Drilling a nice hole

In addition to my welding class I'm taking a machine shop class. Our
project involves a little piston air motor.
To make the cylinder for this we're instructed to drill & ream a
blind hole. I finished and reamed the hole, it's straight and clean,
except that it appears a chip wedged itself between the reamer and the
wall and scored the inside of the cylinder, making my piece scrap.
Is that just a danger of making a hole this way, or is there
something I can do to prevent such a problem? I did run the reamer a
bit faster than I was supposed to because the mill I was on had a broken
low range. Could that lead to this sort of thing?
Reply to
B.B.
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I suspect that you didn't use any lubricant when reeaming the hole.
-- Why isn't there an Ozone Hole at the NORTH Pole?
Reply to
Bob May
As another poster said, using a lubricant is important -- but it still won't prevent the odd errant chip from wedging, if it moves around enough.
One way to help prevent this is to ream upside-down, so the chips fall out as you work. This isn't very practical on most work, and only good for dry reaming, like on cast iron.
Another way is to fill the flutes of the reamer with a sticky wax (like toilet bowl ring wax) to catch and hold chips as they leave the cutting edge. This works nicely, but if you do it, you must clean and re-wax frequently to prevent loading the reamer.
Some old guys swear by wrapping the reamer in waxed paper. But I've never been able to get the thing started in a hole without its shedding its wrapper.
LLoyd
Reply to
Lloyd E. Sponenburgh
Reamers, at best, are not a great tool for such things. As you progress, you'll learn about boring. That's the preferred method of establishing a hole that you want to be straight, round, and on location. Reamers are notorious for cutting multi-sided holes and creating bell mouth in the bargain.
When reaming, make sure you lubricate well, and take out less than .015". Honing the cutting edge often pays dividends, but only if you know and understand how a reamer cuts. You can render one useless very easily if you don't know cutting theory.
The scoring you experienced is more likely from some chip welding on the reamer. If you look closely at the reamer, where the chamfer is on the end, you're likely to find one flute that has a chip firmly attached right at the point of contact, the cutting surface. By taking lighter cuts and good lubrication, plus clearing the reamer adequately, you can usually avoid that condition. It also helps to not be machining mild steel, which is well known for tearing.
Good luck!
Harold
Reply to
Harold and Susan Vordos
How much material did you remove with the reamer? IOW how much smaller than the nominal diameter of the reamer was the hole?
Jim
Reply to
jim rozen
Responding to this and all the other questions I've seen in this thread so far: I drilled undersize by 1/32", as our teacher told us is generally correct for any hole over 1/2", I used lubricant, (though it's anyone's guess what was in that bottle) and the material is aluminum. The cutting tools are all in one cabinet that sees quite a lot of abuse, so I wouldn't be too surprised if I picked up a tool with a bad edge or a chip welded on already, though I did try to inspect anything I got out of there and I am fairly certain my reamer was in good shape when I used it. Oh well, it was my first machined part ever, so no surprise I hosed it. I still prefer blacksmithing as I can just hammer the damned thing back the other way when this kind of thing happens. (:
Reply to
B.B.
B.B.
Hmmm, did he now? The Dormer Twist Drill and Reamer handbook goes into considerable detail about reamer stock removal. You need to remove some stock to prevent the reamer rubbing and blunting rather than cutting but the less stock you can get away with the better. That's why the stock removal table differentiates between reaming holes that have just been rough drilled with a 2 flute drill and those pre-finished by boring or core drilling. In other words part of the stock removal recommendation is to make sure the hole gets cleaned up all round by the reamer even if it's a bit oval or on the piss and the other part is what is needed for the reamer itself to work properly.
For reamers in the range 3/8" to 3/4" it recommends 1/64th (16 thou) for drilled holes and 10 thou for bored holes. Half to a third of your teacher's advice. Aluminium is a bugger for sticking to the tool if you take too big a cut, run too fast or don't have plenty of coolant so in conjunction with you reaming at a high speed it isn't surprising you chewed the hole up with a 1/32" ream. Blind holes are even worse than normal because there's nowhere for the swarf to go.
