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  1. #46
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    Ok that sounds reasonable.

    You saying you remove the workpiece, measure and then remeasure after some time and get a different result in the order of tenths?

    Simon

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  2. #47
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    Yes. terrible repeatability...

    Michael

  3. #48
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    I'm hesitant to offer any help as the chances are, if I have thought of it then you would have anyway.

    But, do you have any reason to doubt your measurement equipment?

    I'm now running a mile.......
    Girl, I don't wanna know about your mild-mannered alter ego or anything like that." I mean, you tell me you're, uh, super-mega-ultra-lightning babe? That's all right with me. I'm good. I'm good.

  4. #49
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    Equipment - no
    Technique - perhaps a bit
    Environment - Most likely the biggest contributor as I'm dousing something in coolant then once I've finished, letting it sit in ambient air in an insulated and heated dwelling.
    Expansion coefficient for steel is 1.2 x10-5 m/m/C, so at 17mm diameter that is 0.0002 mm per degree C or 8x10-6 inches per degrees C. A temperature change of 12.5 degrees C will grow 17mm by a tenth of an inch a thou.

    I normally laugh at guys who claim to measure things to (absolute) micron accuracy in their sheds but I'm almost as bad although I'm only measuring diameters relative to other diameters. I find that if I measure as soon as I come in from the shed and then say 15 minutes later there will be a difference. If I measure say 15 minutes later from that, I get much better agreement. Do it again and it is almost spot on what it was before.
    In reality I'm probably as good as I'm going to get it without going overboard.

    Michael
    Last edited by Michael G; 16th Aug 2016 at 07:52 AM. Reason: Dean is right!

  5. #50
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    A temperature change of 12.5 degrees C will grow 17mm by a tenth of an inch.
    You are mixing imperial with metric. Are you sure you want to do that, oh and you also might want to check your figures.

    Dean

  6. #51
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    Quote Originally Posted by Michael G View Post
    Yes. terrible repeatability...

    Michael
    Michael I'm a little confused now. Is it also not repeatable? My understanding was that it was precise but not accurate ie it was grinding a waist in the work, but that waist was repeatable. The two would likely be caused by very different causes. If the error is repeatable then I'd say it's in the machine, if it's not repeatable then I'd say it's environmental or some other cause. Possibly still in the machine, but I'd broaden my net if it were me.

    While Schlesinger may not have produced specifications for machines that particular size (and I haven't honestly checked that), the principles and process remains the same irrespective of the machine size. By all means set up the machine as well as you can, but if it starts giving grief I'd suggest going back and following the logical process through that he describes. Otherwise you may finish up chasing your tail endlessly trying to correct something that simply can't be done due to "up-line" errors.

    If it's not repeating however that's a whole different ball game, and that type of error can be hugely frustrating. I'd say it then becomes a process of, well, repeating the process, but doing so as systematically identical as is possible. Alternatively making deliberate changes (ie to the length of the work) and seeing what happens to the error. That would especially be the case in waisting, as it could be a clue as to where the error lay depending on where the waist moved with a longer part.

  7. #52
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    Apologies to all for the confusion. Strictly speaking the measurements are repeatable but are influenced by conditions, so stability is probably more the issue. I can however change the packing under the tailstock and that will change the waist dimension but not in a highly predictable way. I think at the moment I'm up against several things.
    • I'm up against the limits of my measuring equipment
    • Measuring conditions are not uniform
    • There are (as Pete suggests) possibly other factors influencing the finished grind
    • There are still items on the grinder that need to be fixed/ altered to avoid influencing results


    I'm relatively close to where I want to be though so I'm going to leave it and work on some of the other issues. Still having fun though.

    Michael

  8. #53
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    Quote Originally Posted by Michael G View Post
    Apologies to all for the confusion. Strictly speaking the measurements are repeatable but are influenced by conditions, so stability is probably more the issue. I can however change the packing under the tailstock and that will change the waist dimension but not in a highly predictable way. I think at the moment I'm up against several things.
    • I'm up against the limits of my measuring equipment
    • Measuring conditions are not uniform
    • There are (as Pete suggests) possibly other factors influencing the finished grind
    • There are still items on the grinder that need to be fixed/ altered to avoid influencing results


    I'm relatively close to where I want to be though so I'm going to leave it and work on some of the other issues. Still having fun though.

