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rfrey
02-05-2008, 05:01 PM
Professor Slocum released a new version of his book FUNdaMENTALs today.

http://pergatory.mit.edu/2.007/resources/FUNdaMENTALS.html

The download includes all the design spreadsheets for bearings, linkages, etc. There's also a SolidWorks tutorial available through the course site.

A truly awesome resource for novice machine builders if you haven't seen it already. If you have, you may wish to replace your copy with the newest version.

Rod

R W
02-05-2008, 05:31 PM
Where can I buy this book.

rfrey
02-05-2008, 05:33 PM
Where can I buy this book.

Sorry I was unclear. It's there on the link page to download. It's free, as are all the design spreadsheets.

Truly a gift from MIT and from Prof. Slocum. I think Evan used this book or part of it when he was studying to begin his mill.

ptjw7uk
02-05-2008, 06:17 PM
Thanks for the link.
NIce download - had a read but one of the first design things he says I would disagree with - he says ' get rid of all unnecessary weight - if it is not required then design it out'.
I've always thought that designers only used the bare minimum of materials in so doing keeping manufacturing costs lower therebye reducing the longevity of the machine etc.
My own view is that if it is a machine then the more mass the better and the longer it will last and keep its accuracy, I may be wrong (usually I am well so the wife says)
Peter

rfrey
02-05-2008, 06:21 PM
Thanks for the link.
NIce download - had a read but one of the first design things he says I would disagree with - he says ' get rid of all unnecessary weight - if it is not required then design it out'.

Hi, Peter. Read a bit further - Slocum's talking about all sorts of machine design, not just machine tools. He probably would not consider damping weight in a machine tool unnecessary.

The 2.007 course is centered around a competition where excess weight would likely be a liability.

toastydeath
02-05-2008, 06:51 PM
I've spoken with Socum, discussing this exact quote. He is not talking about saving money, he's talking about increasing performance. It's a rehash of something somebody long dead said to the gist of "What purpose has it there?" in reference to weight in design. Even when our forefathers were building massive machine tools, they understood raw weight was not an advantage. It had to be carefully placed, and trimmed where possible.

If you can make a machine tool the same static rigidity but decrease the mass, you increase the natural frequency of the machine and thus increase dynamic stiffness. You can accelerate lighter bodies faster with less structural deformation, longer bearing life, and hold greater tolerances.

rantbot
02-06-2008, 12:22 AM
In engineering design you use what you need to do the job. Anything more is a waste.

It's one of the things which separates engineers from hacks. Anybody can throw in everything up to the kichen sink. Much of the trick is in realizing what to leave out.

Joel
02-06-2008, 01:12 AM
It has been said that any idiot can design a bridge that is strong enough.
The real skill is in designing one that is just strong enough.

barts
02-06-2008, 02:35 AM
As much as I agree about efficiency, particularly w/ mass designed and produced products, I feel obliged to point out that for many of our projects the downsides of extra material and increased weight are dwarfed by the advantage of avoiding failures due to inadequate strength, unanticipated loads or long term corrosion. If it doesn't have to fly, extra strength is often very cheap insurance, indeed.

Design the shape for overall best efficiency, but don't stint on thickness.

- Bart

dp
02-06-2008, 03:00 AM
Some things are not built to service - they are built to last while giving service. I've been on this bridge and it is magnificent. It was built of simple materials on a risky foundation by people who had nothing that approaches our capability. It perseveres and it is beautiful.

http://www.destination360.com/europe/italy/ponte-di-rialto.php

My visit to Ponte di Rialto: http://thevirtualbarandgrill.com/italy/Page8.html

rantbot
02-06-2008, 06:54 AM
Interesting. I've been following the 2.70/2.007 courses for over thirty years now. They've gradually become more of a spectacle than a class, for better or worse.

I remember in the mid-1970s when Prof Flowers, who was running 2.70 at the time, showed me his secret stash of scrounged parts for the next term's competition. He was particularly pleased with a closet full of electric motors for the SX-70 camera which Polaroid had donated. I wasn't too sure that was progress; the motive power for things made in the class had traditionally been gravity or rubber bands. One can learn a lot about machine design by working with simple stuff like gravity or rubber bands. It's harder to try to construct something using electric motors in half a term when one is simultaneously carrying a full course load. The end result is that even fewer of the things are finished and in working condition by the time of the contest than had been in the old days. Of course, that's a valuable lesson right there.

agrip
02-06-2008, 09:34 AM
Good question that was, Peter.

The term "unnecessary weight" coming from an August individual may seem daunting. There-in lies assumption.

Occam's Razor is better served by always remembering "necessary and sufficient". Either alone will imply lack of the "necessary and sufficient" {thinking, experience, data} or what have you

There is NO point in ever exceeding "N&S" unless the overage is certain class of gift. i.e., To use this gift, it is necessary to use it untrimmed, because to trim would make the cost exceed the N&S limit from scratch.

