In a message dated 08/03/03, beezaur@.......... writes:

> > temperature changes. I would be most surprised if 1:1 sand and cement 
> > was not strong enough in practice, . . .
> >       Have you considered making a T base frame out of say 1.5" black 
> > steel angle? . . .
> >       Regards,
> >       Chris Chapman
> 
> Chris,
> 
> Thank you.
> 
> I think you are right about the grout mix being strong on its own. It isn't 
> a trick to get regular concrete to 6000 psi compressive strength (garden 
> variety being 2000-3000 psi), and in fact the stone is what usually breaks 
> first in the really high strength stuff.  I still worry about brittleness - 
> this device will have to survive a couple of moves in the next few years. 
> 

       Unless you drop it or hit it with a hammer, I would not expect 
brittleness to be a serious problem. I don't know about the water. It needs 
to be saturated. I used an eccentric dowel on an electric drill to agitate 
the mix and get the air out. It has a surface which is just liquid.

> Since rebar is a problem in concrete for the base, I assume I don't have to 
> ask about pouring the pier over 8 ft R/C pilings to better couple with more 
> stable ground at depth.  It sounds to me like the soil/piling interface 
> would have the same noise problems as rebar/concrete.  But please correct 
> 

       I do not know. The moisture variations could be considerable, but the 
temperature variations are likely to be small. You also have tilt to 
consider. This may be a limiting factor in the period you can use. I would 
place a plinth on the earth / subsoil clear of any piles.

> Steel or aluminum is "plan B" for the base of the instrument. The support 
> for the boom will be a metal tripod, bolted to the base. I would like to 
> keep the natural (resonant) frequency of that structure very high, kHz if 
> possible. I want to put my pickup and damping hardware on the same base as 
> the boom, hence the 40 in length. I hadn't expected a "T" to get me there, 
> but the design you describe may well be stiff enough, especially with 
> 

       I suggest that you stick to steel. It is much more rigid. A completely 
rectangular  design is the easiest to construct. A simple design with the 
least problems is more likely to get built! You can fit 6 to 8" lengths of 
flat plate between the horizontal and vertical parallels to greatly increase 
the rigidity and provide a pan for mounting sensors or damping devices. You 
could use 1" L to brace the top of the U hoop to the end of the T arm. Could 
one of the drive on steel ramps sold for car / truck maintenance be converted 
- they come ready welded? Just a wild idea!

> Another thought: has anyone considered using carbon arrow shafts or kevlar / 
> spectra bowstring for boom components? They are pretty cheap now, and their 
> strength-to-weight ratios give them very good vibration characteristics, 
> and some bowstring absolutely does not creep.

       Carbon could be OK, but how do you plan to make the fittings? Metal is 
easier to fabricate and fit. You can easily get nickel plated piano wire down 
to 8 thou. from a music shop, it does not have any 'bending' memory and it 
works just fine. See http://www.daddario.com/ Suggest that you forget kevlar 
or other plastics. A ball bearing in the end of the arm gives an excellent 
bottom suspension up against a stainless steel razor blade glued flat onto 
the crossbar.

       Regards,

       Chris Chapman 
In a message dated 08/03/=
03, beezaur@.......... writes:



> temperature changes. I=
 would be most surprised if 1:1 sand and cement=20

> was not strong enough in practice, . . .

>       Have you considered making a T=20=
base frame out of say 1.5" black=20

> steel angle? . . .

>       Regards,

>       Chris Chapman



Chris,



Thank you.



I think you are right about the grout mix being strong on its own. It is=
n't a trick to get regular concrete to 6000 psi compressive strength (garden=
 variety being 2000-3000 psi), and in fact the stone is what usually breaks=20=
first in the really high strength stuff.  I still worry about brittlene=
ss - this device will have to survive a couple of moves in the next few year=
s. How much water is typically used?



       Unless you drop it or hit it with a=
 hammer, I would not expect brittleness to be a serious problem. I don't kno=
w about the water. It needs to be saturated. I used an eccentric dowel on an=
 electric drill to agitate the mix and get the air out. It has a surface whi=
ch is just liquid.



Since rebar is a problem in=
 concrete for the base, I assume I don't have to ask about pouring the pier=20=
over 8 ft R/C pilings to better couple with more stable ground at depth. &nb=
sp;It sounds to me like the soil/piling interface would have the same noise=20=
problems as rebar/concrete.  But please correct me if that technique ha=
s proven reliable.



       I do not know. The moisture variati=
ons could be considerable, but the temperature variations are likely to be s=
mall. You also have tilt to consider. This may be a limiting factor in the p=
eriod you can use. I would place a plinth on the earth / subsoil clear of an=
y piles.



Steel or aluminum is "plan=20=
B" for the base of the instrument. The support for the boom will be a metal=20=
tripod, bolted to the base. I would like to keep the natural (resonant) freq=
uency of that structure very high, kHz if possible. I want to put my pickup=20=
and damping hardware on the same base as the boom, hence the 40 in length. I=
 hadn't expected a "T" to get me there, but the design you describe may well=
 be stiff enough, especially with channel or square structural tubing.



       I suggest that you stick to steel.=20=
It is much more rigid. A completely rectangular  design is the easiest=20=
to construct. A simple design with the least problems is more likely to get=20=
built! You can fit 6 to 8" lengths of flat plate between the horizontal and=20=
vertical parallels to greatly increase the rigidity and provide a pan for mo=
unting sensors or damping devices. You could use 1" L to brace the top of th=
e U hoop to the end of the T arm. Could one of the drive on steel ramps sold=
 for car / truck maintenance be converted - they come ready welded? Just a w=
ild idea!



Another thought: has anyone=
 considered using carbon arrow shafts or kevlar / spectra bowstring for boom=
 components? They are pretty cheap now, and their strength-to-weight ratios=20=
give them very good vibration characteristics, and some bowstring absolutely=
 does not creep.




       Carbon could be OK, but how do you=20=
plan to make the fittings? Metal is easier to fabricate and fit. You can eas=
ily get nickel plated piano wire down to 8 thou. from a music shop, it does=20=
not have any 'bending' memory and it works just fine. See http://www.daddari=
o.com/ Suggest that you forget kevlar or other plastics. A ball bearing in t=
he end of the arm gives an excellent bottom suspension up against a stainles=
s steel razor blade glued flat onto the crossbar.



       Regards,



       Chris Chapman