The Art and Science of drilling the perfect hole...Need help from the folder makers

In 1941 the Battelle Memorial Institute published a book, "Tht Prevention of Fatigue of Metals" for the US Naval Ait Command. In this book they describe the stress raisers emmanating from round holes in metal.

Your discussion intrigues me, have any of you noticed methods of drilling holes that weakened the metal to a more or less degree than others?
 
Ed Fowler said:
In 1941 the Battelle Memorial Institute published a book, "Tht Prevention of Fatigue of Metals" for the US Naval Ait Command. In this book they describe the stress raisers emmanating from round holes in metal.

Your discussion intrigues me, have any of you noticed methods of drilling holes that weakened the metal to a more or less degree than others?
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That's an interesting question.
I've always thought of a hole as a feature that would be somewhat less likely to cause problems compared with other shapes--obviously sharp or abrupt ends or inside edges are likely to be more problematic even if you did somehow impart a little less stress making that shape compared with a hole.

I would expect that punching a hole causes some of the greatest trauma to the surrounding material. I have an iron worker that is great for punching holes of practically any shape in steel up to 1/2 inch. It is great for general-fabrication jobs and other courser work. Upon close inspection, you can see the classic signs of this trauma around the edges even when using a sharp punch/die.
I know laser cutting can impart strange characteristics known as heat affected zones, the seriousness of which varies with the material being cut.
Even waterjet is know to impart stresses.
I would GUESS that drilling/milling a hole with sharp tools, the right speed/feed and lubrication would introduce the least stress of the most common methods people actually use to make holes.

.... But like you, I'm sure there's more to this topic than meets the eye.
 
One other thing that will make it difficult to hold a tolerance of less than .0003" is the runout tolerance of the bearings in your drill or mill. These machines could have ABEC 3, 5. 7 or 9 tolerance bearings in them. In the bore sizes for these types of machines, ball bearings with ABEC 3 tolerance the runout tolerance is .0006", ABEC 5 is .0002", ABEC 7 is .0001" and ABEC 9 is .00005". As you can see, ABEC 3 is not a close enough tolerance to hold the type of tolerances that we like to hold. Unfortunately, many drills and mills are manufactured with ABEC 3 bearings.

If the machine has tapered bearings in it, Class 3 bearings have a runout tolerance of .0003", Class 0 runout tolerance is .00015", Class 00 is .00008", and Class 000 is .00004". Most drills will have ball bearings, but some of the larger ones do have tapered bearings. Those will usually have Class 3 bearings with a tolerance of .0003". So it is going to be a borderline machine for holding tolerances that we would like to hold on folders. Class 0 is common on mills, but some of them do have Class 3.

The difficulty is that the runout is also controlled by the runout of the shaft that the bearings are mounted on. You can minimize the runout somewhat by having the high points of runout on the bearing mounted 180 degrees from the high point of runout on the shaft. All of the higher precision bearings have the high point of runout marked on them. You can find the high point of runout on the shaft with a dial indicator.

If you drill or mill is not holding the runout that you desire, you can rebuild your tool with bearing with a higher runout tolerance. The draw back is cost. Any step up in runout tolerance is a 5 to 10 times increase in bearing cost. For instance, if a Class 3 bearing costs $10, a Class 0 bearing will cost up to $100.

Whatever you do, don't put ABEC 1 or Class 2 or Class 4 bearings in your machine tools. You'll save a ton of $$$$$$ on bearings, and ruin every piece of steel you drill with it. The tolerances on those bearings are for wheels, gear boxes, and other applications that don't require very tight runout tolerances.

Can you tell that I'm a Service Engineering Representative for a Bearing Manufacturer? I assist customers in rebuilding machine tools on a nearly daily basis, or teaching mechanics how to rebuild them.
 
I whole heartedly agree with JCaswell about honing after HT. The hole will change size during HT. If you hone before HT, then the hole will not be the correct size after HT. Honing after HT is the acceptable practice in industry.
 
scottickes said:
I whole heartedly agree with JCaswell about honing after HT. The hole will change size during HT. If you hone before HT, then the hole will not be the correct size after HT. Honing after HT is the Standard practice in industry.
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Final reaming and lapping/honing should be done after HT. You need a good machine to do this, and the part should be held freehand. Plan on breaking a few reamers doing this. It's a PITA, but, you can get a really nice finish in your hole (practically mirror finish) this way with a carbide reamer.

As to stress risers, they are virtually eliminatedby drilling at the proper feed/speed with a sharp bit, and then by chamfering the hole by .010/.015 before HT using a 90 degree C'sink that produces a chatter free cut.
 
The concept of stress raisers and round holes is very tought provoking. New - drill vs old dull drill? 2 flute mill drill vs 4 flute? polishing or reaming after or before heat treat? Bark or decarb is known to propigate stress raisers through stress if not removed.
If we all keep our eyes open we may learn more.
I just learned a lot about bearings, thanks!
 
