Drilling accuracy question

Hi all,
my question/problem is;
I need to drill 7/8" holes in homebrew plexiglass and aluminum bearing blocks (to fit R6Z bearings -a silly Van deGraff Generator project).
If I chuck my 7/8" bit (1/2" shank) in my el-cheapo drill press, the concentricity is something akin to a spirograph.
I have a drill chuck for my 3-in-one lathe/mill, but was thinking that the concentricity/alignment with a drill chuck on the mill still isn't as good as say, an end mill holder?
Mill is an MT3, & I only have a 3/8" end mill holder right now.
Would it be silly to buy a 1/2" end mill holder & grind out a bit of the 1/2" shank (of the 7/8" bit) - for the setscrew - to be more precise in the drilling?
Or should one buy the end mill holder and a 7/8" end mill? (unemployed bum budget considerations here)
Opinions?
Thanks in advance! :-)
B
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On Wed, 31 Jan 2007 05:39:25 GMT, Bart wrote:

How about a cheapo boring head? Then you can make any size you want accurately. Well, at least more accurately than using a 7/8 drill.
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Skuke
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How are you getting a center? Eyeballing with a center punch? Assume you have a good center dimple, (optical center punch) start with a small center drill (combined drill countersink) working up in drill sizes to under 7/8" then ream to finished size.
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Spot, Drill and Ream. Clay

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Getting a proper bearing fit without boring is going to be very difficult. You might consider buying the holes...
<https://fa.misumiusa.com/gwos/catalog/catalog_list_pc050.aspx?CATALOG_ID 01 &CATEGORY_ID7>
Ned Simmons
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If you've got a lathe(3-in-1), don't drill, chuck it up and bore it out! A drill(or endmill) alone will never give you the precision you need.
Stan
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Thanks for the input everyone! Guess I'll try the center punch, then drilling sequentially larger to undersize & then try an adjustable hand reamer to fit. Only ~$24.00 for a 27/32 - 15/16 reamer. Never played with one of those puppies before. A new tool! :-)
Thanks again!
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wrote:

That should prove to be an interesting excursion for you, then. Hand reamers are one of the worst tools ever invented, at least in my opinion. The chance of ending up with a hole that is straight, round and perpendicular is close to zero.
The best advice you were given was to use a boring head. Anything less will yield the luck of the draw.
If you insist on the hand reamer, the best possible setup you can make is to use your 3 in 1 machine with a center in a drill chuck to follow the hand reamer as you turn it with a wrench. That will help keep it vertical. Do all the work without moving the piece so you are properly aligned.
Harold
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to
Do
And, don't leave very much stock in the hole. Run your reamer through the hole, check your size, make a very small adjustment, then run it through again. If I had .001 to take out, I'd do it in about 4 passes if possible. Don't make the mistake of turning it slowly with your drill chuck. They call them hand reamers for a reason.
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Harold and Susan Vordos wrote:

I thought it was just me. Thanks for the comment.

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On Wed, 31 Jan 2007 19:18:42 GMT, "Harold and Susan Vordos"
Thanks Harold, Didn't realize they were that scary.
Perhaps I'll either ask the retired friend (that owns a huge milling machine) to do them for me or $ave up for a boring head.
Checked around here for milling heads ~$155.00 not including all the accoutremounts..and the fact that I've never touched one makes one more inclined to ask the friend.
Thanks again! :-)
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Oops, meant "Boring Heads"
.. found one on the 'net for ~$50.00, .and a little more in my price range
Thx!
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wrote:

I've always had such good luck with hand reamers that I never thought it was luck. What am I doing wrong?
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wrote:

I have a better question. How is it you're answering a post that doesn't appear on my computer?
Hand reamers open holes, I have no qualms about that. It's all a matter of what level of precision for which you're willing to settle.
Have you ever inspected a hole so opened? If you're concerned about roundness, bell mouth, and general irregularities, you may get the surprise of your life. There's no better way to show what a reamed hole looks like than to introduce it to a rigid hone, where it defines its every feature. Even chucking reamers make lousy holes-----although, admittedly, better than drills.
Harold
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I just don't know!

So, you're saying that I've never had unrealistic expectations. I'll buy that! My opinion is that precision should be bought from some supplier's shelf. If we have to do something super accurate, we sneak up on it. Better yet, we design so that precision is unnecessary.
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snip-
It is strange----but stranger things have happened. Didn't say I didn't post it-----I just don't understand why it's not showing on my end.

Exactly! With a boring head.

That is the best of all worlds, when possible.
Harold
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Could you drill your holes with a little slop in them, and install the bearings with some epoxy?
Dan
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The mill boring worked! :-) A preliminary test on a chunk of scrap plexiglass seems to have turned out OK! Didn't shell out the big bucks for a boring head either!
Remembered that I had a low-buck 1/2" toolpost boring set for the lathe. The boring bar is one of those allen screw type adjustable units that takes 1/8" HSS bits. (3/8" dia. bar)
Drilled a pilot hole, increased the drill sizes up to 13/16", then chucked up the boring bar & gradually bored & adjusted the bit out a little at a time to the final bearing size (~a thou under 7/8" actually).
Where there's a will..
Thanks again all! My apologies for dragging out the thread.
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