Thread: Measure twist with a pendulum ?

Originally Posted by Stustoys

What do the rollers turn on?

Though really a 50m pendulum isn't likely to need a bearing, I'm guessing even if it was fixed at the "pivot" it would flex enough to cover the range required. So that's one problem solved

Stuart

I'm going to go out on a limb and say a 50m pendulum won't work for what you want to do -- perhaps I'll go as far as suggesting that that size pendulum hasn't a snowflake's chance of being useful as a level




google Foucault's Pendulum

Originally Posted by RayG

Speaking of alternatives to bubbles... Last year, I designed an electronic level. I based the design on the SCA103T series MEMS inclinometers. You could build one with adjustable sensitivity for less than $200.

Ray I'd like to see how it turned out, any chance of a picture? Was it a differential style?

Originally Posted by RayG

Well... to a first approximation

I'll have to have a think about all that......my first thought is a long pendulum makes things worse?

Originally Posted by RayG

I think the DTI idea is a non starter, but maybe a capacitave or inductive sensor detecting the pendulum, like a LVDT sensor perhaps.

Agreed.

Originally Posted by RayG

but maybe a capacitave or inductive sensor detecting the pendulum, like a LVDT sensor perhaps.

Arent we just back to RC's Talyvel?

Originally Posted by ian

I'm going to go out on a limb and say a 50m pendulum won't work for what you want to do -- perhaps I'll go as far as suggesting that that size pendulum hasn't a snowflake's chance of being useful as a level

Well no I never expected it to work. I was just demonstrating how long the pendulum had to be to allow reading to the stated level of accuracy.(excluding repeatability issues)

Stuart

Originally Posted by ian

I'm going to go out on a limb and say a 50m pendulum won't work for what you want to do -- perhaps I'll go as far as suggesting that that size pendulum hasn't a snowflake's chance of being useful as a level

Well no I never expected it to work. I was just demonstrating how long the pendulum had to be to allow reading to the stated level of accuracy.(excluding repeatability issues)

I'm sorry, the reference to Foucault's pendulum was about how pendulums that size can be used to demonstrate the rotation of the earth -- a minute or so after you set the pendulum swinging it's no longer swinging along the bed of the machine

Setting the pendulum swinging is the last thing we want. In fact dampening would be required. But I'm pretty sure no one is going to try building one.

Stuart

Originally Posted by Pete F

Ray I'd like to see how it turned out, any chance of a picture? Was it a differential style?

Hi Pete,

It was a design exercise to see if the solid state inclinometers were going to be sensitive enough for surface plate flatness measurements.. sadly these ones aren't quite there yet, maybe one day they will.

As it stands they are good for 3.6 arc seconds, which is precision level territory, need another factor of 10 to get to talyvel territory. ( or the laser interferometer )

The chip alone costs around $65, so it's not a cheap replacement for a bubble...

Ray

Thanks Guys,
I've really enjoyed reading this thread. I continue to be amazed at the ingenuity of the human race.

Originally Posted by RayG

Hi Pete,

It was a design exercise to see if the solid state inclinometers were going to be sensitive enough for surface plate flatness measurements.. sadly these ones aren't quite there yet, maybe one day they will.

Ray

Great ! My next post was gonna be about flatness

I've just done Phils & Markos scraping class in Melbourne and was wondering, if I bought the Carbatec granite surface plate for $70 (or any surface plate) how does a know nothing mug with almost no tools check to see if his new surface plate is flat ? ............ I think the brick we made would be too small to prove the new granite plate is flat all over ? Or is there a clever way using that brick ?

Did our Grandads have a simple way of doing this .............. what should I be googling for ? All of the things I've found so far assume that you have access to another flat plate or some fancy gear.

............. and heres a hint ............ the following is beyond my budget .................. "Build a big shed, find a 400 kg weight and mount it on a 50m long pendulum ................"


Bill

Originally Posted by steamingbill

Great ! My next post was gonna be about flatness

I've just done Phils & Markos scraping class in Melbourne and was wondering, if I bought the Carbatec granite surface plate for $70 (or any surface plate) how does a know nothing mug with almost no tools check to see if his new surface plate is flat ? ............ I think the brick we made would be too small to prove the new granite plate is flat all over ? Or is there a clever way using that brick ?

Did our Grandads have a simple way of doing this .............. what should I be googling for ? All of the things I've found so far assume that you have access to another flat plate or some fancy gear.

............. and heres a hint ............ the following is beyond my budget .................. "Build a big shed, find a 400 kg weight and mount it on a 50m long pendulum ................"


Bill

Hi Bill,

The 400Kg only needs a 200mm pendulum.. You should have got a certificate with your plate that shows the calibration, that's only good if you mount it on the same three points that were used in the original calibration. I calibrated a second hand plate using the laser interferometer.. not something I'd be rushing to do again.. but interesting just the same ... https://www.woodworkforums.com/f65/calibrating-surface-plate-168893

Flatness is one of those things that can be self calibrating, you need 3 surfaces and rotate them in order, so you lap A against B, then A against C, B against C when all three are flat, then that becomes your master flatness reference. I think I read somewhere that was how the Egyptians did it when building the pyramids.

Ray

http://what-when-how.com/metrology/s...tes-metrology/

Seems to be interesting and easy enough to understand - maybe some of their other tutorials are good as well

I havent seen that site in any of my previous workshop related searches.

Bill

Originally Posted by steamingbill

Great ! My next post was gonna be about flatness

I've just done Phils & Markos scraping class in Melbourne and was wondering, if I bought the Carbatec granite surface plate for $70 (or any surface plate) how does a know nothing mug with almost no tools check to see if his new surface plate is flat ? ............


Bill

They come with an inspection chart.... I just purchased one recently, I should check it... I know it is not flat on the bottom as it rocks, nor is the very square...

Mine is supposed to be flat within two microns..

A pendulum must be ok as a level of sorts.
In the late sixties/early seventies I worked on many slipped formed buildings, and the system they used to ensure the buildings were vertical was a cable attached to a pulley at the top, so that it could be made longer as the building went up, with a large concrete block (maybe 100kg) dampened in a tank of water at the bottom.
Some of these building were in excess of 50m high although the average would have been about 40m.
This system seemed to work very well, although I don't know if it would be any good for setting up a lathe. I think it would be too cumbersome, too difficult, and probably too inaccurate.

Hi Guys,

FWIW I use a 12 mm thick 13" inch square porcelain floor tile as a poor mans surface plate. They are quite accurately ground flat and a pair will cling together making them difficult to separate, particularly if the air is damp. Unlike a glass plate they will break before they will bend.

Originally Posted by BaronJ

Hi Guys,

FWIW I use a 12 mm thick 13" inch square porcelain floor tile as a poor mans surface plate. They are quite accurately ground flat and a pair will cling together making them difficult to separate, particularly if the air is damp. Unlike a glass plate they will break before they will bend.

You should check out the price on a 300x230x50 chinese granite surface plate, I don't know about the UK, but here they sell for around $70 http://www.carbatec.com.au/granite-surface-plate_c20499

Floor tiles are best left on the floor.

Ray

PS.. This one looks to be the UK equivalent, http://www.axminster.co.uk/small-granite-surface-plate