Increasing surface finish on counterbore

aerodark · Nov 8, 2019

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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litlerob1 · Nov 8, 2019

Good to hear your typing Dark.

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

R

Larry Dickman · Nov 8, 2019

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.

plastikdreams · Nov 8, 2019

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

EPAIII · Nov 8, 2019

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.

bigjon61 · Nov 8, 2019

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

Milland · Nov 8, 2019

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.

DMF_TomB · Nov 9, 2019

aerodark said:
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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.
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
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other option is to use 50 grit sandpaper typically that is done in a cross hatch pattern or X pattern

aerodark · Nov 9, 2019

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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DMF_TomB · Nov 9, 2019

aerodark said:
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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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.
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if rough surface needed for adhesive bonding usually doesnt matter about .00005" removed to roughen the surface for better bonding

MattiJ · Nov 9, 2019

aerodark said:
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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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?

Bluejeep · Nov 11, 2019

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

aerodark · Nov 11, 2019

Co-worker said he did this before. Threaded a 64 pitch about .001 deep. [emoji849]

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rickseeman · Nov 12, 2019

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.

eKretz · Nov 12, 2019

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.

gustafson · Nov 12, 2019

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

Noah Katz · Nov 14, 2019

DMF_TomB said:
if rough surface needed for adhesive bonding usually doesnt matter about .00005" removed to roughen the surface for better bonding

Yes, and I believe optimal epoxy bond thickness is .001 - .006".

In aerospace we used a few % .005" glass microspheres to maintain a minimum bond thickness.

edwin dirnbeck · Nov 14, 2019

I cant tell you how many times,guys have have come up to me and said "hey ,I need you to open up this bore a little ,but don't take no material out.In other words ,they screwed something up,but want to put it on you. Edwin Dirnbeck

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