Author Topic: Help with cutting imperial thread  (Read 2028 times)

Offline vcutajar

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Help with cutting imperial thread
« on: September 26, 2018, 10:30:51 AM »
Hi

I need to make a threaded brass plug for an existing threaded hole.  The thread is 3/8" x 26TPI.  I have an appropriate die on order but as I have never cut imperial threads I need some help.

What should be the diameter of the brass rod to cut this thread?

Thanks
Vince

Online Jasonb

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Re: Help with cutting imperial thread
« Reply #1 on: September 26, 2018, 10:46:53 AM »
3/8" or 9.53mm though 9.5mm will do.

Offline vcutajar

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Re: Help with cutting imperial thread
« Reply #2 on: September 26, 2018, 11:33:16 AM »
Thanks Jason.  I suspected so but as this is my first ever imperial thread I thought I had better check.

Vince

Offline gbritnell

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Re: Help with cutting imperial thread
« Reply #3 on: September 26, 2018, 01:53:25 PM »
When I would make a threaded rod or bolt and it would need to be a certain size like 3/8-26 I would turn the material to .375. In most cases the die would cut the thread but sometimes with a lot of effort in some part by the quality of the die being used. When I started doing some models with very tiny threads, down to M.6, that's .6, my watchmakers tap and die set specifies what size drills and material to use and for threads that are only .008 deep they recommend turning the material.001-.002 smaller. Now for these tiny threads that's quite a large percentage smaller but they are still very strong, twist wise. I know that full sized screws have reduced diameters due to the flattened crests but never really gave it much thought until I started doing the tiny ones so the next time I made a screw that had a .375 diameter I turned it to .370. What a difference that made when running the die onto the material. When you look at a drill chart they always spell out 75% thread for tapped holes so I apply the same reasoning for bolts and screws and they work just fine.
gbritnell
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Offline jonesie

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Re: Help with cutting imperial thread
« Reply #4 on: September 26, 2018, 02:32:45 PM »
when  i cut external threads ,single point or with a die i always turn the stock down .003 -.004  smaller and put a nice chamfer on the end .like has been said you are going for 75% thread.

Online Jasonb

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Re: Help with cutting imperial thread
« Reply #5 on: September 26, 2018, 03:13:33 PM »
I'm never happy with going undersize on both. If you go for 75% when drilling and tapping and then also do the same with the male part that's 75% of 75% which is only 56% engagement so usually make the male full size though suppose it depends on the application and what loads you are expecting the thread to take.

If you want to make it easier to run a die down then open up the split die on the first pass and run it down again if needed to get the required fit.

Offline jadge

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Re: Help with cutting imperial thread
« Reply #6 on: September 26, 2018, 09:05:48 PM »
I'm never happy with going undersize on both. If you go for 75% when drilling and tapping and then also do the same with the male part that's 75% of 75% which is only 56% engagement so usually make the male full size though suppose it depends on the application and what loads you are expecting the thread to take.

I'd agree. For internal threads formed with a tap I normally aim for 65-70% thread engagement. For screwcutting internal threads I start at the core diameter. For external threads I start on nominal size whatever method I use. Although I don't use dies that much; either I screwcut or use a Coventry diehead.

Andrew

Offline derekwarner

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Re: Help with cutting imperial thread
« Reply #7 on: September 26, 2018, 09:26:30 PM »
26TPI is a British Brass Thread ......   it has a standard 26 TPI ranging from 1/8" to 1 1/2" diameter.............very rarely used theses days.......domestic light fittings are an exception

As an apprentice [~~ 50 years ago], using an adjustable button die, I  made two 1/4" x 26 TPI Brass thread [in aluminium material] slotted head adjustment bolts for a machine .....the aluminium stock material was 0.250".....loaded up the die with Tallow & Kerosene as the cutting fluid....the thread cut like butter....[but 3 or 4 chip breaking clearings per revolution]     

https://britishfasteners.com/threads/bsb.html

Derek
« Last Edit: September 27, 2018, 12:14:52 AM by derekwarner_decoy »
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Offline john mills

