Drilled hole straightness and centerline drift, reasonable expectations?
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    Default Drilled hole straightness and centerline drift, reasonable expectations?

    This problem seems to pop up here quite often:

    Is there any rule of thumb or guide what to expect from like 10xD drilled hole straightness and centerline drift(exit location)
    Plain vanilla HSS drill?
    Carbide twist drill?
    Gun drilling?
    workpiece rotating or drill rotating?

    I only have some rough ballpark idea, ie 1/4" and 30" long hole is definitely gun drilling category if you want the exit on the opposite side...

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    For gun-drilling I was able to find this:
    Graf + Klett GmbH - Deep hole drilling and deep hole drilling machines - Accuracy

    From what I have heard it takes very homogenous and even heat treatment to get straight holes like gun barrels.

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    A gun driller I used was .001"/1" normally, and .002"/1" guaranteed if he could spin both the part and the drill. That was on a 65:1 L/D. I would think .005"/1" would be good for a HSS twist drill.

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    Quote Originally Posted by MattiJ View Post
    This problem seems to pop up here quite often:

    Is there any rule of thumb or guide what to expect from like 10xD drilled hole straightness and centerline drift(exit location)
    Plain vanilla HSS drill?
    Carbide twist drill?
    Gun drilling?
    workpiece rotating or drill rotating?

    I only have some rough ballpark idea, ie 1/4" and 30" long hole is definitely gun drilling category if you want the exit on the opposite side...
    .
    .
    metal like cast iron is not the same it is harder where it cooled off quicker and often has smaller hard spots in it. so i often see drill bits bend toward the softer center of thicker sections away from the skin or outer surface.
    .
    feed rate can influence drill bit drift. often high feed tends to push and bend drill and so it drifts out of position more. i have found where i have problems of drill bit drifting i reduce feed and normally it is less or hole is straighter. a dull drill tends to drift out of position more too.
    .
    how much depends on length to diameter ratio and drill material. in general hss bends more than carbide. i have seen over 0.100" drift over 8" before with a .531 drill

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    Spinning both the workpiece and the drill will normally produce the most accurate/least drifted holes. Second is spinning only the workpiece. Last is spinning only the drill.

    Many factors to this, mainly flexing of the drill or the work causing misalignment. Spinning both the workpiece and the drill averages these out.

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    Drills often (usually) move in a spiral. Drill a 3/32" hole in a Plexiglas block and watch the drill tip move in an orbit. If the drill starts the slightest bit orbiting off center, the cutting forces tend to exaggerate the movement. Slide a Diatest bore gauge in a drilled hole and watch the needle wiggle. You can often feel the clicks as the split ball goes from one groove to the next. All a twist drill is good for is getting unwanted metal out of the way.

    Bill

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    Quote Originally Posted by DMF_TomB View Post
    .
    .
    metal like cast iron is not the same it is harder where it cooled off quicker and often has smaller hard spots in it. so i often see drill bits bend toward the softer center of thicker sections away from the skin or outer surface.
    .
    feed rate can influence drill bit drift. often high feed tends to push and bend drill and so it drifts out of position more. i have found where i have problems of drill bit drifting i reduce feed and normally it is less or hole is straighter. a dull drill tends to drift out of position more too.
    .
    how much depends on length to diameter ratio and drill material. in general hss bends more than carbide. i have seen over 0.100" drift over 8" before with a .531 drill
    Good points. EDM is probably only way to get straight holes if you workpiece is anything else than very homogeneous.

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    Quote Originally Posted by MattiJ View Post
    Good points. EDM is probably only way to get straight holes if you workpiece is anything else than very homogeneous.
    "D" drills served me so well as reamers OR drills that I was past 30 years of age before I even invested in a set of CTD "store bought" HSS-Cobalt Parabolics.

    Naturally ... I had always used whatever "the company" provided. Triple-ought to 3".

    But.. one of those companies made and used "D" drills and reamers a LOT. Where I learnt their value where straight and ON SIZE matters. Great-Grandfather to the "gun" drill in most respects, "D" drills can be.

    Cheap enough. Fast? Nooooooo... not so much...


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    Default hard spots

    metal hard spots. slag or oxidized silica "sand" and just metal not mixed too well. its not like they put molten metal in a blender and of course as it is exposed to air slag forms on top
    .
    usually they are grain of sand or small in size. as pictures show when they get grain of rice size they can be seen more easily. hard to see when drilling 8" deep. often can hear sound differences
    .
    sometimes when milling you see occasional sparks. usually cause it is milling small size hard spots
    Attached Thumbnails Attached Thumbnails hard-spots.jpg   hardspots2.jpg  

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    Yes this is very common on poorly made castings especially. I had a few years ago with all sorts of sand in the middle of the bearing bore areas. That was fun.

