You need to practice on smaller threads, say 12mm, until you are confident

You may find it easier to do it by hand, instead of under power, especially if you only need a one-off item

GL

Make a crank-handle to fit into the spindle from the headstock end. The sort of expanding clamp used to attach bicycle handle bars is suitable. Turn an aluminium rod about 50mm long to just fit inside the lathe's spindle. Drill it through to take a suitably long bolt or length of studding, loose fit (say 1mm oversize). Cut the 50mm rod in the middle at an a sharp angle ( less than 45° ), so the two halves wedge into the spindle when the bolt is tightened. Put a handle of the other end. Although slow it gives plenty of control for thread cutting, which is an advantage in this case.

3.5mm pitch is a very coarse thread and cutting it will stress the gears. Take gentle cuts to avoid breaking anything. The handle lets you feel excessive force being applied.

Dave

Edited By SillyOldDuffer on 31/07/2020 10:06:23

+1

If you can get a tap for 22quid, grab it, a die too

Do most of the work on the lathe and only finish off with the tap/die

Starting the thread on the lathe also ensures it is parallel/straight, not as easy  to achieve as you would imagine, especially on a big piece

They become part of the lathe if you ever sell it and are a huge help for useful bits if you hang onto it

Edited By Ady1 on 31/07/2020 10:42:35

Captain Barnacles

I can offer four thread set ups for this pitch

For a Myford ML7 with gearbox the Mandrel gear will be 33 teeth; it replaces the 24 tooth gear of the standard imperial gearing. Match that to 10 TPI selected on the gearbox—the pitch generated will be 3.493 mm

For change wheel machines here are three alternative choices. They all need a 63 tooth wheel in the gear train

Mandrel 38—drives 63 on the first stud which is linked to 55 teeth. Idler(s) will be needed to couple to 30 teeth on the leadscrew. Pitch outcome is 3.51 mm

Mandrel 50—drives 63 on the first stud which is linked to 50 teeth, Again idler(s) will be needed to couple to 36 teeth on the leadscrew. Pitch outcome is 3.499 mm

Mandrel 60—drives 63 teeth on the first stud which is linked to 44 teeth. Once again idlers will be need to couple to 38 teeth on the leadscrew. Pitch outcome 3.501 mm

Others have suggested you practice on easier pitches to start with and use a mandrel handle for manually cutting these coarse threads

Finally you ask what hole size is needed to tap your 33 mm x 3.5 mm thread. It should be 29.5 mm diameter

Kind regards Brian

Edited By Brian Wood on 31/07/2020 20:52:46

Edited By Brian Wood on 31/07/2020 20:53:19

Machining a chuck backplate or faceplate can often be one of the early jobs that a new lathe owner will have to tackle, a good read up on the way to approach the job will be very helpful. Screw cutting is a very useful skill to acquire and a good understanding of the features of a screw thread is a good start followed by how to sharpen a tool for the job and set the lathe up, measuring what you are doing and knowing what your target is, is important. ReadIng up and watchIng some videos will help to get a clear plan of how to do the job. A few practice runs will help get a feel for the task and boost your confidence, hopefully.

Mike

Captain, most things have been said already however the first thing I would make is plug gauge to test the internal thread you are cutting. Measure the mandrel thread using the 3-wire method then replicate the thread on a piece of steel bar so you can be sure the thread in any attachments you care to make will fit the mandrel on the wood lathe.

Also remember you cannot disengage the half nuts when cutting a metric thread on an imperial lathe, you must travers the carriage back between each cut. However if you do have a chasing dial there is method of using it but not in the normal way, I do have a written explanation if you want it but have a look at this link for a pretty good video of the method **LINK**

Lastly you asked the bore size for the thread and Brian is correct at 29.5mm but I would recommend you truncate the thread by 0.1 – 0.2 mm so make the bore 29.6 – 29.7 thus ensuring clearance on the root of the mandrel. Also remember to take this into account when cutting the thread to depth.

John

Hi Captain, while I don't have a myford 7 and therefore am unable to comment on if it is able to cut a 3.5mnm pitch thread. I can help with some general information about cutting threads.

As Brian Woods said your starting hole needs to be 29.5mm, it's easy to work out in metric just take the pitch away from the nominal diameter. Ie 33-3.5=29.5. Also you will need a lathe tool with a 60 degree cutting tip as all metric threads are 60 degree.(a lot of imperial threads are 55 degree)

If you decide to use taps to cut your thread you will most likely need a set of three taps: taper, second and plug. Because the thread will need to go all the way to the bottom in order to screw on the shaft.

Hope some of this helps, Tom.

One of the newer things these days is live tooling. So a router is used and is set up on the tool post with an adaptor and with a 55 deg, or 60 deg multi flute cutter. And away you go. For external threads is mounted in line with the work piece and a D bit will work with a 60 deg or 55 deg depending on the thread form of course. With multi point cutters, one idea being used is the cutter has 4 teeth, 2 at the full form and 2 at less than full form. The lesser form does a rough out while the full does the final finish . These work better than just all 4 as a full form.

Here is a link to Stefan Gotteswinter on youtube who shows some things with his set up..

Neil

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.

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.