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Re: Help with cutting imperial thread
« Reply #8 on: September 27, 2018, 08:24:18 AM »
The out side dia should always be full size in 50 years as a machinist i have never used under size out side dia.
The out side is the maximum size measured.if a flat is on the crest that is the size if it is a radius that larger size measured is the size.Tapping the 75% is a starting guide only thin brass and soft materials
should be drilled smaller for more thread.I have had to drill larger in tool steel so the tap would hold together but that was 3/4" u.n.c in a old c.n.c machine.It would drive the tap but the taps would not stay together but that thread has a good depth of thread.so tapping drills should be adjusted depending on material and use regarding
strength required for the finished part

Offline toolznthings

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Re: Help with cutting imperial thread
« Reply #9 on: October 21, 2018, 04:10:19 PM »
I start with the full size diameter of the thread size when chasing the threads. After deburring the thread o.d. with a file the finish o.d. ends up being undersized a few thousands. Gets you where you need to be when finished. Just another way of getting the job done.
Thanks for the visit !
Brian

Offline Lew Hartswick

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Re: Help with cutting imperial thread
« Reply #10 on: October 22, 2018, 03:48:42 AM »
:-)  I've been helping high school students learn to thread on the lathes for 20 years . They do a small "hammer" (more decorative than useful) that uses a 3/8 -16 thread for the head . I have them turn the handle shaft to about .370 to .371 and then single point the thread. I haven't used a 3/8  - 26 thread but think .373 or .374 would be most appropriate.
   ...lew...

Offline Mosey

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Re: Help with cutting imperial thread
« Reply #11 on: October 22, 2018, 02:24:51 PM »
What is an "Imperial? thread?
Mosey  :wallbang:

Online Stuart

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Re: Help with cutting imperial thread
« Reply #12 on: October 22, 2018, 02:44:45 PM »
You call it ‘standard’

UK thing
BSW
BSF
BSB
BSP T
ME

and BA although that’s metric

Our standard is metric  :stir:
My aim is for a accurate part with a good finish

Offline Don1966

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Re: Help with cutting imperial thread
« Reply #13 on: October 23, 2018, 12:32:52 AM »
Vince this sheet calculates the different threads like sharp crest sharp root, flat crest flat root,sharp crest flat root and flat crest shrp root.

don

Offline Mosey

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Re: Help with cutting imperial thread
« Reply #14 on: October 23, 2018, 02:52:27 PM »
The "Official" thread system in the US is Metric. Think NATO. All military and most manufacturing uses it.
Since the 80's most and now all cars made here are fastened in metric.
Our fractional system is ASA, American Standard and has always been so. It is a 60 degree thread form. We have never used the Whitworth "Imperial" system of 19th century England.
Our construction industry still uses a fractional inch system mostly.
If it tickles someones fancy you can say Imperial.
Mosey

Offline crueby

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Re: Help with cutting imperial thread
« Reply #15 on: October 23, 2018, 03:28:41 PM »
Aren't standards wonderful. Everyone has their own, lots don't follow it to the letter...  Used to have a large poster in my lab at work with all the different communications standards from all the manufacturers, most totally incompatible.

Offline Admiral_dk

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Re: Help with cutting imperial thread
« Reply #16 on: October 23, 2018, 08:33:53 PM »
Quote
Aren't standards wonderful. Everyone has their own, lots don't follow it to the letter...  Used to have a large poster in my lab at work with all the different communications standards from all the manufacturers, most totally incompatible.

 :ROFL:  :ROFL:  :ROFL:

Thank you for that one Chris - that really lifted the spirit here  :cheers:

Best wishes

Per

Offline petertha

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Re: Help with cutting imperial thread
« Reply #17 on: January 01, 2019, 04:30:38 PM »