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Thread: How to repair crooked thread
01-06-2011, 11:41 AM #1
How to repair crooked thread
Well, I messed up and need your help. I tapped a 3/8-16 hole about 1/2" deep in some 303SS. The parts are about 1" square and 6" long. The hole is centered on one end, so the parts had to be stood up in the mill to be drilled. Well, some of the threads are off axis a little bit.
The hole is for a shoulder bolt, which needs to tighten up square. I guess my choice is to bore the existing hole, and install helicoils. Obviously being a little more carful with the setup this time. What I'm curious about, is the possibility of using an oversize tap to loosen up the threads, thus when the bolt tightens down, it will draw up nice to the face of the part. The original tap used was a 3/8-16 GH3.
What are your thoughts?
01-06-2011, 12:10 PM #2
Helicoil / timesert it, loose threads can mean loose bolts etc etc.
01-06-2011, 12:25 PM #3
Not sure how long your parts are but this might help...
Get some long bolts that match the thread you tapped.
Cut the heads off the bolts, and clean bot the tapped threads and bolts to remove any grease and oil.
Loctite the bolts into the tapped threads leaving a shank extending from your parts. Allow the loctite to cure.
Chuck the shank of the bolts in a collet on the lathe and dress the contact face of your parts as required using a left hand tool.
This will get your parts facing true with the threads.
Bolts can be undercut to allow complete facing your the parts.....
Once finished, heat the assemblies and bull the bolts....
01-06-2011, 01:31 PM #4The hole is for a shoulder bolt, which needs to tighten up square.
If that's not an option, scrap the parts and start over...
01-06-2011, 03:14 PM #5
If that bolt has to actually hold anything, turn the part around and drill and tap the other end..
Then he could just make some threaded plugs to screw in the bad hole and face it off. (If that really matters to the customer. Although it would look better and keep someone from using the wrong end ).
01-06-2011, 05:16 PM #6
Counterbore to suit a longer shoulder bolt.
01-10-2011, 09:41 PM #7
Thanks for the replies. All interesting thoughts. Flipping them over isn't an option. Looks like I need to install helicoils. 3/8-16 helicoil should be plenty strong enough for the maybe 200lbs tension on the bolts?
01-10-2011, 09:47 PM #8
Make sure you put Locktite on the Helicoil.
01-11-2011, 02:56 AM #9
If you helicoil it it will be the strongest hole in the piece. But why did it happen? Why didn't you tap it while you were over the hole and do it all at once?
01-11-2011, 03:03 AM #10
If the part is only 1" square and 6" long, do all your work in a 4 jaw on the lathe. Bore the hole out first to the helicoil size and either single point the threads or start the tap in the tailstock.
01-11-2011, 03:26 AM #11
This thread is actually pretty scary. There is usually a reason parts are made from stainless. The comment about the threads being stronger is throwing alarms in my mind that your helicoils will likely not be stainless. The only right way to fix this mistake would be to start over, though I can think of one and only one possibly acceptable repair.
Use a four jaw chuck in a lathe.
Bore and tap the holes out(being careful to center them this time) for a 5/8 fine thread bolt which should be @ least 1/4 inch deeper than your specified hole.
Locktite some 5/8 stainless bolts into the newly threaded parts, and allow the locktite to cure.
Chuck the parts back up in the four jaw chuck, and again being careful to center your part, face the head of the 5/8 bolt off, spot drill, drill and tap the parts back out to the originally specified dimension.
(then keep your fingers crossed that the inspector has his eyes shut when he checks the holes with his go/nogo guages)
01-11-2011, 04:21 AM #12
Helicoil inserts are available in a variety of materials, so you can match the base material characteristics such as strength. You will find that a stainless helicoil is stronger than any 300 series bar stock. The on-line catalog engineering section will give you that info.
Proceed with the inserts and do not worry about strength.
01-11-2011, 05:02 AM #13
Make It Like The Print
01-11-2011, 05:20 AM #14
I would have to agree with scrapping the part/s. Not trying to be rude in any way, but this should be done right the first time(Don't know the rest of the parts features, but one tapped hole is a walk in the park). In a situation like this, i would prefer to eat a little of the material cost, so as not to bring to the attention of my customer that i was trying to fix a simple tapped hole i messed up.
I have made many mistakes myself, and have had scrapped parts, so i feel your pain. But depending on how many you have tapped out of square, i would just start over.