Author Topic: Help! Up against the wall  (Read 8837 times)

Offline sshire

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Help! Up against the wall
« on: July 05, 2014, 02:49:50 PM »
I’m about to order stainless tubing to replace the steel that came with the Rider-Ericsson castings.

The drawings spec a 1.85 diameter with a .0625 wall.

The closest tubing dimension that I can order, in the length that I need, is 2.0” OD with a 0.134 wall thickness.

Both the OD and wall have a +/- .010 tolerance.

If my calculations are correct, the worst case would be a 2.01 OD with a 0.124 wall. (+ on the OD, - on the wall)

Turning the OD down to 1.85, would leave a .034 wall which seems too thin.

Option 2 would be a 2.0” OD with a .156 wall. Again, worst case would be a 2.01 OD with  A .146 wall.  (+ on the OD, - on the wall)

Turning the OD down to 1.85 would result in a .056 wall. Better.

However, this option, going the other way with the tolerances (- on the OD, + on the wall)

This would be a 1.99 OD and .166 wall.

The result here, after turning the OD to 1.85, would be a .096 wall (which could be bored to .0625 if necessary)

My inclination would be to order the .156 wall tubing. Since worst case would be .056 wall thickness. The best case scenario with this tubing would be a 1.99 OD with a .146 wall.
After turning the OD to 1.85, I’d have a .076 wall.

If you’re confused, join the club.

Someone just tell me which tubing to order. Please!
Best,
Stan

Offline cfellows

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Re: Help! Up against the wall
« Reply #1 on: July 05, 2014, 03:40:48 PM »
Given the uncertainty of the exact size you'll be receiving I would probably go with the .156" wall.  It may be more work but you have a better chance of getting the size you want out of it.

Chuck
So many projects, so little time...

Offline b.lindsey

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Re: Help! Up against the wall
« Reply #2 on: July 05, 2014, 04:09:18 PM »
Yep, I agree with Chuck also Stan.

Bill

Offline mklotz

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Re: Help! Up against the wall
« Reply #3 on: July 05, 2014, 04:16:10 PM »
Quote
The closest tubing dimension that I can order, in the length that I need, is 2.0” OD with a 0.134 wall thickness.

Both the OD and wall have a +/- .010 tolerance.

If my calculations are correct, the worst case would be a 2.01 OD with a 0.124 wall. (+ on the OD, - on the wall)

Turning the OD down to 1.85, would leave a .034 wall which seems too thin.

2.01 - 1.85 = 0.16 removed from the diameter so that means 0.08 removed from each side/wall...

0.124 - 0.08 = 0.044, not 0.034

Quote
Option 2 would be a 2.0” OD with a .156 wall. Again, worst case would be a 2.01 OD with  A .146 wall.  (+ on the OD, - on the wall)

Turning the OD down to 1.85 would result in a .056 wall. Better.

Again...

0.146 - 0.08 = 0.066, not 0.056

What am I doing wrong?  I've only had one cup of coffee and I have the distinct feeling that I've overlooked something obvious and will be very embarrassed when you reveal my error.
Regards, Marv
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Offline sshire

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Re: Help! Up against the wall
« Reply #4 on: July 05, 2014, 04:54:52 PM »
Thanks, Marv. Coffee or not, you're correct.
I had a page of all permutations for +- .01 of both OD and wall thickness.
The wall thickness range was .066 to .096

Not the neatest page of calculations. It's a wonder I posted any correct numbers.

The second question is that this tubing is for the displacer piston. Is the wall thickness (within the .066 - .096 range) that critical?

An aside:
Jerry Pournelle (you'll know who he is if you're a reader of science fiction) said that he would have gotten his PhD in Physics instead of Political Science if calculators had been available in 1964. He couldn't do the math.
« Last Edit: July 05, 2014, 04:59:46 PM by sshire »
Best,
Stan

Offline Tennessee Whiskey

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Re: Help! Up against the wall
« Reply #5 on: July 05, 2014, 04:58:44 PM »
Stan, kinda like Marv here, except, I've absorbed way too much beer and hickory smoke, but, best I can remember from the last real job I had, there is a difference in pipe and tubing as far as O.D. and wall thickneses. If I can find my old briefcase I have some charts somewhere. We had to deal with it concerning piping vs. Instrument tubing. God, I thought I'd never forget that stuff. Thanks for the memory jog.

Cletus

Offline sshire

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Re: Help! Up against the wall
« Reply #6 on: July 05, 2014, 05:00:43 PM »
Thanks, Mr. Ribs and Pulled Pork. This is definitely seamless tubing.
Best,
Stan

Offline mklotz

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Re: Help! Up against the wall
« Reply #7 on: July 05, 2014, 05:02:29 PM »
Quote
The second question is that this tubing is for the displacer piston. Is the wall thickness (within the .066 - .096 range) that critical?

Perhaps for ultimate efficiency but, for hobby/demonstration use, I doubt it matters much.  The atomic level physics going on in the displacer chamber would probably need a supercomputer to model with sufficient fidelity to discover a difference.
Regards, Marv
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Offline sshire

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Re: Help! Up against the wall
« Reply #8 on: July 05, 2014, 05:07:16 PM »
Since this is the first hot air engine I wanted to be sure before ordering a $35, 6" piece of tubing.
Additionally, my Grizzly Atom Analyzer (G475443)  is waiting for a part under warranty.
Best,
Stan

Offline Mosey

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Re: Help! Up against the wall
« Reply #9 on: July 05, 2014, 05:08:53 PM »
Stan,
Take a look at Zoro Tools for tubing sizes that may match what you need better than domestic sources, and for good prices. I bought some from them to make a draw tube for my jewelers lathe. Watch out for stuff made in India.
no, I don't own them, haha.
zoro.com
Mosey

