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Craft Forum / Wood/Metal / Metalworking / February 2010American hex key, Indian hex socket screw
I'm in the process of mounting a DRO on the quill of my Millrite MVI, which involves replacing the connector block that goes between the quill and the bronze nut that's moved when the quill feedscrew is turned. To the new connector block is attached a 0.125" thick mild steel blade that replaces the indicator and to which the sensor of the DRO will attach. The connector is attached to the blade with two 10-32 hex socket flat head machine screws. There is some scraping and hand filing involved in getting the blade to be parallel to a machined reference surface. The blade moves up and down parallel to this surface. So, there is lots of assembling and disassembling of connector from blade, and the 10-32 screws are very tight, and let go with a crack. So far so good. The first time I tried to undo the screws after really tightening, one screw came out without difficulty, but the other one didn't, and didn't feel right. I figured that the hex socket was rounding out because the steel wasn't quite up to it. But the socket looked OK. Pressed on. The 3/32" hex wrench (key) turned 60 degrees and felt soft, but the screw finally let go with the usual crack. The hex key had twisted axially into a nice spiral, like a piece of decorative wrought iron square bar. Now, I got this US-made hex key set in the late 1950s or early 1960s from a local hardware store. I'm not completely sure, as things may have gotten mixed up over the years, but from appearance (these were zinc plated) I think they were made by Indestro Mfg Co. Never had one of these keys twits before. The hex socket machine screws are made in India, recently purchased from MSC. My 1970s Sears Craftsman hex key set has no such problem. And the Indians are learning how to make steel. Joe Gwinn 10/32 hex bolts should not be torqued so tight that even a soft steel hex wrench should twist. Many times, when a hex fastener is installed with a powered tool, excessive torque is applied. Hex fasteners need to be 'cracked' loose with a shock load on the hex driver. Applying hand twisting torque is almost guarenteed to strip the hex or twist the hex wrench JR Dweller in the cellar. >I'm in the process of mounting a DRO on the quill of my Millrite MVI, which >involves replacing the connector block that goes between the quill and the [quoted text clipped - 31 lines] > >Joe Gwinn -------------------------------------------------------------- Home Page: http://www.seanet.com/~jasonrnorth If you're not the lead dog, the view never changes Doubt yourself, and the real world will eat you alive The world doesn't revolve around you, it revolves around me No skeletons in the closet; just decomposing corpses -------------------------------------------------------------- Dependence is Vulnerability: -------------------------------------------------------------- "Open the Pod Bay Doors please, Hal" "I'm sorry, Dave, I'm afraid I can't do that.." > 10/32 hex bolts should not be torqued so tight that even a soft steel > hex wrench should twist. Many times, when a hex fastener is installed > with a powered tool, excessive torque is applied. Hex fasteners need > to be 'cracked' loose with a shock load on the hex driver. Applying > hand twisting torque is almost guarenteed to strip the hex or twist > the hex wrench. The hex socket screw seems OK, and has been done and undone many times so far. If I wear the 10-32 screws out, I'll just replace them. The Craftsman hex key is standing up nicely too. The weak link is the old hex key. I wonder if the plating had something to do with it, as they may have had to heat the keys to allow hydrogen from plating to diffuse away, to avoid hydrogen embrittlement. The Craftsman hex keys are black oxide finished, as are the hex socket screws. <http://en.wikipedia.org/wiki/Hydrogen_embrittlement> Joe Gwinn > JR > Dweller in the cellar. [quoted text clipped - 55 lines] > "Open the Pod Bay Doors please, Hal" > "I'm sorry, Dave, I'm afraid I can't do that.." > In article <e7pgo5tb3sst7hl972nuqleegs0qbum...@4ax.com>, > [quoted text clipped - 67 lines] > > > >Joe Gwinn The Unbrako line was SUPPOSED to do that rather than shearing off or rounding off, had a demo of that by the counter critter when I got my set in the late 60s. I've taken out some pretty tight capscrews with them, but have never had one so torqued up that it pretzeled the wrench. The alternative is to wedge the hard wrench into the socket, shear it off flush and have a nice time trying to either remove the stub or drill out the remains. You might want to look up the torque limits for that size capscrew. Stan <stans4 wrote... > I've taken out some pretty tight capscrews with >them, but have never had one so torqued up that it pretzeled the >wrench. The alternative is to wedge the hard wrench into the socket, >shear it off flush and have a nice time trying to either remove the >stub or drill out the remains. And is that stub hard or hard? G/f bought a 750 yamaha and when we went to change the steering races we discovered the remain of a bell-end key in the fork yoke bolt, drills wouldn't touch it, ended up with dremeloid and a diamond burr and an hour scooping it out, realised why the previous owner put up with the sloppy steering.... Dave H.  