Cutting an M33 x 3.5 thread on my ML7

Hello all,

I am taking delivery of a new wood lathe in the next couple of weeks and it has an M33 x 3.5 thread on the headstock. I was wondering if it is possible (given my very limited experience) to produce items like face plates etc for the wood lathe using my ML7?

I have a selection of change gears and I understand that it is possible to cut metric threads but I am a bit hazy on the set up. Looking at the chart It looks like I need to use B50 D75 A30 C45 E63 Is that correct? Presumably I then need a thread cutting tool on a bar? Can I then cut 3.5mm pitch threads on anything I can fit in the chuck? What size should I hole should I have before I start cutting internal threads?

Many thanks in advance for any advice.

I was wondering if it is possible (given my very limited experience) to produce items like face plates etc for the wood lathe using my ML7?

It is possible but some experience would be beneficial... I expect lots have done it in the past, while not particularly experienced.

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

Thank you. So manually rotate the chuck, I can see the sense in that. It's difficult to do much damage at 6 RPM I suppose .

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

There are plenty of taps available. £22.61 could get you one (don’t know what quality it might be) from epay.

While I would not suggest you try to use one just like that to cut the thread, you could rough it out the thread on the lathe and finish it off with the tap.

+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

3.5 mm pitch is not too far removed from the 8 tpi of the Myford Leadscrew. You need to set up a gear train so that the Leadscrew runs at 1.1023622 times that of the chuck . A 33:30 will give the closest approximation (33.07087) , but using standard gears 55:50 will be the next nearest approximation (55.11811 equating to 0.2147454% error ).

By using a compound train, it should be possible to reduce the error even more. But you are looking to provide a ratio of 1:1.0021475, which is not possible with standard gears, incrementing by 5T..

Making a 22T gear, to drive a 20T on the Leadscrew, via one or more idlers will not improve matters. (You would probably need to use the tumbler reverse ) The ideal gear with 22.09459T, would give an error of 1.0042995, or 0.4299545% which is worse

Have a play with an EXCEL spreadsheet to see if you can come up with a more accurate gear train..

As already said, practice on smaller material to get the pitch right, before attacking expensive large diameter material!

HTH

Howard

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

Do remember that it is the register surfaces that are, by far, the most important part - the thread is only there to keep those registers together precisely, so generally doesn’t need to be absolutely perfect in form.

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

Thank you all so much, there's some fabulous information for me to digest.

I hadn't considered using a tap, I suppose that's a great example of Occams Razor at work. It's an approach that would give me the result that I'm looking for quickly and easily. I was rather looking forward to trying thread cutting on the lathe and it's a skill I would like to become competent at in case I need to machine something for which I dont have a tap or die. I think in the case of M33x3.5 though a tap would be a good investment as I plan on making several accessories.

I really like the idea of using a hand crank to rotate the work at a very controlled speed. I have a VFD on my lathe and can slow it right down but the torque seems to drop off quickly at the lower end of the speed range and I imagine that when cutting threads torque is all important.

My ML7 doesn't have a gearbox unfortunately. It looks as though I may need to buy additional change gears as mine all are in increments of 5. I have downloaded Duncan's app for calculating change gearing so I'll be off down the workshop this morning to take stock of exactly what gears I have available and see what I can do with them. I can see that it may be worthwhile purchasing additional gearing as I'm much more likely to cut metric threads in future.

One further thought, what material choice would you recommend? When I have used face plates in the past (for wood turning) the small ones, say 2-3", seem to be steel but larger ones seem to be lighter, perhaps some alloy?? Would standard grade aluminium be too soft (not to mention expensive)?

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 Cap'n!

Some thoughts; I wouldn't see a tap as a quick and easy way of cutting an accurate 33mm x 3.5mm thread unless it's used purely for final finishing to size of a thread previously cut in a lathe. It's far to much metal to remove IMO and will be very difficult, perhaps impossible, to drive, esp. in steel.

Aluminium might be OK. Cast-iron should be good, although still fairly tough going. With a mandrel handle you will find steel very hard work with that size of thread, even aluminum won't be that easy. Don't forget also that, for a metric thread on your machine, you can't disengage the half-nuts to return the carriage after each cut. It will have to be wound back. I'd favour using the VFD with the lathe back-gear and belt settings to give lowest speed with adequate torque and ability reverse the motor.

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.

Tom,

It would most certainly would not need three taps. It is a through-hole, so even a first tap would suffice, if long enough.

A 33 mm thread may need more grunt than is available if starting from scratch (particularly if cut in steel🙂 ).

A 60 degree cutter would be necessary if the OP cuts it all on the lathe. Cutter geometry is not an absolutely overarching necessity, if roughing out and finishing with any one of the three tap set - just being careful not to exceed 60 degrees is most important (but harder finishing with a tap, if roughed out at less than 60 degrees).

Depth of cut would clearly need to be less than half the pitch, if roughed out on the lathe - even if only to leave just a small finishing cut, if the cutter were 60 degrees, and obviously less if the cutter angle was ground slightly under the target angle.

Edited By not done it yet on 01/08/2020 12:36:22

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..

https://youtu.be/6_1Nlf-X4bw

Neil

Neil The OP has stated that he is cutting internal threads.

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.

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.