Self-Drilling Screw Used With Standard Nut

Consider also the corrosion possibilities between aluminium and mild steel were these metals to be simultaneously in contact with the same moisture bead.

Thanks. Forgot to mention that the mild steel part will be hot-dip galvanized.

Think I'm missing something here.

What's the subtle difference between drilling a clearance hole and using a standard machine screw, and using a self-tapping or self-drilling screw?

Self-drilling screws (including self-taping threads)

Normal ISO screw

The thread is different.

There are also some with smaller pitch thread (the one on the right), but the thread profile is still much sharper than a standard ISO thread, which has no "sharpness" in the thread.

Thread has to be sharper; it is after all self tapping.

I realize that - what I don't get is why it's OK to make a hole with a screw, but not for a screw.

See my latest comment. It is of course feasible to drill holes but this takes more time and time costs about 60 Euros per hour. I am trying to avoid drilling.

Outsource this. 60€/h for simple drilling is ridiculous.

Better yet, punch it.

I dont't think there is such a thing as a self-drilling screw which isn't also self-tapping. If the drilling part had a big enough OD to make a clearance hole for the threaded part, you couldn't fit a nut.

Good point!

But, if you use a self-threading screw in two sheets, it won't clamp them together. You must have at least one clearance hole, and then you may as well have two. And, I certainly worry about the sheet thickness for thread forming.

My recent experience driving about a thousand self drilling/tapping screws in my barn roof suggests that three sheets are clamped together just fine. When the top sheet hits the head, it strips. If your application needs as much as 10k fasteners, it might pay to have a special fastener designed for you. Try Fastenal Co.

The self drill screws I used for the attic floor fit a 10-32 nut just fine.

You've got two identical sheets. How do they know which one should strip? If it's the bottom one, you're SOL.

If I understand you correctly you want to take two sheets of aluminium with a sheet of steel in between. The sheet of steel has a hole in it but the aluminium sheets don't. You want to use a self-tapping screw to both create the holes in the aluminium and create a secure connection (all without drilling a pilot or clearance hole).

Hmmmm.

I don't recommend this with 3mm thick aluminium, especially without a pilot hole, regardless of physical stresses. Standard self-drilling screws are likely going to time-consuming and deform and make a mess of the surface (especially if the joint is not solidly supported when they are inserted).

I would strongly suggest looking into alternative bonding methods such as industrial double-sided tapes or adhesives, both of which are commonly used for bonding these types of materials together (even aircraft bodies).

What exactly is the application? Can you describe it in more detail please.

Since we don't know what you're up to (you won't tell us), let's assume that you're supplying some kind of "home-assembly kit" (for want of a better name). Purchaser buys the ally components from some third party, and the steel bit from you. They now have to assemble the kit, using only hand tools (screwdrivers etc.).

Why not supply a self-drilling/tapping screw to be used as a 'drill bit' to form a clearance hole (doesn't matter much whether it's tapped or not) into which a (supplied-by-you) machine screw could be inserted, and fitted with appropriate washer(s) and nut?

Here is some more detail.

I am installing Photovoltaic panels on a frame. The frame is nothing more than a horizontal square beam (120x120) with vertical legs to the ground foundation every a few meters, I have no choice over this.

I also have no choice over the regulations in the country of installation which require significant safety margins, over and above what we would consider "adequate" strength. The operation is to be repeated many hundreds of times as this is a small PV park.

I am looking at different ways to create some kind of "adaptor" between the panels and the beam. I have already developed some designs which will do the job but which require changes to the existing frame design (addition of welded-on lugs for e.g.), pre-drilling of the PV panels and significant installation work, so I am trying to come up with something simpler which will not require design changes to the frame and no off-line work on the panels.

In this example, I am using a "blade" of steel 3mm thick. Here is a quick schematic

The PV panel aluminium frame is blue, surrounding the PV cells, which are in orange. The square beam is light gray. The "blade" is dark gray.

The blade has a square hole in the middle to slide over the beam (plus a PE retainer to prevent chafing but that's out of the scope of this discussion). There will be one blade sandwitched in-between two panels' aluminium frames. The key question is to find a quick method to join the panel frames to the blade.

In this variant of the concept I have added two holes on the blade. The Al frame has no holes in it, so I am looking at self-taping screws, in order to avoid adding a drilling operation on site. However the country regulations will probably not accept a joint with self-forming thread in Al and will require to add a steel nut on the other side for strength.

