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Is countertop granite flat enough for surface plate?

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  • Forrest Addy
    replied
    There's a couple of points to conisder when selecting a planar references - that is a flat or a surface plate. One is flatness that is certifiable. This may be of no particular importance to most home shop machinists but more than a few have legitimate need for a accurate flatness reference. Another is stability and rigidity. Granite tiles and thick float glass have been low cost convenient expedients for generations. Much of it is quite flat but its flatness of a byproduce of the finishing process not a product attribute in itself. Also these materials are thick enough to give the impression of rigidity but when work of any weight is placed on them the apparentlt stiff material may sag somewhat perhaps enough to allow error to creep into work where greater than usual precision is desired.

    Those of you on a budget may find a 12" granite tile 1/2 thick or a hunk of 3/8 float glass is well suited for your model steam engine work. A piece of granite countertop an inch thick may be even more satisfactory. Most of the time I use my milling machine table for quick and dirty layout. The T slots are there and much of the equipment I need are handy to it. My DaAll Class "A" 24" x 36" granite flat is always buried under stuff (it IS a flat surface after all and so it's the default repository for stuff I haven't put away) and the smaller granite flats are always in the scraping class box. So I use expedients too.

    If it works for you fine but there is always a "but..." . Many of us do work where a certified or at least a certifiable flatness reference is required. Also weighty work and inspection requipment has to remain in a plane if accurate work is to be accomplished. Everything deflects to some degree under load; what was flat enough when unloaded on a bench may deflect excessively when loaded with an indexing head with a chuck on it, a 12" height gage, and a height reference. This application requires greater stiffness that a granite tile or a piece of float glass can provide.

    I'm always scraping things that require better than machined flatness as examples for my scraping classes, restoration of machine tool components etc. I don't think anyone will quibble my choice of a granite flat as a reference for this class of work.

    These days of cheap but very serviceable granite flats and promotions offering free shipping for smaller sizes means no US home shop owner needs to resort to budget expedients unless he does without on principle. Sad to say our European and Southern Hemisphere brethren haven't our access to low cost import machine shop goodies. I wish there was a way we in the US could send them a share of our bounty but shipping of granite surface plates, chucks, and other heavy stuff is prohibitive.

    You who use granite tiles, countertop remnents, and plate glass as flatness reference do so with my blessing combined with a caveat: all that appears flat may not be adequate for all classes of work.

    Parenthetically speaking: I won't allow mystery metal in my shop; only materials whose identity can be supported by factory markings, invoice description, calalog reference, color code, etc, will be placed in my stock racks. No measuring tool in my posession is without a standard of some kind for in-shop checks. That includes my straightness and flatness references. This may be a fussy way to run a shop but I was well trained in an exacting work environnment and a bit paranoid about tracible accuracy. There's been too many instances in my past where work was screwed up because tooling that was presumed accurate was, in fact, not.

    So if I sound off excessvely about accuracy and certifiability allow me a little slack. OTH, It's been a long long time since any product of my shop was defective as a result of inaccurate tools. I've screwed up plenty of times but my personal shop equipment has been very much up to snuff since the early 1970's even when I was not.
    Last edited by Forrest Addy; 12-31-2008, 04:49 AM.

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  • Evan
    replied
    Interferometry doesn't require a coherent light source, but does need a monochromatic light source
    True. But, if a coherent source is used it must be a stable single mode source. Because all of the modes produce coherent light but of varying phases it causes the position of the inteference fringes to vary. With incoherent monochromatic light the fringes are an average of the jumble of phases and are stable.

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  • Guest's Avatar
    Guest replied
    Heh. So, this Indian stone products supplier thinks their Indian produced machine tools are of inferior quality.
    Or, they understand and accept our prejudices and have altered their business model accordingly.

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  • Jpfalt
    replied
    Interferometry

    Interferometry doesn't require a coherent light source, but does need a monochromatic light source. When I was first learning metrology, the standard was a green filtered mercury vapor lamp. The method i learned was using gage blocks to stack up to a particular dimension and then set the optical flat across the gage block and onto the workpiece. You could then count the interference lines across the top of the gage block to figure out the tilt angle on the optical flat.

    My problem with using interferometry is that it first requires an optical flat, which I don't have.

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  • Evan
    replied
    yep but use a lens to spread the beam out .. and if you want to get real fancy, use a pin hole to do some spatial filtering
    The average dollar store laser pointer won't work well. My experiments show that they are very unstable and usually run mixed mode. To do interferometry with a laser you need a stable laser that runs in TEMoo mode with no mode switching. This especially includes green lasers. Green lasers are feedback controlled by an integral photodiode that regulates current by sensing brightness. External lightsources, especially reflections of the lasers own light will cause instability in the regulation function and can even shut down the laser. There is a very big difference between just any old laser and a stable mode controlled laser. When a laser changes modes constantly the phase of the light wavefront changes and that makes it unsuitable for inteference measurements.

