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Cutting an M33 x 3.5 thread on my ML7

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Hopper02/08/2020 01:11:11
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Posted by Neil Lickfold on 01/08/2020 22:39:04:

Not on the home lathe, but many cnc lathes with live tooling cut internal threads with a thread profile wood ruf cutter.

The advantage is a very clean thread that is de burred and has the leading feather edge removed. It also allows for a timed start of the thread relative to other detail on the part.

Many who do woodwork also thread mill internal threads on the lathe with small cutters down to 5/16 threads.I am looking at making a set up for my Myford lathe, with a variable speed trim router to drive the cutter based on either 6mm or 1/4 inch collet, and getting some cutters made in Carbide from a company in Auckland.

If you look on YouTube at old films of British factories making Howitzer shells and the like you can see the next step in this internal thread milling process. The cutter instead of being like a profiled woodruff cutter is multi rowed, looking more like a gear hob. So it is plunged straight into the job, which is then rotated one turn to cut all 10 or so turns of thread in one hit. Very time efficient and a perfect result every time.

not done it yet02/08/2020 08:46:15
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Yes, but we are hobbyists, not making thousands, or millions, of the same item on a production line (with several processes being carried out simultaneously or in quick succession) such as in a munitions factory. KISS principle should apply - at least for new starters - leaving the more difficult/specialised manipulations for the others.

Captain Barnacles02/08/2020 13:52:21
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Posted by Thomas Cooksley on 01/08/2020 16:29:01:

Hi Everyone, not done it yet I'm sorry, not being familiar with wood lathes I assumed that the centre hole in the face plate would be smaller than the size of the shaft leaving a lip to come up against the end of the shaft, thus requiring tapping right up to the lip.

Tom.

Good point. I believe that most faceplates do have through holes which allows the face of the threaded cylinder to mate up to a flat face on the headstock spindle. However, I have seen faceplates which do not have through holes and I do not know if these have a thread which is long enough to provide mating surfaces (as the through hole type would do) or if the "stop" is provided by the end of the threaded spindle coming up against the bottom of the faceplate's threaded cylinder. Apologies if my terminology isn't accurate, I'm not an engineer (yet!).

More good information to digest, I am most grateful for all the input. One thing I am realising is that it's not quite as simple as I first envisaged but I'm stubborn. Difficult/Challenging = Worthwhile in my book.

Thomas Cooksley02/08/2020 18:06:47
32 forum posts

Hi Captain, the more you do in this game the more you will find that there are many different names for things and processes as well. There are many modern names for things that more traditional engineers call something else, none of them Wong, just different.

The simple fact is that there will be a: stop, lip, register (call it what you like) to tighten up against. If you had a straight main spindle and a through hole it would just get tighter and tighter while you were turning your wood. The end of the spindle may come right through and damage what you are making. Also you would find difficult if not impossible to unscrew the two parts.

Tom.

Howard Lewis02/08/2020 19:50:26
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As a backplate for a chuck, there is no reason why the hole needs to be blind. So the thread can pass right through, making life so much simpler..

Having cut the thread, and measured the diameter, and depth, of the register on the Mandrel, the backplate can be screwed onto the mandrel in reverse, so that the register can be accurately bored into it.

Once this has been done, the backplate can be screwed on to the mandrel in the normal way. It is now ready for the O D to be turned to the size needed to provide an accurate fit in the chuck. Once this has been done the backplate can be faced so that its face is square to the axis of the mandrel, and is the correct thickness for the chuck in question.

The only job remaining is to mark out and drill the holes for the fixings..

Chuck and backplate can then be united and used a s complete unit.

Howard

Mike Poole02/08/2020 22:19:29
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My thinking on a backplate or faceplate thread is that a loose fit is more desirable than a tight fit, my thoughts are that the register should not be fighting the nose thread, the threads job is to hold the faceplate against the register and does not play a part in the true running of the faceplate. The effort needs to be put into getting the registers right rather than a showpiece thread.

Mike

Howard Lewis03/08/2020 14:23:41
3536 forum posts
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As Mike says, the register is the all important part of the job. A bit of clearance in the threads will allow the registers to perform their function and locate the chuck accurately.

Howard

Thomas Cooksley03/08/2020 21:39:11
32 forum posts

Hi Captain, we have all got tied up in cutting this thread and how it mates with spindle on your new lathe but how are you going to hold a steel blank large enough to turn a face plate out of on your Myford ML7? it's not going to fit in the chuck. IMO your only chance would be if you already have a face plate for your Myford then you could bolt your work piece to that. It would need to be very carefully balanced as well, that lump of steel is going to weigh several kg's and you don't want that getting out of balance on your lathe.

Tom.

Hopper04/08/2020 04:00:41
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Posted by Thomas Cooksley on 03/08/2020 21:39:11:

Hi Captain, we have all got tied up in cutting this thread and how it mates with spindle on your new lathe but how are you going to hold a steel blank large enough to turn a face plate out of on your Myford ML7? it's not going to fit in the chuck. IMO your only chance would be if you already have a face plate for your Myford then you could bolt your work piece to that. It would need to be very carefully balanced as well, that lump of steel is going to weigh several kg's and you don't want that getting out of balance on your lathe.

Tom.

Bolt it temporarily to a piece of smaller diameter round and hold that in the chuck.

Captain Barnacles06/08/2020 13:09:01
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Well I got around to trying thread cutting yesterday and it went ... OK-ish indecision

Actually, for my first time I was quite happy with the results. With my lathe I inherited a mountain of tooling, most of which means nothing to me (yet!). I had a rummage around, found what looked like an external thread cutter and mounted it the lathe. I set up the appropriate change gears and loaded a piece of 12mm mild steel bar in the chuck.

I suspect that the cutter wasn't a sharp as it could have been but I still got a result of sorts.

Onwards and upwards with a few more practice runs before commiting to a proper lump of material. I may also invest in a couple of replaceable tip thread cutters to make life a bit easier.

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