Increasing surface finish on counterbore
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  1. #1
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    Default Increasing surface finish on counterbore

    An engineer approached me today with a project requiring rework of a 3.500/3.501 counterbore that is finished to size but needs to have the surface finish increased from 20 to 63 to 125 Ra. I'm not sure this can be done without ID knurling it and then boring it back to size. Anyone have any thoughts on this?

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    Good to hear your typing Dark.

    I think knurling is probably the way I would do it.

    R

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    how deep? And I assume you mean the ID of the bore, and not the face. An old trick to roughen up the surface finish of lee plug holes is to very lightly run a thread mill around it.

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    I understand the whole knurling thing, but wouldn it leave low spots?

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    OK, if I am reading the question properly, he wants a rougher finish, not a smoother one.

    If it is worth the effort, you could use a boring head in a vertical mill to provide a radius adjustment. Mount a small, Dremel type, fine grit stone in a bearing on that boring head. The bearing would allow it to free wheel so it would only impress small dimples in the ID, not grind it down. Run it around the ID while making small adjustments to the radius until you have the desired result. Recheck the diameter.

    Of course, that stone should be dressed before doing this. If the bore is too deep for a small, 1/8" shaft stone, there are larger ones available. And centering on the existing hole is critical.

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    Would media blasting be an option? Might have to play with different medias. Just a thought.

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    Would abrasive grit blasting of the existing bore be acceptable? You might need to test the process on a spare part to ensure not wrecking a good piece.

    Is this for adhesion properties? If so, I'd definitely grit blast after cleaning, then a second cleaning after blasting.

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    Quote Originally Posted by aerodark View Post
    An engineer approached me today with a project requiring rework of a 3.500/3.501 counterbore that is finished to size but needs to have the surface finish increased from 20 to 63 to 125 Ra. I'm not sure this can be done without ID knurling it and then boring it back to size. Anyone have any thoughts on this?

    Sent from my Pixel 2 XL using Tapatalk
    .
    .
    typically when rougher finish wanted for applying epoxy you machine at much higher feed rate. rather than .0030" ipt or less you got .010" to .030 ipt feed. or use a corncob or serrated type roughing mill and after side milling it will have the lines on surface from the wavy flutes. try on scrap piece of metal first of course
    .
    other option is to use 50 grit sandpaper typically that is done in a cross hatch pattern or X pattern

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    My thought is there is NO material to remove so raising material and then truncating the raised material is the only way. Still hoping for the simple solution.

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    Quote Originally Posted by aerodark View Post
    My thought is there is NO material to remove so raising material and then truncating the raised material is the only way. Still hoping for the simple solution.

    Sent from my Pixel 2 XL using Tapatalk
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    sandpaper even when trying to remove .0001" takes a lot of effort. obviously I have had to remove .0001" from surfaces before over the decades.
    .
    if rough surface needed for adhesive bonding usually doesnt matter about .00005" removed to roughen the surface for better bonding

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    Quote Originally Posted by aerodark View Post
    My thought is there is NO material to remove so raising material and then truncating the raised material is the only way. Still hoping for the simple solution.

    Sent from my Pixel 2 XL using Tapatalk
    Abrasive grit blasting with suitable grit and pressure might do the trick. Coarse grit blasted only for short time so that it raises some burs but removes minimal material?

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    I would contact flex hone. see if they have a something.

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    Co-worker said he did this before. Threaded a 64 pitch about .001 deep. Increasing surface finish on counterbore

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    I don't know how big the part is but if the hole was deep enough for a hone I think there are stones that would give you that. I've never used a profilometer after the 70 grit stones, but it's rough.

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    If it is for adhesion purposes and there is access to a CNC mill, I'd run a thread mill around the hole in effect making a shallow spiral groove and leaving a core of untouched original finish to provide positive location of whatever fits in the hole. If for other purposes maybe something different. If that was already stated (for adhesion or not) I missed it. Either way, increasing the Ra to a set number without increasing the bore size will be difficult. If the spiral groove is no good (and probably isn't if you need 63-125Ra) I too think best bet would be gentle grit or bead blasting.

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    I think the very fine thread idea is the best.

    Tell me they did not call a one thou tolerance for a glued joint.

    If not telling us why it needs to be roughened would be instructive


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