Dimple Die Rules of Thumb Testing

We’ve done multiple iterations of dimple designs over the years and put together a sort of in house “bible” on what should and shouldn’t be done.  This is a snippet from our bible on when cutting a hole in a panel to dimple die or flare it.

Do not place flared holes in panels situated in high stress areas, it will reduce the overall strength of the design.  The idea is that if you have a seamless panel (no through holes or flared holes) in a high stress area it will be stronger in most cases due to the lack of stress risers/concentrations. A correctly produced flared hole in most cases will be stronger than a through hole in a panel.  If a flared hole must be used in a high stress area the flare should protrude to the side that is bending outwards under load, this will minimize the reduction in strength from the removal of material.

Example:

• 12” x 12” panel
• 11ga 1020 mild steel
• 1.5” flanges
• 5 x 1.5” flared holes
• 1000lb load exerted downwards on top flange surface
• Fixed on bottom flange

This example forces the high stress location to be where the flared holes will be located.  It offers a good comparison in terms of flare direction in regards to overall part strength.  The strength in this case improved 4.5% by having the flare in the direction of the bending panel.  However keep in mind it is still 30% weaker than if the part did not have any flared holes to begin with.

Left – “wrong” dimple die direction (flared inwards)

Right – better dimple die direction (flared outwards)

Locate the low stress areas, such as web components, and remove weight from those areas by flaring.  Removing the mass and flaring the holes in these areas will reduce the weight while providing greater strength.

Example:

• 6” wide, 6” tall, 12” long
• 11ga 1020 mild steel
• 1.5” flanges
• 1.5” flared holes (when applied)
• 1000lb load exerted downwards on top flange surface
• Fixed on bottom flanges

Here is the part without any modifications under load:

Now you can see the majority of the stress is on the top flange surface, it would not make sense to dimple die this area.  Here is what would happen:

There is a lot less red visible since there are extreme stress concentrations located on the edges of the flared holes.  Having the flared holes in this location reduces the part’s effective strength by 20%.  Therefore it is a terrible misjudgment to locate flared holes in this high stress region.

Now notice how the sides have very little stress as opposed to the top surface.  Having flared holes there would:

The interesting bit is that the part is 6% stronger than the part without any modifications.  In this case the flared holes improved the strength of the part, while also reducing the weight by 6%.  As you can see strategic placement of flared holes can produce a lighter part, as well as a part that can take a greater load.  If the flares were inverted in this example (having the flare protrude to the bending side of the panel) the part would be even stronger.

Don’t take the exact #’s too literally, just understand that flared holes are only beneficial in low stress areas and can offer greater overall part strength when done effectively.  To do it as effective as possible only FEA (finite element analysis) can be performed and the optimal setup can be determined.  Otherwise it’s a “best guess” in where and what size flared holes to put in a panel.

If you are designing or just building something that you know has an extremely high factor of safety, then having flared holes in conservative locations should only improve the design by removing un-necessary weight.  If designing a structure, don’t let the use of flared holes allow for a sloppy/overbuilt design.

Please note, this article does not consider the removal of material to be substituted with extra material in the high stress areas.  Also this article does not consider panel deflection due to load.  Those two are entirely different (but related) topics that are documented in our full “bible”.  So rest assured when we punch or cut a hole in a steel part to flare it, we know what we are doing!  Please take a look at our dimple die service that we offer.

Steel mannequin

We had a neat project this week, design and build an interlocking steel mannequin for modeling scarves.  We normally use Solidworks as our primary CAD design source, however Autodesk has come out with a pretty neat piece of software called Autodesk 123D.  It takes 3D models (.stl and .obj files) and allows you to manipulate the model in many ways, in this case we are creating an interlocked sliced 3D model.  This will allow us to slice the model into sheets and allow us to cut in on the CNC table and weld the pieces together.  This 3D model we are dealing with is a steel female mannequin for mocking and displaying infinity scarves for our sister company, Möbius Threads which is run by Jaylene Andres.

Before we cut any steel we cut out a small 4″x4″ template with various notch sizes.  That will allow us to find the correct notch size and ensure that when we cut out all the pieces they will fit up tight, but still have some room to slide together easily.  Along the back where the horizontal sheet meets the vertical sheets, they will be tacked in place via the TIG welder.  This will lock all the pieces together and allow this structure to be transported from the sewing/cutting room floor to trade shows and open houses.

Here’s what the template looks like:

Based on the kerf width of the plasma a notch of 0.108″ (in CAD) is ideal.  Just enough room to be loose for assembly but tight enough that once all the pieces are in it’ll be a solid structure.  Another way around this would be to measure the actual kerf width, then update the CAM software, then make a template.  It’d come out with a more realistic CAD notch size, as the material being cut is 14ga steel (which is 0.074″ in thickness), but the method we took is quicker and worked just fine.

