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The Artful Bodger
09-23-2011, 03:58 PM
I do not have a flat reference such as a surface plate, I dont even have anything better than a hardware store straight edge, neither do I really see the need with the sort of work (play?) I do.

I assume my mill table is likely to be the most flat surface in my shop, how would that compare with, say, plate glass?

Presumably I would be able to buy something like a stick of key steel and check that against my mill table for use as a portable straight edge?

gary350
09-23-2011, 05:58 PM
I need a flat surface too, to cut glass so I guess I will just have to build me a special table with 3/4" plywood top. The hardware store wants $1.50 per cut 324 mirrors 2" square = $486 plus the mirror glass plus sales tax. I have cut glass before the table can not have any humps, bumps, dips or defects. It needs to be just as smooth and flat as the glass itself. I wish I could borrow a professional glass cutting table for about 2 hour.

KiddZimaHater
09-23-2011, 06:11 PM
Probably your mill table.
Give it a good stoning to remove any burrs.
I had to use my mill table as a surface plate when making a few rods today.
Customer wanted them 15 inches, +/- .005.
And I only have 12" calipers. (oops!)
So I mounted an indicator in the mill spindle, set it at zero on 5 stacked - 1,2,3 blocks, and BINGO!! Instant height gage!:)

firbikrhd1
09-23-2011, 06:20 PM
One of Dave Gingery's books talks about making a "surface plate" from pieces of glass. His method is to obtain two pieces (thicker is better) and lap them with valve grinding compound. I think using three pieces might be better; as with scraping, when all three can be interchanged and mark the same they are flat. I made these years ago of 3/4" plate glass but unfortunately eventually dropped and broke them.
In any case, it could be done cheaply. Valve grinding compound is very inexpensive and I see glass table tops for free on Craigslist often. One table top could easily be cut to make two or three pieces if decent size.

A quality table saw probably has a fairly flat surface as well.

Your Old Dog
09-23-2011, 07:42 PM
I need a flat surface too, to cut glass so I guess I will just have to build me a special table with 3/4" plywood top. The hardware store wants $1.50 per cut 324 mirrors 2" square = $486 plus the mirror glass plus sales tax. I have cut glass before the table can not have any humps, bumps, dips or defects. It needs to be just as smooth and flat as the glass itself. I wish I could borrow a professional glass cutting table for about 2 hour.

You're over thinking this one gary! It just ain't that big a deal. I worked for a glass shop and can tell you your work table doesn't need to be nearly as flat as the original poster is talking about:) More important you remember to snap the glass immediately after scoring it with the cutter and not to let the cut get "cold". Don't forget the drop of oil on the cutter before making the cut.

Evan
09-23-2011, 08:23 PM
I assume my mill table is likely to be the most flat surface in my shop, how would that compare with, say, plate glass?

Plate glass isn't common anymore. Plate glass is made by grinding the surfaces fairly flat but not to very tight specs. Nearly all window glass is now float glass and it is nearly optically flat. It is around 4 wavelengths flat (530 nanometres per wave) within any 2 cm radius and over longer distances it has a curve equal to the curve of the Earth.

Float glass is very flat but that doesn't mean the thickness is constant. Thickness may vary by 10% or so but will be very consistent for any particular piece. If evenly supported it will be flat as a surface plate over similar distances.

boaterri
09-23-2011, 08:24 PM
I generally use my table saw as a flat surface when using a height guage etc. It may not be as good as a granite toolmakers plate but it works well enough.

Rick

The Artful Bodger
09-23-2011, 08:38 PM
Plate glass isn't common anymore. Plate glass is made by grinding the surfaces fairly flat but not to very tight specs. Nearly all window glass is now float glass and it is nearly optically flat. It is around 4 wavelengths flat (530 nanometres per wave) within any 2 cm radius and over longer distances it has a curve equal to the curve of the Earth.

Float glass is very flat but that doesn't mean the thickness is constant. Thickness may vary by 10% or so but will be very consistent for any particular piece. If evenly supported it will be flat as a surface plate over similar distances.


Thanks Evan, I do have some glass, I assume it is float glass, on a desk top which I can use if I want to get 'fussy', otherwise it appears the mill table will be my 'shop reference'.

Thanks for comments everyone.

J Tiers
09-23-2011, 09:02 PM
Float glass is likely to be pretty flat, indeed, depending on how made (how fast and how fussy they were).

The problem is that it is so thin that it is bendy, where a reference flat should be solid and unmoving.