I ream all of my folder pivot holes in the hardened blade after heat treating and final surface grinding. I use a Gühring carbide reamer and the blades are firmly held in inch thich steel fixture plate. The plate can move on the table but its weight keeps it from spinning. This allows you the "feel" the reaming as RJ mentions. I use cutting oil. The hole finish is mirror-like. I use a H7 tolerance reamer.

Then I carefully hand lap the sintered bronze bushing to fit into the hole in the blade I just made. My pivots are pre-made and have been turned, heat treated and lapped to fit the bushings.

This works every time.

I strongly suggest NOT trying to hold hardened blades by hand across a machine vice for reaming after heat treat. When they helicopter with the $100 carbide reamer, it's not pretty.
 
Ed,
I've learned a whole lot more from you over the years! From visiting your shop a few years back, to reading your articles (over and over). I'm grateful for your easy going manner and willingness to assist me when I've had questions. I've just forged my first blades from 52100 and will be using your methods for heat treating them.

By the way, if you or anyone else ever gets a large spherical bearing (the kind of bearing with the "barrel shaped" rollers, these are the ones that are fatter in the middle than on the ends), it is most likely bearing grade 52100. This is both the races and the rollers.

If you have any doubt, just email me the bearing part number and I'll be able to tell you if it's made of 52100. There are a few places that have case carburized spherical roller bearings (for instance, some crusher machines).
 
scottickes said:
One other thing that will make it difficult to hold a tolerance of less than .0003" is the runout tolerance of the bearings in your drill or mill.
Click to expand...

True. I have a cheap HF drill press, and a HF mini-mill. Using the same cutters and technique I can NOT make a decent hole with the drill press (about 9 thousandths runout on a chucked shank). The mini-mill has adjustable bearing preload and runout on a chucked shank is about ~0.0004" (four ten-thousanths). The mini-mill makes really nice holes, but it took some tuning - the chuck that came with it was junk, but with a good precision chuck and the bearings snugged up it really shines (in a home shop on-a-budget sense).

One of the things that also helps is using reamers and laps that are on full-length shanks. If we each held onto opposite ends of a bar and one of us moved his end, how much of that movement would the guy at the other end feel? It depends on the length of the bar. The longer the bar the less movement is detected at the other end. A long reamer masks some of the wobble in your drilling machine (however small it may be). That's a Don-ism I learned when someone else was saying to cut down the reamer shanks.

-Ben
 
Next time you check out the drill presses in your local stores, run the shaft down and see how the play is. Was stunned at how sloppy many of the higher end drill presses were!!!
 
Robert Hankins said:
Next time you check out the drill presses in your local stores, run the shaft down and see how the play is. Was stunned at how sloppy many of the higher end drill presses were!!!
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I went a step further and took my dial indicator and stand with me last year when I went drill press shopping at the local stores. What an eye-opener. I came home empty handed. I couldn't find any decent presses locally. I'm convinced the 'consumer grade' box-store stuff is just always going to be geared toward the guy who's going to use it to build a bird house, not us odd-ball types trying to do precision metal machining at home.

-Ben
 
Did anyone mention making sure that the drill bit is correctly sharpened? Making sure the cutting edges are equal is extremely important.
 
Dang...this is a GOOD thread....

I learned a BUNCH!

But noobody mentioned the chant...

While rocking back and forth on the balls of your feet...

"C'mon Baby...C'mon Baby...":p

Sorry for the humor.:D ...hole drilling is tough for me...

And it seems like such a basis detail...and kicks my butt moe often than not...:confused:

It often seems as much like voodoo as does HT'ing....:eek:

You guys have helped me see some of what I been doing wrong...:thumbup:

Thanks so much,

Shane
 
I also appreciate all the input. Some makes my head spin. Some makes real sense.
I know quality equipment is important, but I gotta believe that outstanding skill and knowledge can trump lesser quality equipment......at least alot of it, because skill and knowledge can utilize less than the extreme best epuipment to produce extremely outstanding knives.....
Remember one of the finest steel carvers in the world (I know... opinion) works with files and sanding paper at his (normally) kitchen table in eastern Europe......
One of who I consider one of the extremely finest engraving knifemakers in the world "push engraves" his knives and does a tremendous amout of hand work on his folders.....while listinening to to "classical music of the Baroque era"... and "overlooking a ravine filled with birdlife"..... what a wonderful life for a fellow who appears to be about 30 years younger than his chronological age......
But if he didn't have a drill press or a mill, I'd bet he would still create wondrful ,wonderful knives......
And ther are many others who are so skilled and knowledgeable that I'm not sure that the quality of equipment is of a consequense to the final product...excuse...creation......just my opinion.....all $0.02 worth of it.......
 
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