A head scratcher here …

[poorboy]

I have an interesting issue with my PM-1340GT that I am trying to figure out.

I put a 2”x12" bar in the chuck and performed a collar alignment test (with no tailstock). Took light cuts (.002") -- I then put a .0001" indicator on the carriage and ran the indicator over both collars -- I am off by .0003" over 10 inches. Everything looks great - but ...

If I use a micrometer to measure the collar closest to the chuck and then measure the collar farthest from the chuck and the difference is .010". There seems to be a taper in the bar. How can I have a taper and not have the indicator on the carriage show that there is a taper ?

Any ideas ?

 

[Shotgun]

The bed is twisted. As the carriage moves down, it move in or out .010" over that ten inches. The indicator riding on the carriage doesn't see anything move.

 

[poorboy]

I still dont understand. I just removed .002 from the collars using the same carriage - i dont understand how there can be a taper and an indicator not show that

 

[woodchucker]

I still dont understand. I just removed .002 from the collars using the same carriage.

you need to put a precision level on the bed ways and get them level (from twist).
if your ways are twisted (not level) you carriage is not going to run straight. if your ways are lifted at the tailstock on your side, then you will cut closer at the tailstock end, if your ways are lifted at the tailstock on the back end, you will cut farther and it will look like the reverse of a taper, more like a trumpet.

 

[Shotgun]

I still dont understand. I just removed .002 from the collars using the same carriage.

The same carriage that you mounted the indicator to? What you proved there is that the tool did not move relative to the carriage as you made the cut.

The micrometer on each end was independent of the carriage. It showed that the carriage does not maintain a specific distance from the workpiece as it moves down. The carriage's distance from the workpiece is controlled by the ways. The ways are twisted. You generally take that twist out by adjusting leveling feet. Several videos online demonstrating the phenomena with lathe mockups.

 

[poorboy]

I have leveled my ways using a precision level (6 months ago). Let me take another look at things again. Thanks

 

[Shotgun]

Things can move a lot in 6mos, especially when we consider .010" to be a lot. Even with a precision level, the true test is still going to be the test you just did.

 

[benmychree]

What order of precision is the level? .005" is not accurate enough for finish leveling of a machine tool, I.E. a Starrett #98 is not accurate enough. If you cut the 2 collars to the same diameter as evidenced by a micrometer reading and then used the indicator on the carriage, you should see evidence of the probable twist in the bedways.

 

[mksj]

The bed would be a pretzel if it had that much twist, seems like it would more likely be a headstock alignment. Just about every 1340GT I have encountered needed hedstock alignment, might be due to its size and light weight. I would check the bed is level, I use Rolle's Dad's Method for headstock alignment and then I do the 2 ring test. The headstock needs very small adjustments of the alignment screws where the motor is located, after you lightly loosen the headstock bolts.

Rolle's Dad's Method

Rolle's Dad's Method (RDM) is a brilliant way to accurately measure the alignment of a spindle to the bed/column, and is extremely useful fo...

benchtopmachineshop.blogspot.com

 

[Shotgun]

The bed would be a pretzel if it had that much twist, seems like it would more likely be a headstock alignment. Just about every 1340GT I have encountered needed hedstock alignment, might be due to its size and light weight. I would check the bed is level, I use Rolle's Dad's Method for headstock alignment and then I do the 2 ring test. The headstock needs very small adjustments of the alignment screws where the motor is located, after you lightly loosen the headstock bolts.

Rolle's Dad's Method

Rolle's Dad's Method (RDM) is a brilliant way to accurately measure the alignment of a spindle to the bed/column, and is extremely useful fo...

benchtopmachineshop.blogspot.com

How do you distinquish headstock alignment from bed twist? If the bed were twisted such that the carriage moved .010" away from the work, but the head was nodded down such that the work move .010" toward the cutter, could you detect a taper? How does RDM know the difference?

Not an academic question. I'm working on setting up a 80yr old lathe, and I think I've come to the conclusion that it doesn't matter as long as it cuts straight.