It all depends on whatcha need. For me, no way even float glass would be good enough, so I use a granite flat reference. Doesn't bend when I set a reasonably heavy machine piece on it to blue up.

For you, might be fine.

The Artful Bodger
09-23-2011, 09:13 PM
Evan says float glass is real flat so I guess if one was to put a couple of inches of concrete in a tray and 'float' some float glass on top that would make an easy surface plate, it would support plenty of weight but still not like things being dropped on it.:)

Don Young
09-23-2011, 09:16 PM
At a Harley shop years ago we used a piece broken from a solid glass door, about 1 1/2" thick as I recall. Nothing we measured or measured with could detect any lack of flatness in it. I have often wondered if a bullet-proof window might be pretty good.

GNO
09-23-2011, 09:20 PM
used to be the fuel inj. guy for a gen plant big diesels, used heavy glass for a lapping plate.Could wring the parts togetter after lapping & cleaning [that's how I knew they were flat enough]

Evan
09-23-2011, 09:56 PM
The main problem with glass is that it scratches so easily. Rather than going to the trouble of setting it in concrete maybe float it on some plaster of Paris. If you can find some in 12mm or so you won't need to do that. Of course, surface plates are pretty cheap. Another possibility is some countertop granite. It isn't as flat as a surface plate but is usually better than .001" over a good distance.

.RC.
09-23-2011, 10:07 PM
surface plates are what? $50 or so...

Why would you bother with glass then they are that cheap...

Scottike
09-23-2011, 10:13 PM
Why not just spring for a granite surface plate? they're not that expensive unless you get into inspection grade. And from the sounds of it, your not looking for that much accuracy. (I know it would be nice, but I know I don't need that much accuracy)

Mcgyver
09-23-2011, 10:18 PM
Evan says float glass is real flat so I guess if one was to put a couple of inches of concrete in a tray and 'float' some float glass on top that would make an easy surface plate, it would support plenty of weight but still not like things being dropped on it.:)

depends what you're working on...if its really flat it'll work for all but the most challenging tasks, although sometimes you need very flat. Often quite flat is good enough; some say you can get by with pretty flat :)

come on guys, these descriptions are meaningless...the top of a table saw is really flat to a carpenter.

AB, what matters is what you're trying to do.....layout with a scriber in a surface gauge is different from checking squareness with a 10th's indicator or scaping.

imo small shop grade import plates are cheap enough its not work going for any thing less, they'll do most everything you might encounter.

The Artful Bodger
09-23-2011, 11:06 PM
I dont really need to spend anything on a surface plate, my question was what is the most flat surface out of the stuff home shoppers typically have on hand. In my case that seems to be my mill table.

Float glass on concrete, well I am rather partial to concrete right now...:D

darryl
09-23-2011, 11:13 PM
I've done a couple different things for a 'flat', including embedding a transmission valve body into cement. The best size and most trustworthy is the surface plate, 12x18 in my case, and it was less than $50 including taxes. It's kind of a no-brainer once you have it- are you going to get a glass plate to be as flat? How would you know? Why not buy a small surface plate-

Other things I've used are pieces of granite countertop and floor tiles. Either one can be checked to a degree of precision by looking across it and angling it to catch a reflection of things in the background. As you angle the flat back and forth you can see whether there's any rippling in the reflection of straight lines on things. If things look good visually, it might be more than good enough for the application. But that' the thing- how accurate do you need it to be? A piece of glass is probably flat enough over a half square foot or so, but maybe not over the entire surface, corner to corner both ways. How can you know- for fifty bucks a working grade surface plate is a pretty good deal. Mine has a certificate saying it's good to .0002 or something over the entire surface. You will have some peace of mind using something that you can feel sure is flat.

As a backup for sandpaper, etc pretty much any of the alternates would be good enough. I trust my piece of granite counter top for that, and the same for the floor tile, although the tile needs to be mounted to a 'flat' surface to help it stay as flat as it is.

Bill736
09-23-2011, 11:46 PM
Stone surface plates are inexpensive, and I bought mine during " free shipping" specials. I have two small, 9 x 12 inch plates, and I'm amazed how often I now use them. I put wet or dry sandpaper on top of them, and use them to surface carburetor parts, touch up ruler edges, and with an angle jig I sharpen my chisels . In the process I also found out that my table saw top was not very flat after all.

dp
09-23-2011, 11:49 PM
The flattest thing in my shop is a granite surface plate I bought from Forrest Addy when I took a scraping class. 18"x12"x2", it wasn't all that expensive.

