Problem- Threading Standard/Metric

[Bill308]

Brain teaser question..... Can you use a standard 60 Deg Threading insert to cut metric pitch threads? I was told you could. So I tried it, (non critical part) and it seamed to work. However the part I threaded was a stop for the spindle bore that basically screwed into the back of the bore. Now best guess was that it was a M56x2.0 Thread. I say this because the thread was made in the bore and then bored flush with the ID of the bore so the crest were turned off.

Now having that in mind.... you lose a certain amount of engagement. After threading, testing and recutting several times it finally went in. These threads were cut with a standard 60 Deg insert with a 29.5 plunge angle and a .07874 pitch (had to convert everything from M to S).

Bottom line question... Do you need metric threading inserts to cut metric threads or just use standard inserts the whole way through? This is to settle a debate at work. I looked the thread profiles up in Machinery's Handbook and the angles were the same for both profiles, 60 Deg. Aside from that the thread profile was different.

So any help would be awesome!!

[One of Many]

I figure if it fits use it.

To the purist this might matter on a print so it can inspected and pass. Yes, there are full form inserts for metric threading. What really matters in your application is functionality of the pitch diameter and final fit. No real need to debate root and crest form to prove right or wrong. That is just being anal for no other reason!

Over/under sized minor/major diameter threads that do not need full strength are common. Full form threads are a bit vulnerable to damage in harsh environments. This also gives clearance for grime displacement reducing the risk of galling and other ham handed abuses. Typically it is one thread or the other(internal or external) and NOT both. You might even see a higbee cut or blunt start threads to help mimimize cross threading in these applications. Moreso on coarser threads though.

DC

[martinw]

Dear Bill308,

It really boils down to to your application, but generally speaking, it is probably a good idea to cut the threads correctly IMVVHO.

If it is a water feature in your back yard, by all means bodge it with incorrect thread forms, and bung in some chewing-gum if it leaks a bit above 5 ps1 . That will work.

If it is a 50 bar pipeline containing oxidising gases, you might want to take a more serious approach..LOL

Best wishes,

Martin

[Geof]

If you want to get edicated on threads go here:

http://www.boltscience.com/pages/screw1.htm

If you want a thread for your 750psi air compressor go here:

Dryseal threads. (Covered by the SAE standard J476).

[martinw]

Dear All,

Speaking of bunging up stuff with chewing gum, has anybody got any experience of Loctite 55? It has a great specification, and I really like the idea of winding the stuff over the threads rather than round them.

http://www.loctite55.co.uk/

Does this stuff really work on threads up to 4"??

Any feedback will be most welcome.

Best wishes,

Martin

[juergenwt]

Bill - metric and standard threads use the same inserts - 60 deg.
Was this done on a manual lathe?
If yes - check your compound first. 29.5 deg from the straight in position. Most lathes have the zero setting parallel to your work. That means you compound will have to be on the 60 1/2 deg. mark.
But just how did you get the 2mm (.07874) pitch unless you had a lathe capable of cutting metric thread and than you would use the 2mm pitch setting.
If you had a standard inch lathe you would need a 120/127 gear combination to convert to metric. So I think your problem is most likely with the pitch not being correct.
M56x2 is standard metric fine thread.

[Bill308]

Martin, thats pretty much the same answer I said...almost word for word...lol I had just never experienced the the whole single point metric threading with a regular threading insert before now. And the other guy had never heard of a metric threading insert before so we're pretty much even...lol All 3 of us learned something.

juergenwt, its not a manual lathe it is a Prototrak 1630, not a bad lil lathe, but has some features I dont really like. Anyways instead of putting in a standard pitch say 1/2-28 of .0357 I took the 2.0mm and converted it over to .07874 and used it on the inch side instead of changing all my offsets to mm and in which case I would put 2.0 where is asked for pitch. As far as the 29.5 deg plunge angle.... thats just a default setting on the threading cycle. It can be changed depending on how much of a cut you want to take per pass.


Thanks! Good info from everybody!