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    Quote Originally Posted by eKretz View Post
    Yes this is very common on poorly made castings especially. I had a few years ago with all sorts of sand in the middle of the bearing bore areas. That was fun.
    .
    i machine thousands of bigger castings often over a ton. there is always a certain percentage of castings with hard spots. some foundries have a higher average percentage of castings with bad hard spots.
    .
    i have gone months and over 100 castings with no problems then you get a batch from a bad pour where you get many hard to machine castings.
    .
    my company often ultrasound checks smaller 200 lb castings that must pass a pressure test. usually ultrasound will detect bad castings saving doing a lot of machining only for it to fail a pressure test after all the machining.
    .
    same casting can cost $300 or $2000 but the $300. castings can have more problems. ultrasound testing will usually find the bad ones. above my pay grade if the cheaper castings are worth it. the $2000 castings are usually at least 95% good castings. never ever saw any foundry always supply 100% good castings

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    Yeah the budget stuff is always the worst. That particular batch of probably 20 very large castings contained a sh*tload of sand inclusions that were a serious pain to work through. Basically it was one of those times where you just have to grind a giant negative on the top of the boring insert and bang your way through. Oh, forgot to mention they were stainless as well, which really worked out well (instant work hardening once the tip would get dulled on a sand spot). Regrind the negative t-land every other cut or so, then for finish I'd leave a small amount of stock, and before finish cut take a die grinder and grind out any visible bad spots just below finish depth and finish with fingers crossed. Got 'em done, but that job must have died time wise.

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    Quote Originally Posted by eKretz View Post
    Yeah the budget stuff is always the worst. That particular batch of probably 20 very large castings contained a sh*tload of sand inclusions that were a serious pain to work through. Basically it was one of those times where you just have to grind a giant negative on the top of the boring insert and bang your way through. Oh, forgot to mention they were stainless as well, which really worked out well (instant work hardening once the tip would get dulled on a sand spot). Regrind the negative t-land every other cut or so, then for finish I'd leave a small amount of stock, and before finish cut take a die grinder and grind out any visible bad spots just below finish depth and finish with fingers crossed. Got 'em done, but that job must have died time wise.
    yes i have gone through over 6 sets of carbide inserts and every time they hit grain of rice size hard spot inserts would go dull. eventually took small hand held air grinder and ground away the hard spots (slag) and then it machined ok.
    .
    trouble is a drill deep in a hole often cannot drill through a hard spot. it will instead go side ways and around hard spot if it can. thats why deep drilled holes are not straight.
    .
    i have seen metal not mixed well. that is not slag but just slightly different alloy mix in front of drill bit. again drill will drift to side thats softer. thats why deep holes are often not straight. on some finish milled parts you can see color difference in the metal. white iron is very hard and grey iron is softer. the gray is from carbon that come out of solution when metal cooled forming graphite specks
    Attached Thumbnails Attached Thumbnails hard-spots.jpg   hardspots2.jpg  

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    Castings and hardened parts are probably class of its own difficulty. But even if you have annealed 4130 cromoly the drills are going to drift somewhat.
    Gun barrels are apparently made of double-tempered steel with extremely low amount of inclusions. Not all are straight but from what I have heard the best match grades are pretty darn close.

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    Quote Originally Posted by MattiJ View Post
    Castings and hardened parts are probably class of its own difficulty. But even if you have annealed 4130 cromoly the drills are going to drift somewhat.
    Gun barrels are apparently made of double-tempered steel with extremely low amount of inclusions. Not all are straight but from what I have heard the best match grades are pretty darn close.
    .
    .
    steel is a mix of elements and is more like concrete. if you look with a microscope you see it is not all the same. sometimes they use a acid to get the different parts to change color more so more easily seen.
    .
    with steel iron oxidizes on exposure to air. the oxidized iron often is in the process of having oxygen removed by other deoxidizer elements like silica, aluminum, manganese, carbon. you get silicon dioxide, aluminum oxide, etc which is what grinding wheels are made of. carbon amount changes can effect steel properties. strength hardness etc
    .
    just saying steel is not thoroughly the same. you got often sand size pieces in the steel. you can get sections bigger than a pea that has different hardness. drill bit going through side by side sections of different hardness will slowly drift to the softer metal. often thicker sections that cool slower are softer and the skin near surface can be harder. for example cold rolled steel can have a hard skin on the surface but thats from cold working which increases hardness and strength

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    To reply to the OP..
    I typically get 0.1 - 0.2 mm location error all-manually vs "true" in start / DP.
    And much less via jigs/spotters/locaters etc.

