Stripped screw holes in my bandsaw -- now what??

[Jason White]

I was replacing the lower guide bracket on my JET bandsaw with a new Carter unit and used the new machine screws supplied by Carter. Unfortunately, the threads in the old cast-iron holes disintegrated as I tried to snug up the new bracket screws (I did not over-tighten, I swear!!).

Short of re-tapping the threads and using larger diameter screws, is there anything I can do? Is there something I can wrap around the screw threads that will allow me to tighten the screws in the stripped out holes?

Jason

[Rick Gauthier]

Go to your nearest hardware store and pick up a helicoil set. It generally consists of a drill bit and something that looks like a small coil spring. You drill out the hole and then screw in the thing that looks like a spring (helicoil). Snap off the piece in the center of the helicoil when its in place. You can then use the original screw. Make sure you take the screw with you so you buy the right size helicoil. Cast iron is a little "iffy" to drill so use some sort of lubricant.

[Brad Wood]

I could be wrong, but I think, to use the helicoil, you still need to re-tap the hole. Use of the helicoil allows you to use the same size screw when you are done, rather than a larger one.

Maybe there is a "shop trick" to avoid the tap, the only ideas I would have would sort of make the screw a permanent fixture, and I don't think that would be a good route

[John Coloccia]

Definitely you want to drill and tap before installing the helicoil. Personally, I would use the Helicoil. You can probably buy an appropriate size at the local auto parts store (hardware store too, maybe), and it'll take about 10 minutes to install....and it's done RIGHT. Just use a lubricant with the drill and tap, and go slow. Drill slow. Back the tap out when you feel too much resitance and clear the chips. A broken tap in there will have you buying a tap extractor. LOL....I can't tell you how many tools I've aquired simply to fix my screwups!

[Brad Wood]

LOL....I can't tell you how many tools I've aquired simply to fix my screwups!

amen brother.... good to know I'm not the only one :rolleyes:

[glenn bradley]

+1 on helicoil (http://www.emhart.com/products/helicoil.asp). You really want to fix this well. A shortcut now will haunt you later.

[Jay Jeffery]

If the tapped holes are deeper than the screws are long, perhaps you could try using longer screws. Otherwise, you are probably stuck rethreading.

[harry strasil]

helicoil kits come with the special tap and the proper size drill bit, but be warned helicoils have a tendency to stick to the screw when you attempt to loosen them after installation.

I was a blacksmith, weldor, machinist for 55+ yrs and have never installed a helicoil, I always got the job after the helicoil attempt had been made, and I drilled the hole larger and tapped for a standard bolt thread, locktighted it in,then drilled and tapped the proper size thru the dummy plug.

[David DeCristoforo]

I have used Harry's method with good results. There are two other options. One is (assuming that there is enough iron) to simply drill the hole deeper, tap it and use a longer bolt. The second is (if the condition allows it) to drill all the way through and use a through bolt with a lock washer and nut.

[Jason White]

Not really enough room for a nut underneath (unfortunately).

Will give the helicoil a try. Thanks!

Jason


I have used Harry's method with good results. There are two other options. One is (assuming that there is enough iron) to simply drill the hole deeper, tap it and use a longer bolt. The second is (if the condition allows it) to drill all the way through and use a through bolt with a lock washer and nut.

[Sean lennon]

There is another option other than a Helicoil, IF, you have enough margin around the old hole.

Keenserts
http://www.thorintl.com/Keenserts-Keyserts-Keylocking-Inserts.htm

They install with standard drill and tap sizes.

I have had some really bad experiences with Helicoils backing out on castings I have had to fix.

I Drill,Tap, coat the Keensert with Loctite, screw it in and stake it.


they don't work well in castings with very little material around the hole and they are the only way to go on aluminum and other soft materials.


just my .02 worth


cheers,
sean

[Mort Stevens]

Everyone is saying "helicoil" but, because a helicoil requires you to drill a larger, much larger hole - your next step should be going to the next larger bolt size and tap the existing hole, then if that doesn't work step up to the helicoil, because once you go the helicoil route there' no turning back.... just my 2-cents.

