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drum shaped sheet metal production

Luke Rickert

Hot Rolled
Joined
Oct 24, 2007
Location
OSLO
Hi I am working on product (a small coffee roaster) that uses a stainless steel drum around 300mm in diameter and 150mm deep (that is 12 and 6 inches) made from 2mm 304 stainless steel. It is not a complete ring but rather 3/4ths of a circle or a C shape if that makes sense and it is supported on both sides with welded tabs. There are number of cutout, holes etc in the part and what I would like to figure out is what is the best production approach and with that approach what sort of tolerance is reasonable. The the dimensions of interest are basically how round it is as we use a paddle inside that rotates and stirs the beans. The challenge is we want to lip of the paddle close to the outside without hitting and if that surface is all over the place that isn't easy and the temperatures are too high to use any normal flexible material on the ends of the paddles. Also this thing is has a good bit of hot air blown into it so there is going to be some amount of movement from the heating (The air enters at around 400C and brings the beans up to a bit over 200C.)

We have had a few different prototypes made and most of them use a series of bends (ever few degrees) but this is not ideal as there is a section with many small holes so these turn into a bit of a cheese grater with the distinct bends. We also have a version that was rolled and while the curve is smoother it seems to be really inaccurate in terms of roundness and straightness. (radially I see a difference on the order of 4mm as you go around and side to side) These samples were made by a rather poor quality supplier so I am not sure if these results are representative or not.

The other methods I have considered are things like spinning and deep drawing but I think spinning isn't well suited and for deep drawing and other forming processes (hydro forming, super plastic forming, the tooling costs are far too high for the projected volume (400 per year and prices that real people, rather than the military are going to buy).

My preference would be to redesign and make everything out of cast and machined iron but that isn't likely to happen.

I can share some pictures etc directly but would rather not post them online.

thanks

Luke
 

memphisjed

Stainless
Joined
Jan 21, 2019
Location
Memphis
Roll a laser/water jet part (holes and features) with a sheer roller. Not a plate roller. Large tack to small weld flange on the outside edges to get and help keep a even radius.
The tangents from rolling can either be pre bent, coined bent, or maybe even ignored and placed into the flange profiles.
You could also do semi closed die bending via a coining concept. This might give you the most constant bend with features pre cut. You will not have to fight the tangents either. Still need to flange the edges.
 

Luke Rickert

Hot Rolled
Joined
Oct 24, 2007
Location
OSLO
Roll a laser/water jet part (holes and features) with a sheer roller. Not a plate roller. Large tack to small weld flange on the outside edges to get and help keep a even radius.
The tangents from rolling can either be pre bent, coined bent, or maybe even ignored and placed into the flange profiles.
You could also do semi closed die bending via a coining concept. This might give you the most constant bend with features pre cut. You will not have to fight the tangents either. Still need to flange the edges.
Thank you memphisjed
Not being completely conversant in the jargon of sheet metal fabrication let me check if I understand everything. I take it that if the correct type of roller is used with proper care and techniques it should work. Would the normal course of action be to oversize the blank and then cut it back (on both ends) after rolling as the end curvature is going to be an issue or is the point of the sheer roller that it will avoid that issue? Anyway that seems solvable but I am a bit confused about what you are saying about weld flanges.

I have been searching the term and variations and am generally finding machined flanges that would be welded onto the sheet metal but I don't think that is what you are talking about so I guess it is some sort of detail added to the edges of the laser cut sheet before rolling but I am having trouble see how that would work if the edge thickness wasn't the same as the rest.

thanks

Luke
 

swarfless

Cast Iron
Joined
Aug 31, 2005
Location
South Australia
First up I wouldn't use stainless, rather mild steel, BUT if smitten by stainless I'd hunt for seamless tubing .. online metals.com list 12" od 1/8" wall seamless 304. Alibaba would probably supply exactly what you want. OR you might find stainless saucepans that could be de-bottomed & de-handled. Weld on CLOSE-fitting substantial flanges, maybe even prep them with an annular groove one tube wall thickness out from inner edge & TIG a filler-less edge weld for minimum distortion. Then do all perforations, cut-outs, etc, by the method of your fancy.
 

Luke Rickert

Hot Rolled
Joined
Oct 24, 2007
Location
OSLO
First up I wouldn't use stainless, rather mild steel,
This is just one of many questionable choices that were made before I became involved.

I didn't know there was thin walled tubing that large, I did a bit of looking around and there are tubes in close to the desired size available in Germany. I imagine with a bit of volume we can find most anything. It shouldn't be too hard to find someone with a 4th axis laser cutter that big I hope? The tolerance on the tube I have found so far isn't great +/-1% on diameter but perhaps I can find some D4 tolerance which is half of that and probably workable.

Any idea how much seamless tube moves when you do cutouts?

thanks for the suggestions

Luke
 

Ries

Diamond
Joined
Mar 15, 2004
Location
Edison Washington USA
all food grade stuff needs to be stainless. Plus, roasting, the heat will oxidize mild steel and make a mess.
So it pretty much has to be stainless. And even that will need to be designed to be replaced, because it will wear out, and oxidize.

