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SeanFarr
Posts: 475
Registered: ‎07-25-2012
Message 1 of 14 (1,612 Views)

NPT Issues

1612 Views, 13 Replies
10-10-2012 06:40 AM

Good Day All,

 

I am modelling a pin that is ported in one end for lubrication purposes using Inventor Pro 2012. A 1/4" pilot hole is drilled to a depth until the intersecting hole is reached. Then the same hole is drilled 1" to allow for a 1/8 NPT fitting for the autogreaser to attach to. I know the NPT threads only go down about 3/8".

 

After researching what size to drill for a 1/8 NPT fitting, I found that drill size R (.3390) is to be used. So using Inventor hole command, I tried to create this thread. But for some reason inventor uses .313 as the tap drill size?

 

I am creating another pin that will require a 1/4 NPT fitting and the tap drill size to be used is 7/16, but inventor again uses a different value .422 (27/64)?

 

I don't require the model to show the eact thread, and on my drawing I will just use the hole annotation to show what I want and the shop can determine what tap drill size to use from their references....But why are these number so different?

 

 

Sean Farr
Product Designer at TESInc.ca

Inventor Professional 2014-Update 2 - AutoCAD Electrical 2014
Win7-x64 | ASUS P8Z77-V | i7 3770 -3.4 GHz | 32GB RAM |
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JDMather
Posts: 28,056
Registered: ‎04-20-2006
Message 2 of 14 (1,595 Views)

Re: NPT Issues

10-10-2012 07:20 AM in reply to: SeanFarr

When I worked out on the shop floor - we always used this chart

http://www.brokenbolt.com/images/starrett-inch-metric-tap-drill.pdf

with minor variations for class-of-fit considerations.

These values seem to be close the Inventor values.

 

I found a reference to Whitworth pipe taps in Machinery's Handbook that was closer to your values.

Please mark this response as "Accept as Solution" if it answers your question.
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karthur1
Posts: 4,312
Registered: ‎04-27-2005
Message 3 of 14 (1,590 Views)

Re: NPT Issues

10-10-2012 07:26 AM in reply to: SeanFarr

SeanFarr wrote:

... 

After researching what size to drill for a 1/8 NPT fitting, I found that drill size R (.3390) is to be used. So using Inventor hole command, I tried to create this thread. But for some reason inventor uses .313 as the tap drill size?

 

I am creating another pin that will require a 1/4 NPT fitting and the tap drill size to be used is 7/16, but inventor again uses a different value .422 (27/64)? 

 


This is something that AD has wrong about the NPT threads. I dont have a good answer for you on this. Its a catch 22.
If you use the values in the table, then the threads will model correctly, but the tap drill will call-out wrong in your thread notes. Thats just what you ran across.
You can change your table to get the correct tap drill callout, but then some of the NPT sizes will not be modeled correctly. This is the way that I have my table.
Its been like this ever since they included NPT threads, and will probably be this way forever. They need to use a different column for the tap drill information.
Valued Mentor
SeanFarr
Posts: 475
Registered: ‎07-25-2012
Message 4 of 14 (1,565 Views)

Re: NPT Issues

10-10-2012 10:01 AM in reply to: SeanFarr

Ok Thanks JDMather and karthur,

 

It seems that there is nothing wrong with my model then,reading from your replies. Whichever value inventer decides to use for tap drill size, it is ultimately up to the machinist to decide what tap drill is best to produce the wanted NPT. As long as my detail drawing states what NPT is required.

 

Correct me if I am wrong but when annotating a thread, tap drill size is not required, just the thread info, ex. 1/8 - 27 NPT, unless of course there is a class of fit consideration as you mentioned JD.

 

Thanks Again


Sean

Sean Farr
Product Designer at TESInc.ca

Inventor Professional 2014-Update 2 - AutoCAD Electrical 2014
Win7-x64 | ASUS P8Z77-V | i7 3770 -3.4 GHz | 32GB RAM |
240GB SSD | nVidia GTX 670 4GB - 320.49
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mrattray
Posts: 2,517
Registered: ‎09-13-2011
Message 5 of 14 (1,558 Views)

Re: NPT Issues

10-10-2012 10:13 AM in reply to: SeanFarr

If there is a non-standard class of fit then you still shouldn't call out the tap drill. You would call out the fit. I have never once called out a tap drill size.

You don't tell the machinist what to do, you tell the machinist what product you need made.

Mike (not Matt) Rattray

Distinguished Mentor
john.retzlaff
Posts: 947
Registered: ‎11-03-2003
Message 6 of 14 (1,554 Views)

Re: NPT Issues

10-10-2012 10:15 AM in reply to: SeanFarr

We don't specify tap drills on our drawings at all.

We mostly use roll taps, which require a larger tap drill than cutting taps.

So my advice would be to leave the tap drill off the drawing and let the machinist decide, as you said.

 

John Retzlaff
Valued Mentor
SeanFarr
Posts: 475
Registered: ‎07-25-2012
Message 7 of 14 (1,550 Views)

Re: NPT Issues

10-10-2012 10:28 AM in reply to: SeanFarr

Thanks All!!

 

Still kinda new to the forum, can I accept more than one reply as solution or only one?? haha

 

Sean

Sean Farr
Product Designer at TESInc.ca

Inventor Professional 2014-Update 2 - AutoCAD Electrical 2014
Win7-x64 | ASUS P8Z77-V | i7 3770 -3.4 GHz | 32GB RAM |
240GB SSD | nVidia GTX 670 4GB - 320.49
Distinguished Contributor
kstate92
Posts: 906
Registered: ‎05-09-2005
Message 8 of 14 (1,536 Views)

Re: NPT Issues

10-10-2012 11:34 AM in reply to: karthur1

I suspect at Autodesk, 'NPT' stands for 'New Part-Timer', as in: give the thread stuff to the newbie, 'cause it's boring and not part of the next sales campaign bulletpoint list.

 

On a personal note, I wish NPTF were outlawed.  Making them consistently right seems to be more art than science, and pressure testing with them means you're wearing the peaks flat before they even ship.  I suspect the real reason they persist (like in-laws that won't leave) is that the fittings are cheap (like the in-laws) compared to the alternative systems.  I have seriously thought of advocating line-item charges, at least on our quotes.  Oh, you want NPTF ports? Here's the extra cost you'll have to pay, and yes it is just slightly more than the price difference between an NPTF one-piece screw machine part versus a real, modern hydraulic fitting!

 

Grease fittings I guess we're stuck with, but why isn't at least 1/4-28 straight thread the standard by now?

KState92
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mrattray
Posts: 2,517
Registered: ‎09-13-2011
Message 9 of 14 (1,528 Views)

Re: NPT Issues

10-10-2012 12:11 PM in reply to: kstate92

Is it just me or have you gone on this anti-NPT rant at least once before?

Mike (not Matt) Rattray

Distinguished Contributor
kstate92
Posts: 906
Registered: ‎05-09-2005
Message 10 of 14 (1,525 Views)

Re: NPT Issues

10-10-2012 12:19 PM in reply to: mrattray

Yeah... a lone voice, crying in the wilderness.  Or a crackpot to be avoided like a skunk that wants to cuddle.

KState92
Inventor Professional 2014 64 SP2
Windows 7 Pro 64 bit - SP1
Xeon E5-1620
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