[Phil Thien]

Everyone is saying "helicoil" but, because a helicoil requires you to drill a larger, much larger hole - your next step should be going to the next larger bolt size and tap the existing hole, then if that doesn't work step up to the helicoil, because once you go the helicoil route there' no turning back.... just my 2-cents.

The benefit to the helicoil, though, is that it gives you steel threads. If this is a machine screw that will be adjusted quite frequently, the helicoil will outleast cast iron.

[Jason White]

Since this screw is holding the lower guide, the blade will be pushing against it (against the thrust bearing) as I push wood through the blade. Therefore, I imagine I'll have to adjust it from time to time.

I just ordered a Helicoil kit from Amazon. Tried a couple of different auto parts and hardware stores, but nobody has one small enough (an M6). Oh well.

Jason


The benefit to the helicoil, though, is that it gives you steel threads. If this is a machine screw that will be adjusted quite frequently, the helicoil will outleast cast iron.

[David Epperson]

I could be mistaken, but I seem to remember my college machine shop instructor saying do NOT use oil or lubricant to drill or tap cast iron. It powders up and traps the chips rather than clear them out. Use compressed air and wear eye protection.

[Pete Bradley]

You're going to have to drill and tap no matter what. Unless these screws are going to move regularly, I'd drill for the next size screw rather than helicoil. Don't try to tap without tapping fluid or you'll be back asking how to get broken taps out of holes. If you've never used a tap before, the keys are to not bottom it out, and to turn back regularly (every half turn forward or less) to break the chip.

The tap drill chart will ususally have two drill sizes, one for a close fit, one for a loose fit, use the close fit. You probably want to stay metric, but there's no reason you can't use a standard thread for this. Google up "tap drill chart" or "metric tap drill chart" for the appropriate table.

Tapping fluid and drill bits in sizes you don't have are available from www.mcmaster.com (http://www.mcmaster.com)

Pete

[bob hertle]

I agree with David, normally cast iron is tapped dry. The free carbon in the form of graphite in the iron structure, serves as a lubricant. That said, I've seen a lot of Chinese and Taiwanese iron castings that are of poor quality, and machine terribly. Chilled spots, sand inclusions etc. If the tap drives hard under these circumstances a little lube can help.

I also agree with Harry in that I'm not a huge fan of Helicoils. If it were mine, I'd first run a 1/4-28 tap thru it and try a 1/4-28 bolt. (assuming the longer M6 bolt idea won't work). Next try would be to go to 5/16-18 tap requiring the hole to be drilled out to either "F"drill (.257), or 17/64. This assumes enough meat in the casting and the mating part.

Bob

[Jason White]

I've drilled and tapped cast-iron without oil before with no problems. I usually back the tap out every couple of turns to clear the chips. Has worked quite well.

Jason


You're going to have to drill and tap no matter what. Unless these screws are going to move regularly, I'd drill for the next size screw rather than helicoil. Don't try to tap without tapping fluid or you'll be back asking how to get broken taps out of holes. If you've never used a tap before, the keys are to not bottom it out, and to turn back regularly (every half turn forward or less) to break the chip.

The tap drill chart will ususally have two drill sizes, one for a close fit, one for a loose fit, use the close fit. You probably want to stay metric, but there's no reason you can't use a standard thread for this. Google up "tap drill chart" or "metric tap drill chart" for the appropriate table.

Tapping fluid and drill bits in sizes you don't have are available from www.mcmaster.com (http://www.mcmaster.com)

Pete

[David Epperson]

http://bbs.homeshopmachinist.net/showthread.php?t=31012

While tapping "dry" is what I was taught - that does not necessarily mean without lube. Wax the tap to keep it from sticking, back it out and clear the dust often.
Also if you go to the next larger size screw, cast was traditionally tapped with coarse threads instead of fine threads.