Personally I ream out even less than the Dormer book suggests and I don't find my own reamers getting blunt or cutting out of tolerance. When I make bronze valve guides on the lathe I pre drill at 7.9mm before reaming at 8mm so that's no more than 4 thou stock removal. Hundreds of guides over the years and I'm still on the same reamer, which was a used one when I got it, and it's still reaming at cock on 8mm. I've tried bigger cuts but you stand a good chance of the flutes getting packed with swarf and chewing the bore up.
In short, the less stock you remove with a reamer the better IMO. Also the slower the speed the better within limits. In rigid high speed production setups it is often suggested to run at half the drilling speed but I always find a much slower speed does the job best for the home machinist. Maybe 80 rpm for a 1/2" reamer and 150 for an 8mm one. Much less chance of chatter or taper developing and less heat build up in the job. -- Dave Baker - Puma Race Engines
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Reply to
Dave Baker
[...]
Well, then. Mystery solved! To get down to specifics, I dug up my notes. I needed to ream a hole .625", or 5/8. So, following the instructor's instructions I wound up with a 19/32 drill, which the project cabinet just so happened to have a plethora of in various stages of devastation. IIRC, I wound up running the reamer around 700-800 RPM, where the proper speed (according to the teacher) was closer to 600. I'm guesstimating the "good speed" would have been more around a tenth of that. Attempt #2 (which I think I'll be doing next Monday) will get a much gentler treatment. What would you say about feed rate? When he demonstrated he pushed the reamer through a 2 1/4" deep hole in about two seconds' time. I assume a much slower RPM would mean an equally slower feed?
While I'm here, we had to lay out the part prior to machining it. What, exactly, is the point of that? Is it really just so you can double-check yourself and make sure you don't do anything stupid, or am I missing something obvious? We didn't have to center-punch the part before drilling, we simply used a center-drill first, so I don't see much benefit to this. Then again, maybe we're supposed to do this just for the sake of it because later class (which I'm not taking) will do it "for real" and they're just giving up practice.
Reply to
B.B.
snip---
Because reamers are multi-toothed, they accept fast feed with ease, but it has to be within reason. What you need to do is some practice reaming of various materials and learn what works under your circumstances. I'd have fed the reamer slower if I had to use one, but I would have bored instead. When reaming as much material as you did (1/32"), there's usually no room for the large volume of chips to accumulate, and that alone can be troublesome. There's no way in hell you should have been reaming so much material in such a small hole. My advice would have been to use the 17/32" drill to establish the rough hole, then to follow up with a 35/64" drill (or bore the hole), then ream for size, assuming you had to ream, not bore. You'd have been moving far less metal, thus loading the flutes less. It's also not a bad idea to stop mid bore and clear the chips when you're reaming deep holes. The chips stack up at the cutting end and eventually resist flowing in the flutes otherwise.
In my opinion, that's a good thing. It not only teaches you proper layout for when you really must have it, it shows any mistakes you may be about to make. Learning to use a scale with each move will eventually take the place of the layout for finding mistakes, assuming you learn good habits and always have one near.
We didn't have to center-punch the part
If he's teaching to work without a center punch, I'm all for this guy. It's more than time for that relic of a way to locate holes to be abandoned. Regardless of what most guys insist on doing, center popping holes that you want located properly is a poor way to work. You generally introduce more error with the punch than you'd otherwise get, and it also prevents the use of a wiggler with decent results if you want to pick up the cross points of a layout. When you want to drill holes on location and don't have the luxury of a mill with calibrated dials or a DRO, you can use your layout and a wiggler to locate the holes with outstanding results. Pick up a punch and a good deal of that is lost. Using this method requires that your part be firmly clamped on location, and that can be done even on a drill press. You don't have to use a mill, although it's much nicer if you have one.
