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  #11  
Old 04-08-2011, 10:39 AM
bordontn2 bordontn2 is offline
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Default frustum

I believe a frustum is a cone with the pointy end removed......like a funnel or tapered roof jack....
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  #12  
Old 04-08-2011, 01:57 PM
striker12300 striker12300 is offline
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Quote:
Originally Posted by kazoo View Post
The frustum should have a 48 inch circumference on one end,rising one foot to 8 inches circumference on the other end.
Are you talking diameter or circ? If you're talking circ. then your reducer, thats what we call it, would be 15.278" dia. to a 2.546" dia. That's mean diameter also.
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  #13  
Old 04-08-2011, 03:59 PM
john_galt john_galt is offline
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Check out "The Metal Worker Pattern Book [1881] in the library at the top of the page.
Start at [for now] page 99 in pattern problems [part one] and read on. Good Stuff.
Also read the first post in this thread by pricer and the later one by bud, that's Good Stuff Too.

Last edited by john_galt; 04-08-2011 at 04:05 PM. Reason: add bud's post, good advice
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  #14  
Old 04-10-2011, 07:28 AM
kazoo kazoo is offline
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It's a cone with the smaller end cut off. Think of a pyramid with a flat top. How would I go about a layout on a flat piece of metal?
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  #15  
Old 04-10-2011, 02:35 PM
sharpscriber sharpscriber is offline
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Default frustum

Part of a cone where both ends are parallel to one another but you can have them at an angle to one another and still lay them out easily using a true length line to reference. For the first couple of years of layout it was as hard for me as trying to fly a space shuttle. Now I can beat anyone starting from scratch laying out a pattern faster than they can especially when it's a complicated layout such as a frustum of a cone with the ends not perpendicular to the center line or when the ends are not parallel. Now this might confuse a CAD programmer but still it takes many years to learn how to do it quickly and accurately. CAD seems to be more practical if the revenue is there but when it comes to complicated cornice work or irregular products I don't know how good it would be. You can make just as easy a mistake either way.
Layout is just developing a plane or line and finding the relationship between all the rest. If on a slant or different plane you just radial line or triangulate it.
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  #16  
Old 04-11-2011, 07:46 PM
kazoo kazoo is offline
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I'm still not there on the frustum yet, so maybe it would be easier for me to make the cone and cut the top out. How would you lay out the cone?
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  #17  
Old 04-11-2011, 08:53 PM
sharpscriber sharpscriber is offline
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Default Cones Cad and Calculators

Cones we learned in high school so I was lucky and blew through that lesson in apprenticeship school.
If you want the mathematical way to do it then see if you can understand the diagram and angle at stretch out. Or you could cut out the outside bottom first and roll it along the proper circumference length, which some say delivers a true stretch out for after forming which makes up for material thickness final diameter. You would do this on an angle against a marked distance.
It's easy to think of it as a simple layout, but in the real world of boiler making and special industries if you had to weld or have a certain thickness exact so that the quality control guy doesn't trash it it has to be perfect. If you miss a step just a sixteenth of an inch will not let a radial component fit properly on another to be welded properly or fit properly. Shipfitting work also.
Formulas and CAD and calculators are good for this but in the end if you don't know what to allow for the nature of the material's behavior after forming you won't know what to allow for shrinkage, stretching, etc...
It does take on the job experience to know and understand this. Sometimes you have to run a trial piece through the process to determine the formation changes in a material's behavior to include the required differences during layout to obtain the wanted results.
Even a test piece say 2" wide vs. a 12" wide strip of .250" material will end up with different outcomes when formed the same way. There is a reason for this. Lots of things to consider when doing this work right that CAD and calculators have to figure out to get it right.
Yeah if you're just doing some sheet metal ductwork and you want to hammer on the collars and always know that it never is proper and correct and that your CAD program or calculator almost got you there then it will work for you and the customer won't know with all that sealant you can put on all the joints. But if you want to work in industries where the stuff matters there is a real sense of pride and satisfaction that comes with knowing that there IS a proper and indisputable method that gives the proper layout and forming method to get the exact end result needed.
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