A.K. Boomer
09-24-2011, 09:24 AM
I need a flat surface too, to cut glass so I guess I will just have to build me a special table with 3/4" plywood top. The hardware store wants $1.50 per cut 324 mirrors 2" square = $486 plus the mirror glass plus sales tax. I have cut glass before the table can not have any humps, bumps, dips or defects. It needs to be just as smooth and flat as the glass itself. I wish I could borrow a professional glass cutting table for about 2 hour.



Please tell the board your not building a giant disco ball and then we all can get this massive elephant out of the room and get back to the discussion...

A.K. Boomer
09-24-2011, 09:43 AM
right now my mills table it the flattest thing I have - it's better than 2/10th's for the entire length and might be quite a bit better than that but I lack the proper measuring tools to find out, all I can say is at first I had to check and see if the gauge was even on the table cuz there is zero movement the entire length.

Before I had a mill I bought a piece of thick glass - I sprayed contact adhesive on one side and tore three long brand new high quality 80 grit belts for a belt sander at the seam and sprayed them all on the backside, and carefully put them on the plate and this was actually the way I "milled" cylinder heads on small aluminum head car engines for like a decade or more --- god I used to hate it when someone overheated their engine and the head was warped by .004" or more --- took allot of elbow grease to bring it back around but what a nice finish...

I had a flat workbench with stops built in to hold the glass and used the heads weight to help sand off the material. I would go from straight on lengthwise to radical criss/cross but never sideways.

The glass eventually broke and then I built a thick plexiglass one that worked just as well - it's all worn out now but still comes in handy for sharpening gasket scrapers and such...

Metalmelter
09-24-2011, 10:31 AM
Surface plates (used ones) tend to be rather cheap as the sellers have an issue with the costs of moving them (think free!). So you probably can find a decent 24" X 24" or similar for next to nothing - all you need to do is move it. Keep your eyes open and put the word out that your looking for one. I have 3 of them and passed up some rather big ones. There's not too much that beats a decent surface plate I would guess.

You would be amazed what you can get for free just by asking :D

Good luck in the search!!

KEJR
09-24-2011, 10:47 AM
There is alot of granite where I live, so it tends to be cheap. That said, the 12"x16" or so granite surface plate I bought from the woodworking store was only $35USD or so.

Personally I'd use your mill table until you can save up to buy a surface plate, they are handy for tricky measurements.

KEJR

edit: Sadly/Ironically the granite plate I have came from China even though I live in the "Granite state".

Forrest Addy
05-22-2012, 08:16 PM
Suckers! Flatness in the home shop has been a favorite Artful Bodger troll for years and you guys fell for it.

lakeside53
05-22-2012, 08:37 PM
Err.. Forrest.. looks to me like you were trolling, and resurrected an 8 month old cold thread:D

Forrest Addy
05-22-2012, 10:13 PM
Well, um, yabbut...

You know it's hard to type with your foot in your mouth

oldtiffie
05-22-2012, 10:36 PM
Not really - as many will have noticed - I do it all the time - both feet at a time.

I can give you lessons if you like Forrest.

oldtiffie
05-23-2012, 01:01 AM
Its really a matter of how big and how flat - and how portable? - a flat surface needs to be for the job in hand as opposed to what you want - and taking that into account whether the "flat surface" has to or needs to be a surface plate.

It can be a PITA if your surface plate - if you have one - is too small or is covered up with "junk" - or is too much trouble to move to the job if needs be.

Perhaps a "Plan "B"" is needed.

I'd be surprised if a mill or surface grinder table would not be quite adequate in many cases.

saltmine
05-23-2012, 02:30 AM
My mill table used to be my flattest spot.....Until I finally gave in and bought a real granite surface plate (24" X 24") In order to save the money on shipping, I drove to Glendale, California, and purchased the plate right from the manufacturer. Even though it was a 700 mile round trip, it was still cheaper than Fed Ex or UPS delivering it....Something about weight and miles. Once I had the plate in a steel frame, I noticed my accuracy improved measurable. My layouts, with a height gage and a scriber were way closer than measuring from the edge with a pocket ruler and a square.
Inspections are more accurate too. But, not to throw a wet blanket on the campfire...I suddenly began noticing that a lot of stuff I thought was flat....wasn't. It's a good thing Michael Ward is doing a series of articles on scraping. ....

Paul Alciatore
05-23-2012, 02:39 AM
Saw tables and mill tables can vary a LOT in flatness or the lack thereof. I have seen mill tables off by +/-0.002" or more and some saw tables can be as much as +/-0.010". If you don't have a real granite or CI flat, then a piece of thick glass is probably your best bet.

