tapping a screw hole

[jussi]

Went to use my used Dewalt 621 I got a while back and realized one of the screw holes for the knob that held the edge guide in place was partially stripped. The top half of the hole was fine but the bottom half had no threads at all. I don't have any experience with tap and die sets. I have an inexpensive one I bought a while back because I thought I might need it one day. Are there any trick to it? Do I need to add oil if it's just going to be such a small hole (probably only 1/4" that needs to be tapped). Will the correct tap screw in easily in the part of the hole that still has the threads?

This is probably something simple but I just don't want to do something stupid and regret not going here first and asking. Thanks

[pelligrini]

I'm certainly no machinist, but I've used them some.

Cutting oil wouldn't hurt, but if you're just chasing small diameter threads I don't think it's necessary. A good rule of thumb is 1/3 to 1/2 turn in and a 1/4 turn out. The back turn should break the chips so they'll fall into the flutes of the tap.

[Uncle Cracker]

Because half the hole is stripped, you will have to drill out the entire hole to a larger size (this means removing the threads that are still good. I'm assuming your threads are in steel...?... To tap the new hole, make sure you have used the right sized drill bit for the new tap, as too big or small of a bit will be a problem. Then you can put a drop of oil in the hole, and start the tap, trying to keep it at right angles. Its tip will be small enough to start easily, and then will ream out the hole as it gets further along. Once the tap has "stretched" the hole a little, you can start turning in a quarter turn, and back it out an eighth, continuing this until the threads are fully cut. You should not need more oil during this process, but can add a drop here and there if you feel the need. Make sure the whole thing is blown clear before you put the screw in place.

One more thing... The tapered shape of the tap will prevent the threads to be expanded to full size at the very bottom of a blind hole, so if yours is not a clear-through hole, you will either have to drill deeper than what you need (careful!), or open the very bottom of the hole with a hardened bolt of the proper size. Hope this doesn't sound too complicated, because it really isn't.

You may also be able to get away with simply replacing your screws with longer ones, in order to take advantage of the good threads you still have. Not looking at what you are, but this may be a better way.

[jussi]

Because half the hole is stripped, you will have to drill out the entire hole to a larger size (this means removing the threads that are still good.

Hmm. So there's no way to keep the hole the same size huh? Reason I ask is because I'd like to be able to use the stock knobs that hold the edge guide in place.

[Uncle Cracker]

Hmm. So there's no way to keep the hole the same size huh? Reason I ask is because I'd like to be able to use the stock knobs that hold the edge guide in place.

If the hole has stripped threads, then it's already too large to hold the stock bolts. A tap of the same size would just rattle around and fall out. If you need new threads you gotta have new metal...

There is such a thing as a heli-coil, which requires a larger tapped hole, but has threads of the original (or smaller) size already tapped inside it, but that's for things like stripped spark plug holes and the like... probably not suitable here.

If I were you, I'd look for new knobs with longer bolts, because you won't have to be milling your machinery. If you can find a knob with a bolt that is too long, you can always hack a bit off the end, or stack a couple washers.

[phi1l]

Isn't it strange that the bottom of the hole is stripped but the top is OK??

Like maybe it was a bad casting to start with??

It really depends on how much trouble you want to go to. It looks like it is an an aluminum casting, so I guess you could find someone that is experienced working with aluminum, to fill in the hole, then drill & tap again.

There are some "Hail Mary" things you can try, but that probably won't work.

One thing would be to put a few drops of something like a polyurethane glue in the hole. Urethane glue would be a good one to try since it expands as it cures. Clean the hole well with alcohol & then mineral spirits. then screw in the proper sized bolt. Coat the bolt threads with a wax or grease first so that you will be able to get it out. The original metal + the new plastic just might be enough to hold it for a while if you are careful. Probably not, but you don't loose anything by trying.

[Uncle Cracker]

One other thing you might try is filling the hole completely with JB Weld, and then redrilling and retapping to original size.

[phi1l]

One other thing you might try is filling the hole completely with JB Weld, and then redrilling and retapping to original size.

ya, there's another thing to try that stuff is really hard, when cured.

[LCHIEN]

is the bottom of the hole open or closed (e.g. a through hole?) If its closed end i don't see any way it can be stripped at the bottom and not the top. I'm not even sure how you can tell that.

