Put a decent chamfer on the clearance bolt-holes. Some screws, in fact most screws, have a root radius, which is why you place a washer under the head.
Try a proper supplier, like Fastenal or somebody's "Nut & Bolt" Company locally, or any mill supply house. Ask for "Grade 8" in whatever style you need. Be prepared to spend more money than at Lowes, but depending on what and where and how many you buy, the best Holo-chrome's might not be a lot more bucks. Personally, I find that Allen-head bolts are less likely to break than cap screws. Not sure you can buy "machine screws" in Grade 8, but purchased from a good supplier they will be at least Grade 5.
Twice (at least) in previous replies you mentioned using stainless bolts rather than the zinc-plated machine screws used at present. You should note they S/S are NOT stronger that good steel bolts. In fact, may be quite a bit softer if they are not also Grade 8 or better. "Stainless" is a quality of the material, and not directly related to strength or holding/breaking capacity.
I have done many time what you are describing, only using self- drilling sheet metal screws and an electric drill to drive them. You and I were both over torquing the screws. Somewhere there is a table of allowable torque on the screw you are using. If you can find a toque indicating screwdriver and limit your indicated torque to the amount allowed for that screw, you will be home free.
The Lowe's screws are undoubtably made in China qand are usually junk. Try a company like Fastenall for good screws.
But -- by "halfway through" to you mean with the screw half-way into the aluminum? You are asking about machine screws, not self-taping screw, based on the subject line of your posting, so I have to ask: "Are you tapping the holes prior to trying to install the screws?" Machine screws are *not* designed to make their on threads.
If by "halfway through" you mean "when you have about half of the screws the project requires installed and tightened" then there is something else seriously wrong, and junk metal is quite likely.
:-)
What I would suggest is black oxide finished screws, unless you need to have light colored screws.
Go to someplace like MSC:
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and register there (you have to register before making your first purchase) and then they will send you a massive catalog ("the Big Book") which you can use to look up tools and supplies more easily than I find their web based presence to allow. Then you can phone in a purchase (remembering the account number you got when you registered) and typically they will deliver by the next day. (Hopefully you are not in Canada, because they insist on using UPS for delivery, and UPS insists on charging a customs brokerage fee on packages to Canada.)
Note that you will find the prices for a box of 100 screws better than the prices for 10 screws of the same size in Home Depot. I don't have a convenient Lowes to compare to for prices, but I would expect them to be similar to Home Depot.
Hey Tim. Are you spewing ignorance just so you can spam your website in your e-mail address and in your excessively long signature on USENET? Does spewing nonsense while claiming to be logical help get you into discussions about logic that (you think) helps promote your website?
"Halfway through, the machine screw twists apart just below the head."
leaves us uncertain whether you are saying that each screw breaks before the head bottoms (in which case there is some problem other than simple quality of the fastener) or that you start getting broken screws before you have half of the screws installed and tightened.
We are trying to figure out exactly what the conditions are under which the screws are breaking, and a lot of the initial answers were addressing the implied condition that the screws were breaking before they were fully into the workpiece -- that is, before the head was even clamping down the other part.
We make suggestions, and you attack. ISTR that you came in with a similar behavior a couple of months ago or so.
We're *trying* to help, and to do that, we need to understand the precise conditions under which the screws are breaking. You seem to have later suggested that the holes are properly drilled and tapped, but it is not totally clear that was what you said.
I took two interpretations and answered both of them with my initial response before reading more than your original post.
Huh? A correctly tapped hole is the starting point for using a machine screw -- either in the workpiece, or in a matching nut.
BTW -- it does *not* take Superman to wring off 6-32 screws. They are the weakest for their size of any common screw. The threads are too deep relative to the size.
A machine screw should *not* be asked to do that into aluminum, or any other material. There are screws specifically made for the task, but the standard zinc-plated screws are not.
So -- does this mean that you are not drilling and tapping the holes? Just drilling them? Do you assume that the term "tapping" means to mark the location with a center punch? I've seen people make that assumption.
Little pressure -- or before you even get the screw in far enough for the head to contact? This is the kind of detail which can make a difference in you getting useful answers.
Of course you can get better screws. I posted MSC as a possible source. Others have pointed to a couple of other sources. I even suggested the black oxide finished screws instead of the zinc plated ones. They tend to be much harder screws.
None are if you are trying to form the threads with the screws, unless you purchase specific thread forming screws.
Now to read the rest of this, and *try* to resist commenting again.
Any steel screw will have greater strength than the aluminum they are threaded into. If you are twisting the head off I'd agree with previous posters that there's a problem with the threads in the aluminum or that you are over toqueing the screw.
And I'm trying to tell "we" that it really doesn't matter.
Better to go with the stated condition, that the screws were breaking with too little torque. Since you don't know how much torque I apply, you'll just have to take my word for it.
And you're planning to tell your imaginary kill file friend on me.
I've said this several times already. I'm asking about bolt strength, not about methods. Talking about methods is misconstruing my original post. I mentioned the method because it happened to be how I determined that the bolts on using are crap. Doesn't matter if I am holding the small end of the bolt in a vice, the bolt breaks too easily. How else am I supposed to determine bolt strength? Why isn't that method okay? What other method would you suggest for me to get an idea of bolt strength? Screw it into a correctly tapped hole?
I've gotten plenty of useful answers about screw strength, IMO, probably at least enough already. Some people aren't inclined to give useful answers. Some people just don't know (for example, folks living in the UK when asked about "USA chain stores"). Some people just want to talk about fastening stuff, and that's okay with me.
