Longish small holes in gunmetal

I need to drill eight 5/64" holes up to 1.5" long in a gunmetal casting (part of a model kit I was bought for Xmas !). The holes are steam
passages so dimension is not critical, but there is not much scope for wandering off line, and a broken drill down the hole would be v bad news.
I was thinking of setting it up in a lathe with the drill chuck in the mandrel and gently hand feeding the casting onto it using the leadscrew (can't really rotate the casting) . All I have to hand is regular (new) HSS bits, and I was going to go for 5/64ths straight off. I have no idea what feedrate or rpm would be best, and I woudn't have any 'feel' - except for listening and watching. I could use my 7/8" pillar drill instead, which might give a little more feel.
So before I bodge the whole thing I thought I would ask the experts.
Thanks Steve
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Cheshire Steve wrote:

2mm diameteer, 38mm deep. Is your drill bit that long? I would make a D-bit out of spring steel and do it that way. Gunmetal is soft enough to make this a quite easy task. Start with a pilot hole and then proceed with the D-bit, peck-drilling all the way through.
Nick
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Nick Mueller wrote:

Thanks for all the advice guys, job done. I cranked the 40 year old Meddings up to 4000rpm and with plenty of lube and gentle pecking it all went well. Managed to peck beyond the drill flutes OK to get the required depth (couldn't find a source for a long series drill nearby). Main learning point was to use the drill guard to prevent the drill leaving a horizontal stripe of grease across my chest - the high speed meant it could throw it 6 feet !
Have just been reading about D-bits in a really old lathe turning book, and see a new thread opened on the topic. Will comment there, but I can see D-bits in the near future where I need a small precision hole - and I'll need one as part of this job.
Thanks, Steve
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wrote:

Steve     I'm not sure if I've understood you correctly, which is probably why is wiser heads than mine have yet to respond. Are you going to hold the casting in your hand? If so you couldn't have devised a better recipe for breaking a 5/64 drill one and a half inches deep - to say nothing of safety considerations or whether you stay on line!. Is there *no* way you can fix the casting at the appropriate angle beneath the business end of your pillar drill? Can you not contrive some sort of jig....even something out of (shudder) MDF with a couple of clamps. A cheap universal vice? Arc Eurotrade (http://www.arceurotrade.co.uk /)do a reasonable one for about 45.
    Perhaps I've misunderstood. In any event, standard HSS bit are fine for gunmetal, you don't *need* lubrication, of course. The ideal speed would be somewhere in the region of 3000rpm but it isn't critical. You will doubtless be drilling less critical holes in your casting, use this as practice and you'll soon get the feel for the feed rate vs the rotational speed you have available.
HTH
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Chris Edwards wrote:

<snip>
FWIW, I have always found a problem with gunmetal is that the deeper you drill, the warmer it gets, it expands then catches the drill. In my experience is is always far worse with gun metal than any other material.
Is that other peoples experience? Or am I just showing up my bodging skills!
Cheers
Peter
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Oh, goodness, it never occurred to me that Steve might be contemplating holding the casting by hand. Don't even think about it, unless you fancy being called "one-hand Steve". OK, probably not with a 2mm drill, but expect a certainty of broken drills unless the workpiece is held with complete rigidity in a vice.
David
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Chris Edwards wrote:

Er .. no. I was going to clamp the casting in a vice attached to the vertical slide on my lathe, or in a vice on the XY table on my stand drill.
Steve
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wrote:

    Ok, now I understand. My advice is generally the same as that which others are offering. The key to success, assuming a rigid setup, is likely to lie in getting as much 'feel' as you can. with frequent pecking to clear the swarf and minimize heat build up. Personally, I'd go with the pillar drill. Do you know anybody local who's built George Thomas' Universal Pillar tool, with sensitive drilling attachment? See Hemingway www.hemingwaykits.com/ ....pity you're not a bit further south.....!
Keep us posted. --
Chris Edwards (in deepest Dorset) "....there *must* be an easier way!"
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Well, there are others here far more expert than I, especially on gunmetal (which I use only rarely) but I have drilled a fair few small holes, so FWIW....
First, you will probably have noticed by now that a 5/64" (~2mm) drill is typically only fluted to about 7/8" (22mm), so you may need to look for a long series drill to finish the holes.
Second, the recommended drilling speed for 2mm drills in GM is about 4800 rpm. It is highly unlikely your lathe goes up to that speed, but you need to use as high a speed as you can get, below 4800.
Third, GM is notoriously sticky, grabby stuff to drill, so you will need to go very carefully. The "feel" you get from shoving the casting towards the rotating drill in the headstock is likely to be very poor indeed, and this is about the worst way of avoiding broken drills. If you can swing the casting and make/borrow a sensitive tailstock feed, or make use of a sensitive drilling machine, you have a better chance.
Fourth, 37mm is pretty deep for a 2mm hole. In my experience, when drilling deep holes it is vital to get a good start to the hole. I would start the hole with a smallest-sized centre drill (centre pip only) then go in with a stub drill to its maximum flute depth (stub drills are about half the length of normal jobber's drills, thus about 4 times stiffer and correspondingly less likely to wander off course). For steel, I would use a 1.8mm or so stub drill, but I'm not sure whether GM likes being drilled out with a 0.2mm larger drill - perhaps someone more familiar with GM will comment.
Fifth, are you at all familiar with spark erosion machines? You may well need to be - removing broken drills from GM is not easy by any other method.
David
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I should have mentioned that it is also important to withdraw the drill very frequently to remove chips. Clogged flutes are a major contributor to drills wandering off course (and breaking). Do it about twice as often as you would think, don't forget that the swarf takes up a lot more room than the solid metal you removed.
G H Thomas, in one of his superlative books, has some very good notes on deep drilling IIRC.
David
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David Littlewood wrote:

David,
Thanks for the advice. I am of course always on the lookout for an excuse to buy more toys :-), but was hoping to get away without in this instance. A mini-drill stand might be an idea though.
Sorry to have been confusing - by 'hand feeding using the leadscrew' I meant mounting it in a vice on the vertical slide, precisely lining it up, then engaging the leadscrew and turning the leadscrew by hand to gently feed the work onto the drill (centre drill first) - not holding the casting by hand. Likewise on the pillar drill, I have a vice and an X-Y table, so its not going to wander around - and fingers are not going to be in the machinery !
If I have to swing the casting in the lathe, then I will have to mount the vice on the faceplate, and I don't fancy twirling that lot around at high speed. It will also deny me the precision X-Y motion to line the drill up.
I assumed if I was pulling the drill out frequently to clear swarf, then maybe I could drill deeper than the extent of the flutes. Forever going in and out suggests to me the pillar drill would be best. It will also do 4000rpm, though I am not sure why the speed is important if you are not in a hurry. Aren't the quoted speeds based on the needs of production, and so are upper speeds consistent with life and damage to the tooling ? Won't slowing the speed help keep the GM temperature down and stop this grabbing you refer too? The pillar drill will give me a vertical hole so I can pour lube down it every time I clear swarf.
So it looks like the pillar drill.
Steve
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Cheshire Steve wrote:

If you go this route use the rack and pinion if your lathe has one instead of the leadscrew to advance the cut. That hole needs to be Peck drilled not continuously cut. Say drill 1 x dia fully retract and repeat till you are to depth.
You kind of mentioned this in the text i snipped below, but this pecking needs to be done as quickly as possible, hence the reason I suggest the rack rather than leadscrew.
Wayne...
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Yes, that's the way I read your original post - but when Chris raised the point I wondered if I had misunderstood you.

Make yourself a sprung-loaded pointer: take a nice straight piece of ground steel, bore a clean hole in one end for a spring plunger (from an appropriately smaller piece of ground steel). One end has a point, the other has a centre hole. Place in lathe with a dead centre (a live centre may have too much lateral movement) in the centre hole and the point in the marked hole location (having previously centred it by eye and nipped up finger tight). Rotate by hand only, of course, and use a dial gauge with an elephant's foot to centre it, then tighten fully Can be done as accurately as you want, assuming your lathe is true of course.

I've tried this, and it takes an *amazingly* short time for the flutes to clog and the drill to stick. You could try it, but take the very smallest of pecks. How much will it cost you to replace the casting versus the cost of a long series drill though?

The high speed means the drill cutting edges are not trying to take too much per revolution. If you are forced to use a slow speed you have to be really sensitive not to push too hard, which is more difficult to "feel" with tiny drills. 2mm is just about big enough to feel with a normal lever feed if you are careful, but using a feedscrew gives you no feel at all.
Best of luck!
David
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