Where to buy machine screw assortment?

I assume that you're tapping the holes first, right? If not, that's your problem.
Screws should thread into tapped holes very easily. If your tap is extremely worn, that might be the problem. Or you could be using the wrong size of tap. Or you might not be cleaning out the swarf from the hole after tapping.
But I doubt you could break even the cheapest screws by simply threading them into correctly tapped and cleaned holes.
Best wishes,
Chris

I need more strength than just enough to thread the machine screw through a correctly tapped and cleaned hole. If the screw is going to break so easily, it probably won't hold the pieces together either. That's my problem.

You shouldn't be breaking even the poorest screws just threading them into a tapped hole without tightening them. Something is seriously wrong here. Either that or you're trolling :-).
Try McMaster-Carr. They may have sets of screws.
Best wishes,
Chris

Like by ignoring the question and for some strange reason insisting that threading a machine screw into a properly tapped hole is the only valid application for a machine screw. Plastic screws into properly tapped holes. Threading screws into a hole is not the end objective. The objective is to keep the pieces together. I can tell that the bolts aren't strong enough, that's why I ask for better.
If they are better than the apparent crap I get at Lowe's.
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Try this site.
Bruce

I agree with Christopher here in that something is very wrong with what you are doing or the you are seriously failing to provide the whole picture. I would suggest though that you may want to look at thread forming fasteners which are often used for ductile materials such as Aluminium, drill the hole fit the fastener, but you need to work out it is suitable for your job which sounds like a problem at the moment .

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Drill and tap the "bottom" part correctly for a machine screw of your choice. Drill a hole in the "top" part slightly larger than the body of said screw. Place the screw through the top part and turn it into the threaded hole of the bottom part. The top part will now be held tight to the bottom part by the head of the screw.
Randy

Heya Bert.
I know that. Unfortunately it has nothing to do with the question or my current situation.
Maybe I should have just asked for better machine screws without any description, but some people prefer to know what's going on.
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Bingo.
This is a discussion forum, not just a free information service. Most of the members are very knowledgeable and appreciate polite give-and-take discussion.

When I bought my lifetime guaranteed screw drivers from Lee Valley, I asked if the guarantee still applied, even if the Phillips drivers were only used for removal and never for installation. Gerry :-)} London, Canada

Is there something wrong with asking for better machine screws in the UK?
Considering how badly the picture is being distorted, more is clearly not better for a troll.
I need better/stronger machine screws. I'm seriously not understanding why anyone would suggest that machine screw quality makes no difference, and that all that matters is threading the screw into a correctly tapped hole. How to use machine screws sounds like a completely different subject.
And that is something stronger. Besides a stronger machine screw (at least stronger than junk steel), maybe something different will work too. I might go with stainless steel. The fitting must be extremely tight. A correctly tapped hole will not work. I will look into thread forming fasteners, but I've heard that a machine screw into aluminum can do that. Unfortunately, as I keep trying to get the message across, the cheap machine screws I have give way under too little pressure. I know they aren't going to be strong enough for my application.
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I would try to buy bolts that are properly graded for anything important (above golding a picture frame on the wall).
i

I didn't say it was. You've misread what I wrote.
Best wishes,
Chris

That's another possibility, that he might be tapping the holes in both parts. If so the starts and ends of the threads may not be aligned, so that the two parts act like a locknut. If this is the problem, only tap the hole in the part furthest from the head of the screw. Drill a clearance hole in the other part.
Best wishes,
Chris

Okay. It would really help if you could post a picture of the two parts and the failed screw. Then it would be much less a case of guesswork.
If you don't have any web space, e-mail the picture to me and I'll host it for you: cdt22 AT cantabgold. DOT net
Best wishes,
Chris

So you want a little discussion before answering my question?
I usually prefer a concise question, from someone asking for help.
Yeah, I'm pissed at crap machine screws from Lowe's messing up my project, and I guess it shows. It's definitely not a beautiful day in the neighborhood here.
Whether it's understood or not, once again. I'm simply asking about where to buy stronger machine screws than the zinc plated type I get at Lowe's. And maybe whether stainless steel is the only way to significantly increase strength from those type of machine screws that break when you torque them.
Are those machine screws acceptable for anything? I'm surprised anyone in this metalworking group would appear to be defending them.
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Your "guesswork" would go out of orbit.
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If you want strong, buy HoloKrome screws. I believe J&L is in the UK.
Pete Keillor

OK, _I'll_ give it a try. I'll give you some facts and some comments:
Fact: Taking perfectly good machine screws and forcing them into untapped, undersized holes in aluminum will break them, regardless of grade or quality.
Comment: So, logically, if a machine screw breaks when you force it into an untapped, undersized hole in aluminum, it doesn't mean that it's a bad screw, it means that you aren't applying it correctly.
Fact: Jamming a fastener into a hole where it doesn't belong doesn't make a 'nice tight joint'. It makes a poorly made, undependable joint. It's kind of like tying a quadruple granny knot: it's hard to take apart when you want it apart, but when you _don't_ want it apart it'll fail.
Comment: So, logically, you shouldn't do that.
Fact: The strength of a bolted joint doesn't correlate well with the fastener torque unless you're using clean, properly dimensioned threads.
Comment: Uh -- you can figure this one out.
Fact: Loctite is your friend.
Comment: So, logically, if you want a properly made, permanently fastened screw joint, you should get decent screws and put them in with Loctite red or something stronger. If Loctite red isn't strong enough for you, see if there's anyone on this group that you haven't alienated and ask (nicely) for advise on the right stuff to use.
I have never had problems with the fasteners that I get from Home Depot. I treat them like grade 2 bolts, and everyone is happy. If I need something stronger I get it from McMaster or Small Parts, and everyone is happy.