[Dan Mitchell]

helicoil kits come with the special tap and the proper size drill bit, but be warned helicoils have a tendency to stick to the screw when you attempt to loosen them after installation.

I was a blacksmith, weldor, machinist for 55+ yrs and have never installed a helicoil, I always got the job after the helicoil attempt had been made, and I drilled the hole larger and tapped for a standard bolt thread, locktighted it in,then drilled and tapped the proper size thru the dummy plug.

+1. I've used them on aluminum engine bits occasionally when there was no other choice, but they're a bit of a PITA. I'd go for retapping to the next size, if available area permits. Quick & straightforward.

Dan

[Phil Thien]

http://bbs.homeshopmachinist.net/showthread.php?t=31012

Wax the tap to keep it from sticking, back it out and clear the dust often.

Actually, just use some dishwashing soap. A few drops. You can also rub the tap against a bar of ivory soap.

[Scott T Smith]

Jason, another option is to chase the threads with a tap and insert a stud. Oftentimes a stud will grip in poor threads where a bolt won't - partially because you can usually screw the stud further into the hole and bite into some undamaged threads.

Additionally, once the stud is loctited in place the forces acting on the cast iron are simply tension, as opposed to the shear/tension placed on them by a bolt. Thus, studs are less likely to pull out than bolts.

Although lubricating oil never hurts, good quality cast iron is indeed "self lubricating" and typically a lubricant is not needed. However, considering the small size of your bolts, coupled with the fact that small diameter taps are much easier to break than larger ones, and it is wise to do everything possible to avoid breaking a tap. Broken tap removal can be a real PITA.

One drawback to heli-coils is that you generally lose a slight amount of thread depth because you have to leave the room to break off the tang in the bottom of the hole. You also need to get your depth pretty precise when installing them, and make sure that the end of the wire closest to the open end is embedded between two threads, else it will tend to unravel, etc.

Having said that, threaded inserts such as these are superior to helicoils when it comes to repairing cast iron.

http://www.mcmaster.com/#thread-repair-inserts/=3syryu

As Harry mentioned, installing a larger bolt and then drilling/tapping it is another way to repair; however the skill set required to do this with precision accuracy is greater than that required to install the heli-coil or threaded insert. Also, you will need to use a softer grade of bolt (such as a grade 3 or grade 5), in order to be able to drill/tap afterward.

Scott

[harry strasil]

There is another option other than a Helicoil, IF, you have enough margin around the old hole.

Keenserts
http://www.thorintl.com/Keenserts-Keyserts-Keylocking-Inserts.htm

They install with standard drill and tap sizes.

I have had some really bad experiences with Helicoils backing out on castings I have had to fix.

I Drill,Tap, coat the Keensert with Loctite, screw it in and stake it.


they don't work well in castings with very little material around the hole and they are the only way to go on aluminum and other soft materials.


just my .02 worth


cheers,
sean

My First experience with those was using them to repair stripped head bolts on a big over the road semi engine Sean. At that time they were only obtainable from Caterpillar way back then. I got the job because the mechanics were afraid they couldn't get them in straight enough, Ibored the holes with my portable drill press for my Sioux hand drill and used a stub shaft with a 60 center turned on it, in the drill with the center in the end of the tap to start the tap.

[Faust M. Ruggiero]

M6 is almost the same diameter as 1/4". Can you swap out to a 1/4"-20 bolt?

[Michael Wildt]

Can one weld cast iron ? then drill a new hole and then tap for original size ?

[Scott T Smith]

Can one weld cast iron ? then drill a new hole and then tap for original size ?

Yes, technically cast iron can be welded, but it is problematic. Typically it cracks while it is cooling.

Harry's method would be a superior solution versus welding).

