Machine screws through thin metal plates


I have been wondering about something ever since I got my electric drill and started drilling all over the place, thus far mostly in wood and masonry.

If I want to fasten together two thin metal plates (2mm thick galvanized steel), I need to drill a hole through both and then fit a machine screw or bolt through them.

My questions are:
1. What should the size of the hole be in relation to the screw size? I always drill into wood with a bit that is 0.5mm or so smaller than the screw I want to put in so that the screw can grip tightly. How does it work for metal?
2. Do the holes need thread for the screw to work properly or is a plain hole good enough?

I could experiment but I don't like to waste material or drill bits - I need a theoretical push in the right direction. And I don't have the bits that make thread. Is it perhaps worth investing in a set?


I am not an expert, but I have made things of metal before. I would think your metal screws could work although it will leave a sharp pointy end out of the back side. I think I would want at the least a bolt and nut with a lock washer so it doesn't come lose. Also I might recommend "pop" rivets if they would work for your application. They would leave a flush finish on both sides of your metal. I don't know what they might be called in your country, but here is a link to a video about using them.

Thanks for the ideas. I was just fixing a mop that had rusted and the part that holds the strands together had to be replaced. In the end I just bent the metal piece and it turned out to be secure enough. At first I tried with machine screws but found it very difficult to drill a hole that suited the screws that I had on hand.

Pop rivets would be an option, but I don't have a tool for that and I'm not sure it's worth the investment.

David Trammel's picture

Without knowing more, I'm going to guess your drill bit was walking around the surface of the metal and not going where you wanted? Here is an old trick if you are having trouble putting a hole into metal where you want it. Find a nail and use a hammer to put a small dent at the place you want to drill. Your drill bit will seat into the dent and will stay until the drill gets started.

That said here are some observations from an old hand at putting screws into things.

What you are describing is loosely called putting a "pilot hole" in. This is a fairly common thing to do, especially on metal or wood. For the size you want a drill bit that is the same size as the shank of the screw, that's the solid center part, not the threaded part. Its especially useful when joining two pieces together. If you don't then as the screw gets thru the first piece, the tip can push the two pieces apart. If you are joining two pieces of wood, like a couple of 2x4s then drill a pilot hole thru both pieces of wood, but only a little bit into the second board. Then go back and drill the hole in the first board, big enough the screw goes through by hand (as big as the threads). This way the screw will grab the second board and pull the two together firmly.

For metal, some of the problems can be solved by using a self tapping screw. This is a metal screw that has a small drill bit type tip on it. Its still useful to pilot hole the site though. I'm working with metal wall studding, which is thin 20 gauge (about 0.9 millimeters) thick. You can still use a sharp tipped screw if you pilot hole first though.

Another factor is whether its "fine or coarse" threaded. If you look at the screws at the hardware store bins, you'll see some with more threads per inch of shank. The fine thread ones are mostly for metal. The coarse ones for wood.

Piloting a hole is especially important if you are going to screw into wood. If you don't then the screw will press the wood material out from where you are installing it. This can split the wood or the screw can "spin out" and destroy the threads the screw is cutting. This happens a lot if you are using a drill motor to install the threads.

For wood, more than metal, there are the typical bolt type heads, or something called "countersunk" heads. These are the ones that go into the wood and end up flush. If you use these, its helpful to get a countersink. These are short triangle bits, which cut a small cone shaped depression on the surface of the wood. Countersinking is nice because it prevents most of the splintering on the surface that normally happens when you drive a screw into wood

For sheet metal a good option is to use what is called "lath screws". These are screws that are typically used to install the metal mesh for putting down on wall studs before you apply plaster. They have a wider and lower head. They come in either sharp point or self tapping. Here's a picture of the two types, sharp point and self tapping.