If you want a quick test of a piece of glass, just look at the reflection of the room lights at a shallow (grazing) angle on it's surface. If you can see any waviness in the reflection then don't use it. Most float glass will show almost perfect reflections when viewed like this.

Granite flats really are fairly cheap. The real problem is shipping costs. Some have gone to local marble suppliers for a piece of polished marble. Apparently these companies can supply fairly good flats. The reflectance test can be used here also for a quick way to accept or reject a given piece.

Forrest Addy
05-23-2012, 05:42 AM
BTW glass needs to be about as thick aas a granie flat thickness for area. Itherwise even "thick" glass (1/4" or 3/8") isn't much flatter that the surface it's sitting on. It only averages out the short pitch waviness of whatever its sitting on. My objection to glass is it's so damn fragile. Drop a clamp on glass or something a granite flat would shrug off and it instantly shatteres into a zillion razor edged flinders leaving sparkly little stabbers every where.

Use your mill table or a piece of virgin MDF for casual layout. Otherwise bite the bullet and get a grante flat.

Forrest Addy
05-23-2012, 05:43 AM
BTW glass if used as a surface plate needs to proportioned about like a granite flat. Even door glass deflect ubder small home shop loads. Thus, so-called "thick" glass (1/4" or 3/8") isn't much flatter than the underlying surface. It only averages out the short pitch waviness of whatever its sitting on.

My objection to glass besides it limberness and readiness to scratch is it's so damn fragile. Drop a clamp on glass or something a granite flat would shrug off and it instantly shatters into a zillion razor edged flinders leaving sparkly little stabbers every where. Drop a wrench a year later and slash your fingertip on a shard stuck upright in the floor.

Use your mill table or a piece of virgin MDF for casual layout. Otherwise bite the bullet and get a grante flat.

Weston Bye
05-23-2012, 07:49 AM
...a zillion razor edged flinders...

Don't see that word often. I admire a man who knows the right word and knows how to use it.

gwilson
05-23-2012, 07:53 AM
Granite plates are so cheap. What's the issue?

SGW
05-23-2012, 07:59 AM
My old mentor, Dan Fay, used a broken piece of stone veneer off the front of a store as his surface plate. It was not particularly flat, but Dan, being the master that he was, worked around the limitations. ("It drops off about 3 thou in the lower left corner, so I have to allow for that.") Eventually he came across a deal he couldn't pass up on a 12x18 granite plate and retired the piece of stone veneer, but I often think of what Dan could accomplish with less than perfect tools.

vpt
05-23-2012, 08:31 AM
Anyone can have a flat surface in the shop. Take a pan and fill with water, ta da, flat surface.

sasquatch
05-23-2012, 08:45 AM
Great suggestion Andy, but first i,d be sure to check it for level.:rolleyes:

Also it may not be perfectly flat,,,,any harmonic vibrations from motors running etc could set up microscopic ripples that would mess up the layout etc.

Also be sure it is located in "Dead Still" air,, no air movement is crucial due to surface distortion.:D

lazlo
05-23-2012, 08:51 AM
Take a pan and fill with water, ta da, flat surface.

Actually, it'll be a curved patch with a radius of 4,000 miles, but as long as you're not making parts for NASA :p

lazlo
05-23-2012, 08:55 AM
The flattest thing in my shop is a granite surface plate I bought from Forrest Addy when I took a scraping class. 18"x12"x2", it wasn't all that expensive.

$45 with free shipping at Enco. I'm going to pave my patio with them :)

http://www.use-enco.com/CGI/INSRIT?PMAKA=640-0120

vpt
05-23-2012, 09:16 AM
Great suggestion Andy, but first i,d be sure to check it for level.:rolleyes:

Also it may not be perfectly flat,,,,any harmonic vibrations from motors running etc could set up microscopic ripples that would mess up the layout etc.

Also be sure it is located in "Dead Still" air,, no air movement is crucial due to surface distortion.:D


An interesting bit on water. Once while fishing I found a rattle can floating around in a back water slu. Obviously I had to pick it up and also check if anything was in it. Yes it was a near full working can. This is when I found out if you spray paint on water the paint will float, dry into a film, and can be picked up with a stick almost instantly. I haven't found a use for this knowledge yet, just found it neat.

Mcgyver
05-23-2012, 09:16 AM
Anyone can have a flat surface in the shop. Take a pan and fill with water, ta da, flat surface.