There will be hundreds of these blades and joints so speed and minimum fuss on the site is critical.

I hope this gives enough information. Any alternative way to attach the panels to this blade would also be welcome. I have no problem to change the blade or design special fastening parts, but modifying the panels or beam should be avoided.

Thanks.

Just curious but are the Blades positioned before the vertical legs are fixed into the ground? If not then it looks from your sketch as though it will be difficult to slide them into place.

Instead of conventional nuts, you could have some small square (or hex, if funds allow) steel plates made with a pilot hole in the middle, and use them in the same way as ordinary nuts but allow the screws to form the threads.

Drew

Yes will have to be entered before the legs are installed. This is OK because the frame is not yet in place.

I like the idea of the pilot-hole steel plate on the other side. The screw will do what it is designed to do (cut thread) but the joint will be much stronger compared to making the thread into the aluminium. The square plate will work best as it will jam inside the aluminium channel of the PV frame and prevent rotation. Thanks ad016!

Also thanks energygod for suggesting more screws closely packed.

I guess I will detail-design for both and choose on the basis of cost and installation time.

Dedalus

Glad to help.

Have you considered pop-riveting? I don't know what your strength requirement is but they are very fast.

Drew

good day Dedalus, could you not drill and rivet?

If your going to have folk using a hand-tool to make several hundred(?) holes through 6mm of ally, I suspect you're going to get some nasty cases of RSI, tendonitis or whatever. It's also going to take quite a long time.

I'd seriously suggest a rechargeable pistol drill (if the frame is deep enough - may have to drill a bit oversize from both sides if you have to go in at an angle), then use an ISO thread screw with a Keps nut:

An alternative-

FIRST- Use self-TAPPING screws rather than self-drilling. They have a sharp, pointed thread end rather than the tap end.

SECOND- Use two different sizes- Start with a #8 x 3/8" (9 mm) stainless undercut screw- the ones that use countersunk holes. The head will be about 3 mm long, so the actual insertion depth will only be about 6 mm.

Then- Use a #6 x 1/2" (12 mm) soft steel oval head screw with a standard flat back. The original hole will clear the second screw to avoid any binding between plies and the second screw will fully penetrate and lock on to the rear aluminum plate. The cleared first hole will allow a reasonable level of clamping force to be produced.

The first screw can be locked to a Phillips driver with epoxy. Use a rapid change-out driver tool with a magnetic bit so you can quickly switch to a standard Phillips driver for the second screw and the screw will be held by the magnetic driver until set.

Depending on how tight you need to make this assembly, you can use a screw every 12 cm or so, or spaced as wide as every 24-25 cm. Installation should be fairly easy and quick with a reasonable appearance.

If you're using a screw to make a clearance hole in one sheet, may as well go through both, then fit a machine screw & nut (as per my #13).

GA

Dedalus is looking for a self drilling screw which will mate with a standard nut. He would be better looking for a nut which mates with a self drilling screw.

flat nuts

Are we sure the problem has been identified?

Remember you will need to isolate the aluminium sheet from the mild steel sheet to prevent corrosion problems arising - thin rubber sheet being the most suitable.

In regard to self-tapping screws, ANSI Self-tapping and metallic drive screws come under B18.6.4-1981, and come in a variety of forms ranging from Type A, AB, B. BP and C when turned into a hole of proper size forming a thread of true size; or Type D, F, G, T, BF and BT when turned into a hole of proper size forming a thread by way of a cutting action.

Screws come in Round Head, Flat Countersunk Head, Oval Countersunk Head, Undercut Flat and Oval Countersunk Head; Flat and Oval Countersunk Head, Pan Head, Hexagon Head, Hexagon washer Head and Truss Head.

Sizes vary depending on designation but generally encompass 0.06" to 0.5" dia, and finishes include Nickel plated, corrosion resistant steel or zinc plated steel.

Hope this is of initial help to you.

Apexmaster

I have to agree with Jack of all trades. have a good look at 3M's VHB tape. I have used it before and it is truly impressive. Have a look at the link below.

3M VHB Tapes video

Not only will the tape isolate the sheets from intimate contact eliminating the galvanic corrosion problem, there is no drilling, screwing or other modification required. Call your 3M technical rep. In my experience with them they have always been very helpful in recommending the correct grade of tape required depending on the material to be bonded , the surface finish, end use environment etc.

