut test method, it gonna be easier and less prone to measuring errors.
 
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aaronpadilla said:
For this method to work, the starting point is the EXACT distance from the center of spindle to surface of bar so, yo would need a way to put your dial rod in the very exact center of spindle close to chuck, then pull and hold your dial rod enough distance to allow the installation of the bar in the chuck and then release the measuring rod.
Click to expand...

I've always just chucked up a bar & then set up the dial indicator & cranked the bug to zero. The distance from the center line is immaterial with this method- what you're actually measuring is the differences on the indicator from one end to the other & averaging them out to get center line variance at the far end as compared to at the chuck.

Re. bumpy bars I'm not sure what you're expecting for accepatble results but any fresh cold rolled bar will be close enough to work. Realistically for hobbyist projects I think if you can get things within 2-3 thou you're golden. And don't forget that if you're using a 3 jaw chuck it's probably going to wobble off the actual center line by that much anyhow,
 
I agree, straight is more important than level, level just makes it easier to recheck later.
Put your level across the bed, like your first picture & run the saddle up & down the length of the bed. You're looking for movement in the bubble or twist in the bed, just a little would explain the error you're getting, a slight tweak to the jack bolts or some shims to get it dialed in. Just don't be surprised when it changes in a week or a month, everything moves in time!
 
Howard E. said:
I've always just chucked up a bar & then set up the dial indicator & cranked the bug to zero. The distance from the center line is immaterial with this method- what you're actually measuring is the differences on the indicator from one end to the other & averaging them out to get center line variance at the far end as compared to at the chuck.

Re. bumpy bars I'm not sure what you're expecting for accepatble results but any fresh cold rolled bar will be close enough to work. Realistically for hobbyist projects I think if you can get things within 2-3 thou you're golden. And don't forget that if you're using a 3 jaw chuck it's probably going to wobble off the actual center line by that much anyhow,
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I really would like make it work but I just don't get it.
I think that this method is missing something, geometry
As an example, If I put a good long bar in my chuck, close to chuck I got 2mils of wobble, which averages to 1mil, the other en gets 8mils wich averages to 4mil, then I twist my bed and I got the exact same wobble at different offsets of my dial which is perfectly normal, the wobble average will not change nor go away twisting the bed.

Anyway I will keep thinking about this may be I will found an answer
 
aaronpadilla said:
I really would like make it work but I just don't get it.
I think that this method is missing something, geometry
As an example, If I put a good long bar in my chuck, close to chuck I got 2mils of wobble, which averages to 1mil, the other en gets 8mils wich averages to 4mil, then I twist my bed and I got the exact same wobble at different offsets of my dial which is perfectly normal, the wobble average will not change nor go away twisting the bed.

Anyway I will keep thinking about this may be I will found an answer
Click to expand...

Dont_touch_dial.png

Absolute readings to the center line are not important here, you're looking for the differences in the average readings of an indicator set to an arbitrary pointless point.

Mount the dial indicator on that carriage and put the anvil in contact with the vertical midpoint of the bar near the chuck. Set the dial to something convenient and then leave it alone. Rotate the chuck & record the high & low readings, calculate the average reading & record it. (most of the variance here is caused buy three jaw chucks lack of perfection).

Rollies_dads_realworld_1.jpg

WITHOUT touching the dial indicator(s) crank the carriage to the tailstock end of the bar.

Again rotate the chuck, again record the average reading.

The difference between the chuck reading and the end of the bar reading is how much the twist has affected the horizontal center line at wherever the end of the bar is.

Example-

Near end readings are +1 & -3 for an average of -2.

Far end is +7 & + 4 for an average of +5.5.

The twist is resulting in the center line at the end of the bar being the difference between -2 & + 5.5 or +7.5

Shim & repeat as necessary.

Check the vertical by mounting the indicator to the high spot on the bar.

Again if you're using a 3 jaw chuck the actual center line will move around a bit anyhow, how much generally depends on the size of the object clamped into it- this is caused by the scroll that moves the jaws not being 100% perfect along it's length) For example with a 17mm bar mine wobbles around by about .75thou, which sounds not too bad but a 5/16" rod shows a ~5 thou wobble & a 2" is about 3 thou.

I got the far end of my lathe shimmed to be about 1.5 thou, that's good enough for me & it will probably move from that whenever it feels like it anyhow.
 
Howard E. said:
View attachment 354677

Absolute readings to the center line are not important here, you're looking for the differences in the average readings of an indicator set to an arbitrary pointless point.

Mount the dial indicator on that carriage and put the anvil in contact with the vertical midpoint of the bar near the chuck. Set the dial to something convenient and then leave it alone. Rotate the chuck & record the high & low readings, calculate the average reading & record it. (most of the variance here is caused buy three jaw chucks lack of perfection).

View attachment 354699

WITHOUT touching the dial indicator(s) crank the carriage to the tailstock end of the bar.

Again rotate the chuck, again record the average reading.

The difference between the chuck reading and the end of the bar reading is how much the twist has affected the horizontal center line at wherever the end of the bar is.

Example-

Near end readings are +1 & -3 for an average of -2.

Far end is +7 & + 4 for an average of +5.5.

The twist is resulting in the center line at the end of the bar being the difference between -2 & + 5.5 or +7.5

Shim & repeat as necessary.

Check the vertical by mounting the indicator to the high spot on the bar.

Again if you're using a 3 jaw chuck the actual center line will move around a bit anyhow, how much generally depends on the size of the object clamped into it- this is caused by the scroll that moves the jaws not being 100% perfect along it's length) For example with a 17mm bar mine wobbles around by about .75thou, which sounds not too bad but a 5/16" rod shows a ~5 thou wobble & a 2" is about 3 thou.

I got the far end of my lathe shimmed to be about 1.5 thou, that's good enough for me & it will probably move from that whenever it feels like it anyhow.
Click to expand...


Interesting, let me check that.
Thanks!
 
In cant believe how much of a project you guys are making out of leveling a lathe.
If it is bigger than a bench top level it and make a test cut.
 
sdelivery said:
In cant believe how much of a project you guys are making out of leveling a lathe.
If it is bigger than a bench top level it and make a test cut.
Click to expand...
Leveling and test cut was my very first test and the result was so bad.
In the kingdom of klunkers bed twist a serious topic, belive it.
 
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