Cheap and different lathe leveling method

[Ben Dono]

Hey guys,
I just stumbled across a YouTube video on a different method of lathe leveling using a plumb bob. It looks like a good concept and thought you all might have different opinions about it.

It is called "Cheapest way to level a lathe!!!" by roadracenut .

Sorry, I don't know how to imbed the video but it's worth searching for it.
I think I might give this a go as I don't have a machinist level.

[Michael G]

Cheapest Way To Level a Lathe!!! - YouTube (http://www.youtube.com/watch?v=wANmXuwowqw)

(Just cut and past a link)
In theory that may work but I suspect that in practice the errors that he would have stacking up mean that his lathe is "level" more by luck than scientific measurement.
5 thou over 12" is not all that flash as a target either - the usual target is better than 3

To each their own.

Michael

[Bryan]

Points for ingenuity I guess - and cheapness - but the main point of levelling is to get both ends of the bed the same, so you need to keep swapping the instrument from end to end as you make adjustments. Seems like that device would be cumbersome and slow for the bob to settle. Better than nothing though.

[Ben Dono]

Thanks for posting the video up Michael!

I agree it looks cumbersome but I like the concept! When I get around to it I will post a review.

[Michael G]

I think the biggest source of error is the suspension of the plumb bob. You would have to make sure that it is hung in such a way that the pivot point, centre of mass of the bob and the indicating point are all co-linear. A slight deviation and the bob may not behave repeatably.

Michael

[Steamwhisperer]

I think the biggest source of error is the suspension of the plumb bob. You would have to make sure that it is hung in such a way that the pivot point, centre of mass of the bob and the indicating point are all co-linear. A slight deviation and the bob may not behave repeatably.

Michael

Would a pendulum give better repeatability?

Phil

[Greg Q]

How the heck can he resolve the out-of-plumb condition with any degree of precision? Master levels detect 0.0005" over ten inches (approximately, depending) There's no way that you could resolve that fine a value sighting a plumb bob.

On the other hand: if you could layout a projection of the spindle centre line precisely beneath the spindle, and had a plumb bob damped in a viscous medium which you could read precisely, you might have something. Meh.

Greg

[Steamwhisperer]

Basic physics Greg. The longer the vertical rod the further away the pivot point is and the higher the accuracy. It then can become a comparator.
Unless I have completely stuffed it up. I wasn't real flash at physics.

Phil

[Greg Q]

Well, yeah, I suppose, but you'd need a pretty long pendulum in order to see what a level shows, wouldn't you? Plumb bobs and levels are all measuring the same thing after all, and with the same accuracy, just not the same precision.

And I am not claiming that plumb bobs don't have their places in machine set-up. I have three beautiful ornate ones for reasons I can't recall. My wife won't let me hang them on the Christmas tree...cast iron and bronze ornaments of a bygone era would look great I reckon.:wink:

[Michael G]

The sort of thing I was alluding to was this -

219358

You'd need to use a fine wire and make sure the suspension was a point rather than just twitching it through a hole. Similarly with the bob, the attachment point would be something similar rather than the old tyre weight tied with a bit of string that I use. The weight would need to be symmetrical of course.

To get 3 thou in 12", you would basically need to double his 72" (so it's now 12ft) to use a 32nd scale to discriminate. The other thing I would be worried about would be air currents. You'd almost want to run the bob line down a piece of PVC pipe to avoid drafts.

Of course, with a longer wire are you starting to move into the realm of Foucault's pendulum and so start getting movement from the motion of the earth?

It's all getting too complicated...

Michael

[Stustoys]

The plumb bob is an interesting idea and while it might not be perfect, if you have nothing better, hopefully its at least heading in the right direction.


if you could layout a projection of the spindle centre line precisely beneath the spindle, and had a plumb bob damped in a viscous medium which you could read precisely, you might have something. Meh.

But then wouldn't you be twisting the bed to line up with any error in the bed/spindle alignment?

You could use wire and read off it with a scope like Phils........although that will likely cost you more than a level.