Ag

Mcgyver
02-06-2008, 10:07 AM
As much as I agree about efficiency, particularly w/ mass designed and produced products, I feel obliged to point out that for many of our projects the downsides of extra material and increased weight are dwarfed by the advantage of avoiding failures due to inadequate strength, unanticipated loads or long term corrosion. If it doesn't have to fly, extra strength is often very cheap insurance, indeed.

Design the shape for overall best efficiency, but don't stint on thickness.

you nailed it, bang on. i think its more cost driven as opposed to bad engineering, however overall the design and engineering of products is trending toward the atrocious. In the quest to fill shelfs with bubble packaged crap at 19.99, seems like half the crap at Christmas this year didn't last 48 hours.

onto the designers of our GM Suburban; now there's a bunch who know how to carve out extra weight. They 'd done so to the point where it's just a POS. Ever had an internal door handle break off of in your hand? more than once on the same car? To save an ounce of steel they've destroyed the brand...in my family at least. The kids view GM the way my generation did the Lada. too bad, what a waste.

They would do well to also teach that most important of engineering maxims; build it twice as strong as it needs to be and it'll only fail half the time.

rantbot
02-06-2008, 02:25 PM
The engineering approach can be boiled down to a simple maxim.


Good enough ... is perfect.

Nearly everyone - especially engineering students - assumes this is a joke, until they think about it.

Those who don't grasp this may try to deliver a steak when the customer ordered a hamburger. But by exceeding the specifications they fail to do the job, if the steak takes longer to deliver and sends the cost over budget. We all know vendors like this; they're too expensive and too slow.

tony ennis
02-06-2008, 03:13 PM
Good enough ... is perfect.

Depending on who decides what 'good enough' is. We had salesmen deciding 'good enough' back when I maintained a chemical plant simulator. They didn't care if it met the clients' needs or not - they just cared that it met the specs so they could get their commission. They also didn't care if the project could be done to any specs in the agreed upon time - commissions were based on sales, not net.

My advice is to make sure the people deciding what's 'good enough' have some skin the the game. Our sales team didn't and it showed.

lazlo
02-06-2008, 05:17 PM
Alexander Slocum's recommendation "if it is not required then design it out" is drilled into every engineering student from the day they take their first engineering fundamentals class.

Or said more eloquently by Antoine de Saint-Exupéry:

“A designer knows when he has achieved perfection not when there is nothing left to add, but when there is nothing left to take away.“

The point is that a professional engineer is required to design around a specific set of requirements, so they can't afford to add superfluous features/weight/cost, or in the context of the HSM, swag things.

That doesn't mean that a professional engineer designs things to a minimum spec: engineering analysis' always includes a design margin.

So to put that in context of the bridge, the engineer would be typically be given a design spec to support X Million Pounds of Dynamic Load. That spec would come from a detailed structural analysis of what the bridge would be required to support in day-to-day operation (a worst-case dynamic load from a convoy of speeding tractor-trailers, for example).

You would then, by good engineering practices, calculate a design margin. For non life critical devices like a microprocessor, design margins of 20% are typical. For life critical structures, that design margin may be 100% or more.

So with the revised design spec of X Million Pounds of Dynamic Load with Y Million Pounds of safety factor, the engineer goes about designing the "optimal" structure that will support X + Y Million Pounds of Dynamic Load, and nothing more.

rantbot
02-06-2008, 06:42 PM
Depending on who decides what 'good enough' is.
Naturally. It's one of the more difficult parts of the project, and far too often is skipped entirely. "Good enough" has to do with what the customer needs (which is not always the same as what the customer wants), what's physically and financially possible, what's up to code, etc.

It does not mean anything simple, like cutting out safety margins. Those are as necessary as any other part of the design.

A good example of violation of the "good enough is perfect" rule is seen here constantly. Someone is looking for a way to get a surplus industrial machine he's just acquired working. It has a 3 phase motor. The helpful types who immediately chime in to insist that he's just gotta have a VFD are ignoring "good enough is perfect." The customer needs to get his motor running in a fashion similar to the way it did when it was wired up in a factory somewhere. The gewgaws available in a VFD, while very real and maybe even useful, have little to do with the task at hand. It's part of the engineer's job to remember what he's trying to accomplish, and not get sucked in by the lure of bells & whistles.

It's very difficult - many engineers fail to do it, and fail right at the beginning, when they decide how they're going to solve a problem before they really know just what the problem is.

Arcane
02-06-2008, 07:04 PM
I have to wonder if the designers of the interstate 35W bridge in Minneapolis thought they had "achieved perfection" when they designed that bridge 40 years ago. http://www.michigan-sportsman.com/forum/showthread.php?t=218178
Maybe they should have paid more attention to the old adage "When in doubt, build it stout."

lazlo
02-06-2008, 07:21 PM
If you don't do proper maintenance on a bridge (or any other machine), it's going to fail.

The Minnesota Department of Transportation engineers repeatedly warned that the I-35 bridge was a disaster waiting to happen. The collapse happened because the Federal Highway Administration didn't want to pay for the repairs.

http://www.dot.state.mn.us/hottopics/35w/06_br_%209340%20.pdf