Sheet Metal Sticker Shock

I just got the following parts quoted (I asked for a ballpark quote, got
a detailed one -- hmph). Here's a rough sketch, with unnecessary detail
_and_
things left out:
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Over the phone we modified things such that I'd be responsible for the
indicated holes, and the finish would be brushed & clear anodized. The
quote came back at around $36 each in lots of 50.
Is this just what I can expect to pay for these things? Did I go to the
wrong shop? What are the cost drivers here?
I'm trying to put together a training device that I can include in the
cost of a seminar, $36 is more than 1/3 of the worst-case BOM cost that I
was contemplating. I need to know if I can beat this down by being
creative, if I need to bite the bullet and accept a higher BOM cost, or
if I should yank my eldest out of school and put him to work in the
garage bending metal!
The principal of this thing is that there's a counterweighted arm that
swings on a shaft that's pivoted on bearings in the large holes in the
frame. A microprocessor monitors the position of the shaft through a
potentiometer and controls the voltage to a motor/propeller, with the
goal of holding the arm at a commanded position. The hole locations are
assumed to need tight tolerancing because of the apparent tolerance needs
of the pot I'm planning to use, but I'll be re-thinking that decision
here soon.
Thanks in advance.
Sorry this is off topic - would it help if I mention that I'm putting my
vote behind Hillary McBama, with Ralph Paul as her running mate?
Reply to
Tim Wescott
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=========== Looks like an interesting project.
The first cost driver is the low volume.
I would take a walk through WalMart, a kitchen/restaurant supply with lots of pans, etc. and see what was close enough to modify. Possibly something plastic, unless you need metal. A plastic box with a snap on lid is good as you can pack everything inside.
These sites also looked interesting (stand on end?)
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(many other types on hammond site)
The brush/anodize finish looks nice but is extra handling and an avoidable cost. If possible get a prefinished container or chassis
It may be cheaper to drill oversize holes for the bearings and then use separate bearing pieces with sheet metal screws to allow adjustment/alignment rather than trying to hold close tolerances on position and size on a single fabricated/folded part.
I would consider plastic for the bearings also. You should be able to make quite serviceable bearings yourself from plastic strip, possibly reaming the bearing hole for a good finish if required.
You can also make a long reamer/alignment tool from a piece of drill-rod to insure that the holes are inline from side to side. Indeed, you might be able to use pop rivets here also eliminating the sheet metal screws and speeding assembly.
If you need it, as a box or pan with closed ends will be considerably more rigid, you may also be able to use off-the-shelf L brackets and pop-rivet to the sides.
A little value analysis and you should be able to knock these out at home with a drill press. For sheet metal a special drill is helpful. For examples click on
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Another possibility is a wood base with the "sides" made from commercial L brackets attached to the base with screws.
Good luck and let the group know how you make out.
Feel free to email if you think I help.
Unka' George [George McDuffee] ------------------------------------------- He that will not apply new remedies, must expect new evils: for Time is the greatest innovator: and if Time, of course, alter things to the worse, and wisdom and counsel shall not alter them to the better, what shall be the end?
Francis Bacon (1561-1626), English philosopher, essayist, statesman. Essays, "Of Innovations" (1597-1625).
Reply to
F. George McDuffee
A student of mine built the prototype, which worked admirably well. Cost- wise, it made oodles of sense as a one-off, but not in any volume over onesie-twosie.
I'm trying to figure out how to achieve the desired behavior, but at a reasonably low cost.
Reply to
Tim Wescott
I'm planning on either using Rulon bearing inserts from McMaster, or just running the shaft against the aluminum.
Dunno about the adjustable bearing idea -- I've considered it, but if I'm paying to have it assembled it may be worth a bit of time on a milling machine to avoid having to find someone who's capable of doing the bearing adjustment right. The whole bearing adjustment issue is being driven by the sensor, so I'm going to look into that part independently.
It's important that it look reasonably professional, but bare aluminum may be good enough -- I may ask for another quote without the anodizing.
Reply to
Tim Wescott
Tim Wescott wrote in news:1M2dndUshPGBsk_anZ2dnUVZ snipped-for-privacy@web-ster.com:
You would be cheaper to make it 3 pcs and use the bracket for afixing the 3. The reason is that the parts can then be cut on a waterjet or laser, at very low cost, including tight location holes. When you get into bending, you cannot put the tight tolerance holes in prior to bending, because of tolerance stack-up. This makes fixturing for the holes to be put in later a difficult task. The savings would more than pay for a snap-in plastic corner strip to cover the sharp edges. I would re-think it along these lines Tim.
