Questions: self drilling/ tapping screws for chassis work

[profdant139]

I'm getting ready to install my underbelly cover -- there will be aluminum C channel beams (to support the foam panels and the coroplast under-skin) attached to the underside of the trailer's frame members. Some of the steel frame is pretty beefy -- either I beams or box beams.

I know some folks have used beam clamps to attach the C channel to the chassis, but I am very risk averse and want it all to be screwed down in a bomb-proof way.

I thought about quarter inch bolts -- yes, that would work, but that would entail a lot of time drilling up into the frame while laying on my back with bits of metal falling in my face. Sounds like fun! Plus it involves drilling big holes into the frame, potentially weakening the beams.

So I am leaning toward self drilling self tapping screws. I am planning to drill a pilot hole -- I will experiment to see what size pilot will give me the best results.

I am leaning toward hex head screws -- I have a tendency to strip Phillips screws when working upside down, and hex heads give me a nice solid grip, so that I can snap the head off a few of the screws by over-tightening them. (I wish I were kidding -- I have a terrible problem of knowing when to stop tightening things. My wife calls me "The Great Twisto" in honor of the things I have destroyed.)

But here come the questions -- first, is there a particular brand of screws that you prefer?

Do you use an impact driver to install the screws, or a drill motor with a socket?

In thin metal (let's say a sixteenth), do you need to drill a pilot hole?

Thanks in advance for your advice. I am guessing that there are quite a few of us who are not experts in using this type of screw, so your comments will be appreciated!

[fj12ryder]

"Snugging the screws without stripping is an art."

Very true, which is why I suggested using a drill with a clutch. Snapping off more than a couple heads, pretty easy to do BTW, makes for a very frustrating job. Especially lying on your back with hot chips falling inside your sleeves and collar. 🙂 Some of those self-drilling screws are fairly brittle due to their design.

[HTElectrical]

Cummins12V98 wrote:
To drill and tap holes for machine screws or bolts when a self drilling screw does the exact same job in a few seconds is simply a WASTE of time.

OH and the job done in seconds works just as well.

x2

[Cummins12V98]

fj12ryder wrote:
Some of the screws will, by necessity, be self-drilling/tapping screws simply because you won't be able to get to the nut to tighten them. If you have to do some, might as well do them all. Just be sure to use the clutch on that drill/driver.

BEST drill for many tasks is an Impact. I have the US Made DeWalt 20V 1/2" drill and impact combo. Get used to feathering the trigger and WaLa. I have built thousands of feet of metal stud walls from light to heavy gauge using pointer and driller screws. Snugging the screws without stripping is an art.

[fj12ryder]

Some of the screws will, by necessity, be self-drilling/tapping screws simply because you won't be able to get to the nut to tighten them. If you have to do some, might as well do them all. Just be sure to use the clutch on that drill/driver.

[Cummins12V98]

To drill and tap holes for machine screws or bolts when a self drilling screw does the exact same job in a few seconds is simply a WASTE of time.

OH and the job done in seconds works just as well.

[twodownzero]

joshuajim wrote:

Let’s see... Metal buildings have structural steel beams, steel purlins and cladding.. Trailers have structural steel beams, crossmembers and cladding. I guess buildings don’t have wheels, but I guess as an engineer you probably should have been able to recognize that.

Then again, many of the engineers I know have a problem thinking outside the box.

Metal buildings do not move down the road, or move at all. Also, there's basically zero chance you'd ever take a metal building apart and reassemble it. The same isn't so for anything on your camper. Just tap the holes and make your life easier. Yes it takes more time, but a quality job often does.

[joshuajim]

Chum lee wrote:

joshuajim wrote:

Chum lee wrote:

twodownzero wrote:
Maybe I'm in the minority, but I would drill and tap every hole and install an appropriate hex head bolt in it, probably with some antiseize.

It's a minority of at least two. Sheet metal screws are not generally used to attach to mild steel structural members. That's a job for finer threaded self tapping machine screws which require a pilot hole. I like the idea of drilling and tapping if you ever plan to remove them for future service/repairs. (wet location corrosion)

Chum lee

Self drilling and tapping sheet metal screws are exactly how metal buildings are put together. I’ve installed thousands of them into high strength purlins with probably a half a dozen failures. Galvanized and painted.

Years later they come right out because they have their own sealing gaskets

I guess you've missed the point here. Were not discussing buildings, were talking about vehicles/trailers. Do you know the difference between wheels and no wheels? That is a rhetorical question that you needn't answer. You've already embarrassed yourself enough. It's a completely different industry/application. Surprise, . . . . . the fasteners are different too!

