Fixing A Stripped Thread On An Adler 67

[Uwe]

I picked up this nice Adler 67-GK373 yesterday and as I was going through it I notice a missing screw on a clamp piece. Turns out the screw is missing because the thread inside the hole is stripped. It's supposed to be an M5x12mm but with a finer pitch than the normal M5 (0.8) (Adler part number is 993 91 101 7, I believe).

There's a second, identical screw that holds things together adequately for now and the machine actually works just fine, but I'd like to fix the thread and put that second screw back in place to keep it working fine. I'm sure I'm not the first one to encounter a stripped thread on a sewing machine.

Replacing the actual clamp part is an option I suppose, but it seems to involve taking apart a good portion of interconnected parts in the machine, which I'm trying to avoid.

I'm considering tapping the hole for a slightly bigger size screw or using one of the thread fix approaches (e.g. Loctite thread repair kit)

If anybody has done this type of repair before, I'd love to hear what works best, as I'm trying not to screw this up (pun alert!) on a nice machine.

Here's an annotated picture of the part that's in trouble:

Edited July 3, 2015 by Uwe

[Ken G]

Ideally it would seem that tapping new threads for a larger diameter screw would be the best repair but you would probably still have to disassemble the machine to drill and tap the hole accurately. Where my limited machining skills would stop me is I have never tapped a blind hole and since my taps are tapered I don't know if you could drill deep enough to get enough of the correct sized threads to hold the new screw.

If it were my machine, and I've never used the Loctite thread repair product, I would put a small hose clamp around the clamp assembly to the right of the missing screw. I would probably cut a small sliver of wood dowel to shim out the flat area of the clamp. Then, since the thread repair product is expensive, I would degrease the screw hole with brake parts cleaner then install a new screw with a glob of JB Weld in the hole then never touch it again. It might just outlast all of us.

[DrmCa]

+1 for re-tapping into M6. The machines use finer thread pattern than standard M as they are continuously oiled and there is a possibility of a faster thread to undo itself under stress and vibration. I am pretty sure you can get a slower 6mm tap from the ebay and re-tap.

I routinely use locktite 262 which holds like nails, but not sure how it would hold in a continuously oiled environment. Slower thread sounds like a better option.

Edited July 5, 2015 by DrmCa

[Uwe]

Thanks for the input folks! Re-tapping seems like the "proper" way to do it if I can access the hole without disassembling everything. The hacker in me is intrigued by the external clamping suggestion - definitely worth investigating. I'll do some tinkering and report back.

[Northmount]

Several grades of Loctite are available. And they do hold in an oil environment if the oil was cleaned off the surfaces prior to adding the Loctite. May years ago I had to do a temporary fix on a mechanical accounting machine. Didn't have the correct size nut for an eccentric adjustment. Applied some Loctite, let it set up. Came back a week later, all was still in the correct position. Replaced the nut with the correct size. I tend to use the less permanent type that allows you to unscrew the screw/bolt/nut later if necessary. In a pinch, have also used Loctite "bearing retainer" for screw threads. Works fine.

And wrt bottoming taps. A bottoming tap should work in this case since you are not trying to jamb the screw tight against the shaft.

Tom

[Cascabel]

I have fixed stripped holes by welding the damaged hole shut, and re-drilling and tapping new threads. This takes special welding equipment and skill so as not to melt the part. When tapping a blind hole, use a standard tap with a tapered lead section to start with, and then finish with a "bottoming tap" You can make a bottoming tap by carefully grinding the end of a standard tap flat.

[stelmackr]

I have fixed stripped holes by welding the damaged hole shut, and re-drilling and tapping new threads. This takes special welding equipment and skill so as not to melt the part. When tapping a blind hole, use a standard tap with a tapered lead section to start with, and then finish with a "bottoming tap" ....

...then grind the screw tip to clear the last thread so you can snug it to the shaft.

Bob Stelmack

[Uwe]

Thanks again for everybody's input on this!

I fixed the stripped thread and I'm quite happy with how it turned out.

The original screw was actually a 11/64" screw - a common sewing machine screw size but impossible to find at your local hardware store. I originally thought it was an M5, but I was mistaken.

Since 11/64" is about 0.434mm, I decided to re-tap the hole with a 5mm M5-0.8 tap, and then remove the new threads in the top part of the clamp piece by re-drilling with a 7/32" drill bit.

I did the tapping in two steps as suggested in a previous reply to get he full M5 thread as deep as possible into the blind pocket hole.

The first tap is done with the regular tap bit, but the tip hits the bottom of the hole and it doesn't get to create a full-width thread on the entire length.

The second tap pass using the bit with the broken-off tip uses the previous tapped threads to guide cleanly into the hole and then create a full threads to the bottom.

I bought two identical M5-0.8 tap bits and purposely broke off the tip on one of them by clamping the tip in a vice and snapping it off. The tap bit tip (lol) broke quite cleanly.

Luckily, I was able to do all this without having to take anything apart. The new screw has a solid feel and does its job of helping clamp down on the rod. I feel better now that two screws are doing that job instead of just one.

I'm attaching a few annotated pictures that hopefully illustrate what I'm talking about.

[DrmCa]

The fastener companies may have those odd screw sizes.

Here in Toronto area, Canada, I go to Brafasco if I cannot make a screw myself.

If they had a screw, it would be way cheaper than at Home Depot etc.