Phht, hardly, it'll have a pronouced curve. depending on where you are the radius will be about 3960 miles, but you'll have to do some math or get out the big radius gauge set out to know precisely....but you could be out by as much as 8 micro inches over a 2' tray.

:D

vpt
05-23-2012, 09:18 AM
Phht, hardly, it'll have a pronouced curve. depending on where you are the radius will be about 3960 miles, but you'll have to do some math or get out the big radius gauge set out to know precisely....but you could be out by as much as 8 micro inches over a 2' tray.

:D


Might have to give it a quick flycut to get the "curve" out. :D

lazlo
05-23-2012, 09:34 AM
Might have to give it a quick flycut to get the "curve" out. :D

Michael Morgan talks about this in his scraping book -- the error due to the curvature of the Earth can get pretty substantial on a long bed (i.e., 20') machine.

MrFluffy
05-23-2012, 09:42 AM
I have a camel back because shipping is less than a square plate, but I use my mill table for most, and I can test the table against the camelback (although I'd like to test that on a calibrated plate one day!)

Having said that, I'm always on the lookout to get a decent largish surface plate as part of my long term plans to achieve the nirvana of a stevenson unseeable floor in my workshop :)

Paul Alciatore
05-23-2012, 09:58 AM
An interesting bit on water. Once while fishing I found a rattle can floating around in a back water slu. Obviously I had to pick it up and also check if anything was in it. Yes it was a near full working can. This is when I found out if you spray paint on water the paint will float, dry into a film, and can be picked up with a stick almost instantly. I haven't found a use for this knowledge yet, just found it neat.

If you want a use for floating paint, just look at the many items in sporting goods stores that have a camouflage pattern painted on them. Put object in water, add two or more colors of paint and stir into a pattern, lift object through paint film.

Paul Alciatore
05-23-2012, 10:11 AM
A few thoughts on Forrest's comment on the lack of flatness of a piece of glass. He states if it only 1/8" or so thick, is not much flatter than the surface it rests on. This depends on several factors.

First, is the size of the glass. A 10 x 12 sheet will stay a lot flatter than a 24 x 36 sheet. But then I doubt that anybody would use such a large sheet of glass as a flat. Any metal object that needs that size would probably crush it. So for glass of practical sizes, you can probably get away with more.

Second, even in large sizes, the error in such a sheet will be long term. It will almost completely eliminate any short term irregularities of the surface under it.

On the negative side, such thin sheets of glass will definitely react (warp) under the weight of almost anything you set on them. I would expect several tenths just from the weight of a height gauge.

If you are just laying out a project that is under 6 or 8 inches in size where +/- 0.002" is OK, then go for it. If you are working on larger parts or are going to scrape, then get a real flat. Know what you are working with and stay within it's limitations.

vincemulhollon
05-23-2012, 11:12 AM
Please tell the board your not building a giant disco ball

Quick in the head estimating:

2 inch on a side thats 4 sq in, 324 mirrors, thats about 1200 sq inches

radius = sqrt(sa/4pi) = sqrt(1200/4pi) = sqrt (300/pi) = about sqrt (300/3) = sqrt (100) = 10 inches

diameter = 2r = 10*2 = 20 inches = "about a foot and a half across"

I'm surprised, you'd think 300 "hand size" mirrors would make something much bigger. Not in the "giant" category at all.

Now cnc mill the ball the mirrors will be glued to, using a 4-axis and that would be cool.

dian
05-23-2012, 11:19 AM
"hardware store straight edge"

take it to the surface grinder.

alternatively make a thick plate out of epoxy granite and pour epoxy on top of it. voila, your new surface plate. now you can scrape in the straight edge.

John Stevenson
05-23-2012, 11:46 AM
come on guys, these descriptions are meaningless...the top of a table saw is really flat to a carpenter.



LOL :) :D

When I'm doing carpentry [ read wood butchering ] ower drive way is flat enough for me.

oldhat
05-23-2012, 02:16 PM
Picked one up $24.00 free shipping for the smaller plates only.
Used gage .0005 needle did not move from one end to other but for one small dip round 1/8 or less.


G9648 9" x 12" x 3" Granite Surface Plate, No Ledge
Grade "B" Granite Plate has a bilateral accuracy of plus or minus .0001"

http://www.grizzly.com/search/search.aspx?q=surface%20plate&cachebuster=5087149786352390


Bill

Forrest Addy
05-23-2012, 04:04 PM
Actually, it'll be a curved patch with a radius of 4,000 miles, but as long as you're not making parts for NASA :p

Which leads to a story.

