cut an internal thread

[j45on]

I am trying to make a light for my push bike I use for work and I need to cut an internal thread in the end of a now shortened maglite 3D body.
Trouble is I have never machine cut a thread before    the closest I have come is watching a mini lathe video  where the guy shows you how
to cut a thread without a threading dial.
The only bit i'm unsure about is where to position the compound I know the angle has to be 29.5 am I correct in guessing the compound has to come in from the rear ?


This used to be a scruffy corroded maglite
I bought the led's and lens last winter after seeing this thread http://www.thehubsa.co.za/forum/topic/23048-diy-cree-led-bike-light/ but never got around to building anything 

[atwatterkent]

To cut an internal RH thread, I rotate the compound so that the dial end faces the back of the bed, 12 o'clock, then rotate it 29 1/2 deg clockwise. You can then use the same compound infeed amount as if you were cutting an external thd.            Good luck,     Bob

[mardtrp]

 

To cut an internal RH thread, I rotate the compound so that the dial end faces the back of the bed, 12 o'clock, then rotate it 29 1/2 deg clockwise. You can then use the same compound infeed amount as if you were cutting an external thd.            Good luck,     Bob

But why move it AT ALL?

You can cut it just as easily with the compound left alone, why confuse yourself by moving the compound, it just adds confusion and does not help at all.

j45on, if you have never cut ANY thread before, then may I suggest you try cutting an EXTERNAL thread first, get it so you know what's going on, then try for the internal thread.

Just leave the compound alone and cut straight in, it works without all this black-magic art that others will try and shove onto you.  OK, OK, so the book says to do it this way, well, when was the bloody book written and maybe the tooling has improved slightly since then too.

For those that are going to say you have to move the compound around, then please send an email to Sir John, Earl of Suds-Pump-Water and tell him he's been doing it ALL wrong for many many years, just sit back and see what sort of a response you'll receive.

Mark

[No1_sonuk]

I use the angled compound infeed, BUT I use it on THE SAME SIDE as for external threads (29.5 degrees anti-clockwise from 90).
Just put the tool in upside down, contacting the BACK of the hole, and infeed in the same way as for external threads.

[atwatterkent]

I don't care to be drawn into an arguement about the better of two methods for threading that I am familiar with. I have never used the third method suggested and have no opinion about it. Both standard methods work and the direct infeed method is easier. I have used both and taught both methods and found that the offset compound method, although a bit more tedious, gives a better finish in my humble opinion, so that is the method I recommend.
Learning to thread externally first is a very good suggestion and I would urge anyone who is unfamiliar with threading operations to master that first. Bob

[Dean W]

But why move it AT ALL?

For one thing, it's easier on smaller lathes. 

You can cut it just as easily with the compound left alone, why confuse yourself by moving the compound, it just adds confusion and does not help at all.

If it confuses you then do it the way you like.  Most of us aren't confused by such a thing, and don't have any
trouble doing it with the compound set to 29.5.  There's nothing hard about it.

Just leave the compound alone and cut straight in, it works without all this black-magic art that others will try and shove onto you.  OK, OK, so the book says to do it this way, well, when was the bloody book written and maybe the tooling has improved slightly since then too.

Nothing "black magic" about the simple act of slewing the compound around.  And, the people who wrote the book also made the machines.
I don't really understand your attitude on what is just your second post to this forum.  You don't need to go knocking the way
other people do it, which is probably the way they've been taught, and which has worked for long before you were born.


Jason;  It would be best to practice on some outside threads first like Mark said.  At least you will be able to see what is
going on and learn what to expect.  Then move to the internal threads.  A few practice runs will save you some scrap.  You
don't want to ruin your flashlight piece while learning this.  Cutting threads is a common thing, but like anything else, the
first time you do it you will be feeling like you have to watch four things at once.  Run your lathe slow.  Slowest speed you
have will probably be best. 
Pay attention to the threading dial when you first engage the lead screw, and leave the tool bit
a little way to the right of the cut when you are engaging the screw.  That way, if you miss your mark on the threading dial,
you will have time to disengage before the tool meets the work piece.
Make sure you have some space at the end of the thread for the threading tool to run off a little.  You don't want it crashing
into a corner in the work piece.


I set my compound like this for doing inside threads.

