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  • Inaccurate sine bar

    Would you buy a sine bar from this seller? Ebay UK 373201945695.
    Since the new edge came out and I now use Firefox more often, I have been unable to cut and paste ebay links, sorry for the long winded number.

  • #2
    What makes you think it's inaccurate? Or more more to the point made to a tolerance which is larger than you would prefer? Without a link to a description of the product it's pretty tough to say one way or the other. Deciding to deal with the seller or not is likely based more on the feedback scores. A small percentage of negatives is to be expected but higher negatives would be a warning.
    Chilliwack BC, Canada

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    • #3
      Because the sellers are listing it as 6", 150mm.

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      • #4
        TheSine Bar. distance between two bolls 200 mm. Parallelism +/-. 0002 on both top & bottom. Flange on end for resting work.
        Helder Ferreira
        Setubal, Portugal

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        • #5
          How do you expect us to know if it is accurate ? ? ?
          DZER

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          • #6
            I've seen hundreds of ads like that
            It happens because the sellers are ignorant
            They don't realize that the critical dimension is the center-to-center distance between the rolls
            And not the overall length
            Most likely they are a general sales outfit and not tool specialists
            That, or the ad was written by someone with a business degree and no shop experience

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            • #7
              6", 150mm
              and then

              distance between two bolls 200 mm
              "bolls"? What are bolls? And where did the 200 mm come from? I do not see it in the ad.

              I would not buy this sine bar at all. Inch based sine bars are commonly made in 5" and 10" sizes for a reason. They either do not require any math or the math is very minimal. For the 10" size you look up the sine of the angle you want and simply move the decimal place over one digit to the left to get the height of the spacer or Jo block stack. This effectively multiplies the sine value by a factor of 10, which is the distance between the two cylinders. Any machinist, even those who are math-challenged, can do this on the shop floor without error or confusion.

              The 5" size is a compromise between the somewhat large 10" size and the easy math. With the 5" size you look up the sine of the angle and again move the decimal point one place to the left. Then you divide by two to get the size of your stack. That's a bit more difficult because of that additional step of dividing by 2, but still most machinists can do this while standing by the machine in the shop.

              But with a 6" or 150 mm, they don't even say which and they ARE two different lengths. There is no where near as easy procedure for calculating the height needed. Sine value times 6" or sine value times 150 mm; neither is very easy to do in your head.

              For a metric sine bar, 100 mm or 200 mm would be nice values. For 100 you simply move the sine's decimal point over two places (multiply by 100) and you have the stack height in mm. Or for the 200 mm you do that and then multiply by two. Either of these two values returns the math challenged machinist to an easy calculation.

              And that is why I would NOT buy either a 6" or a 150 mm sine bar. Not for my shop.

              Edit: Upon a second look, I see that it is indeed a 5" sine bar. It is not a 150 mm nor even a 125 mm one. So I guess it fits my criteria. But a 5" sine bar being sold in EUROPE, which is mostly/all metric? Now that does not make any sense. It makes an English/metric conversion a part of the calculation if the shop does not have English value Jo blocks or spacers. I guess digital calipers and micrometers help with this, but even so, things are more complicated.

              The ad shows a +/-0.0002" tolerance for both the distance between the cylinders and for parallelism. This would imply a number like +/-0.0003" overall accuracy with an upper limit of +/-0.0004" as an upper limit. If it is out by more than that amount then it is out of spec.
              Last edited by Paul Alciatore; 09-10-2020, 05:28 PM.
              Paul A.
              SE Texas

              Make it fit.
              You can't win and there IS a penalty for trying!

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              • #8
                The aspect of 6" or 150mm is more to do with the fact that there are now millions of youngsters who were brought up on metric and are too dumb to appreciate that a) precision is important on machine tools, and b) old stuff will be imperial and probably a round figure in imperial units. Even worse are the hundreds of 'garage traders' who are buying up workshops from widows and putting it on ebay with a tape measure roughly in the vicinity expecting you to guess the dimensions. Collections of change wheels where they can't be bothered to list the numbers of teeth and don't know what DP means, collets where you are expected to peer at the blurred photo to read the size, chuck backplates " lathe thing" with no thread information.

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                • #9
                  The listing says "Distance between center of rolls: 5" ± .0002" - title is for a 6" bar. Photos show two different bars. It's a lucky-dip!

                  Looks rough, made in India. I know this is a huge generalization, but the machining stuff from India seems to be worse than China.

                  Link to listing https://www.ebay.co.uk/itm/6-Inch-Si...m/373201945695
                  Last edited by pinstripe; 09-10-2020, 06:27 PM.

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                  • #10
                    Hover/zooming on the first photo it looks like they marked the recesses for the pins with center punch marks, and didn't machine/finish upto them - doesn't inspire confidence that anything was done to the level of accuracy claimed.

                    There are too many alarm bells in that listing for me:
                    • ebay seller with no reputation
                    • country of origin known for selling factory reject items via ebay.
                    • the finish when hover/zooming on the photos looks pretty poor for a new item.

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                    • #11
                      Originally posted by old mart View Post
                      Would you buy a sine bar from this seller? Ebay UK 373201945695.
                      Since the new edge came out and I now use Firefox more often, I have been unable to cut and paste ebay links, sorry for the long winded number.
                      No, I would avoid at all costs "machine tools" from india.
                      Are you actually considering a purchase?

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                      • #12
                        The distance between the rolls makes no difference. It could be 5.0000 or 5.0005. What is important is that the value be known and used as a constant in your calculations. Calculators don't care and neither should you. Measure it once and it becomes your basis. If you're worried about other features being accurate such as flat, square or parallelism, that's something else to be considered. Make your own if you have the means.

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                        • #13
                          Originally posted by Paul Alciatore View Post
                          I would not buy this sine bar at all. Inch based sine bars are commonly made in 5" and 10" sizes for a reason. They either do not require any math or the math is very minimal. For the 10" size you look up the sine of the angle you want and simply move the decimal place over one digit to the left to get the height of the spacer or Jo block stack. This effectively multiplies the sine value by a factor of 10, which is the distance between the two cylinders. Any machinist, even those who are math-challenged, can do this on the shop floor without error or confusion.
                          Do you actually use a table to look up the sine of an angle?

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                          • #14
                            They are actually selling a 5" sine bar, of course I knew that all the time. We have three at the museum, the 5" that was there before me and the two I bought from an ebay seller, a 5" and more interestingly for me, a 2 1/2" baby one, both made by the sellers father who was a toolmaker.

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                            • #15
                              Originally posted by old mart View Post
                              They are actually selling a 5" sine bar, of course I knew that all the time. .
                              Yes, I was sure that you knew that, but it is worrying that the seller doesn't.

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