Offline sshire

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Re: Help! Up against the wall
« Reply #10 on: July 05, 2014, 05:11:10 PM »
Rob
Thanks. I've gotten things from them but never thought to look there for tubing
Best,
Stan

Offline Tennessee Whiskey

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Re: Help! Up against the wall
« Reply #11 on: July 05, 2014, 05:18:59 PM »
Okay, look at 1.5" Sch5 seamless stainless PIPE. 1.9" o.d and .065" wall. I think the polish will take up the difference. I've welded Sch.5 before and you had to make sure there were no scratches in you lens, because they looked bigger than the joint, you. Didn't know which to follow :Jester:

Old Pork Butt

Or as Marv said, in local government terms, what's a few thousands here OR there going to matter :lolb:

Offline mklotz

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Re: Help! Up against the wall
« Reply #12 on: July 05, 2014, 05:46:33 PM »
According to Wikipedia, Pournelle has a BS/MS in [spit] psychology and a doctorate in [spit^2] political science [the ultimate oxymoron], which means he shouldn't be allowed anywhere near a calculator or, in fact, normal people.  I'm not much for science fiction but I used to read him regularly in Byte and there he seemed remarkably human-like for something with those credentials.

Porky mentioned digging up his stuff on pipe versus tube.  For the benefit of the lurkers who may read this my ancient writing on the subject is offered below.  [I really have to stop wasting my time on lurkers.]


              PIPE AND TUBE

What is the difference between pipe and tube?  At first glance, the naive
might say "none".  After all, they're both just hollow cylinders.  That's
wrong, of course.  There is a difference (in the metalworking arena) and I
hope to clear up a bit of the confusion here.

For any hollow cylinder, there are three important dimensions - the outside
diameter (od), the inside diameter (id) and the wall thickness (wt).  Since
these three are related by a simple equation:

      od = id + 2 * wt

one can completely specify a piece of pipe/tube by supplying any two of these
numbers.

Tubing is more frequently used in structures so the od is the important
number.  Strength depends on the wall thickness.  So tubing is specified by
the od and the wt.  Very logical and simple to measure.  The id is simply
whatever falls out of the equation above.

Pipe is normally used to convey gases or fluids so the internal
cross-sectional area (defined by the id) is important.  It's therefore not
surprising that pipe is specified by the id.  Although anyone who's ever done
any plumbing knows that the id on the pipe label is only a *nominal* id.  As
an example, a (nominal) 1/8 wrought steel pipe will typically have a
*measured* id of 0.269 (schedule 40) or 0.215 (schedule 80).  (More below
about those schedule numbers.)

While the designation for tubing is straightforward, that for piping is
obscure for some perverse reason unclear to me.  All pipe of a given nominal
size has the *same od*.  An abbreviated list:

nominal
size      OD

1/8      0.405
1/4      0.540
3/8      0.675
1/2      0.840

Now, the folks (ASME?) who codify this stuff, in an effort to make things
difficult for us, instead of specifying the wall thickness directly, decided
to use (seemingly arbitrary) schedule numbers to specify the wall thickness.

For instance, a (nominal) 1/8 schedule 40 pipe will have a wall thickness of
0.068 (id=0.269) while a 1/8 schedule 80 pipe will have a wall thickness of
0.095 (id=0.215).

And, no, these schedule numbers do not reflect a constant wall thickness.  For
instance, a (nominal) 1/4 schedule 40 pipe has a wt = 0.088 while the same
pipe in schedule 80 has wt = 0.119.

Schedule numbers range from as small as 5 up through 40, 80 (common) to as
high as 100, 120 and 160.  There may be others.  This is not my area of
expertise.  Larger schedule numbers correlate one-for-one with thicker walls,
which seems to be the only predictable thing about schedule numbers.

To the best of my knowledge, there is no mathematical relationship that can be
used to translate schedule number into equivalent wall thickness.  You're
forced to consult a table.  Machinery's Handbook has such a table for wrought
steel pipe (pg. 2378 in the 23rd edition).  Do these tables also apply to pipe
made of other materials (e.g., plastic)?  I don't know, but I doubt it.  That
would be too simple.  Since I don't want to make a career out of plumbing
minutiae, I'll let you research it for your application.

I can only guess that the schedule number relates to some burst pressure and
thus the relationship to wall thickness is non-linear.  But that's only a
guess - anyone who knows the real story please correct me.

Why do you care?  Well, beyond the satisfaction of simply knowing some obscure
metalworking stuff, this should help you in selecting and specifying hollow
cylindrical elements for that project you have planned.  It should help you to
understand why you won't have much success trying to bend tubing with a pipe
bender.  On the latter, the bending dies are sized to the (constant) pipe ods
mentioned above.  It's unlikely they'll fit any tubing you buy since tubing od
generally comes in straightforward sizes like 1/4, 3/8, 1/2, etc.  If you want
to bend tube in a pipe bender, count on making some purpose-built dies - a
tricky lathe job.  Or buy a tubing bender.
Regards, Marv
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fcheslop

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Re: Help! Up against the wall
« Reply #13 on: July 05, 2014, 05:52:49 PM »
Used stainless electrical conduit before with no ill affect .Just dont know the sizes you guys use
Its a bit tough going but works
Home depots often have stainless goodies
cheers

Offline gerritv

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Re: Help! Up against the wall
« Reply #14 on: July 05, 2014, 08:10:01 PM »
Marv and other, thank you for all the details, it has given me hope of making a MT2 tailstock for my Taig lathe. Now that I know about pipe I no longer have to machine from solid!
 :cartwheel:

Gerrit
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