Reply to
Anthony
I will venture another opinion. It's your drawing. You are asking a guy to quote on a print with words like "about" and "3-4" with tolerances of .001". From where? Center to center? You didn't say that. The sheet metal guy sees trouble, and he will quote a premium price to compensate for the potential trouble. He or she is not in the engineering prototype business.
You might want to look at extruded C channel, in aluminum or plastic. In plastic you wouldn't have to worry about a bearing material. McMaster has some.
Kevin Gallimore
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Reply to
axolotl
$36 doesn't sound out of line based on my understanding of the sketch. What's the piece marked "bracket"? A welded-in gusset? Depending on the configuration and requirements, welding may be expensive.
Assuming the shop has a Timesaver belt sander, the brushed finish and anodize shouldn't add more than a couple dollars to the price.
You may be able pay for the finish by buying your bushings from Igus.
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Reply to
Ned Simmons
My plan was to get budgetary pricing so I knew if I should even start the business, or use that approach, then to pay a mechanical engineer to finish the design with a nicely detailed and toleranced drawing.
Clearly this shop doesn't understand the notion of budgetary pricing -- I was getting back an honest-to-god quote with four significant digits; a "budgetary price" with more than 1-1/2 significant digits is either a lie of a wildly premature quote.
Reply to
Tim Wescott
Tim, I believe there are several expensive areas in your design. First off, expecting .001" tolerance means it must be milled or wire EDM. Can you tell me why the tight tolerance? Does the mating part have the same tolerance? What size holes in the mating part?
Next, I don't understand the bracket. Is this a purchased item, or is it formed into the sheetmetal? Or, is it to be fab'd separately and attached, if so how?
Material thickness?
Anodize is not a big cost. Here, it's about $50 run charge, plus $1/ part. IIRC. Dave
Reply to
Mechanical Magic
Well, yes. My frustration is compounded by the fact that I'm accomplished at other aspects of engineering. Ask me how to make a low- cost circuit, or how to write software to allow for a processor that costs 1/2 as much, or to design a control system that maintains performance with a cheaper sensor or actuator -- I can do any of those.
But a feel for how much it costs to put a bend in a sheet of metal, or a hole, or what features of a mechanical design may drive the cost up or down? Not there.
Reply to
Tim Wescott
I knew I should have erased parts of that damn drawing and re-scanned it.
At this point I'm just trying to understand the cost drivers for the part without the holes -- I'll either have the cost to have the holes put in at a machine shop as a second operation, or I'll see if I can eliminate the need for such accuracy altogether.
Fabbed separately, screwed on.
The bearings I'm contemplating require moderately thick material -- it works out to 14 gauge, but I'm damned if I can remember exactly what the bearing spec is at the moment.
That's good to know, and a reasonable price.
Reply to
Tim Wescott
That changes things a bit. You may find "budgetary pricing" is a foreign concept to most working fab shops (the fellows here that run shops could tell us).
You apparently are in contact with engineering students- You might want to slide one of them a few bucks and get dimensioned/toleranced drawings to send out - or - have it given out as a mechanical engineering problem.
If you can give us more detail about what you need to do (having mounted a fair number of servo pots I don't understand the .001" tolerance) the folks here can give you some ideas about how to do it in budget within the design parameters. We can't help it.
Kevin Gallimore
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Reply to
axolotl
I'm a consulting systems engineer, so I'm in contact with any number of working mechanical engineers. Some of them are willing to help out for beer, in little bits at a time. Some of them are willing to expend chunks of time -- but they're the consulting mechanical engineers, who have this strange notion that they should be paid for that sort of thing (it's a fine notion, but only when I'm getting the money :-).
All of this is complicated by the fact that most of the places where I rub shoulders with such people are either instrumentation houses or aerospace houses. For those sorts of places "cost effective" means that you hog it out of 6061-T6 with 1/2 inch sections for rigidity and a nice web of pockets for lightness, and then you don't expend any more engineering time later messing with things falling out of spec. I would get a design that could be used for aligning lasers, but not, perhaps, for selling to individuals.
I could be totally wrong with that, which is why I'm working on opening it up. The tolerance comes from what appears to be a 0.005" off-center requirement from an under-specified but really cheap pot, then the necessary stack up to get from the bearing center to the pot center with the pot mounted on a circuit board.