    Michael
    On the slim chance you haven't got one, would a 1um Cary DI be of use?

    BT

  9. #54
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    cylindrical grinding to 3/10ths in your "backyard" is no doubt an amazing effort in itself either way. However, theorising the nature and cause of that last few 1/10ths is none the less intriguing. Michael, when you cylindrical grind an workpiece and it naturally heats up, have you considered it may not necessarily expand consistently along it's length? The ends may not get as hot as the middle because of the cooling affacts with the air and as such will not expand as much. While the middle, with less cooling will expand just that bit more, meaning more may be ground in the process. Then, when the whole piece cools down, the middle will show as having a waist.

    I have experienced this with surface grinding. I have left pieces on the mag chuck for a few hours after grinding and then when I give it another tiny lick, I notice the ends are higher than the middle. I use coolant always but I can't see any other reason other than the middle heats up just that little bit more, or should I say that the ends have a greater rate of cooling because they have air around them and expand less than the middle. Then when it all cools down, the ends appear higher.

    Just a thought.

    Simon
    Girl, I don't wanna know about your mild-mannered alter ego or anything like that." I mean, you tell me you're, uh, super-mega-ultra-lightning babe? That's all right with me. I'm good. I'm good.

  10. #55
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    Quote Originally Posted by Anorak Bob View Post
    On the slim chance you haven't got one, would a 1um Cary DI be of use?
    I have a 2um Mahr somewhere that I should probably be using

    Quote Originally Posted by simonl View Post
    ...when you cylindrical grind an workpiece and it naturally heats up, have you considered it may not necessarily expand consistently along it's length? The ends may not get as hot as the middle because of the cooling affacts with the air and as such will not expand as much. While the middle, with less cooling will expand just that bit more, meaning more may be ground in the process. Then, when the whole piece cools down, the middle will show as having a waist.
    One of the reasons I had the coolant system plumbed up before I started grinding was a concern about heating effects. The whole test bar carries a film of coolant while grinding so I don't think I have a great range of temperature on the bar. I also do a couple of passes with zero feed to effectively spark out.

    I'll get there. It's great having a few guys like you, Pete and Bob throwing in some suggestions - it gives me ideas and things to look at.

    Michael

  11. #56
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    Quote Originally Posted by Michael G View Post

    I'm relatively close to where I want to be though so I'm going to leave it and work on some of the other issues. Still having fun though.

    Michael
    My highly pragmatic take on this is, does it matter for the class of work you plan on doing with the machine?

    If not, move on to the next project and call it done.... it's damn easy to get to the 'how many angels on a pinhead' problem with this sort of caper. Fine if that's actually the intention, otherwise, not.

    PDW

  12. #57
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    Quote Originally Posted by Michael G View Post
    Apologies to all for the confusion. Strictly speaking the measurements are repeatable but are influenced by conditions, so stability is probably more the issue. I can however change the packing under the tailstock and that will change the waist dimension but not in a highly predictable way. I think at the moment I'm up against several things.
    • I'm up against the limits of my measuring equipment
    • Measuring conditions are not uniform
    • There are (as Pete suggests) possibly other factors influencing the finished grind
    • There are still items on the grinder that need to be fixed/ altered to avoid influencing results


    I'm relatively close to where I want to be though so I'm going to leave it and work on some of the other issues. Still having fun though.

    Michael
    For tailstock height to cause waisting, you would need to go from a condition where the wheel is above/below centre, to on centre, and then below/above centre. I can't see how moving the tailstock up will decrease the waisting, it will just cause it to turn in to a taper. The taper could then be taken out with offset, but all of this is thrown out the window when you work with longer or shorter work.

    As I mentioned, I would try longer and shorter work and see what the results are. Nevertheless I do think you're barking up the wrong tree with the tailstock.