My boss sent me to a largish shop to set up a floor milll having about 60 ft of runway. This was a big machine and the customer gave me a burly helper. To make a long story short about moving a zillion wedge jacks, grouting, and multi-ton machine elements, let me say the customer worried that the runway was flat.

I made it so using 4 Starrett 199 master precision levels and a lot of wrenching and tweaking. In the end whereever you moved any of the levels on the runway ways they registered dead level even on reversal. The customer brought in a laser guy as a third party check; a step anyone has to accept with good grace. He showed the runway rising a bit in the center. Damn, I had it dead level and a check with Mr Starrett's finest showed ti to be so. Level: flat; laser: humped. While it was still in tolerance and we both had gained confidence in each other's good will and readings we were perplexed at the difference.

Then we remembered the world was round. A level registers a perpendicular to local gravity and that can change as you move along the surface of the earth. A laser shoots a straight line independent of gravity. Compare the two and you can measure the height of a chord on a 4000 mile radius. With the instruments we were using we could work the math and determine for ourselves what the radius of the earth was to some presiion. It wasn't 4000 miles but it was close.

A sales guy wandered from the office and listened as the customer, the laser guy and I as we worked out this little discovery. "Who cares?" He dismissed it all. No curiousity. No sense of wonder. I guess that's why he was in sales.

Anyway surface of the earth compliant or not the floor mill went together and was accepted as level, square and plumb.

Who was the Greek who worked out the diameter of the earth from the distance between two wells in Egypt?

The Artful Bodger
05-23-2012, 04:33 PM
BTW, is the force of gravity truely vertical at all parts of the globe?

I expect not as the Earth is not a true sphere.

John Stevenson
05-23-2012, 04:55 PM
Suckers! Flatness in the home shop has been a favorite Artful Bodger troll for years and you guys fell for it.


BTW, is the force of gravity truely vertical at all parts of the globe?

I expect not as the Earth is not a true sphere.

And still falling for it.

http://clouddragon.files.wordpress.com/2009/01/troll.jpg?w=510

justanengineer
05-23-2012, 04:59 PM
Who was the Greek who worked out the diameter of the earth from the distance between two wells in Egypt?

http://en.wikipedia.org/wiki/Eratosthenes

rohart
05-23-2012, 06:09 PM
If I were to use the glass and concrete idea, I'd let the concrete set on its own first, to avoid any temperature effects. Then I'd float the glass on it on a bed of some adhesive, bondo, PVA, mortar, anything would do.

I wouldn't trust the concrete to stay flat as it set/cooled.

I remember a thread a couple of years ago that suggested that some kitchen top tiles, probably marble, were as flat as anything around.

I fully understand and sympathise with the idea of bodging - of making do - and making all your own tools.

lazlo
05-23-2012, 08:02 PM
BTW, is the force of gravity truely vertical at all parts of the globe?

I expect not as the Earth is not a true sphere.

Not even close -- the Earth is a lumpy mess. That's why the Metric system is bollocks: the length of the meridian through Paris doesn't relate to anything :D

cameron
05-23-2012, 10:46 PM
And still falling for it.

http://clouddragon.files.wordpress.com/2009/01/troll.jpg?w=510


Were you that sales guy, John?

The Artful Bodger
05-24-2012, 05:36 AM
And still falling for it.

http://clouddragon.files.wordpress.com/2009/01/troll.jpg?w=510


Thank you for your kind words..

oldtiffie
05-24-2012, 05:50 AM
Originally Posted by The Artful Bodger

BTW, is the force of gravity truely vertical at all parts of the globe?

I expect not as the Earth is not a true sphere.


Not even close -- the Earth is a lumpy mess. That's why the Metric system is bollocks: the length of the meridian through Paris doesn't relate to anything :D

The force of gravity is radial (from the centre of the earth).

The length of the meridian is no longer an issue as the standard metre (on which the standard foot and inch are based) is now:


The metre (meter in the US), symbol m, is the base unit of length in the International System of Units (SI). Originally intended to be one ten-millionth of the distance from the Earth's equator to the North Pole (at sea level), its definition has been periodically refined to reflect growing knowledge of metrology. Since 1983, it has been defined as "the length of the path travelled by light in vacuum during a time interval of 1 ⁄ 299,792,458 of a second".[1]

from:
http://en.wikipedia.org/wiki/Metre



International inch

From July 1, 1959, the United States and countries of the British Commonwealth defined the length of the international yard to be exactly 0.9144 metres.[5][6] Consequently, the international inch is defined as exactly 25.4 millimetres.


from:
http://en.wikipedia.org/wiki/Inch

oldtiffie
05-24-2012, 06:58 AM
It seems that the default position as regards "flatness" in the HSM shop at least is defined by surface plates.

