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Repairing stripped thread in Walker Turner bandsaw

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AJAX26/01/2021 20:25:45
137 forum posts
42 photos

I have an old Walker Turner bandsaw that I'm finally getting around to repairing. I have all the parts but there's one thing I would like to fix before reassembly. The top wheel has an adjuster for tracking and it appears that someone stripped the thread and/or used an M8 screw. I don't trust the thread enough and don't want it to give way during use. Failure might lead to the top wheel making sudden manoeuvres.

Here is a closeup of the tapped hole. As you can see, there's not a lot of material to work with. It is diecast something (pot metal?) that goes with the era. The rest of the machine is very solid cast iron.

h2 h1
I have been tempted to mill this part down and make a replacement "lug". However, there's really not a lot to screw this lug into. If I have to do more complex machining I'd be tempted to make a new assembly of my own design but I'd rather a small job didn't grow into a big one.
If you want to know what it could look like, here is an example (not my own)
http://www.vintagemachinery.org/photoindex/detail.aspx?id=5286
Thanks,
Brian

AJAX26/01/2021 20:28:00
137 forum posts
42 photos

Something else I forgot to add:

Would filling the hole with JB weld, drilling and tapping be a viable solution? I'd much rather fill it with metal.

not done it yet26/01/2021 20:30:15
5626 forum posts
20 photos

Helicoil?

Phil super726/01/2021 20:34:53
14 forum posts

Repair the thread with a helicoil

AJAX26/01/2021 22:14:57
137 forum posts
42 photos

Thanks for the helicoil recommendations. I've never used one before but I knew they exist.

Measuring the internal diameter of the threaded hole (the "minor diameter" indicates about 7.64 mm. There's probably less than 1.5mm material to one side. I'm assuming the original screw was 5/16 BSW.

I just watched a YouTube video to get an idea about how a helicoil could be used. He drilled and tapped the hole, then installed the helicoil. No shortage of taps here but I'm not keen on enlarging the hole.

Are helicoils suitable for a through-hole? The screw must pass through the threaded section.

Any suggestions?

Neil A26/01/2021 22:23:06
87 forum posts

To be honest the remaining wall thickness after using an M8 x1,25 helicoil tap looks as though it will be rather thin from your photo. From the helicoil information it should be a minimum of 3,5mm but may not be enough if your assessment of the metal is correct.

Is there any mileage in machining out the existing poor thread and fitting a headed steel bush which has a plain 8mm diameter section, that is Loctited into the hole, and an M8 x 1,25 threaded end for a nut and washer?

Perhaps you could just fit a through bolt and Loctite that in position, I don't know how much room you have.

There does not look to be much metal to play with, it might require a bit more thought.

I hope it works out.

Neil

Forgot to say, the outside diameter of an M8 x1,25 Helicoil tap is 9,6mm, tapping drill size 8,4mm.

Edited By Neil A on 26/01/2021 22:40:33

AJAX26/01/2021 22:43:18
137 forum posts
42 photos
Posted by Neil A on 26/01/2021 22:23:06:

To be honest the remaining wall thickness after using an M8 x1,25 helicoil tap looks as though it will be rather thin from your photo. From the helicoil information it should be a minimum of 3,5mm but may not be enough if your assessment of the metal is correct.

Is there any mileage in machining out the existing poor thread and fitting a headed steel bush which has a plain 8mm diameter section, that is Loctited into the hole, and an M8 x 1,25 threaded end for a nut and washer?

Perhaps you could just fit a through bolt and Loctite that in position, I don't know how much room you have.

There does not look to be much metal to play with, it might require a bit more thought.

I hope it works out.

Neil

Thanks, Neil.

