Saddle causing .150 taper 3” off chuck no tailstock need help.

[petertha]

I’ll upload some pics from my camera, but here are my findings. New & tight Noga mini DTI mag base. I chose it for test#1 because its the smallest one I have. Mitutoya 0.0001” DTI. Installed on a 3x5 chunk of steel. About 2 thou preload & needle zeroed sitting upright. Rotate the steel base 90-deg I get about 0.0002” deflection. Rotate it 180-deg so completely inverted, get about 0.0010-0.0012” needle deflection. So it’s real. My guess is a longer arm mag stand and/or a heavier Dial Indicator vs DTI may well exaggerate these findings. My gut feel is 99% of this is ‘gravity effect’ is attributable to the indicator holder assembly itself because when I grip the DTI by the short stub arbor that comes with the kit in a lathe collet & spin around a dowel pin I get no measurable difference. So that has eliminated pre-load & inversion on exact same DTI. The only remaining variable is the holder assembly. Maybe if you absolutely have to go inverted then consider a bolt-together holder vs the hydraulic or tension lock style. I'll try my longer Noga next.

 

[petertha]

I know this is detracting from the thread, but I'll post my pics & re-post elsewhere under something like Gravity vs indicator holder.
Noga Mini + Mitutoyo 0.0001: Level = 0.0000, 90-deg = 0.0005 - 0.0008", 180-deg (inverted) = .0011
Noga 14"? + Mitutoyo 0.001: Level = 0.000", 90-deg = .0042", 180-deg = .0055" (suspect a smidge more because it didn't return to zero & there might be some flex in my bar).

Conclusions
- I think the DTI by itself might not see much gravity effect, but the contribution of the DTI holder assembly sure can.
- if you have to use a holder, best to find something significantly more rigid. Or else find some independent way to DTI reference in an angled/inverted position. Reading DTI values at different rotation clock positions is a no-no.
- assembly weight is the enemy here. I suspect a dial indicator would be worse because it weighs more than DTI

 

[bretthl]

I know this is detracting from the thread, but I'll post my pics & re-post elsewhere under something like Gravity vs indicator holder.
Noga Mini + Mitutoyo 0.0001: Level = 0.0000, 90-deg = 0.0005 - 0.0008", 180-deg (inverted) = .0011
Noga 14"? + Mitutoyo 0.001: Level = 0.000", 90-deg = .0042", 180-deg = .0055" (suspect a smidge more because it didn't return to zero & there might be some flex in my bar).

Conclusions
- I think the DTI by itself might not see much gravity effect, but the contribution of the DTI holder assembly sure can.
- if you have to use a holder, best to find something significantly more rigid. Or else find some independent way to DTI reference in an angled/inverted position. Reading DTI values at different rotation clock positions is a no-no.
- assembly weight is the enemy here. I suspect a dial indicator would be worse because it weighs more than DTI

I’m confused. I thought a dial test indicator is used to take a relative measurement. So unless there is flex in the arm caused by the spring tension from the indicator itself then things should be okay.

 

[petertha]

Yes, that's the whole issue. Flex in the apparatus that is holding the indicator, not so much an issue with the indicator itself. Think of it this way - if you press your finger on the articulated arm, the indicator registers displacement. Well gravity is like your finger, its a force acting on the mass of the arm assembly. Its moving under its own weight vs. the assumption that it stays locked in position. So the indicator cannot distinguish this displacement from something that is out of round.

 

[plunger]

Ive bought a lathe like this. It also had a bad taper that I could not get rid of .I had it reground by a very dubios character.He said it would take six weeks but I had to physically remove it six months later.He took a six meter lathe and because his grinder wasnt long enough he ground it from both sides and blended it in. But thats another story.
The remark about the gap should be looked at. I would try a three jaw chuck in a 4 jaw to push it away as far as possible from the headstock and try taking two cuts from the tailstock and headstock side and then compare.
Just out of interest when I removed the headstock I was shocked to find there are no vees under the headstock. But what shocked me about it is that instead of being scraped in you could clearly see that they had used an angle grinder . There were actually big gouges taken out.

 

[Illinoyance]

Many lathes have jack screws that are used to align the headstock to the bed. See if your lathe has them.
Before checking for, and correcting taper issues be certain the bed is not twisted. Check it with a precision level.

Use a test bar as shown in the video. Relieve the middle of the bar leaving a collar about 1/2" wide at the outboard end and next to the chuck. Take a light test cut across both collars (without disturbing the cross slide) . Measure them. If there is a difference, use the jack screws to bring the headstock in line with the bed. It will take several attempts. Adjust, make a trial cut, measure, repeat until both collars are the same diameter after a test cut.

After you have the headstock true to the bed it is time to address the tailstock alignment. Make a test bar with a center in each end and a raised collar at each end. At the headstock end leave room beyond the collar for a lathe dog. Set up the test bar between centers and drive it with a dog. Make a test cut across both collars (without disturbing the cross slide). Measure the collars. Adjust the tailstock until a test cut produces equal diameters on the collars. Save the test bar. In the future you can set the test bar between centers then set an indicator to zero on the collar nearest the headstock, them move the carriage and see if the indicator reads zero at the tailstock end. If it does your tailstock is dead on. If not adjust the tailstock to get the indicator to zero. You might have to repeat that check a couple time to get both collars to read zero.

The sequence of tests and adjustments is critical.
1. correct twist in the bed, commonly refereed to as leveling the bed.
2. adjust the headstock so the spindle is parallel with the bed. A bar held in the chuck should show no taper when turned.
3. Adjust the tailstock using the two collar method.

The tool used to make test cuts should be HSS and very sharp. Test cuts should only be a few thou depth of cut. Heavy cuts or carbide inserts will deflect the test bar making it difficult to achieve good results.

I am now in the process of aligning the headstock on my Nardini. I had about .020" taper in 6". It is a time consuming job.

 

[bretthl]

Just out of interest when I removed the headstock I was shocked to find there are no vees under the headstock. But what shocked me about it is that instead of being scraped in you could clearly see that they had used an angle grinder . There were actually big gouges taken out.

I don't think any Asian made lathes have head stocks that rest on way vees.

 

[Illinoyance]

The quality of Asian tools can vary all over the map. A lot of it has to do with the quality control the importer has in place. I suspect many importers care very little about the quality of machines they sell.

 

[bretthl]

Yes, that's the whole issue. Flex in the apparatus that is holding the indicator, not so much an issue with the indicator itself. Think of it this way - if you press your finger on the articulated arm, the indicator registers displacement. Well gravity is like your finger, its a force acting on the mass of the arm assembly. Its moving under its own weight vs. the assumption that it stays locked in position. So the indicator cannot distinguish this displacement from something that is out of round.

If you don't move the indicator base/arm the gravity acting on the arm should not change.

 

[Al 1]

Check to see if head stock bolts are loose. Al.