Size will define the thickness and weight of the plate.

Accuracy (ie "flatness") of the plate is defined by its Grade.

Grade AA is the most accurate (flattest) and is 2 x as accurate as a Grade A and 4 x as accurate as a Class B.

http://www.shars.com/product_categories/view/5100103/Precision_Black_Granite_Surface_Plates_Grade_AA

http://www.shars.com/product_categories/view/5100102/Precision_Black_Granite_Surface_Plates_Grade_A

http://www.shars.com/product_categories/view/5100101/Precision_Black_Granite_Surface_Plates_Grade_B

Its a matter of how much flatness you need, the size and weight of the plate - and your budget.

darryl
05-25-2012, 09:17 PM
There's also the issue of what you are going to do on this 'flat'. I've cut a fair amount of glass using a piece of plywood laying on the ground as my 'flat'. Not ideal, but it worked ok. I didn't break any glass anywhere except on a scribed line.

I've often needed a relatively flat surface to build something on, where I need the thing to be free from twisting. I made such a table using mdf for the top and a steel framework underneath, shimming as required to get the closest result I could read on my best level. That has been handy. At the time when I get to build my new shop, I will be building a 'flat' worktable for it again, only larger than this current one. The plan is to build it like a door, only thicker, so it will be a box structure with lots of diagonal bracing inside. The plan includes 'topping' it with a lake of epoxy.

For cutting glass, I came up with the idea to make the table surface in two parts. One part is fixed, but the other (call it the out-table) is free to hinge downwards a few degrees. The scribed line would be centered above the hinge pin axis, which would be directly in line with the edge of the fixed table surface. The movable part of the table would be supported on springs and have a stop at the perfectly level position. Something like that anyway.

jhe.1973
05-26-2012, 02:19 AM
A sales guy wandered from the office and listened as the customer, the laser guy and I as we worked out this little discovery. "Who cares?" He dismissed it all. No curiousity. No sense of wonder. I guess that's why he was in sales.



This one reminded me of one of my own funny stories - off topic of course.

I wanted some cast iron arms for an antique oil lamp chandelier. I only had 2 out of the original 12.

Went to a foundry & they sent me to a 2 man pattern shop w/the suggestion that I have them cast in brass or aluminum.

The pattern maker at this shop was holding the 100+ year old part and was explaining to me that it was not possible to get the type of detail he was pointing out to me on the part.

He never bothered to realize that someone made the part he was holding 100+ years earlier. :D

MichaelP
05-26-2012, 10:48 AM
I'd be more than satisfied with a 4,000 mi radius hump on my reference surface. Heck, I could even live with a 3,960mi one! But compensating for the tidal effects would make it absolutely unbearable.

loose nut
05-26-2012, 12:01 PM
A laser shoots a straight line independent of gravity.


Forrest, light is bent (follows the curve of the well) when traveling through a gravity well so wouldn't a laser beam bend to match the curve of the earth or its gravity well.:confused: :confused: :confused:

jhe.1973
05-26-2012, 12:35 PM
Forrest, light is bent (follows the curve of the well) when traveling through a gravity well so wouldn't a laser beam bend to match the curve of the earth or its gravity well.:confused: :confused: :confused:

It's my understanding that light is deflected by the gravity of the earth, but doesn't conform/match the curve of the earth. If it matched the curve of the earth, once it entered from one side, it would bend all the way around to illuminate the entire earth.

I am reasonably sure that laser light is deflected slightly, but for most purposes on earth, it doesn't matter. If we were shooting a laser at Jupiter, now that might pose a need for correction.

Just my thoughts.

loose nut
05-26-2012, 06:15 PM
It doesn't follow the curve of the earth but it is bent some by a gravity well, whether or not it is noticeable, I don't know. That was what I was wondering, how is a laser used to make a straight line in a gravity well. Maybe the deflection is miniscule over short distances.

Mike Burch
05-26-2012, 06:50 PM
Presumably the laser light is travelling at the speed of light, 186,000 miles per second.
The Earth's gravity causes things to fall at 32 feet per second squared, i.e., in the first second of travel, a particle shot out horizontally will fall 32 feet.
If we ignore air resistance, a bullet leaving a muzzle at 1000fps will therefore have fallen 32 feet after it has gone 1000 feet horizontally.
A laser, on the other hand, will have fallen 32 feet after it has gone 186,000 miles. (Of course, it won't in real life, because it will be over half-way to the Moon by then.)
So I deduce from first principles that a laser beam may be considered essentially straight over earthly distances.
Am I right?

