question about offsetting tailstock

[BHQ]

this one has really got me confused
i'm placing my indicator on the tool carriage at joint end of cue
driving the cue from the butt thru weight bolt hole
i put the indicator against the live center, which is approx 2" beyond the end of joint
offset .200 towards me
after running router along the cue several passes, i checked dimensions
joint measured .839
butt measured 1.195
difference of .356 divided by 2 = .178
NOTE: i checked first to make sure tailstock was centered before offsetting
checked digital indicator, set on zero, before offsetting
one thing i'm concerned about, don't know where the problem is, but after making a pass, when i move router back to the joint to make another pass, the tool carriage has moved inward towards cue a couple 1000th's
play in the carriage somewhere? i honestly don't know much about lathes. i'm not stupid , just ignorant of the lathe workings, it's my first lathe. I'M LEARNING.
by putting the indicator against the live center rather than the cue, was that a mistake? does that added distance of 2" change the taper? sure doesn't seem like it would

 

[TellsItLikeItIs]

this one has really got me confused
i'm placing my indicator on the tool carriage at joint end of cue
driving the cue from the butt thru weight bolt hole
i put the indicator against the live center, which is approx 2" beyond the end of joint
offset .200 towards me
after running router along the cue several passes, i checked dimensions
joint measured .839
butt measured 1.195
difference of .356 divided by 2 = .178
NOTE: i checked first to make sure tailstock was centered before offsetting
checked digital indicator, set on zero, before offsetting
one thing i'm concerned about, don't know where the problem is, but after making a pass, when i move router back to the joint to make another pass, the tool carriage has moved inward towards cue a couple 1000th's
play in the carriage somewhere? i honestly don't know much about lathes. i'm not stupid , just ignorant of the lathe workings, it's my first lathe. I'M LEARNING.
by putting the indicator against the live center rather than the cue, was that a mistake? does that added distance of 2" change the taper? sure doesn't seem like it would

What make of lathe are you working with?

 

[BHQ]

What make of lathe are you working with?

it's a sheldon, made in 1942. bought from original owner
i was just out there checking on something. if i grab hold of tool post, i can move it back and forth a smidge
if it would help, i can take a picture?

 

[Murray Tucker]

this one has really got me confused
i'm placing my indicator on the tool carriage at joint end of cue
driving the cue from the butt thru weight bolt hole
i put the indicator against the live center, which is approx 2" beyond the end of joint
offset .200 towards me
after running router along the cue several passes, i checked dimensions
joint measured .839
butt measured 1.195
difference of .356 divided by 2 = .178
NOTE: i checked first to make sure tailstock was centered before offsetting
checked digital indicator, set on zero, before offsetting
one thing i'm concerned about, don't know where the problem is, but after making a pass, when i move router back to the joint to make another pass, the tool carriage has moved inward towards cue a couple 1000th's
play in the carriage somewhere? i honestly don't know much about lathes. i'm not stupid , just ignorant of the lathe workings, it's my first lathe. I'M LEARNING.
by putting the indicator against the live center rather than the cue, was that a mistake? does that added distance of 2" change the taper? sure doesn't seem like it would

The way you did it will get you close but not perfect, as you found out. Since the center is tapered just changing how deep you drill the center holes in the cue can make several thou. difference in the size of the cue. That is because the offset has to be measured from where the centers touch the cue and not relative to each other. Plus if you have already made provisions for the bumper there is a lot of overhang from where the center hits the cue and the end of the cue. That is one reason that I don't like the offset tailstock method. Just make adjustments until you get the taper you desire. Then you can note the offset for future use.

 

[Bill the Cat]

Offset problem

The extra 2" will make a slight difference. Mathmatically, it's a simple ratio. If you make the offset check 2" beyond the end of the cue, your offsetting at 31" instead of 29". The following calculation will show the resulting offset at 29". .200"/31" = X/29" where X is the resulting offset at 29". In this case X = .187". This doesn't explain the full error you are seeing, but does show that your method of setting the offset on the live center needs to be adjusted.

