Thread Recutting?

Hi All,

I have an old motorcycle that I need to make a flywheel extractor for (new ones are not available) but the thread in the flywheel that the extractor screws into is an unusual size - M33 x 1, which is a bummer as I can easily buy an extractor with an M33 x 1.5 thread cheaply. Does anyone have any suggestions as to how I could modify the standard extractor? Would it be possible to cut a 1.0 thread into the 1.5? Would it be possible to build up the thread with weld and recut it to 1.0?

Any suggestions are welcome!

Cheers,

Colin

I doubt that would work as you would end up with little thread as you are trying to cut a shallower thread into only the crests of the 1.5. Easy enough to make one or find a local job shot that can.

Welding and then threading could be very problematic. If you have enough material on the old extractor you could drill/bore it and fit a bush with the correct thread. Or just make your own extractor if you have the facilities.

regards

And the old motorcycle is? I have a puller for an old motorcycle, too, but not the motorcycle. It is a metric thread, for sure…

As for modifying an extractor, it might be easier to go one side smaller and make an adaptor collar, assuming the end of the crank allows.
That means two threads to cut of course, unless you weld the ring to the new puller.

Which bike is it for; reading between the lines 350 Ducati, but single or twin?

The owners club might hire them out.

Bill

The bike is a Spanish-made Ducati 350; although they are similar to Italian-produced ones, they are not the same. I could make one, but it would be a lot of work, as I only have a lathe not a mill, but I can buy the 1.5 threaded type for £13.50. Possibly I could cut off the threaded portion and re-thread it to 1.0 (the thread is external) or make a stepped collar that threads onto it with the correct thread at the bottom, or weld a threaded collar on, although I have no welding facilities either.

Thanks,

Colin

Can your lathe cut 1mm pitch threads? A bit of mild steel would do, and the centre bolt could be any thread which is strong enough, it doesn't have to be exactly the same as the original, just the tip has to fit the crankshaft without damaging it.

If I've understood how this works correctly, could you buy one suitably undersized, loctite a nut on then cut the required thread onto the nut. There seem to be some around for less than £10 at m24/m28

Edit: just realised that's essentially what Bill suggested above

Edited By Tris on 14/01/2023 19:09:56

Edited By duncan webster on 14/01/2023 20:51:37

Having a lathe, this is the way to do it. If you are new to thread cutting, it will take some time to learn but it is time very well spent.

As a last resort, find some one near that will cut the thread for you, watch how (s)he does it and learn something new.

Edited By Huub on 14/01/2023 21:40:53

A picture of the situation would be very helpful. Then a better bit of guidance can be given.

That might be your best solution right there. Personally though, I would make one complete out of mild steel. Usually good enough for occasional hobby use. You don't need a mill to put the spanner flats on it. Either file them on good enough for a spanner to grip, or just hold the flywheel to stop it turning. Or hold the puller with vice grips etc.

As you probably already know, with a mild steel puller especially, don't just keep screwing the central bolt in until one of the two threads strip (usually the large one for some reason). Tighten it up as tight as you dare without risk of stripping anything, then give the head of the bolt a good sharp rap with a 2LB hammer. The shock will usually break the taper loose where steady force will not. And lubricating the bolt thread is a big help too.

Thank you for all your replies, much appreciated as usual. I will go through my scrap bin and see if I have some suitable sized mild steel round bar and will turn up something to do the job.. I have never done thread cutting on my lathe, but will see if it is possible; if not I will get my local engineering firm to cut the thread.

Thanks everyone,

Colin

There are a lot of thread cutting video's on YouTube. This will get you started.

This thread has only a small (1 mm) pitch. To keep it "simple", you can cut this thread at a 0° infeed angle (top slide angle). To avoid overloading the lathe, you can do the cuts using cutting depths of 0.05 mm.

For a self ground sharp tool (60° tip), you need a total cutting depth of 0.65 mm.

If you find the thread to loose, you can make a new one and cut it a bit (0.05 mm) shallower.

Success

Thanks Huub,

This will be a good excuse to practise thread cutting, and the good news is that I have found a piece of 35mm mild steel bar which is about a foot long, so will give me plenty of material to use!

Cheers,

Colin

I'd hold it in a 4 jaw chuck (or a collet if you have one) then if you make it too big, you can set it up again and take another cut. A sheet of white paper on the bed with a bright light shining on it helps, just engage half nut, wind the saddle back with the handwheel to take up backlash and fiddle with cross slide and top slide till you can't see the light

Edited By duncan webster on 15/01/2023 15:26:57

Thanks Duncan, great info.

Don't be afraid to remove the threaded part to offer up to the flywheel. To cut deeper, all you have to do is back off the threading tool, bring it into position halfway along the thread and shut the leadscrew nuts. Turn the chuck by hand to remove the backlash, and bring the tip close to the job. Use the compound to get the cutter aligned with the groove of the thread, using a piece of white paper and a torch to see better. Even a 1mm pitch can be matched closely, and it is just an extractor, not an aircraft part.

Thank you. I have been meaning to try my hand at thread cutting for a long time and this is the perfect chance to learn.

Cheers,

Colin

It would be best to practice on something soft like aluminium or plastic. Even a small thread like1mm pitch will need to be cut in at least 3 passes. The thread total depth will be 0.5mm from initial contact, and I would go 0.2, 0.15, 0.1 and 0.05 depths getting less as the tool has to cut more each pass. An aluminium thread of 8mm or longer engagement in the flywheel would be more than strong enough to make an extractor without resorting to using steel. And a 12mm thread with a decent bolt in it would finish the extractor, it won't be used very often.

Another thing, if you make the extractor body double ended, if the first end is too tight, remember the depth of the last cut and do the other end deeper by about 0.1mm.

Edited By old mart on 15/01/2023 19:07:31