# Fly Cutting Spring Steel

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##### Well-Known Member
I am purchasing some spring steel for my forward landing gear and the plans call out 7mm thickness but I can only get 0.323". I was considering fly cutting the material to the correct thickness but I am concerned that this will create points for stress cracks to develop. Does anyone have an opinion or experience with this?

#### Hot Wings

##### Grumpy Cynic
HBA Supporter
Log Member
No experience, just an opinion:

.323" (8.2mm) is only 17% thicker, and maybe ~20% stiffer. That may not be a significant problem considering most homebuilts end up a little heavier than they should. Are there any other builders with flying examples that could offer real world feedback on the gear stiffness? Can the attach points transfer the added loads from a stiffer gear?

If you do decide to shave the material I'd consider grinding first. Grinding off 1.2mm shouldn't take too much time. If you do fly cut I'd at least finish sand the tool marks. A D/A sander with a bit of water should make those marks disappear pretty quickly.

##### Well-Known Member
No experience, just an opinion:

.323" (8.2mm) is only 17% thicker, and maybe ~20% stiffer.
Wouldn't that be 60% stiffer (I increases with height cubed)? In that case it might be problematic.

Would shipping from overseas be an easier option? Shipping by aircraft is pretty cheap and save a lot of trouble?

#### Hot Wings

##### Grumpy Cynic
HBA Supporter
Log Member
Wouldn't that be 60% stiffer (I increases with height cubed)? In that case it might be problematic.
:emb: That's what I get for trying to do math in my head.

#### BBerson

##### Light Plane Philosopher
HBA Supporter
7 mm ( .27") seems very thin. What is this for? How wide?

##### Well-Known Member
I don't really see how the extra stiffness would negatively affect things. The fork is attached to the bulkhead with a single through bolt and a u bent piece of steel rod. I will have the stock in a mill to get it to 20mm wide so flycutting would just be another operation. I want to thin it due to weight since this is an ultralight.

#### Marc Bourget

##### Well-Known Member
I recommend you verify the material is annealed before you fly-cut. Fly cutting might work harden. May have to re-anneal before sanding/grinding.

mjb

##### Well-Known Member
I don't really see how the extra stiffness would negatively affect things
Extra stiffness will put higher (than designed) loads on your frame, attachment and spine, neither of which you want.

#### Dana

##### Super Moderator
Staff member
7 mm ( .27") seems very thin. What is this for? How wide?
Quite the contrary, that seems pretty thick for an ultralight.

The spring steel is presumably hardened and tempered; better be using carbide tools. Also, if you flycut it down to thickness, it will probably warp all over the place. If you must do this, you will probably want to take it off both sides, a little at a time, alternating sides... it still might warp, but not as bad.

A picture would help... you say "spring steel", but is it actually being used as a spring?

1/4" (.250) stock is a lot closer to your target .270 than .323, and it may well be sufficient.

Dana

"Calvin, we will not have an anatomically correct snowman!"

##### Well-Known Member
The 5160 is in hot rolled form and about HRC30, so it shouldn't be too bad to machine using carbide. I may see if I can weasel someone to put it on a surface grinder for me. I hadn't considered that as an option but I think that may be a better plan.

Here is a drawing of the spring in use:

If the spring is too stiff it will transfer the load into the bulkhead and distribute it over a fairly large surface area (about 5 in^2). The load will also pull more on the mounting bolt on the bottom of the spring. The mounting bolt will have a large washer under the nut to distribute the load over a larger area than the nut.

#### Dana

##### Super Moderator
Staff member
OK, so you're getting it flat and then you bend it afterwards? In that case warpage shouldn't be an issue. Will it be heat treated after bending?

Dana

Always keep clothes and weapons where you can find them in the dark.

##### Well-Known Member
That is a big 10-4 on both.

#### Matt G.

##### Well-Known Member
Here is a drawing of the spring in use:

View attachment 22629

If the spring is too stiff it will transfer the load into the bulkhead and distribute it over a fairly large surface area (about 5 in^2). The load will also pull more on the mounting bolt on the bottom of the spring. The mounting bolt will have a large washer under the nut to distribute the load over a larger area than the nut.
I disagree; from the picture you posted, I think that'll put the top bolt(s) in tension, not the bottom one(s).

##### Well-Known Member
In reviewing the totality of the original design I beg to differ. The strut is not designed to take a load in the direction that you are referencing. A 3/16" u shaped rod is all that attaches the upper part of the spring to the bulkhead and that isn't a design feature that would take a considerable force in that direction.

#### Matt G.

##### Well-Known Member
On second thought, I think we are both half right...when the tire is in contact with the ground the deflection of the spring will put the bottom bolt in tension, but upon touchdown and spin-up of the tire, the bending moment will (momentarily) be in the opposite direction and then the reaction forces at the bolts will be the other direction.

It looks like it would be critical to make sure that the spring is no thinner than as specified in the plans because if that spring can deflect aft too much from tire drag loads, it'll apply forces that, as you say, it probably isn't designed for.

I don't think you can flycut spring steel. Your cutting tool has to be harder than what you are cutting, and you would have to have a very rigid setup to even attempt it. I think you might find a machine shop or a clutch rebuilder who has a Blanchard grinder. The Blanchard has a round table that will magnetically hold the steel to the table and as it rotates, the grinding wheel is lowered until it contacts the part. These types of grinders can hog metal off much more quickly than a surface grinder. Leave a few thousandths on for finishing. Then either have it surface ground ($) or use a handheld belt sander to dress the scratches from the surface and finish the edges. I would think the Blanchard grinding should not cost over$50 if that much.