I've never had much luck using some of the traditional methods for repairing damaged threads in my projects. For various reasons these techniques may not be workable solutions. The thread may be too damaged to "clean up" by running the matching size and pitch tap through. Drilling and tapping to the next larger size screw may not be an option for mechanical or aesthetic reasons. Jamming in a screw of a different size or pitch or squeezing one of the thread "repair" compounds into the hole won't give you the fit and strength you might need.
My favorite option, by far, is to install a stainless steel insert into the hole that will give me EXACTLY the same size thread as the original, damaged thread. I can the use precisely the same machine screw as I had used originally. The inserts, tiny coils of stainless steel, can also be used right from the get go to add strong threads to parts made of softer materials like aluminum or even plastic.
I'll show you how to properly install one of these inserts in the main workshop at TechShop Detroit. You buy a thread repair insert kit or use the basic tools available at TechShop. In either case the drill press in the Metal Shop at TechShop is very useful for accurately drilling/enlarging the holes you need to make in your part to install these inserts.
Step 1: Gather the Tools
In the first photo you'll see my aluminum block with damaged thread. The size of the threaded hole is (was) for a 10-32 machine screw and I'd like to keep it that way!
Pictured below you'll see the kit of stuff you'll need to pull together to accomplish the repair. This consists of:
-The stainless steel threaded insert, size 10-32
-A drill to enlarge the existing damaged hole
-The correctly sized tap to thread the hole to accept the threaded insert
-The special tool needed to install the insert (comes with the insert kit)
-A knock out bar to remove the tang on the insert after it is installed (more on that later)
-The part with damaged hole (but of course)
-The 10-32 screw I intend to use in the repaired hole and a screw driver to drive it in
And If I'm going to make a hole from scratch that I intend to install steel threads in from the very beginning:
-A center punch to mark my hole postion
-a hammer to give the punch a light, firm tap
Step 2: Understanding the Process
The thing about this technique is that we will intentionally be drilling the damaged hole oversize initially. We then tap this hole to permit the threaded insert to "screw" in. In the photo you'll see that the inert is essentially a coiled spring. The outside diameter is going to fit the oversize hole that we drill and tap and when the insert is, well, inserted into this larger hole the resultant thread on the inside of the hole will be back down to the size of the original that was damaged.
In the first two photos, you see the inert itself. The next photo shows the difference between the size of the tap that created the original 10-32 threaded hole that we are trying to repair and the tap that will be used to create the oversized hole to insert the threaded steel insert into. These two taps have exactly the same pitch, or in other words, they will cut the same number of threads per inch. The only difference is the size of the hole they will cut those threads in. The larger tap came with the thread repair insert kit that I bought and is accurately sized for the insert that I am using to repair a 10-32 sized hole. I happen to be using a kit made by a company called Helicoil, but there are a number kits available from different sources that work by the same principle.
Step 3: Drill the Oversized Hole
Using the drill bit from the Helicoil kit we need to drill out the damaged hole to prepare for tapping to fit the insert. In this case the correct drill size for a 10-32 Helicoil is a 13/64 bit.
I chucked the drill in the drill press in the Metal Shop at TechShop, set the appropriate speed range for the diameter of the bit, and carefully enlarged the hole.
If you'd like to add a hole to your part and use the steel insert from the very beginning just mark the location with a firm tap to a center punch (see photo) and drill your initial hole using the drill bit appropriate to the insert. For a 10-32 hole that would be the 13/64 bit again.
Step 4: Tap the Oversize Hole
Fit the tap that comes paired with the threaded insert and tap the oversized hole.
Step 5: Install the Insert
Using the special installation tool that comes with the insert kit (every manufacturer has some version of this tool that works on similar principles) "screw" the insert into the oversized threaded hole.
The fist photo shows how the insert is screwed onto the end of the installation tool. The insert has a "tang" on the end which is a sort of metal hook that engages with a lip on the end of the installation tool. This allows the tool to rotate the insert and screw it into the oversized threaded hole. You can break off this tang later on in your finished hole is a "through" hole, meaning, something will screw completely though the part of out the other side. If you might want to remove the insert later one for any reason or the hole has a bottom you can leave the tang on.
The second photo shows how the insert is threaded into the body of the installation tool such that the very first thread protrudes from the end of the assembled installation tool and is ready to engage the thread in the oversized hole you tapped in your part. The large plastic "body" of the installation tool that surrounds the insert helps hold the insert together (it behaves like a spring) and makes it easier to screw into the hole.
The third photo shows the insert tool fitted to your tap wrench (I got mine as a check out item from the Front Desk at TechShop) and the whole assembly being used to screw the insert into the hole. It's going to feel a bit strange as you go since the insert winds up like a spring as you turn it. Go slowly and carefully while you observe to see that the insert is going in properly. It takes a minute to get used to the feel of it but you'll soon be able to tell whether your installing the insert properly or the insert isn't lined up and going in as you turn the tap wrench.
Good Tip To Follow:
When the insert is threaded at least 60% of the way into the hole you can hole it stationary with the tap wrench as you rotate the body of the installation tool to screw it up toward the tap wrench handle and reveal the insert. With 60% of it in the hole it shouldn't come unsprung and you'll be able to see it going in much more clearly. Resume tuning the tap wrench and drive the insert as deep as you like into the hole. The last photo shows a body screwed up toward the handle and a bit of the insert showing as I drive it home.
Be careful not to go too deep! If you have a through hole and you go too deep you can always screw the insert completely though the hole, thus freeing it to try again. If your hole has a bottom you only get one shot of installing the insert. If a little bit of the insert remains sticking out of the hole you can sometimes remove it with a pair of pliers but you could damage/destroy the insert and /or the hole.
Step 6: Remove the Tang
The first photo shows the installed insert with the tang used to drive it in still in place. If you like you can remove this using a punch, piece of drill rod or the punch that comes with some of the insert kits. Simply insert the punch from the side of the part that you installed the threaded insert from until it rests on the tang and strike it sharply with a hammer.
Step 7: Insert Your Screw
Grab a new screw (the original may have damaged threads itself) and try out your newly repaired hole.
Your parts should go back together and show no evidence of having been repaired. It's all going to look and feel just like the original thread.
This is especially satisfying when you've installed an insert in something like Delrin. Instead of that squishy feeling you can get driving a machine screw into plastic that leaves you wondering if you're chewing up the thread every time you turn the handle on your screw driver you'll have a nice, smooth action as the screw goes in and comes out. This is especially great for parts that are made from soft material that frequently need to be assembled and disassembled.
Hope this has been just as much of a pleasant AH-HA! moment for you as it was for me when I discovered these great little things!