I've had a few more thoughts about a possible solution. If re-tapping / threading the hole is a problem then I could take an entirely different approach and turn a plug that fits in the centre hole (see photo below) and cross-drill and thread that plug. It's not quite as simple as it sounds, as the centre hole is used for the height adjustment, but that shouldn't be too difficult to modify either.

h3

I will probably be back...

bernard towers26/01/2021 22:44:34
106 forum posts
66 photos

Through hole helicoil inserts are different to the blind hole version, they have a center coil that is kinked outwards to hold it in position. I have used them with great success. Have a word with the man at Tracy tools who specialises in inserts.

peak426/01/2021 22:57:43
avatar
1357 forum posts
151 photos

Could you fill the hole completely, with one of the low melting point aluminium solder/welding rods, and the jut re-drill and tap.
Maybe one side smaller to as not to impinge on the thread/solder interface.

Bill

Jeff Dayman26/01/2021 23:15:04
2069 forum posts
45 photos

My suggestion would be not to waste your time with old pot metal (which was just barely up the job at the start due to the small wall thickness at the boss, and may have been cracking internally since). You will just end up with disappointment when it breaks somewhere else, maybe splitting when tapping for helicoil, or strips again if JB weld is used. The geometry looks simple enough that the part could be remade as a steel fab of a central machined block and separate arms welded or silver soldered on the sides. Or if you have machine capacity you could carve it out of one block of 6061 or 7075 aluminum. Either way you will not likely ever have an issue with it again. Do it right and do it once. Just food for thought.

AJAX26/01/2021 23:32:01
137 forum posts
42 photos
Posted by AJAX on 26/01/2021 22:43:18:
I've had a few more thoughts about a possible solution. If re-tapping / threading the hole is a problem then I could take an entirely different approach and turn a plug that fits in the centre hole (see photo below) and cross-drill and thread that plug. It's not quite as simple as it sounds, as the centre hole is used for the height adjustment, but that shouldn't be too difficult to modify either.
h3

I will probably be back...

As promised, I'm now back after having another look at the tilt and height adjustment.

The easiest, cheapest and strongest solution will be as mentioned above. I will turn a cross-drilled and tapped shaft to fit in the centre hole. The tilt adjust screw can then go through the existing dodgy thread before passing through the newly tapped shaft.

The hole is bored to accommodate the height adjustment screw and a spring (adds tension to the bandsaw blade), but springs can be easily added to the rods either side and then all I need to add is a small bush. I tried a couple of valve springs from my junk box and it worked fine, but stiffer springs would probably be even better.

It's actually much easier than I had thought.

I'll try to get it done one evening later this week.

Neil A27/01/2021 02:35:01
87 forum posts

That looks to be a good solution to your problem.

I can now see that my idea was a non-starter, I had not understood how the screw functioned.

Neil

Howard Lewis27/01/2021 11:45:09
4448 forum posts
8 photos

Possibly being pedantic, and sticking to prototype, what was the original thread? Possibly not Metric but Imperial or Unified? In which case it might be 5/16 rather than M8

If BSF or UNF would give a finer adjustment ( 22 or 24 tpi ) rather than 1.25 mm which is nearer 20 tpi.

Howard

AJAX28/01/2021 00:50:50
137 forum posts
42 photos
Posted by Howard Lewis on 27/01/2021 11:45:09:

Possibly being pedantic, and sticking to prototype, what was the original thread? Possibly not Metric but Imperial or Unified? In which case it might be 5/16 rather than M8

If BSF or UNF would give a finer adjustment ( 22 or 24 tpi ) rather than 1.25 mm which is nearer 20 tpi.

Howard

5/16" was my original assumption and subsequently confirmed by measurement. I believe these Walker Turner bandsaws were made in the USA in the 1930s / 40s and that must have an influence on the threads we can expect to see.

Thanks for your contribution

AJAX28/01/2021 00:59:24
137 forum posts
42 photos

As mentioned earlier, I decided to go for the simple and secure option with a turned "shaft" mounted in the central hole and then cross drilled and tapped. I won't bore you with the details as it was an easy job requiring just a couple of new parts and the height adjust screw being shortened. I used brass for a bush and aluminium (type unknown) for the shaft as that's what I had in the junk box. The tilt adjust mechanism now feels rock solid and I have confidence it cannot easily fail.

20210127-200714
Howard Lewis28/01/2021 15:54:47
4448 forum posts
8 photos

Thanks for telling bus the outcome.

Glad that you have it working to your satisfaction.

Howard

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