The Dang Holes Keep Shrinking!!!

C_Claycomb

C_Claycomb

Joined
Dec 11, 2000
Messages
1,128
I am not generally given to profanity in the workshop, but this time...oh yes :grumpy: For a third time, a hole which was a perfect sliding clearance fit pre-heat treat has become something between a line-to-line and an interference fit, post HT.
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The first two times that I had this happen it was with full tang stainless blades, one ATS34 and one 12C27, drilled for Corby bolts. Everything fits together until the blades come back from HT, at which point the bolts won't go. In both cases a carbide burr and rotary tool saved the day, but didn't do anything for the shape of the holes :rolleyes:.

This time its my first shot at making a folder, using O-1 for starters, and the 3/16 hole, all nicely drilled, reamed, and checked with the precision pivot in the annealed condition, has shrunk during hardening and the pivot that used to fit, won't.

It isn't that the hole is all blocked with black gunk from the quench either, or that there are burrs. There were both, but even after cleaning the hole and dealing with the burrs it is still undersized.

So, am I the only one that keeps having this happen? :confused:

I can only suppose that its to do with the expansion that occurs with martensite transformation, but I haven't read about it before, certainly not with respect to folder making. The only thing I can see to do now is go to and invest in a solid carbide reamer in 3/16 and run that though the hole. If this is the standard way of doing this, is there any reason to bother using HSS reamers before HT?

I would love to hear how other folk deal with this stuff. Do you just drill/ream oversize to start, or does everyone use carbide after HT? Are there alternatives to carbide?

Cheers :cool:

Chris
 
C_Claycomb said:
So, am I the only one that keeps having this happen? :confused:
Click to expand...

Nope. It's not your imagination :D You can usually find each alloy's coefficient of thermal expansion, and/or dimensional changes during hardening/tempering in its data sheet, and figure out how much to over-drill based on that. For instance, the Uddeholm O1 sheet says to generally allow for .25% expansion overall.

That's too much thinkings and 'rithmaticks for me. :o. I generally just drill to size and countersink before HT, then use solid carbide bits to clean out the expansion post-HT if needed.
 
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A few inexpensive laps open the holes right up after HT.
With the added benefit of making them real smooth too!

You gotta shoot for that fit post HT, not pre.
 
Thanks guys. I had been thinking that I couldn't see how a hole could be right if reamed, then heat treated, but I hadn't read anything to the contrary on here, so thought folk were getting away with it. HA! About 10 minutes after posting here, I stumbled on the right search string to find a thread by someone else here on BF who had had the exact same problem, and had been given the same answer. Duh! Oh well. :D

Now the challenge will be to find Barrel Laps in the UK :rolleyes:. So far it doesn't look like anyone uses or sells them. 3 hours of google and not a single hit outside the US. Maybe they call them something different here. I like the look of the www.moldshoptools.com laps, the prices look good too, but once the shipping is added on they aren't quite such a good deal.

Thanks for the answers :). I was feeling pretty good about the project up to this point...had the stock worked down to within 0.0015" of the final thickness, had the drill table and spindle all clocked, had the holes drilled perpendicular, and reamed, had the HT done without warpage, had got a nice jig made and working for grinding bevels and even had a carbide drill for doing the detent. I knew I was some way from a finished blade, but dang, I thought that hole thing was in the bag! ;)
 
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FWIW,
The tang holes for rivets and bolts should be at least 10% oversize when drilled. You want a bit of room in the tang fit-up, or there can be problems in final assembly and especially in riveting.
Pivot holes should be drilled at the same size as the pivot, and lapped after HT.
 
I am so glad for this thread. This would have driven me nutz! Specially on a folder.

Thanks everyone!
 
bladsmth said:
FWIW,
The tang holes for rivets and bolts should be at least 10% oversize when drilled. You want a bit of room in the tang fit-up, or there can be problems in final assembly and especially in riveting.
Pivot holes should be drilled at the same size as the pivot, and lapped after HT.
Click to expand...

WOW! Ten percent?!? That's tons of space! I had assumed that the aim, with bolts and pins at least (I don't rivet much) was to get as close a fit with the tang as possible so that any shearing load applied to the handle slabs is resisted by fairly close metal to metal contact between tang and retainer. I have been using metric drills between 0.05 and 0.1mm bigger than the retainer stock to drill tangs. Of course this can be a pain on assembly, but I thought that it was the fit to strive for.

Thanks for presenting an alternative method!
 