You could be getting galling on the threads, where the aluminum gets "gummy" under pressure and stops being a good sliding surface for the steel. You might try lubricating the threads before assembly, using antiseize, or even going to a thread insert (like Helicoil or Keensert) in the aluminum. --Glenn Lyford
On Tue, 26 Feb 2008 05:32:50 GMT, with neither quill nor qualm, John Doe quickly quoth:
I was taught to use Grade 5 or better bolts in the automotive world. Good stuff.
Since I'm using more and more ACQ treated lumber, the need for non-corroding hardware has led me to stainless research. McMaster has half a dozen stainless alloys and their differences astounded me. From their comparison chart + delving into the individuals:
17-4 PH Stainless Steel, 140,000 psi Combines the high-strength of alloy with corrosion resistance of stainless steel.
18-8 Stainless Steel, 70,000 psi Offers excellent corrosion resistance; it may be mildly magnetic. Alloy 20 Stainless Steel, 80,000 psi Also known as Carpenter 20, it's nonmagnetic and especially resistant to stress corrosion.
300 Stainless Steel, 80,000 psi Meet more stringent specifications such as military specifications. All are passivated (a nitric acid treatment that creates a passive film to protect the stainless steel from oxidation and corrosion).
316 Stainless Steel, 80,000 psi Offers even better corrosion than 18-8 stainless steel. It contains molybdenum, which increases resistance to chlorides and sulfates. It may be mildly magnetic.
450 Stainless Steel, 170,000 psi Offers more corrosion resistance than alloy steel.
Unrated hardware, Class 2A, has a 60,000 psi strength. Grade 5 with a zinc plating is double the strength at 120,000 psi.
-- Such is the irresistible nature of truth that all it asks, and all it wants, is the liberty of appearing. -- Thomas Paine
Absolutely the higher grade screws will be stronger than the ungraded "mystery metal" screws at the local hardware store - but on small #6 and #8 screws you can easily surpass the breaking strength of even the highest grades with a hand screwdriver and a "Don't know my own strength!" wrist. Been there, Done that, Have the T-Shirt.
Too much torque is still too much torque. If you get too good of screws that will take being over-torqued without failure, you'll just start pulling the threads out of the aluminum fitment they are screwed into.
They do make torque screwdrivers that click when you reach the proper setting. If you can't feel when it's tight, you might want to invest in one.
And there are design limits for screws - if the assembly isn't strong enough a "super screw" isn't going to buy you much. You need to increase the screw size, use multiple screws, or redesign so that a bracket or tab takes the load, and it's not all on a screw in tension.
I don't know if you can find any commodity-grade stuff that is much better. You can get certified aircraft-grade fasteners from an approved source, but they will cost a lot more. Still have to watch out for counterfeit stuff there, too.
I buy stuff like this in 100-piece boxes. For electronic-style stuff (ie binding head phillips machine screws), I buy it from Digi-Key. Lately I have had to go to stainless for ROHS reasons, the zinc-plated stuff is non-compliant for some reason.
For other stuff like socket head cap screws and button head (Allen) screws I get them from MSC. I have had some with no threads, way off-size allen sockets, etc. But, I have had no problem with screws breaking off - from either source.
6-32 is the weakest standard screw thread, due to the ratio of minor diameter to major dia., so if that is the size you are using, it's an added complication. You might also want to check the quality of your taps, the way you use the taps, the size of the pilot hole, etc. The screws should go in effortlessly. If there is significant binding, then the thread is off in some manner, or the hole isn't clean. I use spiral-flute taps (not spiral POINT) as much as possible, they can be run in in one continuous motion, producing a chip similar to a long, curly drill chip. This makes it much easier to clean the hole after tapping. I rip a little piece of a paper towel off and twist it into a thin cone and poke it into the hole and twirl it to get the chips out. (Professional shops use compressed air, but my family would object to this late at night.)
The point here is that if the screw is binding in the thread, and requires more than a twist of the finger to put it in with NO load, it will bind a LOT WORSE under axial load. This is probably the MOST common failure mode of a screw or bolt. The thread binds up to the point that the shank fails in torsion, not from axial tension.
Without knowing more about what size bolt/screw, what kind of tension you are trying to achieve, what alloy and how much thread engagement there is, it is REAL hard to advise further. If you need 1000 Lbs of axial force out of a 6-32 screw, you need to go up a couple sizes. If a 1/2-13 is breaking off at only 1000 Lbs axial load, you have to be doing something wrong, or the bolts are made out of recycled beer cans.
Yeah, I think so, and apparently others have hinted at that when talking about lubricant. Thanks.
Much to my pleasant surprise, the primary fastener looks like it might do the job without any extra strength, so the hole can be wide enough for just pushing a strong bolt through and securing it on the other side with a nut. And that will hopefully function as a momentary/temporary hold in case things start to fall apart.
And that is something stronger. Besides a stronger machine screw (at
So you are attempting to drive a machine screw into an untapped hole? Please don't see this question as being confrontational; I'm just trying to figure out exactly what's going on.
There could be a number of reasons why you need a close fitting thread. If it's for location, try to rethink the design. Use dowel pins or a milled slot for location, and screws in clearance holes to hold the joint together. But if you must have precise, close fitting threads you can buy special taps for the purpose. Screw threads are assigned various classes of fit, which range from a close fit to a loose fit. You will need to buy special taps for a close fit. They're used in instrument manufacture. They'll cost more, but they are available.
If you want a close fit to prevent the screws from coming undone due to vibration, use spring washers or loctite instead.
Either way, driving machine screws into unthreaded holes will break them. Even if they're really good screws.
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