ML7 Change wheel set up help please!

[Martin King 2Participant @martinking2]

Hi All,

Just managed to set up the change wheels to allow me to cut a 26tpi thread. This had to be done by rotating the chuck by hand as the threaded length was only about a 1/4".

Managed all this quite well and achieved the end result of a couple of new brass caps for a pair of antique spirit cans.

This was about a couple of weeks ago and I am damned if I can remember how to put the wheels back where they were in the first place! Naturally I have lost the scrap of paper that I wrote down the initial positions on…..

Can any one give me a diagram or explain in words of one syllable please.

Regards,

Martin

[Martin King 2Participant @martinking2]

Problem getting back to how they were!

[Jon GibbsParticipant @jongibbs59756]

Hi Martin,

You need pages 24, 25 and 26 of the manual (on pages 29, 30 & 31 of this pdf document)…

**LINK**

HTH

Jon

[Martin King 2Participant @martinking2]

Hi Jon,

Many thanks for that useful pdf.

I see the iilustration but what would be the standard factory feed set up when you get the lathe? Will that be in tpi as if it were a thread pitch?

I can see me driving myself nuts trying all the wheels to try and get the right set. I know that I must seem very dim about this but if you can tell me to put the x tooth wheel on the lead screw and the y tooth on the idler etc I wll understand better.

Still a complete novice here, enjoying the whole process very much most of the time and then something simple ( to you guys!) throws me completely and I feel a real halfwit!

Regards,

Martin

[Brian WoodParticipant @brianwood45127]

Hello Martin,

You want it simple—will this do?

20 tooth driver on the shaft from the reversing cluster, 65 tooth driven wheel on the leadscrew and a 70 tooth idler gear to link them.

Result 26 tpi

Regards

Brian

[Les Jones 1Participant @lesjones1]

Hi Martin,
If by "put the wheels back where" you mean to the position to give a fine feed for turning (As opposed to 26TPI) then you would want one of the last two settings in Fig.37 page 27 The bottom one will give 1.8 thou per rev but you will need the optional 12 tooth gear for that. the next to bottom setting gives 3.7 thou per rev feed.

Les.

[Martin King 2Participant @martinking2]

Hi Les, That wass the one I want, thanks so much for that.

Brian thanks for the info, that is how the lathe is set at the moment, it was getting back to the first setting that I need.

Regards,

martin

[Les Jones 1Participant @lesjones1]

Hi Martin,
Glad your problem is sorted, Jon Gibbs deserves the thanks for the link to the manual. All I did wa read the chart.

Les.

[Martin King 2Participant @martinking2]

It had not clicked in my head that the initial setting for fine feed was just a thread with a really, really fine pitch!

One other thread problem I find is that I have a lot of small jobs where i have a threaded fitting on the end of a can spout or a small filler cap and need to make the mating part for it.

It is easy to find the tpi with gauges and I can measure the diameter of the male thread in order to make the replacement or new part but working out the diameter for the mating female part to be drilled and threaded always seems to cause problems as just measuring the internal diameter does not help. I cannot seem to use the mating male part as a 'template' and keep ending up with a part that is too loose (usually) or sometimes way too tight.

Obviously if these parts are a standard thread diameter the problem does not arise but most of these items that need fixing have odd diameters and thread pitches. They are usually old French spirit or oil cans in many odd sizes.

So far my attempts at internal 26tpi threading have been pretty dismal but I have a lot of brass and plenty of time….

Any thoughts please?

Regards, Martin

[roy entwistleParticipant @royentwistle24699]

Martin You can get 26tpi taps ie brass or cycle thread but usually imperial sizes

Roy

[Neil WyattModerator @neilwyatt]

Do what I do when making a critical thread – keep trying the fit as soon as you get near size.

One thing that causes problems is that cutting a thread from oversize stock gives a profile with sharp crests to the thread. This makes it tight, so you cut it undersize and then when it is used in anger the sharp tips distort and the thread becomes loose. If you haven't got a tool that will not form an accurate top profile, its always worth making the top diameter rather less than the nominal diameter as flat-topped threads won't cause you problems.

Neil

[Jon GibbsParticipant @jongibbs59756]

Hi Martin,

Sorry I should have mentioned the finest thread pitch set-up being likely to be the one you wanted. Glad Les was able to set you right.

Small internal threading jobs are always going to be tricky to cut because of springing in any tool small enough to fit inside the bore without rubbing.

I can thoroughly recommend Martin Cleeve's book "Screwcutting in the lathe" …

**LINK**

…good value too.

HTH

Jon

[Martin King 2Participant @martinking2]

Roy, I have the 26 tpi taps and dies but the French items are all non standard diameters so must be screw cut.

Neil, I get what you are saying but is there a way of calculating the internal turned diameter?

Lets say the external diameter of a male part on a can spout is 9.4mm across the thread tops; is there a way of working out what the internal diameter of the female part needs to be for a 26tpi thread to fit the spout?

I have about 4 of these to do and another 9 cans with hugely differing end diameters but all 26tpi.

Likewise working the other way, I have many that need new filler caps so can only measure the internal diameter at the thread tops and need to work out what the turned external diameter of the cap needs to be before screwcutting the thread.

If I can get my head round all this I anticpate being really good at this before I run out of cans…

Thanks to all for putting up with all this from me.

Regards,

Martin

[Keith LongParticipant @keithlong89920]

Assuming that the thread is of Whitworth form then 26tpi has a thread depth of 0.0246 inches, so a basis for starting from would be for an external thread use nominal od and cut the thread to the given depth which would give you a core diameter of (nominal o.d – 2 X 0.0246) . For an internal thread the core diameter would be the very minimum size hole you need, so as a start use that. That would (should) result in a very tight fitting thread as you're not making any allowance for clearance but it's always possible to open the internal thread up a bit if you can try the male thread in it – far more difficult to machine a bit back on if it's a sloppy fit!

Other thread forms will have a slightly different depth,

Whitworth – thread depth is 0.64 x pitch

BA – 0.6 x pitch

British Cycle Thread 0.5327 x pitch

Also details of crest and root radii differ.

Tubal Cains Model Engineer's handbook has most of the details and shows how the figures can be used to get either the male or female thread dimensions.

 

(spelling corrected)

Edited By Keith Long on 12/02/2015 16:41:15

[Keith LongParticipant @keithlong89920]

Martin, I guess that most of the threads that you are interested in are "Brass Threads" – all 26 tpi regardless of diameter in which case this table might give you most of the information that you're after. Thread o.d.'s between the sizes listed – possible if the items are of French origin and made to metric sizes – should be pretty easy to work out simply by interpolation.

[Martin King 2Participant @martinking2]

Keith, that table is perfect thanks a lot!

martin

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