Controlling a spring clips dimensions 4

[Charon99]

Hi, I hope someone can help me. I’m having problems with our manufacturing partners not being able to control the final dimensions of a spring clip.
The clip is as shown in the image below. The problem is they can’t maintain the 60.54 dimension with in a tolerance of +0/-5mm. After forming the dimension can vary but this isn’t the big problem (but it is an issue); because after heat treatment it can vary again by up to an additional 8 mm in any direction. How can they resolve this?
They have tried widening the clip to get a better cross-sectional area to length ratio (increasing the 6mm dimension). This seems to have little effect.
The material of manufacture is:- GB/T 1222 65Mn Spring steel.
The forming press is a 16T press.
Heat treatment :- Quenching temperature 830°c, tempering temperature 450°c, heat treatment duration 1.5hrs.

Any ideas why the final product 60.54 dimension is so inconsistent?

 

[EdStainless]

Nope, bend radius is the trick.
Forming coils our of 300ksi steel is real slick.

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P.E. Metallurgy, consulting work welcomed

 

[mfgenggear]

so form it from the coil , then stress all done. perfect

 

[EdStainless]

I should say that the trick to doing this is knowing how much to over-form them to allow for the spring-back.
I suspect that the inside radius of your tabs is a bit tight. It is less than 2t and I would rather see 4t for hard material.


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P.E. Metallurgy, consulting work welcomed

 

[mfgenggear]

OK so it will crack as drawn, bigger liberal bend radii, I thought so.
thus trial and error to allow for spring back yes agreed

 

[3DDave]

When any item is cold-formed by bending the way it keeps its shape is that the outer radius surface is stretched and the inner radius surface is compressed. After the external forces are removed the outer surface is in compression (because they are longer than before, and the inner surfaces are in compression (because they are shorter than before.)

These forces balance to retain the spring shape on top of any plastic deformation.

However, if one just drops that into a furnace the stresses maintaining that shape will be relieved, causing the material to relax/straighten out.

To avoid the amount of unwanted deformation, one can bend the part to a tighter radius (as EdStainless wrote) so the relaxation returns the spring to the desired shape or try to hold the part on a fixture so the part is not allowed to relax (as mfgenggear suggested.) A final option is to use a ring-roller to apply a slight post-hardening yielding deformation to tweak the radius to be as required.

My preference would be to tightly control the inner perimeter from tab-to-tab as the part looks flexible enough that local errors in radius are less important than being able to conform to the perimeter of the mating part.

(edit in the "However" part.)

 

[mfgenggear]

quote "These forces balance to retain the spring shape on top of any plastic deformation"

yes I am forgetting, agreed I need to go back and get training lol, in order to retain the geometry plastic deformation must occur right?
so the part has been yielded, spring steel is an other animal and my logic does not prevail.. From EDs post material of the coil is not full hard,
thus it can be formed, how evert the geometry is limited. yes agreed the inner bend is compressed and outer bend surface is stretched.
there is a fine line of the material tensile and hardness requirements, and geometry held. I am stopping here, clearly if this was my project more clear defined
data is required, I am sure there is white papers out there that defines the procedures, and specifications.

 

[mfgenggear]

here is a supplier that supplies 95 spring steel in the annealed condition[highlight #EF2929][/highlight]
95 Carbon Spring Steel
Ninety-Five Carbon, Cold Rolled Annealed Strip Steel, also known as AISI 1095 carbon spring steel, is the very finest of commercial quality (standard grade) High Carbon Spring Steel Strip available. It has been manufactured expressly for us to rigid quality standards for the purpose of greatest uniformity.
It is of fine grain structure, has been completely spheroidized with Rockwell held to the medium or low side of range. Can be easily formed, blanked or shaped, heat treated, hardened and tempered. Intended for springs or other applications such as mechanical springs, computer parts, saw blades, scrappers, etc. - Large tonnages stocked in all sizes. Technical data for AISI 1905 SAE carbon steel is available.
We can Slit - Roller Level - Cut to Length - Round Edge by Edge Rolling or Remove Burr by Filing. Oscillated Coils on Spools or Cores.

 

[mfgenggear]

here is from the same supplier with harden and tempered 1095 spring steel
Blue Tempered Steel & Polished Strip Steel
Hardened and Tempered to meet the most exacting requirements; Polished, Edged and Blued Precision Brand Blue Tempered Spring Steel. May be depended upon for top quality coil after coil. Every single coil on Precision’s shelves has been thoroughly tested on receipt from mill and prior to stocking.
This is perhaps the most universally used of all tempered spring steels. Recommended for various types of coiled and flat mechanical springs such as ignition vibrator springs, springs for timing devices, springs for the electrical and electronic field, steel tapes, rules, etc.
Tempered Storage
One of the largest stocks of Tempered Spring Steel Strip available. Hundreds of sizes of Tempered and Polished are regularly stocked, both narrow sizes with dressed edges and wide sizes for slitting purposes. Stock sizes range from 1/8” wide to 12 3/8” wide. Also available 24 wide on request.
Coil Stock - Color Blue
Carbon .90/1.04
Rockwell C 48/51
Complete Facilities For Slitting - Edging - Length Cutting – Blanking
Available in coils or cut lengths. Sizes other than those listed and in all thicknesses .001 and heavier are obtainable and may be ordered for you.
View 1095 Blue Tempered Spring Steel Tolerance Data

