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Is window glass flat and straight?

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  • Is window glass flat and straight?

    Because window glass is convenient, easily obtainable and economical, is glass flat enough and sufficiently straight that it may be used as a surface on which one indicates when squaring a milling vise with the table?

    Stated differently, if a 24" long piece of glass is held in a milling vise, can one reasonably expect that it is flat enough and straight enough to be used as a surface on which one can indicate so as to square a milling vise on the table?

    Harold
    For those having fought for it, Freedom has a flavor the protected will never know.
    Freedom is only one generation away from extinction.

  • #2
    I don't believe it would be.

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    • #3
      Glass can be pretty flat, but the edges be square and true are a different issue.

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      • #4
        You must have scored that NASA contract

        I just use a nice clean piece of flatstock in the vice. I suppose I could use the roller bar I took out of an old printer. It's about14-15 " long. Hold .001 over the length of that bar and you should be good to go for backyard work.
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        • #5
          I have used a piece of glass to tram a mill, although I am not sure if it was window glass. It was a big oval about 18" on the long side.
          What I did was mark the 4 points where the indicator would go (front back left right) and then moved the mill over all of those points (by using the xy stage, not by rotating the spindle) to check that these 4 point are level.

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          • #6
            Originally posted by beanbag
            I have used a piece of glass to tram a mill, although I am not sure if it was window glass. It was a big oval about 18" on the long side.
            What I did was mark the 4 points where the indicator would go (front back left right) and then moved the mill over all of those points (by using the xy stage, not by rotating the spindle) to check that these 4 point are level.
            You need to rotate the spindle.The method that you've used will always give the same reading because the indicator is still the same distance from the table,it would still read the same if the table was cocked at 45 degrees.

            Allan

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            • #7
              What i meant was that you need to do this test first to make sure the glass is level. THEN tram by rotating the spindle as usual

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              • #8
                oops, the original post said square the vise. In that case, just check the glass piece by flipping it over and measuring again. But how do you use glass to square a vise? Don't you use an edge finder or indicator on the vise itself?

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                • #9
                  Originally posted by beanbag
                  oops, the original post said square the vise. In that case, just check the glass piece by flipping it over and measuring again. But how do you use glass to square a vise? Don't you use an edge finder or indicator on the vise itself?
                  You need to indicate in over the same length that you'll be working with in the vise... so if the piece is 6" long, indicating the vice in is in fine.... but if the piece is longer, than to maintain accuracy one should indicate in a straight bar held in the vice.

                  - Bart
                  Bart Smaalders
                  http://smaalders.net/barts

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                  • #10
                    Squaring a milling vice on the table- just to clarify you must mean to make the jaws precisely parallel to the table travel, whether x or y axis. The glass would be then be held flat between the jaws, essentially extending the jaws so indications can be taken from points much wider than the jaws. This then would require a strip of glass held vertically. In other words, the jaws are only going to be 1/4 inch apart, or whatever depending on the thickness of the glass.

                    I'm pretty sure that's what you had in mind, and in that case the squareness of the piece of glass is irrelevant. Most glass made these days is float glass, and that can be pretty flat. In fact it should match the curvature of the earth's surface, which over a few feet is practically nil. But- take a look at some glass coffee tables, etc- anything that includes a glass panel as part of it. Look across the glass from all angles, and try to catch an angle where you can notice any anomalies. Yikes! I've seen some pieces that are surprisingly warped, even though the surface is clean and smooth. You'd expect it to be flat- but don't count on it anymore.

                    Evan had a sort of grazing incidence test with a laser he did some time back (checking a surface plate I think) I don't recall the details of that, but it seemed to work pretty well to detect deviations.

                    I've squared my mill vice in a number of different ways. One is by clamping a straight bar in the vise as I think you are thinking of, then with an indicator in the spindle, crank the table back and forth and adjust for low to no movement of the pointer. Another is to assume that the side of the table is totally parallel to the x axis, and compare distance to the bar at two points from there.

                    I have made up a jig that bolts to the table in the usual way with t-nuts, etc, and orients to 90 degrees from x axis automatically by using the machined side of the table. Essentially it's a square, but with a vertical sheet of material crossing the table. There's a notch cut out of the center of that such that a vise fits under it with one jaw on each side. The vise is placed loosely on the table, the jig is carefully placed and bolted down, then the vise is snugged to the jig, which orients the jaws 'perfectly'. The vise is secured and the jig removed.

                    Squaring the jig initially was done using an indicator and cranking the y axis to enable perfect squaring of the jig. As long as I don't drop or knock the jig, it will give me a perfect 90 across the table just by holding it against the side of the table. In fact I don't bolt it down when using it anymore- I just hold it in place, snug the vise jaws, then check the positioning once more, secure the vise, remove jig. I used aluminum plate to build it, checking to make sure I had a straight piece.
                    I seldom do anything within the scope of logical reason and calculated cost/benefit, etc- I'm following my passion-

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                    • #11
                      Originally posted by darryl
                      Evan had a sort of grazing incidence test with a laser he did some time back (checking a surface plate I think) I don't recall the details of that, but it seemed to work pretty well to detect deviations.
                      This just got me thinking, what about holding a laser with the vise and making a line on a wall across the room? sort of the same way people do it with the round column mills.

                      I guess the vise would need some sort of pin or something so that it will go back to the exact same spot everytime, but it could make squaring it really easy.

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                      • #12
                        darryl

                        You "nailed" my intention with your description. Should I remove my vise from the table or should I have a need to rotate the vise, I will eventually need to "square the vise jaws, hence the vise" in either the x or y axis for future work. I can indicate the rear jaw (the one that doesn't move) for a distance of 6 inches which may be quite suitable in nearly every case. But should I have a need to work along a length greater than 6 or 7 inches, then I "assume" it would be best to at least check "runout" (or whatever it's called) over that distance in an attempt to correct or at least minimize runout. For this reason, I was thinking that glass might be suitable.

                        I do realize that ground & polished stock having been "certified" for such use would be ideal but I was thinking that a "certified straight piece of bar stock" is subject to dents & dings .... or even corrosion over time and care would be required to always protect the metal. Accidents happen. Of course, there will be someone at the ready to point out that I should *want* to take better care of precision stock ..... and of course I do and would so there will be no need to visit that aspect. But there are variables in my life that are not common to those who are basically stationary. My job requires that I move with frequency. There are inherent problems associated with global moves, e.g., damage by shippers thus, as in my opening statement, I was thinking that glass may be a viable option provided it was flat and straight. My dilemma is, I know little about flatness or straightness of glass though one would naturally assume that glass is flat and straight. All I know for sure is, glass is a super cooled liquid and over a course of time (albeit a LONG time), will "run".

                        Basically I am searching for a substitute that may come close to precision bar material.

                        Darryl, you hit the nail on the head with what I was attempting to ask.

                        Harold
                        For those having fought for it, Freedom has a flavor the protected will never know.
                        Freedom is only one generation away from extinction.

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                        • #13
                          glass has no place in a shop. i used to think elsewise and had a pane of cheap rem glass from the local hardware store that i used for lapping, but then i dropped something on it and i'm still finding shards of it several years later with my hands at any random time i touch my mill...
                          -paul

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                          • #14
                            someone beat me to it , but NO GLASS IN THE SHOP
                            that stuff is bad when it breaks, and it will....
                            please visit my webpage:
                            http://motorworks88.webs.com/

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                            • #15
                              glass is quite a flexible material, as i sit watching a howling gale [well gust] distort my window.
                              mark

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