Signature(The engineer formerly known as Homeless) "Rules are for the obedience of fools, and the guidance of wise men" - Douglas Bader I don't think that zinc plating involves any high temperatures for electroplating. A prep acid bath may have been part of the process, as a cleaning step prior to plating. If surface embrittlement had been a factor, I think cracking, probably leading to a full break/snap would have taken place. Most quality hex keys/allen wrenches are already fairly hard, though I would think of the twisting failure as a feature for tighening fasteners, to let the user know the screw was as tight as it should be. I agree with JR, that screws this size shouldn't be torqued to the tighness of cracking when they let loose. I prefer to use medium-hold blue threadlocker for small fasteners.. easy enough to remove for disassembly when required. In high quality equipment (aerospace), the countersink angles of the screw head and the base material could create a taper lock, but that's probably not the case in this situation. Medium hold threadlocker would likely eliminate a taper lock if applied to the countersink area. All that may have happened was that the key had a soft spot, or that it was one of the Unbrako-type keys. I'd feel fortunate that the key twisted insead of snapping off, which can result in a very sharp projection which fingers may have come in conact with. Additionally fortunate that the screw didn't require drilling to remove it. SignatureWB ......... > In article <e7pgo5tb3sst7hl972nuqleegs0qbumuem@4ax.com>, > [quoted text clipped - 15 lines] > > Joe Gwinn > I don't think that zinc plating involves any high temperatures for > electroplating. A prep acid bath may have been part of the process, as a > cleaning step prior to plating. Plating itself (regardless of the metal being plated) generates hydrogen ions and drives them into the plated item, all at low temperature. One way to remove the hydrogen is to heat the plated item up, allowing enough hydrogen to diffuse away to prevent brittleness. This is done only for critical items. > If surface embrittlement had been a factor, I think cracking, probably > leading to a full break/snap would have taken place. True. Preventing this bad outcome is why one slightly anneals plated items that may be vulnerable to hydrogen embrittlement, or doesn't plate in the first place. > Most quality hex keys/allen wrenches are already fairly hard, though I would > think of the twisting failure as a feature for tighening fasteners, to let > the user know the screw was as tight as it should be. But the Craftsman hex key hasn't twisted yet. > I agree with JR, that screws this size shouldn't be torqued to the tighness > of cracking when they let loose. > I prefer to use medium-hold blue threadlocker for small fasteners.. easy > enough to remove for disassembly when required. The screw and threaded hole (in 1018 steel) don't seem unhappy. The intent is accurate fixing of one part with respect to the other. Great strength isn't required, just great stability. And, it would be a real nuisance if the screw backed out, as it will take some disassembly to get at these screws. I may well use locktite (or epoxy) for final assembly. I designed the assembly to allow for soft soldering, if needed to fix the parts together stably, but it does not appear that soldering will be needed. > In high quality equipment (aerospace), the countersink angles of the screw > head and the base material could create a taper lock, but that's probably > not the case in this situation. Medium hold threadlocker would likely > eliminate a taper lock if applied to the countersink area. From the look of the countersunk holes, some taperlocking may be happening. > All that may have happened was that the key had a soft spot, or that it was > one of the Unbrako-type keys. I buy the soft-spot theory. I think all vendors endeavor to make Unbrako-style keys, but with varying degrees of success. > I'd feel fortunate that the key twisted insead of snapping off, which can > result in a very sharp projection which fingers may have come in conact > with. > Additionally fortunate that the screw didn't require drilling to remove it. I don't think there was any danger that the key would snap off. It twisted at least one full turn axially quite nicely, without pretzeling. Nor did it break when I twisted it back to more or less straight. Drilling would be a nuisance, but not that hard because the whole assembly is easily removed and clamped in the mill vice. A small carbide drill bit would make short work of the screw. Joe Gwinn > > In article <e7pgo5tb3sst7hl972nuqleegs0qbumuem@4ax.com>, > > [quoted text clipped - 11 lines] > > > > Joe Gwinn |
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