 

[mfgenggear]

I would buy a sample piece of the harden material and see if it can be formed to the geometry at that hardness, then stress relieve and see if works, if it does not then
back to the annealed condition, form and heat treat, options?

 

[mfgenggear]

a quick google and these notes of the different types and hardness of spring steels which can be formed as is and which can not
Information about Spring Steel Strip
Imprint




For further information visit our website or our company:

Stahl-Becker GmbH
Ottostraße 9
63150 Heusenstamm
Germany
Phone: +49 (0)6104 / 40 59 60 Fax: +49 (0)6104 / 40 59 70
Email: info@stahlbecker.com Website:

http://www.stahlbecker.com

Data Protection
What is spring steel strip?
Spring steel strip is used to build springs for a great variety of purposes.
Below especially the cold rolled (carbon as well as stainless) spring steel strip will be described in detail.
The most common production width of spring steel strip is about 300 mm (11.811 inches).
As a matter of course spring steel strip can be slit or cut to length (sheets).
Occasionally excess widths up to one meter (39.37 inches) are accessible.
To describe spring steel strip properly there are mainly two important standards:
1. EN 10132-4 Cold rolled narrow steel strip for heat treatment. Technical delivery conditions. Spring steels and other applications (replaces BS 5770-1:1981, BS 5770-2:1981 and BS 5770-3:1981)
D2. EN 10151 Stainless steel strip for springs. Technical delivery conditions (replaces BS 5770-4:1981)
The main difference between those two standards is how the tensile strength has been achieved (except stainlessness of materials according EN 10151):
In the first case (EN 10132-4) the necessary tensile strength is achieved by a heat treatment (thus this material is available annealed as well as hardened and tempered). The tensile strength of material according EN 10151 is achieved by the cold rolling process.
1. C75S+LC, annealed and lightly cold rolled. In this condition the spring steel strip is easily deformable. It has a tensile strength of 490-640 N/mm2.
After processing the workpiece it can be brought to the favoured strength by heat treatment.
2. C75S+QT, hardened and tempered, tensile strengths of 1200-2000 N/mm2. In this case the strip has already been heat treated to the designated tensile strength. Processing those strips mechanically is more difficult and bending with narrow diameters can cause breakage.
On the other side it is especially suited to laser cut it. In this case standard length sheets of 2000 mm are widely used.
Among the stainless spring steel strips according EN 10151 X10CrNi18-8 (material-no. 1.4310) is one of the most common ones.
The cold-worked condition (+C) is the most common one, with a range of tensile strengths from 1100 N/mm² up to 1900 N/mm2.
IFor a lot of purposes where bending to a certain radius is mandatory, strips with a tensile strength of 1100-1300 N/mm2 are sufficient to at one hand have enough strength and on the other side avoid breakage..
This steel grade 1.4310 is also perfectly suited to laser cut it.


If you need information about spring steel in general try

http://www.springsteel.info

 

[Compositepro]

The thickness of the metal has a dramatic affect on how it bends. Thinner strips must be bent more before reaching yield. So precise tooling will not assure precise bends. The only important dimension for this clip is that the opening must be small enough so that it will spring closed onto the mating part.

 

[EdStainless]

You would be surprised how little tooling is involved in forming these. The only hard points are really the corners or tangent points, everything else forms based on stresses.

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P.E. Metallurgy, consulting work welcomed

 

[dhengr]

Charon99:
Maybe the question should be, what can you do to change your clip design a little bit to make it more tolerant of this (some) variation in the manufacturing process. I can imagine that the 60.54mm base dimension, circumferential length and the 32 rad. would be tough to control exactly/perfectly with that material, any minor material variations and the likely means of making the piece.

I would make the 3.15 feet/tabs at the bottom of the clip slightly humped upwards at their mid-length, that is slightly concave downward, a circular arc. I would want those tabs to bear on the connected pieces/parts anyplace along their 3.15 length, without bearing right at the sharp end/edge/corner, where it might gouge or abrade the joined parts, or at the tight radius of the transition to the bigger arc. Maybe it should be dished or dimpled slightly to accomplish this. Then, I would shoot for a minus tolerance on the 60.54mm base dim., so that I always had to spring the clip open when it is to be installed. This might keep the clip tight when installed. While any plus tolerance on the 60.54mm base dim. will cause the clip to be loose or disengage when installed.