1. Tape is bad. After a few months of 110 deg F+ on solar area, it fails, creeps moves etc. Temps sometimes hit 140 deg F. 2. Cone point and Mill an edge slot at the tip of any decent steel screw and it will go through any aluminum. Use a battery drill (at least) on high speed. If over tightened, it will strip out the meager threads in the aluminum plates and the strip chips will assist in binding in the steel plate. It will then have clamped the stack together and will be ready for a standard nut. Any small machine shop can make a set up for the screws and make 20 or 20,000. MAN, you guys really make a simple problem difficult. Been using this for years.

Actually tape is not bad. VHB tapes have shown through testing that short term temp tolerance is 300 deg F and typical long term is depending on the grade between 160-300 deg F. They have also demonstrated 100% bond strength in the long term testing after 2-5 years on outdoor weathering decks.

Just a thought, and it depends on the configuration of the channels that hold the PV pannels. But maybe you could do this without tapping holes? I have a very rudimentary sketch here of what I have in mind. You would extend the steel blade (dark grey) beyond the channel (blue and white) and use two metal "ears" (in light grey) to pinch the channels to the blade with a bolt (bolt centerline in red). You will likely need something supporting the channels from underneath too (maybe an L-angle?) but I haven't drawn that in. All the holes could be predrilled and then just bolted up on site.

If you're worried about the ears rotating out of position...just use two bolts. I have seen this kind of clip used on other projects, and book cases etc.

Sorry for the rudimentary sketch I only have the microsoft paint program available to me at the moment.

Actually, after posting and looking at the picture I realized it may be beneficial to do a bolted bar across the top and the bottom. I have revised the sketch to show what I mean.

It's described as a frame around the panel - which suggests it would be joined to horizontal members top and bottom, rather than being an open-ended channel.

If that's the case then here are two other ways of getting around it. The one would require a bit of metal bending. The second is a hybrid of the bent metal and the original ear clip idea.

There are hudreds of ways you could play around with this. I'm just putting them out there as a little inspiration for ways to skin the cat without tapping the channel.

Not knocking your efforts - just clarifying (I hope). Keep thinkin' .

I have used many self drilling and tapping screws for many different purposes in custom aluminium work. For your purpose the small diameter threads are not good enough to be a fastener to take stress that will be available. In this application you are stuck with using Flange Head Bolts and, Locknuts or Locktite Blue thread "locker" with regular nuts. The self/drilling bolts (with nuts?) are more expensive and will not be something that will be a good expireince especially if you need to replace a broken panel. It looks like you need to pre-drill everything as space to use a drill is real small. Second guessing myself, I would use the Loctite for speed, security, ease of installation and cost.

In one of my previous jobs the fitters made extensive use of self drilling Tek Screws for interior & exterior cladding on buildings. This application does not seem too dissimilar, I would have thought that these fasteners would be up to the job. Obviously the OP has to do the calculations & allow for corrosion, wind load etc.

Try #290 [green] loctite which is for assembled fasteners & consequently faster, easier & cleaner to use. wicks right into the threads. assemblies can be locked down after all the bolts are locked down

The OP wants to use a self tapping screw, which drills a root diameter hole just before the threads penetrate. Yet he claims that he can't drill a full o.d. hole and insert a real bolt and nut. This does not compute. A better picture might help, but probably not by much.

Isn't he saying that he can have a hole but did not want the cost of drilling it? Using a self drill keeps it to 1 operation.

Sorry for taking a couple of days to log in, I was on a business trip.

The main objectives are:

1. Minimize on-site operations

2. Minimize modifications to the ground structure (the beams). Avoid completely any welding on-site.

3. Avoid completely off-site modifications on standard bough-in parts, (for e.g., drilling holes in the PV modules prior to assembly on site). In general try to avoid any changes to them, if at all possible.

I have of course complete freedom to design additional parts.

So far many of the proposed ideas come close to meeting most of these requirements.:

a) A self-drilling screw with pilot-holed steel plate which turns into a nut, or pre-formed nut plate. - only one operation

b) Drilling through-holes on-site and then riveting. - Two operations but the second is very fast and reliable. (One question here: Often rivets fail to form a good joint when there is a small gap between the parts to be jointed. Does anyone know of a rivet type that is strong and provides an element of "clamping force" to ensure any small gap is pulled close?)

c) Also the ideas by kaedence made me think. The blade I am designing could be designed with a kind of "nest" shape at the bottom end, inside which the module slots and then I only need to screw or rivet the top end connection. This is even faster and also provides automatic alignment for the modules.