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Controlling a spring clips dimensions
4

Controlling a spring clips dimensions

Controlling a spring clips dimensions

(OP)
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?


RE: Controlling a spring clips dimensions

Hi
Which way is the clip varying is it going plus the 60.54 or negative?

“Do not worry about your problems with mathematics, I assure you mine are far greater.” Albert Einstein

RE: Controlling a spring clips dimensions

So these are being cold formed after HT and then stress relieved? This is how springs are normally fabricated.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed

RE: Controlling a spring clips dimensions

You are asking for tolerances beyond process capabilities. Learn the process capabilities and design accordingly.

Otherwise... There's the ol' sort-and-scrap.

RE: Controlling a spring clips dimensions

...or maybe a better vendor...

batHonesty may be the best policy, but insanity is a better defense.bat
http://www.EsoxRepublic.com-SolidWorks API VB programming help

RE: Controlling a spring clips dimensions

(OP)
Thanks for your help,

These are being cold formed and then ht (as above) to make them "springy".
Surely they must form them before ht otherwise they would not take form???
They can vary in either direction but normally the gap is opening but without any consistency.

The supplier tells me they can't achieve the tolerance because the material is too thin or the 6mm width is too small.

RE: Controlling a spring clips dimensions

@charon99
The design is not helping, a very thin material with a large radii, it's an uphill battle.

couple of suggestions:
since this is a very inexpensive part it is a matter of cost vs cost of the process.
is this material being formed in the annealed condition can material be purchased in the annealed condition?
the forming induces stress thus during heat treat the part will move. in addition during the transformation of the material causes movement.
a pre-heat temperature which should be specified in the heat treat specified will lessen the movement but not eliminate.

Important information: make stainless steel heat fixture. with the same configuration as the clip, except for easy loading slide the clips
on to the fixture to restrain them. simple and fast. run the parts thru all the cycles , harden and temper, then remove them.
this is and will be a trial and error, it's is what it is. until the right size on the fixture is obtained.

look into the speed of the quenching oil, insist on a slow acting quench oil, most the distortion happens here.

a thin un-restrained part like this will be difficult to maintain geometry. because it is so thin.

RE: Controlling a spring clips dimensions

(OP)
@mfgenggear

I'm confused. I thought you had to 1, Form the shape, 2, Anneal it to release stresses, 3, "harden" heat treat the part to make it springy.

If 65Mn spring steel is already hardened, how do they form it without it springing back to its original shape?

Why is the factory telling me they purchase 65Mn spring steel, cut and form it and then HT it?

What material should you start with?

p.s. With the samples I have seen the material is ductile before it is heat treated but the specification for 65MN includes hardening ht. Why are the un-heat treated samples ductile if they are 65Mn?

RE: Controlling a spring clips dimensions

@charon
It is standard practice to form metals in the annealed condition.
Please advise the condition of the material as purchased, hardness ?
Please investigate if it can be purchased in the annealed condition.
Material will work harden after forming, so I do agree with the post anneal and harden.

Now my expertise is not spring steel.
But basics is basics. Looking at the % elongation is only 5%

RE: Controlling a spring clips dimensions

2
Charon99,

Your parts are sheet metal. My understanding is that sheet metal is bent to a tolerance of ±0.4mm. Your 32mm radius may be the problem. If you lose the big curve, you can do two 90° bends. This may be easier to do accurately.

Are you talking to your vendor about this?

--
JHG

RE: Controlling a spring clips dimensions

(OP)
@mfgenggear

Thanks for your help. I’m trying to get details on the material they are using. They told me 65Nm. But I think that can’t be right.

RE: Controlling a spring clips dimensions

(OP)
@drawoh

This has to sit over a curved face, like a ring clip on a hose. The rad could be made with multiple flat faces (I guess 10). But I imagine that wouldn’t help (lots of bends with a tolerance on each bend to deal with).

RE: Controlling a spring clips dimensions

Charon99,

Can you configure the thing so that there is one sharp bend that controls that final dimension?

--
JHG

RE: Controlling a spring clips dimensions

some thing like this, sorry for the poor drawing

RE: Controlling a spring clips dimensions

(OP)
I couldn’t imagine how I could loose 8mm out of tolerance in a final bend. The part is just too small. Your idea with a screws wouldn’t work because there’s no free space bellow the 3.15 flanges. I have done some tests and I think I can accept a tol as high as +0/-15mm on the 60.54dim. and it still work. I’m getting a large qty of samples measured to evaluate what the scrap rate would be with such a wide tolerance.
But saying all that I’m still confused as to the raw material that should be used. Is 65Mn a hard spring steel? If so surely this shouldn’t be used?

What grade of steel should we start with? Don’t we need a ductile grade that can be hardened after forming. I’m interrogating the factory now but would love to be able to say you should be using x material and performing y hardening. Any additional advice on this?

RE: Controlling a spring clips dimensions

If the tolerance is +/-15mm it makes no sense to specify the dimension to two decimal points.

RE: Controlling a spring clips dimensions


quote "Your idea with a screws wouldn’t work because there’s no free space bellow the 3.15 flanges."

the clips slide on from the end of the fixture not from above. quick and fast, under the flange has a relief for clearance. , the only restrain is on the R32. to keep it restrained.

RE: Controlling a spring clips dimensions

spring steel can be soft while forming, then formed, because of the carbon content then can be heat treat to the spring steel hardness,
it's really simple. analogy: tool steel can be purchased annealed condition so it can be machined, then after machining it will be harden to full hardness required.
go and talk to your supplier under stand the heat treat process. they can help you.

RE: Controlling a spring clips dimensions

No, spring wire is full hard when it is coiled or formed. The only thing done afterwards is a low temp stress relief (350-400F).
I have never seen springs formed while soft.
If you insist on post-form HT then you need to press quench these to hold the shape.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed

RE: Controlling a spring clips dimensions

Ed
with that bend will it crack?
I did mention I am not spring steel expert, most forming I have done it's been in the annealed condition to prevent cracking.
spring steel only has 5% elongation please advise maybe I will learn also.

RE: Controlling a spring clips dimensions

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

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed

RE: Controlling a spring clips dimensions

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

RE: Controlling a spring clips dimensions

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.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed

RE: Controlling a spring clips dimensions

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

RE: Controlling a spring clips dimensions

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.)

RE: Controlling a spring clips dimensions

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.

RE: Controlling a spring clips dimensions

here is a supplier that supplies 95 spring steel in the annealed condition
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.

RE: Controlling a spring clips dimensions

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

RE: Controlling a spring clips dimensions

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?

RE: Controlling a spring clips dimensions

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: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 www.springsteel.info

RE: Controlling a spring clips dimensions

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.

RE: Controlling a spring clips dimensions

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.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed

RE: Controlling a spring clips dimensions

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.

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