The remainder of the difference you are seeing could be the result play in the machine, tool pressure, etc.

One other thing, you mentioned you checked that the tailstock was centered before you offset. Assuming you checked with the tailstock moved up to the headstock, there may be some error in the alignment of the headstock to the machine ways. A .01 degree misalignment of the headstock, will result in an error of .005" when the tailstock is moved out 29".

 

[BHQ]

The extra 2" will make a slight difference. Mathmatically, it's a simple ratio. If you make the offset check 2" beyond the end of the cue, your offsetting at 31" instead of 29". The following calculation will show the resulting offset at 29". .200"/31" = X/29" where X is the resulting offset at 29". In this case X = .187". This doesn't explain the full error you are seeing, but does show that your method of setting the offset on the live center needs to be adjusted.

The remainder of the difference you are seeing could be the result play in the machine, tool pressure, etc.

One other thing, you mentioned you checked that the tailstock was centered before you offset. Assuming you checked with the tailstock moved up to the headstock, there may be some error in the alignment of the headstock to the machine ways. A .01 degree misalignment of the headstock, will result in an error of .005" when the tailstock is moved out 29".

that's a difference of .009 by my math. so, that difference could be like you & murray said, the depth of the center holes, play in the carriage & possible misalignment.
.009 divided by 2= .0045
.200 - .0045= .1955
i'll use that number and see what happens
thanks a million guys

 

[Bill the Cat]

I know you think you know what you thougth I said....

that's a difference of .009 by my math. so, that difference could be like you & murray said, the depth of the center holes, play in the carriage & possible misalignment.
.009 divided by 2= .0045
.200 - .0045= .1955
i'll use that number and see what happens
thanks a million guys

I think you misunderstood. I'm assuming you came up with the .009" figure as the difference between the .187 calculated value and the .178 actual value that you measured. The .005" I gave was just an example. If the headstock to machine ways alignment was off by .01 degree, the resultant error at 29" would be .005". That may or may not be what is causing the additional error.

More importantly is the calculated offset value. If you are going to measure from the live center (2" farther than the end of the cue), we need to go back to the ratio calculation. If you want .200" offset at 29", the following formula applies.

X/31" = .200"/29" where X = .214"

That is the "theoretical" offset. You will still need to adjust for any "real world" error, such as, misalignment, play in the carriage, etc. The original offset yeilded an "actual" offset of .178" when the calculated theoretical value was .187", or .009" less than the theoretical. Therefore, the new offset should be the .214" new theoretical plus .009" actual error, or .223" offset (at the live center).

That's the math, but don't be suprised if it's still off a little. The best way to get a consistant taper is with a taper bar (and don't move the tailstock once it's set).

 

[BLACKHEARTCUES]

this one has really got me confused
i'm placing my indicator on the tool carriage at joint end of cue
driving the cue from the butt thru weight bolt hole
i put the indicator against the live center, which is approx 2" beyond the end of joint
offset .200 towards me
after running router along the cue several passes, i checked dimensions
joint measured .839
butt measured 1.195
difference of .356 divided by 2 = .178
NOTE: i checked first to make sure tailstock was centered before offsetting
checked digital indicator, set on zero, before offsetting
one thing i'm concerned about, don't know where the problem is, but after making a pass, when i move router back to the joint to make another pass, the tool carriage has moved inward towards cue a couple 1000th's
play in the carriage somewhere? i honestly don't know much about lathes. i'm not stupid , just ignorant of the lathe workings, it's my first lathe. I'M LEARNING.
by putting the indicator against the live center rather than the cue, was that a mistake? does that added distance of 2" change the taper? sure doesn't seem like it would

Murray has this one dead nuts. Draw a line with a straight edge for 12". Then draw an angled line, from one end. Measure the space between the lines at 6" & then at 12". You can see that the longer the straight line(length of the Q), the MORE offset the second line is...JER