    On a lathe, both ends, careful, a 6 mm / 100 mm deep or 16:1 or 8:1 hole, done in 2 ops 5 mm + 6 mm drill, new industrial drills.
    Drill rod == Very Good Stuff.
    The hole may/will look perfect, both ends are perfectly centered, the point where the 2 drills met is invisible.
    Similar to gun-barrel visual tests, it is very obvious if there is any distortion in the hole.

    Using 4-jaw chuck of 12", high accuracy clocking, spot drills, about 1 hour to make one piece.
    SO..
    It is pretty accurate overall. Imo.

    I did not bore the start holes.. that would improve the straightness.
    In theory.
    I did not ball gage or make a go/no gage for the hole.

    1. No reaming,
    2. no double reaming,
    This would actually make a hole a bit straighter. Imo.
    3. no roller burnishing for size/finish/surface hardness.

    My feel/opinion is that the hole had == 0.01 - 0.02 mm straightness overall.
    Based on visuals against a light, just like old-time gunbarrel checks.
    My feel is that the hole had 0.02-0.03 mm size accuracy overall.

    It looked *perfect*.
    I was really surprised how well it went.
    It was a test - I really needed the hole, and had time, and just tried to do it as well as possible.
    Don´t remember why/what for.

    But..
    I recently made lots of 16 mm D, 100 mm deep holes, for fixtures.
    Had to use the DP.
    All the holes were very banana.

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    I did some 4" long 1/16" holes (65D) in 6Al4V a year ago straight to about 0.005".

    It can be done. Starting the hole straight, lots of pecking and properly clearing the chips were the keys for me.

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    Quote Originally Posted by MattiJ View Post
    This problem seems to pop up here quite often:

    Is there any rule of thumb or guide what to expect from like 10xD drilled hole straightness and centerline drift(exit location)
    Plain vanilla HSS drill?
    Carbide twist drill?
    Gun drilling?
    workpiece rotating or drill rotating?

    I only have some rough ballpark idea, ie 1/4" and 30" long hole is definitely gun drilling category if you want the exit on the opposite side...
    Here are our worse case experiences and guidelines to answer your question directly
    Gundrilling with Work piece turning.....less than .0005" per inch of travel (depth)
    Gundrilling with Only Drill turning ( carbide )..less than .0015" per inch depth
    Conventional Twist Drill operation ... Less than .015" per inch
    The above data was achieved drilling well over 10 to 20 thousand holes from 1/2" to 1 1/8" and upto 48 inches in depth

    I must caution the viewer that our work and refinements showed that the single most important factor to improve accuracy ( beside sharp accurate tools !) is the starting hole.
    DO NOT use a center-drill. It may make you feel good that everybody does it, but it is wrong. The best thing to use is a ball endmill that is the same size as the twist drill.
    Use it to be sure of a solid wall support of the drill ( 2 Diameters mostly) and in the case of a twist drill, you want a depth that means 180 degree of drill twist depth. This means that you have 360 degree of flute support before proceeding. ball endmills are rigid and maintain accuracy as well as support. Regular endmills do not, and tend to produce oversise holes.

    Almost all our work was done on 1040, 4140 and 4340 forgings

    Rich

    retired manufacturing engineer

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    FWIW, I was just doing some holes in 303 on the lathe, #54 HSS drill, 10 mm deep, thru. Most were very good, but the worst case at the far end was 45 microns. Agree with the above, the start is the most important part. I probably would have done better with a good carbide screw machine drill, rather than the pos garden variety jobber I used.

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    Default Inaccuracy in Oversize

    Quote Originally Posted by MattiJ View Post
    This problem seems to pop up here quite often:

    Is there any rule of thumb or guide what to expect from like 10xD drilled hole straightness and centerline drift(exit location)
    Plain vanilla HSS drill?
    Carbide twist drill?
    Gun drilling?
    workpiece rotating or drill rotating?

    I only have some rough ballpark idea, ie 1/4" and 30" long hole is definitely gun drilling category if you want the exit on the opposite side...
    You might want to look at 'Inaccuracy in Oversize' in your Machinery's Handbook. And I'd definitely spot drill first using a combination drill and csk...


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