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  1. #76
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    Finally pulled my finger out and did some more on this.

    In truth I needed something quiet and relaxing to take my mind off the fires, just for a little while...

    Anyway, i've got to the stage where I feel like I'm chasing my tale a bit with the contact spots. It's easy done getting to 95% but that last little will leave you chasing your tale if you are not good at interpreting the low, med and high blue spots (that's me) and taking the correct scrapes.

    I also found that I was struggling with the blueing up. Sometimes I had too much and it smeared, resulting in false imprint, other times it was too thin and I struggled to see the blue.

    Since I'm using a homemade Prussian blue formula, I decided to add a few more grams of the ferric ferrocyanide to my batch to increase the pigment. I have to be a little careful because too much pigment powder will make the paste too gritty so it's a balancing act.

    So far I'm happy with the result.

    Second pic is the ram base. There are a couple of places that are a bit light on for contact but i think im nearly there...

    Would It be the end of the world if I left it as is and moved onto the dovetail?

    Simon


    Sent from my SM-G970F using Tapatalk
    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.

  2. #77
    Join Date
    May 2011
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    Murray Bridge S Aust.
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    Nice work, I'd call the low areas "Lubrication Retension Spots" .
    One good thing about these Holidays, people are able to get stuck in and get their projects moving, provided SWMBO allows them shed time.
    Kryn
    To grow old is mandatory, growing up is optional.

  3. #78
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    Jul 2016
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    35
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    It would absolutely not be the end of the world, that is a perfectly respectable contact pattern. Congrats!

  4. #79
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    Thanks guys. I may do another couple cycles.

    Now I have to make a jig to hold a DI to measure how parallel the dovetails are to each other.

    I guess I can spot and scrape the dovetail Gibb too.

    Simon

    Sent from my SM-G970F using Tapatalk
    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.

  5. #80
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    Aug 2011
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    Got some more time in the shed the last couple of days.

    I rigged up a dovetail parallelism comparator (for want of a better term) to measure how (un) parallel the dovetails are on the ram.

    The initial results are not pretty to say the least. The years have definately NOT been kind to this machine! The ram is approx. 600mm long and the dovetails are out of parallel by 0.3mm over that length!

    Forget about a heavy initial pass with the scraper. This is angle grinder territory!

    I guess I'm just going to have to grin and bare it. Question is, do I scrape one side flat and then bring the other side flat and parallel or do I bring both side into parallel by taking a bit off each side?

    You may be wondering if it's this far out because I did not scrape straight down on the base. But this is not the case. I have taken some squareness measurements between the (new) base and the front machined mounting where the tool slide sits and it's square to within 0.01mm. This would surely indicate that the out of parallel dovetails is mostly due to wear on the dovetails.

    Any questions, comments appreciated.

    Edit: I will make a braket to fit a DI for measurement as well. I suspect it will tell me the same information.

    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.

  6. #81
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    Been trying to work out if the relative large deviation from parallalism of the dovetails has been from my scraping. I have still come to the conclusion that my scraping, even if it was not done straight down could not account for that much difference.

    If you look at the progress pics earlier in this thread you can see that the blueing pretty quickly covers the entire length albeit on one side. So there was not any significant preferential scraping done at one end or the other.

    In any case, for the dovetail to be fatter at the front, it would need me to have scraped significantly more at the rear. This does not make any sense anyway because you would expect more wear at the rear anyway, meaning the front would need to be scraped more to bring it down same as the rear. If this was the case then the front would be skinnier, not the rear.

    Geeeees. Nothing like a bit of metrology to ward off dementure!

    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.

  7. #82
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    Oct 2011
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    Or to bring it on?
    Understanding what these numbers mean can do a head in...

    Whatever material you have to remove, the aim is to get a flat plane, so whether it is all taken off one end or a bit from both should not matter greatly. A way to resolve the conflict may be to look at another surface/ plane and remove material to get that to a good geometric relation ship - so for example, parallel with another machined surface (such as the one on the top of the ram) or so any plane of symmetry is perpendicular to the plane you are establishing. For 0.3mm, it probably does not matter physically, but as a construct to repeatably measure to, it is worthwhile.

    Michael

  8. #83
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    Hi Michael,

    I knew someone would suggest measuring parallel to the top surface.... Dammit. I surface ground that parallel to the newly scraped base. It was badly beaten up and would have been difficult to get a reliable reading but still, maybe a rookie mistake.