 



It's just convenient for me this way, and have done it like this for many years.  You could also set it over towards the back
of the lathe, but it's a little harder to see the feed dial.
Once you're set up, zero your cross slide before you make your first cut.  Then, after you dial in the compound and make your
first cut, you can use the cross to move your tool back and crank back the carriage for the next chip.  When you have the carriage
moved back, return the cross to zero, put the next cut on the compound and make your next chip.  This way, you don't have to
remember the setting on the compound slide for each cut.  Just dial the cross slide back to zero before each chip and the tip
of the threading tool will be back where it was when the last chip was cut and you can dial some more on the compound.

Feel free to ask more questions about this!

Dean

[Artie]

Dean I agree with all you have posted....  Ithink the response by Marttrp was a little unkind..... yes indeed ], feel free to ask questions before you unload on others..... from one aussie to another.....

[Ned Ludd]

I think for a beginner the top slide left as standard is the best place to start. Later once confidence has been gained then is the time to start more arcane procedures, after all how are you to know the set-over-top-slide method is better without trying the standard way first.
A beginner is used to operating in just two directions, where dials read a known amount, with the set over method you have to calculate the in-feed, which I think might confuse while the operator lacks confidence or experience.

Before I get flamed, I must point out that I use all three of the usual methods depending on how I feel at the time and which of my lathes I am using, and I can say that no one method is superior on all occasions, but some methods are more convenient than others. Tooling, I think, has more to do with the the quality of a cut thread than the method.
Just my views, but then if we all agreed over everything there would be no point in a Forum.
Ned

.

[No1_sonuk]

It's not "black magic".  I have a table which lists all the metric pitches I need with the infeed distances for an angled compound.
This approach is especially handy as I have an imperial lathe - the numbers on my table are in both metric and imperial.

[Jonny]

Might give it a go one of the days set over at an angle but cannot see any advantage unless distraction is an issue, just dont get distracted.
The only other reason it could cut better using one cutting edge and worth a try?

Use good tool holders like Sandvick and Kennametal the finish will pay dividends plus repeatable forms.
HSS pretty good for making your own profile up.

Practice as said above on external threading first.
Internal much the same but more care with clearances with tooling and depth, especially when you cant see what you are doing.
Personally i would be scared stiff if any of my lathes hadnt got a clutch or foot brake and back gear! In this instance rotate chuck by hand and use cutting oil or similar.

[Rob.Wilson]

Hi Lads


Sorry for the crap drawing 

Rob

[Ned Ludd]

Hi Rob
You get the same effect by moving the top slide, for each pass, by half the in-feed on the cross slide. but this way saves you having to upset your carefully set to parallel top slide.
For those who like the set over top slide method, if you haven't made yourself a cross slide stop, do so. it makes life soooo much easier.
The style of tangential threading tool from Eccentric, or Rick Kruger, also works extremely well with the set over top  slide (for external threads) because it has positive rake on the cutting edge. It really is a pleasure to see long curly swarf when thread cutting.
Ned

[Bogstandard]

I think for a beginner the top slide left as standard is the best place to start. Later once confidence has been gained then is the time to start more arcane procedures, after all how are you to know the set-over-top-slide method is better without trying the standard way first.

The way I see it is that we are here to help, and not to confuse. I find that the above statement is a**e about face. The 'standard' way as mentioned above isn't, in fact it is far from it.

The normal method taught on how to cut threads is to use the topslide (compound) offset method. As shown by Rob above. This will produce a well cut thread with good results, with much reduced loading on machine and tooling. Not much to worry about when using large machines, but on some small workshop lathes it can mean the difference between success and failure.

Once you have that under your belt, then you can experiment on using plunge cutting, just to find out just what the weaknesses your machine and tooling has. Sometimes you get away with it, sometimes you don't.


Bogs

[j45on]

Thanks for all the advise guys  very much appreciated 
I shall defiantly be trying some external threads at the weekend   
I would not have to cut the internal thread if I had not ruined the end that already had a internal thread

[techonehundred]

One other thing to remember when cutting Internal threads is where the threads stop.  Are you threading to a bottom? If there is enough room, I bore an area at the end of the threads just deeper than the major dia. of the thread that will give clearance to the threading tool.  Without the clearance, It is easy to break a tool if you don't stop in exactly the same place(ignore if this is a throughhole and you can thread all the way through).  Here is a quick COC drawing with paint to illustrate.
   Once you do it mark the tool for start and finish of relief area.(I use a sharpie or felt pen) That way you know where the end of the threads are without trying to contort to see inside.