Now for using a bolt, you have two choices. You can create threads, which is called "tapping" a hole. That's because the tools to do it are called "taps". These are a type of threaded cutting rod that is specific for the size of bolt you are going to install. I won't go into the details in this comment, because you do need a bunch of tools to do this, and some time/material to practice. Its very easy to snap a tap off inside the hole if you haven't done it before.

Tapping a hole is typically done when the thickness of the material is big enough you can get 4-5+ treads into the hole. Though more threads are always better. Or if you don't want to go all the way thru. Most of the time for simple repairs you can put a bolt thru it and then slap a nut on the backside.

To do a repair that way, measure the diameter of the bolt you are going to install, then drill a hole slightly larger than that thru the two materials. You should also install washers under the head of the bolt, and under the nut, between them and the material. Washers are there to distribute the pressure of the bolt, and prevent the hole from splitting or deforming. For most things use a washer about twice the size of the diameter of the nut. For sheet metal there are bigger washers, called "fender" washers you can use. The wider size sandwiches the thin metal better.

If you don't want the nut to come off easily, you can put a small dab of finger nail polish on the tip of the bolt before screwing it on and tightening it. There are special "locknut" you can get. These have a nylon insert inside the nut, with treads. The bolt cuts the threads deeper when you install it, and then its hard for it to come undone. You can only use those locknuts once though. If you unscrew them you have to replace them.

There are lock washers, which are thin and about the size of the bolt head, and have a split in them. The split is slightly opened. You put these under the nut, between it and the regular washer. As you tighten the nut, the split digs into the nut and applies pressure to keep it from loosening.

Ok at this point I've probably confused you terribly, lol. Feel free to ask questions about anything I've posted. I'll try and further explain or post pictures.

add photo: 

Hi David,

Thanks for the detailed information - it's very useful to know which drill bit to use based on the bolt / metal screw that I will use. I do have a centre punch that I used in my tests but I was using a hand drill (not electrical) and perhaps my aim wandered a bit. Anyhow, the mop parts are fine just being bent around each other with no fasteners. That surprised me a bit.

The lath screws look very interesting - I have a few of them in my box of tricks - screws and nuts that I have collected from broken furniture etc. - and I wasn't sure what they were for. Now I know :-)

I did a week's course of wood work a couple of years ago and my woodwork confidence is definitely greater than my metalwork. How I wish I insisted on woodwork in high school rather than the assumed home economics. That was 35 years ago in the dark ages. One year after me a girl came into our school who insisted that she wanted to do woodwork and managed to persuade the powers that be. I was very envious of her guts and her woodwork skills.

I really do appreciate the extensive and detailed information you have captured here - I'm sure I'll use all of it at some time during my repair adventures.


lathechuck's picture

I think the world would be a better place if EVERY high-school student had a course in home-economics. Household finance, budget, credit cards, small-scale investing; nutrition, relative food value vs. cost of dietary components, cooking techniques, household repair skills, horticulture, civil law, etc. ... all the things that can allow a family (even a family of one) to make the best use of whatever cash income they have. I learned a lot of that stuff in our 4-H club, but a lot of people haven't learned it at all.

Absolutely. Everyone should know the basics of running a home, keeping a budget, cooking, simple home repairs.

They're the foundation of everyday life yet many people don't learn them at all.
Home Economics in school doesn't replace parents and family but it's a great supplement and if the family is disfunctional?

Home Ec is all some kids get.

David Trammel's picture

I grew up around a father who was very handy in general do it yourselfness. We moved extensively, he worked in the US Space program thru a big aerospace contractor so we were always buying a new house when we went to another state. This mean we were doing things like finishing a basement, adding closets or decks, general home improvements. Once I was grown and past college and a brief tour in the military, I spent the next 40 years in manufacturing and machine work. I forget sometimes how most people don't know things I consider just common knowledge like what screws to use or how to fix things.

I'm thinking I'll try and do a blog post for next week that is just an introduction to fasteners with a lot more pictures as references. Bolts, screws, rivets. Once you know the basic tools, then you can usually kludge together a solution.