Then again, maybe we're supposed to do this just
That's my take on it. Try to learn all you can------without a doubt, you'll use all of it over a period of time.
Good luck with the classes!
Harold
Reply to
Harold and Susan Vordos
Holy crap Batman!
One thing you should bear in mind and which will stand you in good stead for your future machining is this. The majority of the recommendations on speeds and feeds for machining different metals have evolved from the needs of high speed production to be as cost effective as possible.
That means the fastest way to remove metal consistent with not killing expensive tooling too quick and maintaining accuracy. It's all about trade offs. Let's say a carbide tip costs £5 and lasts for 100 parts at 500 rpm, 70 parts at 800 rpm and 50 parts at 1000 rpm. Factor in labour rates, wear on the machine at high rpm, extra electricity at higher rpm and you can calculate the most cost effective speed and feed to do each job at. Generally that means going as fast as possible even if it burns tips out a bit more than going slowly.
For hobby machining forget the lot. The best speed and feed is the one that works for you and you find that out by trial and error. Most materials have a wide range of speeds at which they'll cut just fine with the right tooling. Cast iron needs to be turned fairly slowly because it's abrasive and aluminium much faster or it tears but with a nice sharp tip you can get a good finish on anything at fractions of the speed that a book will tell you to turn at.
I always machine things slowly because my time is my own but carbide tips and drill bits cost money and I rarely burn one out. My friend who's a CNC engineer will run his lathe at 5 times the rpm and use 3 tips on the job because taking an extra couple of hours costs him a lot more in wasted earnings than a few new tips do.
Learn to machine things slowly and accurately and you can always speed things up bit by bit as you gain experience.
It's a feel thing. Just push the reamer as fast as it wants to go. However you don't want to leave a reamer turning in a hole any longer than you have to or it'll rub and cut oversize. Aluminium cuts really easily so you won't need to be in there long but if I were your teacher I'd be starting you off on through holes rather than blind ones so the swarf can fall through.
If your only choice of tools is drills in 1/64 increments then get as close to final size as you can. Ideally though you want to bore to within the last 10 thou before you ream but if you have no boring tool then manage without. Try the drills at 400 to 500 rpm and ream at 100 to 150 and see how it goes. It won't matter how fast or slow you ream though if you generate so much swarf it clogs the flutes. You'll still chew the job up. You might need to peck a few mm at a time and then pull back and clear them unless you're only reaming the last few thou out of the job. That's a bad way to use a reamer though and chances are you'll end up oversized at the top if you keep taking it in and out of the job. Reaming should always be a one shot deal and then back out of the job as fast as possible.
To teach you to get your reamed hole in the right place as well as being the right size. There's no point in getting the perfect hole but 2mm out of position.
Is it really just so you can
Good. I've never used a centre punch in my life other than for starting a hand held drill in roughly the right place to fix a shelf up. On a machine you should be laying out with a DRO or the crank dials and starting holes with a centre drill or milling cutter. A centre punch has no place in precision machining. -- Dave Baker - Puma Race Engines
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Reply to
Dave Baker
Sounds like you were taking a bit large of a bite with the first try, and maybe you were running the reamer a bit on the fast side. Harold's comments about keeping the feed rate up there (not too slow) are good.
One thing I learned from a toolmaker at work is, you can tweak the final diameter by adjusting the speed. The faster you run, with a fast feed rate, the smaller the final hole is.
As far as centerpunching goes, he and I disagree to some point. Done accurately and inspected carefully, a centerpunch will give final hole locations between one and five thou which is pretty good for rough work.
If you are not doing 'rough work' and want the hole to be inside a thou location, then drill with almost anything, and *bore* to within five or ten thou of the final size, then ream.
You won't get an accurate location that good (+/- 0.001 or better) unless you bore the hole to start.
Jim
Reply to
jim rozen

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