best way to renew threads in old castings?

[metalmagpie]

I have an old sheet metal brake. Very old (1920s). It has no modern design features and many of the adjustments are fiddly. But it will bend 12 gauge steel 3 feet wide and it fits in my shop.

The problem I have on this machine is that many of the bolts used to make the necessary adjustments are in holes in which the threads are worn to the point where the bolt hardly fastens. The obvious solution is thread inserts, but I would rather not have to buy kits in the three sizes I will need, plus I don't really like thread inserts anyway because the inserts sometimes unthread. Also, some of the holes are near the edge of a casting and enlarging the holes to accommodate the insert tap wrench may make the section between the hole and the edge of the casting too thin.

I'm not too bad with a torch, and I have lots of brazing rod and flux but some of these castings are pretty large and I worry that putting all the localized heat into the area around a hole could crack the iron upon cooling. And I don't have any practical way to preheat the bigger castings; i.e. no oven big enough.

Are there any other options I'm not considering?

metalmagpie

[boslab]

Helicoils, easy things to do
Oops, you don't like them, sorry
You could plate over the hole with a bridge plate, doable
Mark

[Mcgyver]

with helicoils off the table, and I share your concerns on large castings.....that's a toughie.

Were it mine, I'd put helicoils back on the table. Yes they are stupidly expensive (where are the Chinese when you need them?) but they are a great solution. Other solutions might not work as well or will have big cost in time or risk.

where a helicoil can't go, close to edge, could you drill tap and bolt a block onto the damaged casting, said block having the right taped hole in it, directly overtop of the damage one. Whether works will obviously depend on the overall form factor

As a last resort, make custom custom threads. It would be a lot of work though. ie if its 1/2 13, make a 13 tpi tap slight larger to reestablish the thread depth chasing the existing hole anbd threads and make 13 tpi fasteners to suit; ie maybe it becomes a .5625 13 thread.

[RichR]

The problem I have on this machine is that many of the bolts used to make the necessary adjustments are in holes in which the threads are worn to the point where the bolt hardly fastens.

Did you actually check the bolts to make sure they are not undersized or worn?

[lakeside53]

If you don't like helicoils use any number of other reliable inserts. Easy...

Personally, I like Time Serts over all others but the kit is a bit pricey for a couple. Borrow the kits from someone you know... and just buy the inserts. No Issues with "close to the edge".

These will NOT come out : http://www.repairengineering.com/keensert.html

I only use helicoils if I don't need to remove the bolts often. If not, they are "ok".

[LKeithR]

One other possibility not mentioned yet is through bolting--drill the threads out and put in a bolt and nut. If the hole is blind you'd obviously have to drill all the way through but that's not usually an issue in cast iron, especially good stuff. This might not be an option if the castings are really thick at the point where the hole is. Otherwise I would just use helicoils--they're a sollid, proven way to make repairs exactly like what you're faced with. If you're concerned about the possibility of the inserts working loose apply a little Loctite when you turn it into the hole and it will really lock in place. Let the Loctite dry well before bolting things together. Timeserts or Keenserts also work well but they do take up slightly more space...

[danlb]

Two thoughts come to mind.

First is that you might be able to clean the threads and then make oversized bolts/screws that will fit the new size. Each bolt will be custom but they should work.

Second is to use something like a T nut. Drill out just the threads and then fasten the T-nut from the bottom and glue or braze it in place. Then use the slightly smaller bolt that fits the nut.

Dan

[metalmagpie]

Did you actually check the bolts to make sure they are not undersized or worn?

Yes, the guy I bought the machine from replaced nearly all the fasteners.

metalmagpie

[metalmagpie]

I was also thinking about filling the holes 100% with JB Weld or other iron-bearing epoxy like Devcon, then DT
new holes.

metalmagpie

[lakeside53]

If you are going to do that use a new waxed bolt as a "mold", goop up with JB and and insert into the existing CLEANED but gooped up threads. That way you don't end up drilling out more good material.

I'd probably use a helicoil before that though.

[Paul Alciatore]

I don't like the JB weld for this. I know it is great stuff, but I doubt it is anywhere near as strong as CI, steel, or even aluminum.

I would look at the holes individually and

1. Drill and tap for a larger diameter bolt.

or

2. Drill deeper, tap deeper with the same thread size, and use longer bolts

or

3. Drill a clearance hole and use a nut and washer on the back side

or

4. Drill and tap a new hole at an adjacent location that will provide adequate strength in place of the original bolt.

or

5. Bite the bullet and use an insert. If you don't like the price of Helicoil or others that are available, you can always make your own. I had to do this in a situation where the commercial inserts would not work. Making your own allows you to use a standard tap for the external thread and you can often use a fine thread to keep the overall diameter smaller. I used a simple threaded tool with a shoulder to insert them and put a bit of epoxy on the external threads to lock them in place because there would be many assembly/disassembly cycles.

I also do not like the welding idea. You mention that it is iron. I have to wonder just how the original threads became so loose. Normally a bolt that is properly torqued down into threads in CI does not develop any problems. So what happened? Improper assembly in the first place? Excessive stress in use? Or could the CI be of poor quality? This makes me wonder if welding may produce even worse damage.

[Cuttings]

The JB Weld with a waxed bolt sounds like a good idea.
You could also look at metric threads that may be a little larger and have a pitch that is close to what you have " If you are lucky"

[Joel]

I was thinking similar to Mcgyver, except just go the the next closest std oversize if there is still enough meat, and I would think there should be. Between std and metric, there are a lot of sizes and oddball size taps are cheap enough.

If the holes aren't terrible yet, another option is install studs w/red locktite, JB Weld or similar. Or, you could cut oversized threads (which you could cut on the next size up grade 5 bolt) that fit the holes tightly as Dan suggests, or make a stud that is oversize on the brake side and std on the nut end. That wouldn't be too hard and should preclude any future problems.

[Joel]

Neither do I like the idea of welding or using only JB Weld and re-tapping.
If going deeper won't do it on it's own and you can go through, make it a through hole as has been suggested, but add a locktited nut on the backside if possible.

A combination of all of the suggestions might be required, depending on the individual hole situation. Perhaps inserts might still be the best plan on one group of the holes, but one set is a lot cheaper than three.

[_Paul_]

Bit off the wall but.... If you have enough room to use a Helicoil but are worried about them coming out why not just use the Helicoil tap to create new threads but instead of fitting the Helicoil make a set of non standard bolts to the same size as the Helicoil tap.

Paul