With that info of the ideal CAD notch size the model is updated, exported to a .dxf and then tool paths are created.  The total cut time took about 1/2hr.  The metal pieces were cleaned up and assembled.  Here are all pieces after coming off the CNC table.

This is the final product.

And now it’s modeling some Möbius Thread scarves.

Not sure if it’s going to be powdercoated or if a patina will be applied to it.  That’ll be decided shortly.  He’s a video of it cutting one of four panels to create this steel mannequin.

Sewing machine…???

Yes you’ve read it right.  We may have some interesting products leave Mint Design in the future with a new Juki LU-563 sewing machine we acquired.  After a 5 hour drive it needed a good cleaning and tune up before we even plan on using it.  It has a 1/2hp industrial motor with clutch engagement, it can sew basically any fabric that can fit within the feet.  It has a variable pressure walking foot, so it can feed through sticky or slippery fabric with ease that would normally cause irregular stitching or bunching up on a regular sewing machine.  The Juki LU-563 sewing machine was the staple of the denim garment, leather and upholstery trades starting in the mid 80’s.  They were made in Japan and just plain made to last.

Here it is in the shop after we unloaded it from the trailer and lightly wiped it down.  Overall not too bad.

It was having some stitching issues and had some wear and tear to the cosmetics of the sewing machine.  So it was sent for a tune up at Century Textiles and then pieces were removed to get powdercoated (wrinkle black) with Sean at Jamison Automotive Services.  Once we got all the pieces back the sewing machine was reassembled and it’s now ready for service!

CNC cutting up a storm

Tonight we’ve been CNC cutting everything from 10 gauge to 3/16″ to 3/8″ mild steel material.  It’s been busy and we enjoy the variety.  CNC cutting is the backbone of our business and we keep our CNC table as busy as possible.

Another batch of door strike plates (a repeat order) used for the electrical rooms at the new Saskatoon Police Station being built.  The three pieces in the back are the gussets to be used for the new welding table we’re building at Mint Design.  The nice thing about CNC cutting is that we keep our previous cut files and link it to a cut part in the shop, that way when we need to recut more pieces we can easily nest more pieces and know where the previous parts were cut.  This ensures that when the torch is cutting it doesn’t run off the edge of the material or cause an incomplete part.

Here is some of the 3/8″ steel we cut out for a local fabricator, CW Fab.  These were a one day turn around from the moment we received the files.  When we have the material in stock we can get parts CNC cut very quickly when needed.

Welding table retirement

After five long years we have decided to retire our little welding table and build a new small versatile table that will allow us to build more precise parts.  This below 3d model is the design we came up with.  The hoops are for ratchet strapping parts down easily to the table, hanging clamps from or to stick PVC tubes (containing TIG filler wire) within close reach without getting in the way.  The corner gussets also allow for the lack of bracing on the bottom of the legs.  The less things under the table we find the better.

We cut the top on the CNC plasma table and began cutting up some 2″x2″ HSS tubing for the base.

The CNC cuts pretty good detail out of 3/8″ steel, with little to no dross and angularity.

Prior to the table being cut out the plate marker was used to center mark where all the holes would be drilled (4″ on center in both directions).  This will allow for fixtures that can be installed to clamp parts down to the table prior to welding.

We feel like a steel wool factory.  The mag drill with annular cutter made quick work of this table top.  All the drilling was done after the 2″x2″ steel frame was welded on the underside of the table top.  Having this frame welded on ensures that any residual stresses after drilling don’t allow the table top to warp.  We didn’t want to induce any warping during the fabrication process, but the table doesn’t need to be precise enough to be blanchard ground.

With the top drilled it was time to weld up the frame.  Legs are completed and welded up to the 3/8″ steel feet which are bolted up to the caster wheels.

Already being put to good use!

The gussets are formed and ready to be welded on.  The plasma cut slit on the bend line makes it easier to bend the part as well as gives a clean area to weld the gusset to the leg.

CNC cut coasters

Made a few promo coasters for our frequent customers.  CNC cut coasters!  These were cut out of 10 gauge 44w steel, mill scale stripped, lightly sanded, sealed with boiled linseed oil and the bottom was lined with a felt pad.  Here they are prior to the felt lining.

Nothing but a high quality cut edge, even on very thin material.

We usually nest in promo pieces if we have room on a sheet, the cutting time usually doesn’t take very long and the material would otherwise be scrap once the skeleton is recycled.  So we try to make good use of our material and provide our customers a promo item here and there to let them know that we do appreciate the business and that we are always there for them.  These CNC cut coasters are one example of that, we also have other promo pieces that make it out of the shop as well.  We don’t photograph everything that goes on in our shop, so there is always a sense of surprise…that and we don’t always have the time to take photos of what we’re working on.  We appreciate the business from our customers and we show that first with our quick turn around time and quality, but it doesn’t hurt to toss a promo piece in there too!