Stuart

[Michael G]

Of course, if you had a microscope collection you could use a ruler in 64th's or even 100th's and either increase your accuracy or decrease the height of your pole.

(Maybe that's why Stuart has those microscopes - to compensate for having a smaller pole)

Michael

[Stustoys]

Its not poles that they are for looking at :D

[Greg Q]

The plumb bob is an interesting idea and while it might not be perfect, if you have nothing better, hopefully its at least heading in the right direction.


But then wouldn't you be twisting the bed to line up with any error in the bed/spindle alignment?

You could use wire and read off it with a scope like Phils........although that will likely cost you more than a level.

Stuart

Yeah, the whole level thing should be a lot easier and more affordable than it is. When you describe the process of using a level to eliminate twist you make the faulty assumption that it is almost free, since the local gravity vector is nearly a constant and spirit levels are common as dirt. Too bad that such a theoretically simple instrument to check is so expensive.

Greg

[Greg Q]

But then wouldn't you be twisting the bed to line up with any error in the bed/spindle

Stuart

I wanted to mention this alone: i reckon that the average lathe, with its seperate headstock, can be adjusted so that the extended spindle centerline becomes parallel with the bed ways. This is the optimum condition of geometrical accuracy, which yields an absolutely constant tool height compared to the workpiece in the machine. This should yield absolute dimensional accuracy, except for the fact that the world is made of rubber of one sort or another, including cast iron.

My lathe's headstock is part of the massive one piece casting that included the bed, so I have to reference every survey and every correction to the spindle centreline as measured from a test bar which is itself corrected for droop, even if the machinist isn't. :oo:

[Ben Dono]

Hahah! I knew this video would get some feedback!
I do like the idea of it to get you in the ballpark or removing lathe bed twist. Test cutting some journals would be the next step I'm sure.

And I had no idea test bars were corrected for droop!

I live in the country and a lot of my neighbors have machine shops. I think I will politely ask to borrow a machinist level to help with the setup.
Is their any merit to going to the cheaper digital angle gauges? Surely this would be better that a Stabila carpenters level for the initial setup.

[Michael G]

Not much merit to the cheaper digital type. Connelly suggests a level for machine alignment purposes with a sensitivity of 0.0005"/12" this equates to roughly 0.0025 degrees. This is probably over the top for setting up a machine in the average shed but shows the order of magnitude being suggested. I doubt electronic levels from eBay are anywhere near this good.

Michael

[Ben Dono]

Thanks Michael. That makes a lot more sense. The digital ones I have seen are really no better than .01
I have spent many years working with wood so I'm still getting my head around the micro scale of things.

[Stustoys]

My lathe's headstock is part of the massive one piece casting that included the bed, :doh: but of course.
+1 to all things being made of rubber.


a level for machine alignment purposes with a sensitivity of 0.0005"/12" this equates to roughly 0.0025 degrees.
My math says the metric 0.02mm/meter is just about half that. No wonder its so "interesting" trying to get something level.

Stuart

[tongleh]

I've just tried this method using a pendulum rather than a plumb on a piece of string. I managed to replicate the measurements over 3 attempts and by my reckoning, I've got the bed to around .0185 mm over 600mm; mind you, it is difficult to measure due to swing in the pendulum, pointer width and being able to measure .25 of a mm on a S/S rule, etc. but it's certainly better than it was.

[BRADFORD]

I have been reading this with interest and it occurs to me that if the plumb bob or pendulum was attached to the saddle of the lathe, then the whole arrangement can be moved along the entire length of the bed without disburbing the setup. If the suspension point was high enough, I can't see why a fairly high degree of accuracy could not be acheived.
I will be setting up my lathe over the next few days, I'm going to try this method, I'll post the results, or lack of here.

[tongleh]

I was so impressed with the results from my mock up pendulum, I have made a new and improved model. Height of the pedulum hinge point is a little short, but as I said: .0185 over 600mm will do me. Unfortunately, if you only have a short bed, the carriage gets in the way.