Drilling a deep straight hole

I need to make a drill extension guide to finish a project. I want to use a
.090 end mill to manually peck drill a hole in angled face deep in the
cavity of a mold. I can get a shot at it with a long bit. I was thinking I
would make an 1/8 bit holder extensions six inches long to do the job. That
would probably be adequate in itself for this job, but I was thinking I get
get within a few thousandths and reduce risk of spin damage by running the
"extension" through a guide with one end machined to wrap around the
protrusion where the face I want to drill is located.
The thing is I don't have a lot of room to work. About .185 clearance on
one side of the extension from center. I could turn a piece of stock down
to .36 just fine, but then I would be limited in the main diameter of the
extension to about .25 or less.
I need about 6-7 inches of reach. I have some aircraft drills in
appropriate sizes, but my experience is that they tend to flex and wander
after more than an inch or two.
You know, I think I am just going to make the extension for now and get the
job done, but how would you tackle it the other way I described. Remember
there isn't much point if it doesn't start out big enough so one end can be
machined to set in place...

Reply to
Bob La Londe
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Can you attach a guide to the mold?
jsw
Reply to
Jim Wilkins
Well the idea was I would make a guide that sits around the protrusion snugly, and has a long hole through the center to run my bit extension through. I suppose I could make another precision surface and attach the guide "outside" of the mold, but the inside where it rests is a finished surface already except for needing to drill that one hole.
I'll draw up a quick example.
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Reply to
Bob La Londe
I have made bushing for odd size end mills. I figure the extension is just a really long bushing. The hard part is making a guide for the extension.

Reply to
Bob La Londe
Looks like you tilt the base of the mold and drill squarely into the angled face of the boss. I'd consider a drill guide in the end of flat bar stock with a recess, pins, whatever to locate it on the boss, and the other end of the bar solidly clamped to a stack of 1-2-3 blocks etc for the height and to restrain the lateral forces on the locating components. Then you could use the aircraft drill.
jsw
Reply to
Jim Wilkins
Sorry, Guess I wasn't clear. I have aircraft drills, but I do not have a .090" aircraft drill. I have a .090" ball end mill though. Hence making an extra long 1/8 end mill bushing (AKA) drill extension to hold the .090" ball end mill. I was thinking of using the aircraft drill to make a guide for the bushing. The problem is that I have never been able to drill more than an inch or two with a .250" aircraft drill without having flex and off center drilling. I would need to be able to drill 6 plus inches down the center of a piece of rod to make a guide for this.
The hole in the protrusion needs to be pretty close to .090". .125 (My closest aircraft drill) is not close to .090" for this application. .095" would be considered close.
I have found I can't really drill with a ball mill, but I can peck drill with one if I have to...
By appropriate size aircraft drills I meant smaller than the clearance numbers and longer than the necessary reach.
I guess its time to break out a piece of brass rod, make a bushing, and see...

Reply to
Bob La Londe
0.09375" is a standard fractional size, 3/32".
jsw
Reply to
Jim Wilkins
You said you have aircraft bits. How about making a few drill bushings then tack them together to create a long bushing that you could then feed the drill through. Sort of like a mortising attachment for wood? The drill would run inside the guide and the flex and wander would be contained.
Reply to
Steve W.
Is this similar to the issue of drilling gun barrels? Could any of those techniques help?
Reply to
David Lesher
Yes. I don't know how to drill gun barrels either. LOL.
Reply to
Bob La Londe
Gun barrels are drilled with single-lip bits that self-center by bearing against the work with the side that's opposite the cutting lip. In volume gun drilling, both the work and the bit rotate.
I can't figure how you're going to get straight holes in your application, but I don't see how gun drills would help. They're a technique unto themselves, and kind of tricky.
Reply to
Ed Huntress
Well, the idea was to make a guide for the drill busing to make the hole. The protrusion is parallel and concentric on 180 degrees to the path of the hole that needs to be drilled in it (already done by hand with just an extension). The deep straight drilling was to make the guide. When push came to shove my hand was accurate enough for this application, but I can see future applications where it might be necessary to get more precise angular alignment.

Reply to
Bob La Londe
I'm probably missing some of this, but when it comes to drilling long, straight holes, there are a few things to consider. The straightest holes are drilled with single-lip bits that are made for the job. They're balanced around the complete cutting circle in one important sense: the cutting edge is always presented to the work with the same radius and with the same angles. That, plus the back-side support, can produce straight holes.
But you can do Ok with other bits, such as crankshaft bits, used for drilling oil holes, which have length/diameter ratios exceeding 20:1. They're production tools used with bushings.
Holes wander because of some complex geometrical factors. I haven't studied the engineering on this for decades, so I won't venture to give an explanation. I just remember that it was complex and sometimes surprising.
Reply to
Ed Huntress
I worked for years is a shop that had gundrilling as a major part of our work. I made lots and lots of bushings for gundrills to get them started accurately. We looked at machines that rotated the work at the same time as the drill. Not because of higher production but because of the increased straightness possible with this technique. We determined that most of the work we were doing didn't require the extra straightness. We were already drilling holes that were quite straight. For example, .094 diameter holes 19 inches deep with less than .004 deviation. I don't remember the oil pressure used on the tiny drills but it was quite high. I did design and build some special rotary unions that had small but constant flow of oil past the seals in order to keep the seals working. The housings had to catch the oil to return it to the reservoir and keep it from spraying everywhere. The rotary unions worked great. Parker seal company helped in the design by supplying info about their seals and how they worked. Eric
Reply to
etpm
Hmm. I think we may have talked about this before, Eric. That's such an interesting specialty, and so dependent upon experience, that I'm sure you could tell us a lot about the practicalities of doing it.
Reply to
Ed Huntress
I doubt it. I didn't do much drilling. I mostly made tooling. I did research the things though and compared what I learned with practical experience. I would like to build my own gundrill with spinning work and drill to make some .22 barrels. But that would take more time than I have now. Eric
Reply to
etpm

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