Size Change During Hardening

--------------------------------------------------------------------------------

Hardening

Temp.
Tempering

Temp.
HRC
Longitudinal

Size Change
%

°F °C °F °C

--------------------------------------------------------------------------------

1475 800 300 150 64 +.12
1475 800 400 205 62 +.14
1475 800 450 230 61 +.18

--------------------------------------------------------------------------------

This is from Crucible , O-1.
 
There is more chance of a metal to metal hard joint shearing than a joint cushioned by a thin ring of hardened epoxy.

The extra space also prevents multiple bolts from not lining up after HT.....as the whole tang may have moved is several planes. The excess around the bolt/rivet is filled with the epoxy. I often drill 3/8" holes for 1/4" Corby shafts. That sounds oversize by a lot, but only leaves .06" around the shaft.

If you try and rivet a bolster on with a snug fitting tang hole, the bolster will often lift up from the tang due to the rivet mushrooming at the tang. Thus the 10% extra space for the pin to expand.
 
bladsmth said:
There is more chance of a metal to metal hard joint shearing than a joint cushioned by a thin ring of hardened epoxy.
Click to expand...

Hmm, I never thought of that, thank you. While I always leave expansion room for anything that will be peened (yup, I had to learn the hard way), I've been making the holes for my corbies just big enough so the bolt slips through without forcing it. Time to rethink that!
 
Since this is over 30 years old, I guess I can post it.

When I was with NAVELEX, I was called to do a secret construction task. They put a bunch of us with multiple building and electronic talents in a locked warehouse in a non-descript location with three large mobile van trailers,and nearly every metal fabrication tool you could ask for. The vans more or less looked like a medium size cargo container on wheels, with a movable tongue.There were welders who could heli-arc and TIG anything, sheet metal fabricators, and several of us with the clearances and ability to come up with ways to install a lot of fancy equipment in a small space. Engineers in some other location had drawn up plans and had equipment racks fabricated. Other places made up the vans . We had to outfit the vans as self contained mobile command and communications units with crypto capabilities...and some other really neat features. They were air conditioned to maintain 50 degrees, to cool down the massive amount of equipment we packed into them. The entire units were made of titanium and aluminum alloys, with as little steel involved as possible. They were going to be air lifted into a very sandy place as part of a hostage rescue mission that I won't go into. We had a very short time to build them, and worked around the clock.They were built like tanks, with nearly every joint and seam welded together, and virtually no bolts or rivets. The equipment was bolted in with super strong fasteners. One van would be for testing, one for the drop, and one for backup if the drop failed. When we put the first one down the Belgian Block course at Aberdeen, the guy inside opened the door and jumped out while it was moving down the course. The equipment all was flying out of the racks it was securely bolted into. When I tore the thing apart, I discovered that half of the 1/4-28 titanium bolts had sheared off. I surmised that the entire unit was too rigid, and sheared the bolts due to direct transfer of all the shock. After we changed every fastener to run through a rubber sleeve and added rubber washers, with the same titanium hardware, the unit passed the test. The second unit was used, and survived delivery ..... but the mission was aborted because of a sand storm.

Fasteners need to be one of two things -
Either so massive that the object being fastened will break before the shear strength of the fastener is reached
or
Able to transfer the shock energy and/or dissipate it, before the shear strength is reached.

Anyone who has ridden on an antique motorcycle with no suspension knows how well shock can be transmitted.
 
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So the epoxy is there to save the bolts from shearing? I had looked at it the other way, that the bolts were there to protect the epoxy from shearing off. It never occured to me that anyone or anything could hit the handle hard enough to shear a pin or bolt by accident, but I did think it possible that a blow could pop the glue loose if the whole lot had got cold enough.

Thanks for the story, quite telling for when the weights and forces get bigger.
 
I think the "epoxy keeping the bolts from shearing off" thing is a tiny bit of an exaggeration (sp?). We're not talking about thousands of pounds of force, here. For that matter, cured epoxy is awfully brittle and I sure wouldn't use it to replace rubber bushings or washers if they were needed in the first place.

Plus, Jimmy Carter isn't in charge of this top secret operation so we can all breathe easy about the "hostages". :D

Apples, oranges, peaches and a few cherries on top... a real fruit salad going on here.

I re-thought my procedure, and I'm not seeing how a couple thou of epoxy would keep a Corby from shearing off if it were somehow subjected to massive amounts of stress. Nor do I recall any instance where Corby bolts just plain broke off.

Bolts hold. Epoxy seals. That's my story and I'm sticking to it until proven wrong.
 
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I agree with you that there is no need for worry, but if you are concerned about the holes not fitting and lining up, drilling them oversize is how to avoid it. The epoxy will fill the excess hole area. The handle scale will most likely break before a Corby shears.

The memory lane ramble was in response to the query about the difference between a snug and a padded fit.
 
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