What PPI under the headstock?
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  1. #1
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    Default What PPI under the headstock?

    I'm scraping under the headstock of my practice-bed. I am wondering what is the typical scraped PPI that the headstock bears onto? Surely it doesn't need to be as high as on the ways?

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    If that was me. Thats a static surface, your only scraping for alignment, If you went past 12 P.P.I, your over working it. Any thing better than 8 is good. And its a dirrerent kind of scraping to slides. You want more contact, for a mechanically coupled / bolt on assembly, so bigger radiused tip leaving larger lands, for me increasing percentage of contact.

    Regards Phil

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    I agree, but I say 5 to 10 PPI and stone a bit harder then when scraping ways. Around holes I recommend you counter sink the threads or slightly counter bore so when you tighten down the bolts the threads don't pull up and twist the head or unit. I make sure to get more POP percentage of points of contact and I also make sure the bottom isn't high in the middle 40% is .00005 to .002" low. Many builders relieve the middle a lot and only have bosses or a flange that they set on. I was just talking to Vette Bob yesterday on the phone and he fit his EE head to his newly ground bed as he purchased a test bar from a place Jim Thelie told him about. He said he read how to scrape it on the monarch thread. :-) Be sure to tighten the bolts down when there is still bluing under there to know you aren't getting a false reading when it isn't tight. If you think your getting a false read, with the head loose tap on the 4 corners with a plastic covered lead hammer or dead blow hammer to hear if you get a solid thud sound. also I see if I can slide in a .001" under the flanges. Rich

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    10 PPI is my vote too.

    Around holes I recommend you counter sink the threads or slightly counter bore
    Read that again!
    Especially if one side has a thread (means: no through holes where a bolt with nut goes through). You will be shocked how much a bolt that goes into a thread pulls out the thread and thus you only get ring-contact around the thread.

    So if there is a thread in one part, generously relieve it.


    Nick

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    Quote Originally Posted by Nick Mueller View Post
    So if there is a thread in one part, generously relieve it.
    Back in the company where I did my time. They had a standard proceedure for any critical threaded holes, for any where a hardened rail, headstock, spindle housing, turret etc was to be installed.

    That was to drill 3x diameter of bolt. So 36mm deep for a M12 bolt. Tap to 2.5x, then pull the first 3 threads out with a nominal size drill 3 x 1.75mm depth at Dia 12 for M12 etc. Then put a generious chamfer on top of that. They used to grind sub-land drills to do it in one shot. That would be similar to the counterbore Rich mentioned.

    I see this a lot on lessor built machines, where you are replacing a linear guide. You run a stone over the seat, and theres mushroom shaped lands around the holes.

    Regards Phil.

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    then pull the first 3 threads out with a nominal size drill
    That sounds like a very good procedure to me!


    Nick

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    Standard procedure at the company I worked for last. Regards Tyrone.

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    One of the first things I noticed on my practice-bed was a raised atoll-ring of metal around the four headstock mount threaded holes. I never thought to mention it. I had originally relieved it with the scraper, didn't realise you had to go as deep as 3 threads.

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    Quote Originally Posted by Peter. View Post
    One of the first things I noticed on my practice-bed was a raised atoll-ring of metal around the four headstock mount threaded holes.
    Atoll , must be the best ever word to describe pulling up metal. Thanks for mentioning it.

    Regards Phil.


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