And is it stripped or not threaded? Big difference. Not threaded you can remove material and thread it. If its stripped then the threads have been torn out and can't be replaced or retapped.

If the hole is through, you might ream out the hole for clearance, and use a longer screw and a nut and lockwasher if the nut does not interfere with anything.

A tap should re-enter the good thread portion easily and align for the remainder of the hole (if not threaded). A flat bottom tap will be needed if you need to tap to the bbottom of a closed hole. The advise of the other posters on oil and back-and forth turning were all good.

good luck.

[phi1l]

is the bottom of the hole open or closed (e.g. a through hole?)


good luck.

the OP said it it is a edge guide securing screw for a router, so it would have to be a through hole.

[eezlock]

tapping a screw hole

You said it was a DW 621...isn't that a biscuit jointer? If so, that is aluminum
really easy to retap a thread into most times. I would take it to a good local
hardware store and get the proper sized tap and threaded knob and do it myself. You probably won't have $5.00 in all the material to do that simple job.
Threading holes in metal, especially soft material like aluminum, brass, light steel
are easy to do...just take your time and start the tap straight and turn the tap in about 1/4 to 1/2 turn back it out and do it again until you reach the bottom
of the hole. If you decide to buy the tap and do it yourself ask for a bottoming
tap (it will be flat across the end) that way you won't have to worry about cutting the threads too deep at the bottom of the hole. eezlock

[cabinetman]

There is such a thing as a heli-coil, which requires a larger tapped hole, but has threads of the original (or smaller) size already tapped inside it, but that's for things like stripped spark plug holes and the like... probably not suitable here.

A heli-coil is about your best bet for ease of repair and a repair that will last. It will allow the use of the original knob/bolt. I would suggest to chase the threads on the bolt with the right sized die to clean it up. The inside threads (bolt thread) determine the size of the heli-coil. Make sure you get the right thread designation and the the proper sized corresponding drill bit. You may have to use one or two up sized drill bits to get to your final drill size for the coil. Here's a bit of info on heli-coils. You can usually get them at an auto parts store.

I'm thinkin' using a product like JB Weld, or Marine Tex may seem to hold but will fail sooner than later. There will not be enough gap for any thickness to speak of to support the threads. Those items don't hold up well in this type of repair. When it does fail, you have a mess to clean out and you'll be right back where you started.
.

[Black wallnut]

There is also a product called form-a-thread that is marketed by Loctite and available at most auto parts stores. It contains an epoxy like filler and a release agent to put on the fastener. You use the fastener to form the threads. Might not be the best thing to use if this hole has a fastener that is removed and replaced often, if that is the case a helicoil would be best.

[jussi]

Thanks so much for all the advice. Loring, it is a through hole. I'm not sure if it was stripped or never threaded properly. I just know it bottomed out half way down and I didnt' have the same problem with the other side. Eezlock, the DW621 is Dewalt's 2hp plunge router. the joiner is 682 (which I also had at one time).

What I ended up doing was taking a tap just a bit smaller than the bolt on the knob. I didn't put a caliper on it but it was small enough that it thread the top part of the hole easily but stopped when it reached the bottom half. I followed the advice of adding a couple of drops of oil and backing it out every half turn or so. I then put in the knob and it was still a little tight but felt I could get it in with a little force. With the help of vise grips I got it all the way through and then backed it out. Now I can tighten the knob by hand. Still a little more snug than the other knob when I get to the bottom half of the hole, but it works.

For future reference, a through hole = tap with tapered bottom and closed hole = tap with flap bottom ?

[LCHIEN]

...
For future reference, a through hole = tap with tapered bottom and closed hole = tap with flap bottom ?

The standard taps are tapered or even pointed to ensure getting the tap started successfully and the tap entering straight.
you use a "bottomhole tap" with a flat bottom if
you have a closed end hole AND you need to get threads farther down that the pointed taps will allow you to. if you only have a 10-24 screw and you need say 12 threads engaged (this might be a tensile strength calculation to carry a certain load) then in a 5/8" deep hole you would almost certainly want to switch to a bottom hole tap before running the tip of the standard tap into the bottom of the hole. If you had the luxury of a 1" deep hole the you would be OK for 12 turns with a standard tap.