I'm putting threaded inserts into a MDF base. When I start to screw the insert in, the thread digs in at an angle and pulls the entire insert in at that angle. I thought it would straighten out when it got further down the hole, but it still managed to go in kattywompus. Now, the knob and bolt look like some tower in Pisa.
What tricks can I use to ensure a threaded insert goes in straight? Is this just a problem because I'm using MDF instead of hardwood?
There are specialised drivers for these which might help -- or you could make your own by using a bolt with one or two "jam nuts" to keep the insert from gravelling up the threads. Use a long enough bolt to make any tilt more visible, hold it straight, drive the insert, and back the bolt out of the insert (backing off the jam nuts if necessary).
Promised illustration: I didn't have an insert handy, so you'll just have to imagine that the dark cap nut is the insert. Assemble like so, preferably with a longer bolt
Then tighten the nut down firmly against the insert. This will force the nut to press up against the bolt's threads while pressing the insert down; that outward pressure should produce enough friction to lock both against rotating on the bolt. Now drive this assembly into the wood as if it were a screw, using the bolt as both guide and lever to make sure it goes in straight. Once the insert has been set, you should be able to hold the bolt in place while loosening the "jam nut", releasing the bolt and nut from the insert.
If you're driving several, a slightly more elegant version would be to use two nuts, locked against each other, at the right point for a washer (optional but can't hurt)and the insert to slip on below them. This is driven the same way, but you should then be able to back the bolt and locked jamnuts out of the insert as a unit without having to do anything special to the nuts; just leave them locked. Obviously that means your inserting tool is immediately ready to be loaded with another insert.
(Feel free to use a bolt with another head design, whatever you've got a good driver for -- this is just what came to hand first.)
As I said, there are commercially made drivers which operate on the same principle: thread the insert on, drive, back the threads out. If you're doing a lot of these that might be an investment worth considering. But for just a few, this does the job.
Late Addition: Some inserts are designed to be driven with a hex key, which has many of the same advantages.
Speaking from a metal working background, we always used a drill press (not running, of course) with the tap drill inside the insert to keep them straight and to ensure proper starting/seating.
MDF is soft enough that it won't stop cross threading on it's own. That is really what you did. with metal threads on both sides you notice because it becomes much harder to turn in the screw/bolt. So getting it to go in straight you really need to make sure you line it up correctly from the start. Once it is stared correctly it should stay in the grooves and stay straight. Though the inserts keshlam suggests would be a good idea as it will improve the reuse of the threads for multiple inserting and removing of the bolts and have a better hold as well.
It sounds like you're trying to install thread-in inserts in the face of a piece of MDF, which is fine but not ideal. If you're installing the threaded inserts into the edge of MDF, you should strongly consider knock-down hardware instead.
I'll cover some alternatives later, but first let's talk about how to properly install thread-in threaded inserts. If you have a tap, consider tapping the threads before installing any thread-in inserts.
Thread-in threaded inserts
First off, one end may have notches cut into it, as though you're supposed to use a slotted/flat-head screwdriver to drive it. If the threads extend all the way to the end, and the notches cut through the threads, then forget intuition; that's not how you're supposed to drive the threaded insert. The notched end is to help with tapping the threads, and should be the first part of the threaded insert that enters the material.
Note that some threaded inserts do have a slotted drive on the end but the threads do not extend all the way to the end. These threaded inserts are not self-tapping, and you'll have better luck if you tap the holes before installing them.
Instead of using a screwdriver to drive the threaded insert, get a bolt with the same threads as the inside of the threaded insert, screw two nuts onto it, and jam the nuts tight against each other. Then screw the threaded insert onto the end of the bolt, so the notched end is facing away from the nuts. If you have a drill press, cut the head off the bolt and install the bolt in the drill press chuck, then slowly screw the insert into the material.
Pound-in threaded inserts
Instead of using the thread-in type, you can also use pound-in threaded inserts. The outer "threads" on these look more like barbs than a continuous thread that spirals around the outside. For this type of insert, you simply drill the hole to the size of the insert to fully engage the teeth or, if force will be applied to pull the insert out of the MDF, you can drill the hole just large enough so the barbs barely engage, but secure the insert with epoxy.
Hex-head threaded inserts
There are also hex-head inserts, and most that I've seen are self-tapping. They have threads around the entire body except that they have notches down the entire length of the threads instead of only at the end. The threads look like a cross between the threads and barbs. I haven't used these in MDF but I think it would work better to drill a hole that barely engages the threads, and possibly use epoxy along with screwing or hammering them in.
T-nut or knock-down hardware as an alternative
For MDF, you should ideally use a T-nut or some other type of knock-down hardware instead (depending on your application), especially if you're installing the threaded inserts into the edge of the MDF instead of the face. (The edge of MDF is more prone to splitting.) But if you aren't creating a through hole or if the appearance on the back side of your material is important, you can still use one of two types of threaded inserts.
Here's a bush-hack for getting a hole straight might work for your need, depending on how big your insert is and how you drive it in.
Use an old CD. Put it over the hole, silver-side up, and drive the insert through the hole in the middle of the CD. Eyeball the reflection of your driving tool to keep it perpendicular to the CD which is parallel to the surface.
The reflection should be in-line with the tool - your eyecrometer can detect this well enough.
The proper tool is the best answer, but this will do in a pinch or for a one-off.