See my post titled "Anyone with experience with this sensor?" for the part I want to use. It's way cost-effective, so I can't imagine that it would demand tens of dollars in the mechanical assembly to mate up to it, yet I can't figure out how to make it work without holding tolerances that tight.
I'd love to see an assembly where it's used -- that would probably clear up a lot of confusion.
Reply to
Tim Wescott
On Sat, 08 Mar 2008 12:43:26 GMT, with neither quill nor qualm, Anthony quickly quoth:
I picked up a 20' stick of 3/4" x 3/4" angle iron a few days ago and it cost a bleeding $14.90! He told me it had gone up 30% two weeks ago and was going up another 10% today. Steel plate is being rationed!
Hayseuss Crisco, it's getting expensive out there...
-- The best and safest thing is to keep a balance in your life, acknowledge the great powers around us and in us. If you can do that, and live that way, you are really a wise man. -- Euripides
Reply to
Larry Jaques
Tim, I'm a very poor estimater, but::
Initial shearing of the parts is cheap, IF you don't have tight tolerances. If you are OK with +- 1/8" then things are cheaper than +- .010". 1/8" can be done on a cheap shear, .010" has to be done on a laser machine costing $300,000+
Bending tolerance, same issue, looser is cheaper.
A sheetmetal shop cannot hold .001" tolerance on hole position, unless they have CNC controlled equipment. You are asking for the equivalent of .05% resistors. Yes, they exist but is it required? A qualified machine shop will be required if you insist on that tolerance. BTW, how are you going to inspect the part to verify the tolerances have been met?
Dave
Reply to
Mechanical Magic
Seems cheap to me.
First, look at material costs- aluminum is not cheap these days- I figure you get 20 parts from a 4x8 sheet- and bare material alone, in . 080 6061, has gotta be around 10 bucks per part.
Me, I lose money on anything that goes thru my shop that doesnt bill out at at least 5 times materials.
Assume that a decent sheet metal shop, even without any cnc machines, is going to be a minimum of $50/hr- and most are more. With CNC, they are going to be wanting closer to $100/hr shop time, and you would too, if you had a million dollars or so worth of equipment to feed.
An hour per part on these, in small quantities, is not unreasonable- without cnc, you would need to shear 18" strips from the sheet, then cut each one down into 12" wide pieces, then shear 4 more times for the angles- thats six cuts per part. Then, bending, then punching the holes and reaming to size (at least, thats how I would do it for precision)
Fabbing up the angle- thats more money in material, more time in measuring, setup, shearing, bending, and punching.
deburring- every part is gonna need hand deburring.
Then, somebody has to drive it to the anodizer, and pick it up when its done. More time, more overhead on truck expenses, diesel, and so on.
And I have never found anodizers to be cheap- $50 setup and a dollar per part- thats at the extreme low end of anodizing prices- and pretty much out of the picture in LA, with its high cost of pollution controls, or up here in the Seattle area where I live.
In short, I think you got a screaming deal on a quote, and, unless you were buying em by the container load from China, its unlikely to go down by more than 10% or so.
I really doubt you could find a bid that low, up here in Boeing land- when the average house (and nobody I know can find an average house) in Seattle is $500,000.
Reply to
Ries
Any good shop will have a standardized quote system. Just plug in the material, the setup times and run rates for each op, click 'store', click 'print'. You read the print as it is presented, tolerances and all. Any part like this will have a sequence of ops, you need to think about how you would make the part and which machines you would use regardless of the precision of the quote.
In a previous life I had all of the sales and presales for a high volume job shop. I had not the slightest interest in quoting to a low volume entrepreneur type. My large customer could get an on site visit from one of my sales engineers to work with the design engineer and CAD tech to wring every penny out of it. The small guy got ignored.
A quick look at your drawing shows .001" tolerances. On a formed part, that is just not going to happen without tooling, exotic equipment, or some very labor intensive processes.
A couple of comments to cut the costs: 1) Make sure the shop you are talking to does sheet metal and likes small, one off projects. 2) you need to figure out which features need the tolerances, which don't
Tim Wescott wrote:
Reply to
RoyJ
Or, if you can have a top on it, consider 6" square tube, as at
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which show 6"x6"x0.125 6063 Al. With a 4" top and 12" bottom, you would get two pieces per 16" of tube, ie a materials cost of about $7 each.
Reply to
James Waldby

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