[Paul Atkins]

"the threads in the old cast-iron holes disintegrated" I wonder if the Carter bolts were metric. Kinda after the fact, but I wondered anyway. The next size bolt plan is always my first and quickest solution and I even have metric/SAE mixed on a machine (which drives me nuts every time I grab that loose Allen wrench ----)

[mickey cassiba]

Seems like everyone is on this one so here are my two cents:
1. No lube! Back the tap out and blow out the hole if too much resistance is felt(Hard to convey "too much" over the internet, it's a feeling that has to be experienced.
2. If possible, drill your hole all the way through the part. This will keep you from "bottoming out" with the tap. 2d biggest cause of breakage in my experience.
3. I've had good and bad experience with Helicoil inserts, Sticking of the bolt in the coil can be avoided by using an anti-seizing agent such as Nevr-Seize.
4. Use Lock-Tite on the coil, and allow it to cure before snapping off the driving portion. Especially with the smaller diameters. They tend to distort when the breaking force is applied.
That said, I agree that the best way to repair the hole is to plug, re-drill , and tap. The steel will stand up to repeated use of the thread much better than the cast, or powdered metal.
Mick

[Bob Wingard]

One other approach that's not been mentioned, but is a bit more simple. Get a bolt that is about 2-3 sizes larger than the 6MM that is stripped out hole .. LEAVE the head on .. have someone with a lathe drill & tap the other end of the bolt 6MM .. drill/tap the casting to the size of the bolt's threads .. apply Loctite and insert the bolt .. let it cure .. remove the head of the bolt.

It's easier to get the new threads concentric with the old if you drill/tap the bolt before installing it, but it's not too difficult to drill out & retap the old threads in the casting with it still in place because the old hole acts as a guide for the drill.

[Paul Atkins]

Smart, Bob. I'll have to put that in my memory bank.

[Jim Koepke]

One trick that has worked for me on cast iron planes is JB Weld. I put it in the cleaned threaded hole. I use a drill bit sized for the tap to make sure there is enough in the hole. Do not leave the drill bit in the hole. Then run a tap through it after it has dried a day or two.

As far as helicoils are concerned, I learned a long time ago in the world of air cooled Volks Wagens that thread inserts are a better way to go.

jim

[Alan Trout]

I owned an automotive machine shop for many years and then worked in the automotive machine tool industry. I literally have repairs thousands stripped out holes in cast iron and aluminum both. By far the best insert I have ever used is the Timesert. They are better than the original threads and are very easy to install and don't come out. Here is a link to their website. http://www.timesert.com/.

On another note it was brought up that you do not use tapping fluid on cast iron and that is correct. You get a tighter tap without it. Just be sure to break the chip (reverse the tap) on a regular basis and you will be just fine.

Good Luck

Alan

[Jason White]

Yep, metric (M6).

Jason


"the threads in the old cast-iron holes disintegrated" I wonder if the Carter bolts were metric. Kinda after the fact, but I wondered anyway. The next size bolt plan is always my first and quickest solution and I even have metric/SAE mixed on a machine (which drives me nuts every time I grab that loose Allen wrench ----)

[John Coloccia]

I owned an automotive machine shop for many years and then worked in the automotive machine tool industry. I literally have repairs thousands stripped out holes in cast iron and aluminum both. By far the best insert I have ever used is the Timesert. They are better than the original threads and are very easy to install and don't come out. Here is a link to their website. http://www.timesert.com/.

On another note it was brought up that you do not use tapping fluid on cast iron and that is correct. You get a tighter tap without it. Just be sure to break the chip (reverse the tap) on a regular basis and you will be just fine.

Good Luck

Alan

I was going to suggest these but I didn't know you could get them in anything but the big kits. I've never seen them in individual kits but according to the website they're available. These totally blow away helicoils. Where do you buy them? I've never seen them retail anywhere.

[Alan Trout]

I always bought them direct from Time Fastener Co.

Good Luck

Alan

[Mort Stevens]

I owned an automotive machine shop for many years... By far the best insert I have ever used is the Timesert.

Alan must then be very familiar with the Cadillac Northstar? Time-serts are the ONLY thing to use for the Northstar headbolt holes and since they are a solid, one piece insert...unlike the coil that a heli-coil design uses, it can better distribute the load from the bolt into the metal.