Chum lee

Let’s see... Metal buildings have structural steel beams, steel purlins and cladding.. Trailers have structural steel beams, crossmembers and cladding. I guess buildings don’t have wheels, but I guess as an engineer you probably should have been able to recognize that.

Then again, many of the engineers I know have a problem thinking outside the box.

[Cummins12V98]

NormC56 wrote:
If exposed to weather, I would also think about using stainless steel if they may ever need to come out again. Plain steel screws will rust to almost any material, even fiberglass. When I replaced taillights and running lights on my '90 Western Wilderness TC I needed in excess of 30 screws. I had already spent more than $150 on lights I wasn't going to scimp on fasteners. I just bought a box of them.

Completely different conditions. Zinc coated screws will do just fine under an RV.

[pinesman]

In my early years of working, I was blessed to work with several different skilled tradesmen. One thing that I learned is that there are usually multiple ways to accomplish most any project. While each of them usually thought their way was best, as an outsider I could see value in all of their methods. While some of the methods discussed here might be "better" than others, the bottom line is do it the way that is best for you with the talent and tools that you have. Twenty years from now, nobody will know the difference anyway.

[NormC56]

If exposed to weather, I would also think about using stainless steel if they may ever need to come out again. Plain steel screws will rust to almost any material, even fiberglass. When I replaced taillights and running lights on my '90 Western Wilderness TC I needed in excess of 30 screws. I had already spent more than $150 on lights I wasn't going to scimp on fasteners. I just bought a box of them.

[profdant139]

Chum Lee, I'm not embarrassed -- I'm ignorant, and cheerfully admit it! There are things I know (barbecue, for example) and things I don't (working with metal). That's why I am asking all of these questions. And I am getting a lot of interesting answers from folks who have a wide variety of technical backgrounds.

I would bet that there are lots of non-expert "lurkers" who read the postings on the Tech Forum, hoping to glean pearls of wisdom from the mouths of experts. I certainly do that, much more often than I post.

In answer to your question about the chassis, I am pretty sure that the metal of the steel beams is around an eighth of an inch thick, maybe a little more. But not a quarter of an inch.

Based on what I have read, I will have to experiment on some scrap metal to find my comfort zone.

[Ralph_Cramden]

SoundGuy wrote:

profdant139 wrote:
So I am leaning toward self drilling self tapping screws. I am planning to drill a pilot hole -- I will experiment to see what size pilot will give me the best results.

I am leaning toward hex head screws -- I have a tendency to strip Phillips screws when working upside down, and hex heads give me a nice solid grip, so that I can snap the head off a few of the screws by over-tightening them.

I'd drill a pilot hole and use Canadian invented #2 Robertson square head self tapping screws ... with a properly sized pilot hole a cordless drill will easily secure the screws tightly without fear of twisting the head off. Robertsons may be harder to source in the US but are FAR superior to those awful American invented Philips head screws. 😛

And there you have it....Canada's legacy to technology......the head of a screw LOL. 🙂

[ktmrfs]

profdant139 wrote:
Cummins, thanks for the tip -- I see that the underside of the head is serrated, which will serve the same function as a lockwasher, I think??

If your using it on anything soft like corplast, use a large fender washer. That's what the trailer mfg do, and what I've found works well. Otherwise there is not enough area to support the corplast.

If your attaching metal pieces then then you need to decide if you need a washer for surface area support or not.

[Chum_lee]

joshuajim wrote:

Chum lee wrote:

twodownzero wrote:
Maybe I'm in the minority, but I would drill and tap every hole and install an appropriate hex head bolt in it, probably with some antiseize.

It's a minority of at least two. Sheet metal screws are not generally used to attach to mild steel structural members. That's a job for finer threaded self tapping machine screws which require a pilot hole. I like the idea of drilling and tapping if you ever plan to remove them for future service/repairs. (wet location corrosion)

Chum lee

Self drilling and tapping sheet metal screws are exactly how metal buildings are put together. I’ve installed thousands of them into high strength purlins with probably a half a dozen failures. Galvanized and painted.

Years later they come right out because they have their own sealing gaskets

I guess you've missed the point here. Were not discussing buildings, were talking about vehicles/trailers. Do you know the difference between wheels and no wheels? That is a rhetorical question that you needn't answer. You've already embarrassed yourself enough. It's a completely different industry/application. Surprise, . . . . . the fasteners are different too!

Chum lee