Cutting metric threads

[Tony Pratt 1Participant @tonypratt1]

 

Edited By Tony Pratt 1 on 20/05/2013 22:47:01

Edited By Tony Pratt 1 on 20/05/2013 22:47:16

[Ian PParticipant @ianp]

Posted by rebekah anderson on 20/05/2013 22:29:48:

They don't do 120mm.

Yeah they are lacking arent they. I bought two, one very short and one very long. The caps are short.

The tubing is from here http://shop.premacon.com/index.php?cPath=35_37

might change if i feel the need.

Do they do the straight tubing to make your cylinders out of, I can't see it.

Ian P

[Michael GilliganParticipant @michaelgilligan61133]

Posted by Ian Phillips on 20/05/2013 21:52:10:

Michael

As you say, expensive, but they are carbide. It good to know that they are available anyway.

I cannot make sense of their measurements, reads like they can be used on hole diameters less than zero! even with a one thou diameter shank I think thats a wild claim!

Ian P

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Ian,

I think you're reading the tolerances … the dimensions of each tool are also listed.

MichaelG.

[rebekah andersonParticipant @rebekahanderson95322]

Posted by Ian Phillips on 20/05/2013 22:47:04:

Posted by rebekah anderson on 20/05/2013 22:29:48:

They don't do 120mm.

Yeah they are lacking arent they. I bought two, one very short and one very long. The caps are short.

The tubing is from here http://shop.premacon.com/index.php?cPath=35_37

might change if i feel the need.

Do they do the straight tubing to make your cylinders out of, I can't see it.

Ian P

Sorry i thought you meant the rubber tubbing.

I'll be turning the cylinders from solid bar

[Michael GilliganParticipant @michaelgilligan61133]

Posted by Ian Phillips on 20/05/2013 22:43:47:

Michael

I was pointing out that a single point of a 1mm pitch tap will have the wrong thread form (the radius will be much to large). Both 60 degrees though.

Ian P

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Ian,

Fair enough … May be better to start from a 0.5mm pitch tap.

MichaelG.

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For info. … Wikipedia has a useful illustration of the profile.

Edited By Michael Gilligan on 20/05/2013 22:53:07

Edited By Michael Gilligan on 20/05/2013 23:04:18

[Ian PParticipant @ianp]

Michael

I forgot the smiley!

But the webpage did not state they were tolerances.

Ian P

[John Stevenson 1Participant @johnstevenson1]

Change wheels are the same as ML7 but ML7 has a 1/8" keyway, ML2 has a pin to locate and drive.

There were some intermediate gears made post war that had keyway and pin holes but It's a simple jobe to drill a hole in an ML7 gear using an existaing gear as a pattern.

To cut metric threads you need two 21 tooth wheels plus the usual standard set, not quite sure if a small tooth count 21T gear has enough meat on it to accept a pin ?

[Anonymous]

Cutwel do an internal 60° micro threading tool that will cut 6mm internal threads. Downside is that the tool and holder cost about £100.

Alternatively, Cutwel do a 6×0.5mm spiral flute tap at a much more reasonable £9.53p, plus the dreaded VAT. A spiral flute tap should get you within 1-2 thread pitches of the bottom of the hole, which should be fine since you have fairly deep threads. The drill data for the spiral flute tap specifies a 5.5mm drill, so just use a 5.5mm slot drill to give a flat bottom to the hole.

If I was going to screw cut the threads, I'd change the pitch to give a tpi of 48, then it would be dead easy to cut on what I assume is an imperial lathe, without having weird gear trains and needing to keep the half nuts engaged.

The only way to get a full thread running the full depth of the hole is to thread mill or fabricate, ie, make a threaded tube and then screw in an end cap, secure, and machine to size.

Regards,

Andrew

[Ian PParticipant @ianp]

Rebekah

Making the cylinders from bar will be challenging! I certainly would not want to tackle it.

How deep will you be drilling and how will you ream/lap/hone/polish or whatever the long cylinders?

Ian P

[Ian PParticipant @ianp]

Andrew

Thread milling is a subject I have not fully grasped, but a 'one pitch wide' run out groove would enable it to be screwcut normally. For Rebekah's cylinders she has said she wants very short threads.

Suggesting 48 TPI is brilliant, it will make no difference to the finished model but makes perfect sense with an imperial lathe.

Ian P

[rebekah andersonParticipant @rebekahanderson95322]

The cyliders will be emotional but i have a few ideas.

[John Stevenson 1Participant @johnstevenson1]

Becky, did you finishe that little CNC ?