Large diameter thin wall stainless is going to be hard to find, only available in large quantities, and not necessarily any rounder- pipe doesnt need to be concentric in tenths.
A good sheet roll can get accurate circles.
But many shops in the USA buy cheap equipment and misuse it, so its harder to find precision sheet metal shops with nicely maintained equipment.
Seems like in Norway (where the OP is located) there might be some precision shops, but not too many- Italy is where they build a lot of stainless food prep machinery, and, not coincidentally, where some of the nicest modern sheet metal equipment is made.
I would be looking for parts like this to be made in northern Italy. Laser cut, then rolled.

A small coffee roaster is a very tough thing to make cheap, especially in less than 100,000 piece quantities.
I had a friend who built them for a while, and they ended up being pretty expensive- the current incarnation of his company sells their 1lb roasters for approx $3500 US. I have had to do repairs on some of the prototype models, and they are simple, sturdy, and crude.
To make a modern, refined on, in the US, anyway, I would guess you would be talking more like $5000 retail.
This is the current one they sell.
 

Luke Rickert

Hot Rolled
Joined
Oct 24, 2007
Location
OSLO
We are looking at production in northern Italy, also possibly Sweden which has some very high quality sheet metal producers.

What I am trying to do at this point is figure out a basic approach that is producible to the required tolerances, of course with input from the eventual producer. If we show up with badly conceived parts no reputable company will start the conversation it will take to arrive at a quality product.

We are dealing with a price range that is sufficient to do quality work, it is just a matter being smart about it. The competition is built in the US and western Europe.

Thanks for the input, the people I am working for don't seem to understand the importance of designing for manufacturing. They think they can send what we can charitably call "immature" designs to China and get back cheap parts, obviously this didn't doesn't work out well.
 

Ries

Diamond
Joined
Mar 15, 2004
Location
Edison Washington USA
it going to be harder to maintain concentricity with a partial circle as opposed to a complete circle. You will probably need to have a piece at each end of heavier material that its welded or mechanically fastened to, to keep the shape you want. A properly rolled circle will tend to self equalize.
 

JP Machining

Stainless
Joined
Jul 15, 2006
Location
Wisconsin
2 roll urethane roller. I have had parts rolled with these by Acrotech before and on a normal roll the cutouts etc cause diferences in bending as the pc is rolled. The urethane roll pushes the part against a tooling cylinder a little smaller than finish size (to allow for spring back) and all parts are bent equally, also no flat spot at start or pre bend needed. I wanted to buy one for awhile but the price tag never could be justified with th qty I was running.

Like this
 

swarfless

Cast Iron
Joined
Aug 31, 2005
Location
South Australia
Seriously this time, I concede the stainless requirement for foodstuffs BUT why bother to manufacture EVERYTHING. 2mm thick wall sounds like overkill anyway. Why not approach a pots & pans manufacturer who likely has a vessel of the dimensions near enough already in production. All you need is the unfinished article to adapt to your needs, the pots 'n pans maker has already solved some of your problems. You may even be able to utilise the bottom intact. Surely you don't make your own fasteners, etc? I do return to adding flanges to control roundness & perforating afterwards, making your 'missing section' a 'window' rather than complete absence, a gap.
 

Luke Rickert

Hot Rolled
Joined
Oct 24, 2007
Location
OSLO
Why not approach a pots & pans manufacturer who likely has a vessel of the dimensions near enough already in production. All you need is the unfinished article to adapt to your needs, the pots 'n pans maker has already solved some of your problems
Swarfless, I completely agree, there is no point in producing things that we can buy off the shelf (or using stainless for everything) but convincing the guys that I work for is a different matter. There are plenty of more egregious examples to deal with on the product. In this case particular case it isn't quite that black and white as given the details of the part (which I haven't shared) cutting it flat and rolling after has some advantages and I think will be less expensive provided we find someone capable of the rolling work.

Along the lines of not reinventing the wheel I have been looking around a bit to see if anyone produces generic deep drawn parts, the same way you can buy elbows, reducers, flanges etc for welding up pipe but I can't find anything.

I looked at pots and pans and there are plenty that are a similar size so if we found a producer that was interested in working with us with could be an option. I don't want to deal with Chinese logistics for this so they would need to be in Europe but that might be possible.
 

metalmagpie

Titanium
Joined
May 22, 2006
Location
Seattle
In Seattle there are lots of small Ethiopian grocery stores that stock green coffee beans. In their culture beans are roasted, ground and consumed all in one ceremony. I used a hot air popcorn popper from the '70s to roast my own beans. I ground them up at the supermarket, who didn't mind if I used their high quality grinders. I had to quit drinking coffee because I couldn't strike a welding arc in the right place because my hands were too shaky. Haven't had any since 2006. I sold that popper for good money.
 

Strostkovy

Stainless
Joined
Oct 29, 2017
Get a custom made bottoming die in the appropriate radius. Probably $4000 for a set. You do the forming in several steps, but each step forces the correct form of that section, so holes and edges don't matter.
Somewhere like Wilson or FabSupply will be able to have one of their engineers determine the max width to strike at a time and the tonnage required. I have formed large radii with punches like this but have never needed a matching die because our parts were uniform. A urethane lower die may also be possible. Punches exist with radius inserts that can be changed out.
 

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