Sheet Metal Problem Part_ Need New Material or Design?

[Buck61]

Hi,

I have a 301 Stainless steel, 1/2 Hard, sheet metal component AKA, Lift [see attachment] that needs improvement:
The Lift is part of a printer sub-assembly, a somewhat complex mechanism so, for the sake of of your time & available writing space, I won't go into detail.

The Lift itself has been problematic w/r to dimensional consistency, quality of the overall part, and sometimes contributes to the printer door jamming or sticking closed (see bottom of this paragraph).BTW, the 'door' is injection-molded Lexan EXL plastic. The printer sub-assy fits into a relatively tight, recessed space within our device (i.e., minimal clearance is available between part and recessed space)since a packet of printer paper sits inside the Lift, taking up most of the available area within the recess. We know that the extent sheet metal edges of the part, particulary the bottom front and of the Lift can interfere with the front & rear walls of the (Lexan)recess at times, but most of the time does not.

Secondly, [2] tack welded stainless steel pins .125 dia +/-.003" X .100 long mate with a cammed slot (arc) features in the printer door, subsequently causing the Lift (& paper packet)to rise within the recess when the door is opened. This allows the user to load or remove the printer paper pack.

Lastly, the Lift is powder-coated for aesthetic purposes: we sometimes see varying paint thickness & over-spray on pins which can potentially cause problems. I am looking to eliminate the secondary tack-welded pin operation but do not have a solution for a replacement "pin". (Perhaps a drawn feature to replicate the pin surface? or tight tolerance +/-.003 max plastic or alloy rod stock material more compatible with Lexan door material? Not sure how I will attach it to the Lift other than a press fit into a hole where the pins currently reside)
No fastener head is allowable on the inside of the lift since it would interfere with the placement & deployment of the printer paper.

A molded or formed plastic Lift appears to out of the question since the walls must be ~.020" thick, will lack strength & dimensional stability (expand or warp) within -10C to 60C operating environment. I considered inj molded plastic early on since molded pin features would replace the tack-welded pins and no powder coat is required.

Soooo....I need some advice on grade of .020" max thick stainless (aesthetically pleasing, e.g., brushed?) sheet material that would offer greater dimensional repeatability, stronger and stiffer than 301 SST. Ductility is important only if a drawn, +/-.003 tolerance pin feature is viable (but I think tolerance is too tight).

I apologize for the length of this post but needed to simplify illustration yet provide critical information for a very busy design.

Thanks in advance for material/design advice.

Best,
Buck

 

[KENAT]

What drawing standards is the drawing meant to be too?

It doesn't appear to fully comply with typical US drawing standards ASME Y14.100 (especially 14.5) which might be part of your dimensional issue. For instance, you don't define how symetrical is symetrical in note 4.

I assume tolerance information and explanation of the 'obround' symbol is in the title block somewhere?

Are you making the part yourself or getting it from a sheet metal supplier - if the latter they may be able to give you some usefull input.

No.4 unidirectional finish ASTM A480

http://www.ssina.com/finishes/sheetsamples.html

may be suitable. However it will typically only be on one side of the material and can be damaged during fabrication.

One possible issue in going to stiffer material may be holding the same bend radii.

Posting guidelines faq731-376

http://eng-tips.com/market.cfm?

(probably not aimed specifically at you)
What is Engineering anyway: faq1088-1484​

 

[drawoh]

Buck61,

I agree with KENAT that your drawing does not conform to any standard. Most of your dimensions are not toleranced. Sheet metal tolerances are not tight. You need to prepare high quality, unambiguous drawings. For example, your note_4 states that the part is to be symmetrical about the centreline. How does your inspector judge whether or not the part he is inspecting, is symmetrical?

Are your parts conforming as far as you can tell?

In other words, do the parts that fail, conform to your drawing?

If you need to be very accurate, you should consider some finish other than powder coating. You can coat stainless steel with black oxide.

JHG

 

[btrueblood]

Coating ss gets my hackles up, but that's me.

You can't make sheet steel stiffer (i.e. less deflection for a given load) without making it thicker, or adding section details to increase its moment of inertia (think flanges on an I-beam).

You might get more consistent part-to-part results if you switch to annealed condition material rather than 1/2 hard...talk to your sheet metal bender.

 

[MintJulep]

"Tack welding" isn't a final product, and it doesn't mean anything. So who knows what you'll get. Consider a flush head PEM stud, if they are available for such thin material. Otherwise, specify an actual welding process.

"Smooth Powder Coating" isn't a process specification, and doesn't tell anyone what you will and will not accept. Specify what you need and will accept.

Once you have defined what you really want and will and will not accept, have a talk with some benders about their process capabilities.

What are your quantities? Consistency comes from having dedicated tooling. If your quantities are small it's likely that it's being hand bent, thus the dimensional variation you are seeing.

 

[MikeHalloran]

You don't have enough edge distance for a PEM stud, and pressing a stainless PEM stud into a stainless sheet gives inconsistent results.

Powder coating is typically _thick_. .015" is not unusual, and it's impossible to control the thickness to the same degree as paint.
Painting stainless makes no sense. Investigate black oxide as mentioned.

Centerlines don't really exist. Try removing them, and see if the drawing makes any sense. If it doesn't, you need to dimension it differently.

The edge distance may cause problems, but the pins are almost short enough and fat enough to pierce and draw. ... i.e., make it all one piece. You should talk with your stamper about that possibility.

You should allow a maximum, and probably a minimum, radius at the internal corners, where the blank's two major rectangles form a T. An implied zero radius there requires a sharp notching tool, and leaves a stress raiser that you could probably do without.

The part is guaranteed to go out of tolerance when you drop it on the floor. I like things more robust than that.

You may not be able to allow the full +/-.002 tolerance on thickness, just because the part will behave very differently at .018 than it does at .022. If you leave off the powdercoat, you might be able to use .025 or .030.




Mike Halloran
Pembroke Pines, FL, USA

 

[TVP]

I have many of the same thoughts that the others have already mentioned: powder coat is too thick, tack welding is not conducive to dimensional accuracy, and you cannot improve the stiffness of the design by using a different grade of stainless steel.

Some ideas to consider:

1. Get rid of the powder coating. Change to cold-rolled steel and cathodic epoxy electrocoat (ecoat) for reduced thickness variation. Ecoat thickness is usually around 15-20 micrometers.

2. Press fit the pins into the stainless steel lift instead of weld. Tack welding seems to be a large contributor to any dimensional problems with this design, especially since the flange is not stiff (thin metal, relief in the bend, no darts or other formed geometry to increase stiffness).

3. Consider changing the material and process to magnesium and thixomolding. While Mg is less stiff than stainless steel, it is still much stiffer than unfilled (unreinforced) plastic. Thixomolding is used to manufacture many parts that require tight tolerances. Just Google "thixomolding" to get more information on the process, potential suppliers, etc.