    This is however a nicely machined surface inside the ram where the pendulum arm mates to. I will indicate off that to see if it gives me any insight to my other readings.

    Simon

    Sent from my SM-G970F using Tapatalk
    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.

  9. #84
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    OK I just run a DI along the inside part of the ram. The results were a little over the place but the maximum change in depth from front to back was 0.05mm.

    Now I'm not sure if it's safe to assume that the machined part inside of the base was done at the same time as the ram base and as such whether it represents an original datum plane from when the ram was new but it's an encouraging result and suggests that maybe I did scrape mostly straight down. Certainly 0.05mm deviation from parallel after about 35 scraping cycles is a reasonable assumption.

    I guess I'll start scraping the dovtails and see what happens...

    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. #85
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    Hi,

    Don't you have two cosine errors there that will stack?(in fact there may even be a third stacking error but lets not get into that)
    Not even going to try and do the maths.
    How about trying a piece of 0.1mm shim under your bolt head and see what your DTI says. Maybe its close, maybe it isnt, maybe I am full of it. Dont be thinking you are going to turn 0.3mm into 0.1mm, but you might knock a bit off.

  11. #86
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    Hi Stu,

    You're killing me. I don't follow. I can change the height of that bolt to wherever I want. This allows me to measure from a different part of the dovetail. It's a comparator so as long as none of those adjustments change then it's good.

    No?

    Simon

    Sent from my SM-G970F using Tapatalk
    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.

  12. #87
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    Ok. Think I know where your heading with this....

    The height on BOTH sides were reduced. That effectively doubles the result in dovetail width change.

    Also the angle of the dovetail. At 45 degrees it would be a 1:1 ratio. I think it's more like 55 degrees so it's approx 1.41

    So, 0.3mm becomes 0.15mm and then it's

    tan 55 = x/0.15

    where x is my scraping depth

    x = 0.15 tan55
    x = 0.21mm

    Is this what you meant.





    Sent from my SM-G970F using Tapatalk
    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.

  13. #88
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    Quote Originally Posted by simonl View Post
    I don't follow. I can change the height of that bolt to wherever I want.
    Me either. Put the shim in the wrong place for your use but it may have shown my point anyway.

    Quote Originally Posted by simonl View Post
    Is this what you meant.
    I don't think so. but its along those lines.

    I'll try again.


    You want to measure the width of the dovetail.(a line parallel to the flat way you have just scraped just a little offset from it)
    Because you are measuring to a surface at 55 deg I'm seeing a cosine error.
    Because of the way you have the DTI set up there is another cosine error.(though I'm pretty sure this one will be much smaller)
    I think they are stacking.

    You're saying a 0.3mm reading is "out of parallel by 0.3mm" which I think is wrong.
    To test it
    I should have asked you to do is this.
    Set your jig to 0.0 on the large end of the dovetail.
    Slide it to the other end (I'm assuming it returns pretty much to 0.0 when returned to the start point)
    Find a piece of shim close to the reading(0.3mm) on the DTI, slide that under the DTI tip.
    If you're right the DTI will return to 0.0 and I am full of it.
    If I'm right it wont, find a piece of shim that does return it to 0.0 and that's your parallel error.




    Clear as mud?


    *My maths of the error from the dovetail angle alone says a reading of 0.3mm becomes an parallel error more like 0.17mm, but I wouldn't trust it as far as I could throw me.

  14. #89
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    Yep. I think we are both on the same page.

    I fixed up any cosine errors from using a DTI and made a new braket to hold a DI. Thats one thing out of the equation.

    I still get a 0.3mm variation across the length so any error from the DTI not being at the correct angle are minor.

    Now I have done some measuremts;

    Placing a 0.1mm feeler gauge under the bolt changes the reading on the DI by 0.06mm

    Also, placing a 0.33mm feeler gauger under there changes the reading by 0.2mm

    Since tan x = OPP/ADJ

    inv tan(0.1/0.06) = 59 degrees.

    Also inv tan(0.33/0.2) = 58 degrees.

    So, lets call the included dovetail angle 60 degrees, which seems reasonable;

    Still trying to get my head around what height difference would give rise to a 0.3mm change although I understand that it will be half this because it's on both sides.

    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.

  15. #90
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    Yeap sorry about that, he's full of it. Turns out you weren't measuring what I thought you were so cosine error isn't going to help you. Your dial gauge setup makes that clear.


    Now get scraping

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