And Tiffie, I must respectfully disagree. Gravity is not precisely and always radial from the centre of the Earth. It is (slightly) skewed by, for example, adjacent mountains. Gravity is a mutual attraction between two masses, and if you happen to be next to a mountain, its mass will have a slight sideways attraction which will pull the total gravitational attraction slightly away from vertical.

rohart
05-26-2012, 07:17 PM
Mike : A very good analysis. And, yes, I think that gravity is affected by local features, like nearby mountains, or concrete lathe bases.

lazlo
05-26-2012, 08:42 PM
Gravity is not precisely and always radial from the centre of the Earth. It is (slightly) skewed by, for example, adjacent mountains.

...and the gravitational force is proportional to the distance to the center of mass (the center of the Earth). Since the Earth is a lumpy mess (much to the dismay of Pierre Méchain and Jean-Baptiste Delambre), the gravitational force varies all over the planet.

Which is why munitions with precision inertial guidance systems (like ICBM's) use gravity maps.

Red is higher gravitational force, blue is lower:

http://upload.wikimedia.org/wikipedia/commons/thumb/7/78/GRACE_globe_animation.gif/200px-GRACE_globe_animation.gif

lazlo
05-26-2012, 08:48 PM
Presumably the laser light is travelling at the speed of light, 186,000 miles per second.
The Earth's gravity causes things to fall at 32 feet per second squared

Photons are massless. The reason photons are affected by gravitational fields is because a gravitational field warps the curvature of space-time.

dp
05-26-2012, 09:23 PM
So if you search Bing or the evil twin brother google for India Black Granite Tile you will find granite tiles in your neighborhood that while not machinist flat are damned flat enough for layout, holding a sandpaper sheet, and any number of things you will think of once you have one, and at 1/2" thick you can pick it up with one hand and lean it against a wall for storage.

I get mine from Home Depot.

loose nut
05-27-2012, 10:37 AM
Photons are massless. The reason photons are affected by gravitational fields is because a gravitational field warps the curvature of space-time.

And since we live in a gravitational field doesn't that mean that a laser beam will be warped. How can it be used to measure flatness. Is the amount of curvature, in a local area IE: the length of a large lathe bed or a surface plate, so small that it doesn't matter.:confused: :( :eek:

lazlo
05-27-2012, 11:02 AM
And since we live in a gravitational field doesn't that mean that a laser beam will be warped.

Bottom-line: the laser beam will not be affected by gravity in a way you can measure.


Is the amount of curvature, in a local area IE: the length of a large lathe bed or a surface plate, so small that it doesn't matter.:confused: :( :eek

An object with mass is affected by gravity, and like I said earlier: Michael Morgan (a pro scraper/machinery rebuilder) points out that the droop due due to the curvature of the Earth approximately .00003" per foot. Insignificant on all but the largest machines, but has to be taken into account when rebuilding a 20 foot bed:

From Michael Morgan's chapter on Machine Alignment:


"Because of the curvature of the Earth, a level displays an error of approximately .00003" per foot. With this in mind, a surface that is 10' long when scraped or adjusted to indicate dead level along its entire length is actually .00015" low on each end. This is a minor problem with a surface this small but when setting up a machine 60' long, this curve begins to be an appreciable problem. With a machine of this length, the error on each end would be .0009" or nearly one thousandth of an inch. If the machine were required to hold part tolerances of less than .0005" this would be a problem."

oldtiffie
05-27-2012, 09:02 PM
If you have something that is flexible or prone to curving/bending, support it adequately on ajustable pads and use a good builders or surveyors (optical) "dumpy" level or theodolite as once the level/theodolite are set in the required plane (not level if you need it) setting the required surface to be "flat" to a quite high level accuracy in that plane is quite straight forward.

Forrest Addy
05-28-2012, 06:28 AM
Forrest, light is bent (follows the curve of the well) when traveling through a gravity well so wouldn't a laser beam bend to match the curve of the earth or its gravity well.:confused: :confused: :confused:

Yeah, yeah. Pick me no nits. You know what I mean. Our gravitional well would put about a nanoradian tweek to a beam of light passing earth tangent to a 4500 mile radius. .

If you are going to quibble at least quibble on the same scale as the origianl arguement.

loose nut
05-28-2012, 08:00 PM
Wasn't picking on you I just wanted to know if it would have any impact on the measurement.