--------------------------------------------------
From: "Charlie Gallo" <Charlie@TheGallos.com>
Sent: Thursday, November 25, 2010 7:32 AM
To: "Jim Thomson" <jim.thom@telus.net>
Cc: <towertalk@contesting.com>
Subject: Re: [TowerTalk] Cushcraft Parts From MFJ
>
> On 11/25/2010 Jim Thomson wrote:
>
>
>
>> ## My guess is, instead of using 6061-T6 or 6063-T832, they have
>> gone
>> to a cheap alloy, like 3003, which comes in various grades of hardness,
>> listed
>> as H-0 to H-14. H-0 is only 6 ksi yield, and the strongest is no
>> match for
>> 6061-T6 or 6063-T832.
>
> What is interesting is "yield strength" vs ultimate strength - various
> tempers WILL have different yield strengths - "Yield strength" is the
> amount of force to cause the material to BEND and NOT return to it's
> original shape - "Ultimate strength" is the amount of force needed to
> BREAK the material
&&& True...BUT.. the only thing that ever gets plugged into either the YS
2.15 or DX eng spreadsheets [ and also the ARRL mast program] is
yield strength and section modulus. Section modulus is just a function
of ID/OD/wall thickness. This is also mentioned in detail in Leesons
book. Bottom line is... yield strength X section modulus = bending
moment. You are correct, yield strength is the amount of force to cause
the material to take a 'permanent set.' IE: the fellow's mast bent.. and
did a 90 deg. It has not snapped off...yet. It's just bent. Same
deal with al eles. Bottom line is... if the els are bent [ permanent
set].. the ele is toast, and that's the end of that. Now if hygain model
XXX 6m yagi was fine b4... and flexed a bit in a 85 mph wind, but eles
never took a permanent set, then fine, the ele design/tubing
schedule/alloy/temper.....has done it's job.
&&& Now if MFJ takes the same model XXX 6m yagi.... and changes it in
such a fashion, that ele halves are permanently bent...man you got A REAL
PROBLEM. The MFJ 6m yagi in question, is one
step worse than this.... and it's els halves are snapping clean off!!
Assuming the tubing schedule [OD + wall thickness] didn't change..[ so
parts for new ants will also fit old ants].... then they MUST have changed
either the alloy, temper....or both.
>
> Now, IF the alloy is actually the same (hint, a lot of the Al that is sold
> is NOT the same for the different tempers, and some can't be tempered
> after alloying) - the "Ultimate strength" doesn't change - Note however,
> the Alcoa data sheets DO tend to guarantee a higher ultimate strength for
> higher tempers (not all - lets talk 6061 - they guarantee the same
> minimum for T0 and T4 - and of course T6 and T6511 are the same - in fact
> I often see t651(1) sub'd for T6)
>
> I know you take say a piece of 0-1 steel - the ultimate strength is
> totally unchanged by hardening - what IS strange - say you go just under
> yield strength - the hardened steel will flex a LOT less that the
> unhardened, and still return, but they break at the same point
&&& good point. So if the mfj ele's are snapping clean off..... then the
ultimate strength must have been reduced by a whole bunch..which means xtal
clear to me, that mfj CAN'T be using 6061-T6 or 6063-T832.
Other than 6061-T6 and also 6063-T832..... I don't know what else will
work [that's actually readily available]. There is all sorts of junk AL
tubing that obviously WON'T work... and MFJ has proven that point.
Now we have arrived at the point where we have to ask...'and what alloy and
temper did you use for both the boom/els, and anything else' ? If they
can't answer that question,,,,then don't buy it.
>
> If I see my wife's uncle this holiday season, I'll ask him to write
> something up - he's a (now retired) PhD in materials science who
> specializes in Alloys (mostly Fe Alloys), and is considered one of the top
> guys in the field (as in international award winning)
&&&& That's wonderful and everything, but any far out
alloy/material/temper, has to be something that's [A] readily available in
size's and wall thickness's we can use..... and [B] be affordable. Sure
7075 is great stuff, and so is 2024-T3.... provided you just won the
lottery. Both 6063-T832 and 6061-T6 are both 40 ksi yield strength.
6063-T832 is usually the cheaper of the 2 x alloys... and is used by M2 and
other's. This 'new' 6061-T8 temper is good stuff for boom material.
It's 50 ksi yield..and comes in .120" wall.........so it all telescopes.
No more swedges and lathe work.
Later... Jim VE7RF
>
> --
> 73 de KG2V - Charles Gallo
> Quality Custom Machine-shop work for the radio amateur (sm)
>
>
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