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Boring close tolerance hole

clr8ter

Cast Iron
Joined
Mar 23, 2013
Location
South Central NH
I need to bore a hole in some brass. It has to accept another brass part that is .422 in dia. I've noticed that the closest drill bit is .4219", which I doubt would allow you to get the part in the hole without pressing it in, which I don't want to do, as it has to come back out again. Is my only solution to use a boring bar?
 
There are such things as decimal reamers available in nearly any diameter you'd want. Now exactly what diameter you get with a given reamer is sometimes only discovered through trial, so order three, from half a thou under to on size, to half a thou over.
 
Hmmm, OK. This is just a one-off part. So a boring bar, then? I can't really afford to go out and buy several tools to make one hole. Is there a size hole under which it is just not feasible to use a boring bar? Or should I say, what is the smallest BB made? I've seen one 1/4"....
 
Personally I have bore down to around .03 hole. You don't say how deep you need to go and it sounds like you are in need of a quick solution. First drill your hole around 3/8 diameter. Then drill with your .4219 drill . This is called drill reaming. Your hole will most likely be to small yet so find a piece of round stock 6-12 inches long around 1/4 diameter. Use a hack saw and saw a split down the length of the rod for about 1 inch or so. You can them mount the rod in a drill motor and put some aluminum oxide paper or wet/dry paper(sand paper) in the split and rotate inside of your hole. This will smooth the hole out and you can size to fit your part.
 
I may be better off just buying a small BB. I have to go about 5/8 - 3/4" deep. Your idea sounds good, but I don't have the .4219 drill bit.
 
..... but I don't have the .4219 drill bit......
Most drill bits drill oversize , so do not use .4219 bit for .422 hole .
As suggested above , drill 3/8" and bore to size .
To go 3/4" deep in brass , you can make a boring bar from a broken end mill or drill bit .
Could probably sharpen a cement nail and get one hole done .
 
0.4 inch?

If you ask a clockmaker about that he'll tell you they don't
make boring bars that *big*!

Yes. Get a small bokum boring bar and your job is done. You'll
use that tool again in the future, it's a reamer that makes an
infinite number of hole sizes.
 
Peter, what size would you expect a .421 to drill? A couple thou oversize would be just about good. Interesting thought on making a BB out of a drill bit or end mill. Not sure how to do that, though.
 
Interesting thought on making a BB out of a drill bit or end mill. Not sure how to do that, though.
Bench grinder.

Another issue I've run into with my limited machinery, while boring small holes (around .350" ish) I just can't get the surface speed needed for a good finish with carbide. Something to think about if you're thinking on buying a carbide bar.

I'd just take a chipped HSS endmill and make it work.
 
Peter, what size would you expect a .421 to drill? A couple thou oversize would be just about good. Interesting thought on making a BB out of a drill bit or end mill. Not sure how to do that, though.

All depends on the quality of it's ground.
A factory grind may be dead on (centered tip, even cutting edges) and cut the hole the same size as advertised or a hair larger. But more than likely it's been reground so it may be off a few thou, making a few thou oversized hole. A drill can not drill undersize, only oversize, and the amount of oversize depends on who sharpened it.

And BTW, boring bars come in much smaller sizes than 1/4". I have a few 1/16" bars and I imagine they come smaller. Probably for clock work.
 
Hmmm, OK. This is just a one-off part. So a boring bar, then? I can't really afford to go out and buy several tools to make one hole. Is there a size hole under which it is just not feasible to use a boring bar? Or should I say, what is the smallest BB made? I've seen one 1/4"....

I bore small holes all the time with small square HSS tool bits ground to suit and old broken HSS taps which are perfect also, after grinding. Some examples...Bob
 

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On a BB made from a drill bit, end mill, tap, square tool bit; you have to grind off the side of the bit to the depth you want to bore, right? And make sure it's relieved so the bit doesn't get in the way of the bore? Are there any rules of thumb to grinding a bit like this?
 
On a BB made from a drill bit, end mill, tap, square tool bit; you have to grind off the side of the bit to the depth you want to bore, right? And make sure it's relieved so the bit doesn't get in the way of the bore? Are there any rules of thumb to grinding a bit like this?

I grind mine wider at the tip and tapered in at the back for clearance. Also tapered from the top to the bottom so the bit only touches the metal in the right spot where its cutting. I have a drawer full of bits made from junk for cutting slots and grooves and boring. Here is a threading tool made from a sq toolbit...Bob
 

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Wow, that looks like a lot of grinding...

I bought a box of those pre ground at a yard sale for future use. I ground the 60 degree tip on it for the thread cutter as it was a grooving tool. Plus it was 1/2" sq way bigger than you would need. Just an example...Bob
 
A .4219 drill would cut over size from from a little to a lot depending on how far the point was off center.
With pre-drilling the hole close to that and then using the .4219 drill like a reamer it would cut very close to size.
Then a sand paper (emery) stick hone could take the last .001/.002 for feel fit.
For a one-up part that is how I would do it.

Oh just notice upnorth5ax posted the same in post "6... Must be a Michigan thing..Will the world ever catch up to Michigan?.., perhaps not.


Rod with a saw cut.
For paper stick honing one can use a length of paper, then use a little longer strip until the hole size is made with a little feel.
It is actually a high precision hone that way and with abrading the full circle will hold center very well like a Sunnen hone.
 








 
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