How much is acceptable or reasonable

[tpic402]

I have a Grizzly 14 x 40 lathe here is the scenario, lathe bed perfectly level with starret level that has been calibrated. turning a 1.75 inch solid aluminum bar in 3 jaw chuck i get a taper of .0011over 7 inches and i took a spring cut.( not supported by center) My metrology skills are not perfect i am just a weekend warrior when time permits so i measured 5 different times with 2 different tools all came within tenths. My real question is this worth chasing or will i need anti depressants doing so!!!! Thanks Tom

 

[benmychree]

you won't find one that tests better than that.

 

[middle.road]

Having a center in it would help some, but that isn't bad on a 1440. Mine is about .002-004 over 8" depending on how it is feeling that day.

 

[Ulma Doctor]

i'd let that go and move on

 

[projectnut]

You might be able to improve, but only slightly by using a center. Rule of thumb is you can allow 5 times the diameter of the material to extend beyond the chuck jaws without support. In your case you should be able to let the material extend up to another 1.75" without support if your depth of cut is .040 or less.

These "rules of thumb" make the assumptions the machine has enough rigidity and power.

 

[tpic402]

Thank you all, cause i was getting the irresistible urge to mess with it, but i had a deep nagging fear it would
bite me in the keaster!!! MOVING ON!!

 

[C-Bag]

This is what I love about this site. It is way too easy to fall into the metrology black hole and start chasing my tail once you have all the equipment to get into the tenths range.

Keepin' it real

 

[Bob Korves]

Measure it with an autocollimator, mark it with a crayon, and then cut it off with an axe...

Accuracy is only needed when it is NEEDED. Most of the time, good enough is good enough. Learning the needed skills to do accurate work and keeping in practice for when it is truly needed is a virtue. Making everything you make to the highest possible tolerances is a giant waste of time and effort. Sometimes it is good fun to waste your time and effort just for the practice...

What probably makes the most sense is being seriously caring about when and where to pursue high accuracy and when a lower tolerance will be adequate. Be honest with yourself about tolerances in your own hobby shop, have appropriate standards for each job and stick with what you think is the correct way to go for each and every job -- and then follow through with it.

 

[mmcmdl]

We were taught to always shoot for nominal . If the print called for 1.000 +.005 , -.000 , you shot for 1.0025 . Didn't mean you had to achieve it but it taught everyone how to arrive there when necessary . It comes down to form , fit and function .

 

[Mitch Alsup]

I have a Grizzly 14 x 40 lathe here is the scenario, lathe bed perfectly level with starret level that has been calibrated. turning a 1.75 inch solid aluminum bar in 3 jaw chuck i get a taper of .0011over 7 inches and i took a spring cut.

Bending is quadratic with distance (from chuck). So if the center is measuring less than 0.000,3 different from the chuck end, all you are seeing is the bending of the bar due to the forces being used to cut the metal.