You could thread mill them on that if you did.

[rebekah andersonParticipant @rebekahanderson95322]

I havent yet. Mainly because i haven't got a pc for it. Plus i need to re-do the set uo.

[Michael GilliganParticipant @michaelgilligan61133]

Posted by rebekah anderson on 20/05/2013 23:04:09:

The cyliders will be emotional but i have a few ideas.

.

I would suggest starting with oversize bar …

First get the bores right; then skim the outside diameter to size, with the job mounted between centres.

MichaelG.

[JasonBModerator @jasonb]

You could use a 7/32" slot drill to give a flat bottomed tapping size hole. Then grind up a small tool to put an undercut at the bottom of the hole which would allow you to thread with a M6x0.5tap with the end ground down with no part formed thread at the bottom of the hole.

Alternative would be to omit the undercut and turn say the first 2mm of the male thread to core dia so it will pass right to the end of the cap even if its not threaded.

J

[David JuppParticipant @davidjupp51506]

I think that I might be tempted to try using loctite (or similar) to hold the ends on – would save all that very fine threading – unless the screwed cap also pinches the rod seal…

Or maybe try to find some off the shelf brass steam fitting nuts that could be modified.

Or drill/tap thru, add male threaded plug – solder plug to seal, drill for rod.

[KWILParticipant @kwil]

Cutwel's Micro Threading tools themselves are around £28 + vat, you can easily make the holders. Expensive but needs must sometimes.

[rebekah andersonParticipant @rebekahanderson95322]

Can I just say cutwell is a great site. price is reasonable compared to most. I shall be using them for this and buy the relevant tools.

now I just need to sort out the gears on the lathe I will get a picture up of my current set up and hopefully you guys can help me out

thank you so much so far for your help.

[Ian S CParticipant @iansc]

I build hot air engines, and on occasions I screw the cylinders together, I use 32 tpi regardless of size, up to 2 1/2". Select a thread, 32tpi or finer, cut a few practice threads, then go for it, just use the lowest speed in back gear, you'll soon get the hang of it. Ian S C

[Grizzly bearParticipant @grizzlybear]

Hi Rebekah, What a brilliant project you have. I can't wait for the next instalment. Not a model maker myself, but I'm in awe of the people on this forum. Regards, Bear..

[Sub MandrelParticipant @submandrel]

For internal threading I have a boring bar that takes 1/8" HSS bits. It's easy to put a 60-degree point on the end of one, I also made one for the end that hold the bit at 45 degrees, so I can cut a thread up to an internal shoulder (lid of the fuel tank for an unfinished Sparey 0.5cc diesel).

It's been mentioned that the thread can be any convenient pitch, but the next MEW will haev some advice on changewheels for metric screwcutting.

Neil

[Michael GilliganParticipant @michaelgilligan61133]

Posted by Stub Mandrel on 21/05/2013 21:46:02:

For internal threading I have a boring bar that takes 1/8" HSS bits. It's easy to put a 60-degree point on the end of one, …

Neil

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Neil,

'tis true … but the tricky bit is that Rebekah wants to cut a 6mm diameter internal thread

Making a suitably sized boring bar to take such bits might be difficult [where "difficult" rapidly approaches impossible].

MichaelG.

Edited By Michael Gilligan on 21/05/2013 22:09:25

[Clive HartlandParticipant @clivehartland94829]

I have read the posts with interest, I had to make some fibre optic terminations and they were very small indeed. I bored out the cavity and then using a ground off tap threaded the hole for the clamping nut. I did cut a small recess at the base of the cavity and it all worked fine. The metal was 306 S/steel as it was going to be used under water , the dia. was 4mm x 0.35 thread.

We had minute boring tools, and cut at a fairly high speed and low feed.

Clive

[Anonymous]

Posted by Ian Phillips on 20/05/2013 23:03:36:

Thread milling is a subject I have not fully grasped, but a 'one pitch wide' run out groove would enable it to be screwcut normally.

Neither have I in a practical sense. However, I do have the CAD model created, CAM toolpath created and G-code generated and checked. For a practical experiment I've been put off by the cost of thread mills, expecially if it goes 'ping'. However, I have recently ordered some single point thread milling tools from Maritool in the US. Watch this space for the results!

Andrew

[John Stevenson 1Participant @johnstevenson1]

Andrew, get a tap of the correct pitch and grind all but one flute off it, instant threadmill.

For larger threads you can use one inset from a Coventry diehead.

[Author]

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