.004 Vertical Drop...How does that translate to the part?

tobnpr · Oct 29, 2018

Decided to measure vertical drop at the cross-slide/toolpost.

.004 consistently for about a foot in front of the chuck, and then rises to O quickly and remains 0 consistently for the next couple feet to the end of the 4-1/2' bed; measured with a drop indicator on the compound and the Noga on the tailstock as the carriage pushes it to the far end of the bed. There is a very light ridge on the tailstock v-way and base, but it has been shimmed and aligned with a long test bar to the headstock.
Most of my work is 2-3 feet from the chuck in the steady and my tailstock is aligned for that area of the bed.

Mathematically, what would I expect as a variation in diameter of the workpiece from .004 of drop?

Milland · Oct 29, 2018

If I understand your question correctly, the answer is "not much", as the tool is moving in a tangent to the OD when it drops. So expect virtually no change with larger OD's, maybe a few tenths to a thou with smaller diameters. It does get worse the smaller the part becomes.

10KPete · Oct 29, 2018

This will help:

Pete

CarbideBob · Oct 29, 2018

2-3 feet from the chuck so 4 to 6 foot diameter part?
That is a 0.0038 degree triangle.
Literally sub millionths.
Your surface finish will way swamp any concern here.
A different deal and problem if you are making .010 dia parts or are top skiving.
Bob

Milland · Oct 29, 2018

CarbideBob said:
2-3 feet from the chuck so 4 to 6 foot diameter part?
That is a 0.0038 degree triangle.
Literally sub millionths.
Your surface finish will way swamp any concern here.
A different deal and problem if you are making .010 dia parts or are top skiving.
Bob

He means away from the chuck in Z, not diameter. Hence the reference to a steady rest. But we agree that the diameter effect's not much in most circumstances.

10KPete · Oct 29, 2018

Sorry about that image, guys. I posted that once before and I thought it was more readable.
If you click on it, it will enlarge but be fuzzy. If you view it in, like Preview or something, it improves
significantly.

Pete

CarbideBob · Oct 29, 2018

Milland said:
He means away from the chuck in Z, not diameter. Hence the reference to a steady rest. ..............

My oops.

Bob

Cole2534 · Oct 30, 2018

It's a right triangle where one leg is the part's radius, one leg is the height change, and the hypotenuse is your resultant radius. Call up your buddy pythagorous and bam.

For a 1" diameter part and 0.004" drop you get- 0.5^2 + 0.004^2 = c^2 = .500016". Subtract out your original 0.5 to find your delta and you'll notice that it doesn't amount to shit.

10KPete · Oct 30, 2018

A member contacted me and at his request I have emailed that chart to him and he was able to open it and read it. If you would like the chart, contact me by private message and I will send it to you.

Pete

tobnpr · Oct 30, 2018

PM sent, thank you.

Seems like this would be an "acceptable" level of wear for a nearly 70 year old machine.

SLK001 · Oct 30, 2018

10KPete said:
This will help:

Pete

It probably would, if it could be read. Any chance you could reload it? Or post it in a ZIP file?

10KPete · Oct 30, 2018

I don't think I can do anything with the file as it's the original I downloaded. But that was some time ago and I don't know if I can find it again.. I'm going to try and find it...

Pete

johansen · Oct 30, 2018

It depends: the carriage doesn't just drop straight down. it pivots on the rear way (which is usually worn far less than the front way.)

which means the "drop" translates approximately directly to the diameter. now of course, the lathe has probably already been twisted to compensate for that near the headstock. and the tailstock would always be adjusted for turning the same diameter at both ends.

10KPete · Oct 30, 2018

New chart for lathe bed wear

Yeah! I found the original. Much better.

Pete

SLK001 · Oct 31, 2018

Thanks, Pete. It's much better, but I still have to strain to read the small print. There is just not enough resolution in the original jpeg. I'm not complaining, just stating a fact.

Here's what I mean:

UNKS.jpg (29.1 KB)

I can't read the small print that the arrows point to.

10KPete · Oct 31, 2018

I open the chart in Preview (I have a Mac) and can enlarge it considerably then it's very readable. The upper arrow is a small 'm' but there is a sawtooth squiggle under it. I don't know why. The lower is a capital 'R' small 'n'. Again with that squiggle under it.

Try reading the chart in Preview or similar. With Preview I can enlarge it many times and it stays quite legible. I don't know what other program will do this but I'm sure that there are many in the PC world.

Hope this helps,
Pete

Technical Ted · Oct 31, 2018

I would think, in addition to the calculations provided above, that it would depend on where your cutting tool is on center height. For example, it would vary depending on whether you set your cutting tool a little above center at the start of your cut (as I typically do when turning a diameter) or if you are starting out right on center. The closer the cutting tool "averages" to being the closest on center height over the total travel up and down your longitudinal ways the less effect the slight variance in height will have.

Ted

tobnpr · Oct 31, 2018

johansen said:
It depends: the carriage doesn't just drop straight down. it pivots on the rear way (which is usually worn far less than the front way.)

which means the "drop" translates approximately directly to the diameter. now of course, the lathe has probably already been twisted to compensate for that near the headstock. and the tailstock would always be adjusted for turning the same diameter at both ends.

Bed has not been twisted, leveled with a Starrett and it turns without taper. I had thought about the drop not being truly vertical, and it would be interesting to know the differences in both bed and saddle wear from rear way to front. No rock whatsoever in the saddle, but someday I'd like to blue the ways to see what the contact pattern is after all those years.

moonlight machine · Oct 31, 2018

tobnpr said:
Decided to measure vertical drop at the cross-slide/toolpost.

.004 consistently for about a foot in front of the chuck, and then rises to O quickly and remains 0 consistently for the next couple feet to the end of the 4-1/2' bed; measured with a drop indicator on the compound and the Noga on the tailstock as the carriage pushes it to the far end of the bed. There is a very light ridge on the tailstock v-way and base, but it has been shimmed and aligned with a long test bar to the headstock.
Most of my work is 2-3 feet from the chuck in the steady and my tailstock is aligned for that area of the bed.

Mathematically, what would I expect as a variation in diameter of the workpiece from .004 of drop?[/QUOTE

That will amount to 5/8 of f*ck all.

Conrad Hoffman · Oct 31, 2018

I do a lot of very small work. That means I also do a lot of very short work! Thus, once I'm set up with the tool on center, bed wear doesn't amount to anything over any practical cutting distance. I think my lathe also has about 0.004" of wear.

.004 Vertical Drop...How does that translate to the part?

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