Thread Recutting?

[COLIN MARTIN 2Participant @colinmartin2]

[COLIN MARTIN 2Participant @colinmartin2]

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

[HOWARDTParticipant @howardt]

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.

[OldironParticipant @oldiron]

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

[not done it yetParticipant @notdoneityet]

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…

[peak4Participant @peak4]

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

[COLIN MARTIN 2Participant @colinmartin2]

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

[old martParticipant @oldmart]

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.

[TrisParticipant @tris]

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

[duncan webster 1Participant @duncanwebster1]

removed

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

[HuubParticipant @huub]

Posted by old mart on 14/01/2023 18:02:58:

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.

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

[Neil LickfoldParticipant @neillickfold44316]

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

[HopperParticipant @hopper]

Posted by COLIN MARTIN 2 on 14/01/2023 14:44:17:

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

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.

[COLIN MARTIN 2Participant @colinmartin2]

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

[HuubParticipant @huub]

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

[COLIN MARTIN 2Participant @colinmartin2]

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

[duncan webster 1Participant @duncanwebster1]

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

[COLIN MARTIN 2Participant @colinmartin2]

Thanks Duncan, great info.

[old martParticipant @oldmart]

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.

[COLIN MARTIN 2Participant @colinmartin2]

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

[old martParticipant @oldmart]

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

[COLIN MARTIN 2Participant @colinmartin2]

OK, thanks, that's very useful.

Cheers,

Colin

[HopperParticipant @hopper]

To test the thread for fit in the flywheel, take the whole chuck off the lathe with job intact and try screwing it into the flywheel. Then if the thread needs more cutting, just put the chuck back on the lathe where it came off. Assuming of course you have a small hobby sized lathe and not something with a 12" diameter chuck etc!

And you can buy a 30 x 1mm die on Aliexpress for about 10 Quid and use that to final finish the thread. Just takes a few weeks to get to you from China.

Edited By Hopper on 16/01/2023 09:33:03

[COLIN MARTIN 2Participant @colinmartin2]

I thought that I would give an update on this thread.

A few weeks ago I decided that as it is Winter I would commence the strip-down of my 250 Ducati engine, as I knew that the bottom camshaft drive bevel gears were badly chipped and I had managed to find a good used pair as a replacement; unfortunately to do this the crankshaft has to be removed, which is a pain. Thus I needed to finally make the flywheel extractor that I asked about at the start of this thread.

I had some mild steel round bar which I turned down to 33 mm, but I happened to come across a 33 x 1 die on eBay very cheap! So I bought it and luckily found someone selling a cheap used die holder (which was obviously almost new). I bored the id of the steel to fit over the end of the crankshaft, parted it off, drilled and tapped a hole for a fine thread 12 mm bolt and used the die to thread the end to 33 x1. It wasn’t pretty, but it worked, the flywheel came off much easier than expected, so I was pretty pleased with myself.

After stripping the engine it was obvious that it was not in such good condition as it had appeared – there was a lot of rust evident, probably from being laid up for years before I bought it, and a couple of the bearings were pitted, so it made sense to change them all, then after that it made sense to have the roller big-end apart to check the condition of that as it was almost 45 years old. Unfortunately the two local firms that I would have used were both out of the question, one because the owner is in hospital, and the other because it has changed hands and has gained a poor reputation. So as I had to send the crank away, it made sense to send it to the best, Lacey Ducati in Wales.

The big-end was found to be rusty too, so a new one was fitted and the crankshaft returned to me, and I am now in the process of putting the bottom end together, and I hope that after careful assembly it will be better than new. After this I will have to think about doing the 350, and I will be needing to ask for some more engineering advice from this forum!

Thank you to all those who offered advice on making the extractor.

 

 

Cheers,

 

Colin

[DiogenesParticipant @diogenes]

Thank you for following up with a ‘conclusion’ to round out the thread – good luck with the rest of your rebuilding..

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