    I noticed you edited your comments about measuring to "micron" level, so I similarly deleted my comments in relation to that before posting. However measuring to 0.001 mm in the home workshop is extremely doable and shouldn't pose too many issues. I'm fortunate to have a workshop that's, while very weeny in size, is very stable in temperature, so I may be taking that factor a little for granted. Actually creating work to that standard is of course another factor entirely, and a whole different level of difficulty. However with general care and attention, it should be entirely realistic to at least measure to 0.001 mm in the home workshop and get repeatable results. Given that the emphasis at this point should be in correct and appropriate machine alignment, that's all that should count at this stage. Conversely, trying to achieve machine alignment purely off finished work is on a hiding to nothing in my humble opinion.

  13. #58
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    Michael,

    If it would help, the Cary could be in tomorrow's mail. The gaps between the ums are wide enough for a bit of sub micron measurement.

    BT

  14. #59
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    Quote Originally Posted by Anorak Bob View Post
    If it would help, the Cary could be in tomorrow's mail.
    Thanks for the offer Bob, but at the moment I'm good. That may change of course, in which case I'll get back to you

    Quote Originally Posted by Pete F View Post
    For tailstock height to cause waisting, you would need to go from a condition where the wheel is above/below centre, to on centre, and then below/above centre. I can't see how moving the tailstock up will decrease the waisting, it will just cause it to turn in to a taper. The taper could then be taken out with offset, but all of this is thrown out the window when you work with longer or shorter work.
    Pete, you are correct, in that I would need to go above to below to get waisting, but you are also thinking of a lathe where the angle and height of the tool is important.
    Some time in the past, a plate was added to the bottom of the (existing) HS. I don't know why. All I can think of is that a little more centre height was needed/ desired and so the HS (and I assume the TS when it was with the machine) got this treatment. On a lathe that would be a bad thing as that changes rake angles as well as centre heights. On a CG it's not so bad as the contact point does not have to be exactly in the horizontal plane for it to work (yes, there is a sine error creeping in but...), because a tangent is a tangent.

    P1030604 (Medium).JPG
    Whether I get a taper by shimming the TS will depend on whether the two centres are the same and where the contact point with the wheel is. (This why I value these discussions, because I have to start thinking and prove my assertions). I went out and clamped a business card between the wheel and the ground centre in the existing headstock. As you can see from the second photo, the card is not quite square, suggesting that the HS centre is slightly higher than the wheel spindle. Please excuse the square - it is the one normally reserved for welding so it is a bit battered.

    P1030642 (Medium).JPG P1030643 (Medium).JPG
    So to get the bar parallel, ideally I need the TS the same height. The plot below is the diameter deviations for the test bar, corrected to be zero at both ends. The bottom plot is for the TS not shimmed at all. The middle plot is for a T/S shimmed with 50 thou of shim. The upper plot is for a TS with 32 thou of shim. My thoughts are that with no shim, the TS is lower than the HS, with 50 thou shim, the TS is now higher than the HS. With 32 thou of shim I'm much closer to even height.

    Test graph.jpg
    I think with 32 thou I still have the TS higher than the HS but (to my frustration) I have not been able to get a nice clean grind that shows that. 120mm is probably longer than most (if not all) surfaces that I will grind but makes any error easier to spot.
    I do need to go through the whole machine but before I started painting my 'castings', I wanted to at least get close.

    My main desire for this machine is to be able to grind pins and tapers for gauging, so although this is chasing angels on pins to a degree, knowing that a cylindrical surface is parallel is important for me to be able to produce those with confidence, particularly as I can adjust for taper but not for waisting.

    Michael

  15. #60
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    You are not actually measuring, but comparing. I doubt too many of us can measure to single digit micron levels, but we can compare to that level.

    With the experience with using my cyl grinder for the inexperienced like myself sometimes it grinds straight other times it does not. I even found differences happen between pulling and pushing the drive dog on the workpiece. How much tension you have the right side dead centre putting on the workpiece, etc etc etc.

    Ideally you would have a known straight test bar and use that and an indicator with the machine off and test it with a DTI.
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