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  • lazlo
    replied
    Originally posted by radkins
    The $25 free shipping is still in effect? Do you have the code?
    One more day:

    Hurry, you have just 2 days left to get the best deal of the season at Enco! Get Free UPS Shipping* on your order of $25 or more today through Wednesday!

    See below for details and Act Fast...this Free UPS Shipping* offer expires Wednesday, December 31, 2008 at 11:00pm EST!

    * To get Free UPS Shipping on your order, apply promo code RFS85 when you order $25 or more online or mention the code to your friendly sales associate when you call 800-USE-ENCO.

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  • derekm
    replied
    Originally posted by Teenage_Machinist
    Laser Pointer?
    yep but use a lens to spread the beam out .. and if you want to get real fancy, use a pin hole to do some spatial filtering

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  • Teenage_Machinist
    replied
    Laser Pointer?

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  • radkins
    replied
    Originally posted by lazlo
    The Enco 12x18, Grade B surface plate is $24.95 with free shipping:

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


    The $25 free shipping is still in effect? Do you have the code?

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  • Evan
    replied
    Not all LEDs are the same. Some LED colors have multiple wavelength emissions which is responsible for the color we perceive. The best for optical interference work is what is called Pure Green. These are emitters at 560 to 555 nanometers. They are single mode LEDs and have a narrow bandwidth. It is also the wavelength where the human eye is most sensitive.

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  • fasto
    replied
    Originally posted by derekm
    now made a lot simpler by cheap lasers
    ...
    illuminate with a coherent light source
    A perfectly acceptable coherent light source is a COLORED LED flashlight. A white LED flashlight WILL NOT WORK as it's closer in operation to a fluorescent tube (uv light changed to visible by phosphor).

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  • derekm
    replied
    Originally posted by Jpfalt
    ...
    After taking the scraping class from Forrest last June, This has launched me on a path to see how accurately I can scrape and then measure to verify the accuracy. So far that's been limited to the accuracy of my best dial indicator and the measuring tools i've made to use the indicator.
    I think you need to start researching interferometry... It sounds scarier than it is ... but its just a standards room practice dating back many many decades etc... now made a lot simpler by cheap lasers. Essentially you have an optical flat separated from the surface under test whih you illuminate with a coherent light source (originally a sodium lamp behind a fine slit) and you observe the shape and count of the fringes formed by the interference patterns.

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  • Jpfalt
    replied
    Some comments on granite

    Granite is basically a cast silicon dioxide and aluminum oxide with some other minerals thrown in. Grain size, shape and color is driven by the composition and cooling rates. As a result, granite has all the issues normal for a casting. Granite is solidified liquid magma and the grain size and residual stresses are going to depend on the thickness and cooling rate of the original slab when it solidified. I've seen some writeups on granite surface plates that talk about using specially selected granite from specific locations. I have to assume that a particular quarry is tapping into a specific granite "casting" that just happened to come out better than the rest. Arkansas sharpening stones are similar in that the type and quality was determined by the quarry they came from in Arkansas.

    I've made some tools for scraping and measurement using countertop material. A lot of what you can use depends on what you need from it.

    I made two 12" straightedges from emerald pearl granite and the results were OK. I was told that emerald pearl comes from a specific location in Norway and is selected for large grains (up to an inch across) angle of reflection and color. I polished it mostly with harbor freight diamond plated hones and used yellow oil based artist color thinned with WD40 as the transfer spotting compound. I also made a 6" x 6" flat from paradiso granite sample, which is fine grained and pinkish red. The top surface was initially flat within about .002 and finished at about .0001 flatness.

    Cutting was done using a 4-1/2" wet diamond saw blade (also Harbor Freight) on an angle grinder with a trickle of water put in at the entry to the cut. Messy, but effectice. A tile cutting saw would probably work better. I also use a 7" saw blade under flood coolant on a tool and cutter grinder.

    You would be really surprised at how quickly the diamond plated hones in water will flatten granite. It's basically the same process as scraping, but you are snading rather than scraping. With the plastic back removed from the hone, the metal mesh domes slightly where you press aginst it, which works well for removing a local spot of material. With stiff backing, the mesh is stiff enough to avoid rounding over edges.

    After taking the scraping class from Forrest last June, This has launched me on a path to see how accurately I can scrape and then measure to verify the accuracy. So far that's been limited to the accuracy of my best dial indicator and the measuring tools i've made to use the indicator.

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  • dp
    replied
    Originally posted by darryl
    I guess it would be naive to think that granite wouldn't have stress in it. So it's millions of years old- what's going to happen when you mine it and cut it up- you might be relieving it of a lot of pressure- ah there's more to this than I care to think about right now. Except for one thing- is there anything industry does to stress relieve it?
    Dunno, but my wife just showed me a blowout on our kitchen counter. A dime size shallow divot showed up under an area we keep our cook books in. No abuse possible there, it just let go.

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  • Evan
    replied
    I actually considered using them as paver blocks out to my shop.
    Sure would make a nice hearth.

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