Text of the description

The present invention relates to a cutting blade enclosure and its manufacturing process, particularly the present invention relates to adoption of titanium as construction material of said blade enclosure.

The opportunity for making the most of the titanium mechanical and chemical strength characteristics in lawnmower manufacturing, and more specifically in manufacturing of lawnmower parts such as the frame which is the enclosure wherein the cutting blade are housed, up till now has not been taken into serious account by manufacturers from this technical field mainly due to the raw material high cost, which would make economically unprofitable use thereof for manufacturing structural parts of lawnmowers. Although it is not possible to direcdy act on the cost factor of the subject- matter material, it is possible to adopt manufacturing solutions that can justify the expense necessary for purchasing the raw material with technological advantages involving the whole service life of the so-manufactured lawnmower. Nevertheless it is to be noticed the progressive decrease of the raw material cost in the future potentially imputable to the change from the marketable titanium Kroll manufacturing process— from iron minerals and/ or igneous rocks - to the FFC-Cambridge manufacturing process which in respect of the previous process has a lower complexity and, therefore, a lower cost.

With the economic burden of adopting titanium as construction material for lawnmower blade enclosure, the dimensioning of the latter gains a huge benefit therefrom: in respect of a like steel blade enclosure, for example, a titanium blade enclosure has a mechanical strength, both in terms of ultimate strength and yield strength, slighdy lower but a whole weight lower than 40%, for which reason it is possible to adopt in the titanium blade enclosure greater sections and, all things considered, a strength (or greater if desired) like the steel blade enclosure strength, by keeping a minor weight in respect thereof as well. Moreover, in respect of the aluminum on one hand titanium has a weight greater than about 60% but on the other hand it has an almost double strength: essentially by manufacturing the same blade enclosure with titanium, rather than with aluminum, it is possible to adopt thicknesses more than four times smaller, while keeping a noticeable strength, but decreasing the whole weight of the part also down to 50% in respect of the same manufactured with aluminum. In the following the main mechanical and thermal characteristics of the most used metallic materials in this technical field as well as of the titanium for a numerical comparison:

Carbon

Mechanical properties Stainless steel Aluminium Titanium steel

Density [gl crn \ 7.8 7.9 2.7 4.5

Ultimate strength [MP ] 458 610 320 475

Yeld strength [MPa] 305 480 210 390

Elongation [%] 30 54 12 20

Young's module [MPa] 21 Ό90 20 300 7 00 10 500

Thermal conductivity

65 90 236 19

[Wm ¹ °C ¹ ]

A noticeable simplification of the manufacturing process, coming from the adoption of titanium in order to manufacture a lawnmower blade enclosure, is given from the absence of surface treatments after machining, since titanium is aesthetically nice to the sight— being polished by nature — but also to the touch due to its light superficial roughness. In addition, the surface treatments generally provided for the steel or aluminum blade enclosures for increasing their strength against the structural ageing, or chemical and mechanical etching of the weather are no more needed while adopting titanium, since it is naturally resistant to oxidation, due to the surface passivation just triggered by the oxidizer acids, but also by sulphuric acid, hydrochloric acid, ammonia, alkali, brine, chloride solutions and the most of the carboxylic acids. Furthermore, thanks to a relatively high titanium melting point and a low coefficient of thermal expansion, a blade enclosure manufactured with such material keeps its mechanical characteristics intact even after a long exposure to the sun rays or, however, following up even high thermal ranges, which indeed almost slight distortions correspond to.

As each person skilled in the art could easily notice, the clear absence of surface treatments after machining for manufacture of the part causes the titanium blade enclosure to be wholly recyclable with a subsequent decrease of the environmental impact both during manufacture and disposal at the end of the lawnmower service life.

Finally, the titanium biocompatibility makes the lawnmower blade enclosure completely inert and, therefore, not contaminating thus allowing, in fact recommending, its use especially in areas subject to vegetable cultivations with food purposes and in play grounds.

Main object of the present invention is therefore to provide a lawnmower blade enclosure obtainable by a manufacturing process in which the used construction material is titanium.

A detailed description of the manufacturing process for obtaining titanium lawnmower blade enclosure according to the present invention with reference to the annexed drawings in which:

fig. 1 is a flow diagram showing steps of the manufacturing process of lawnmower titanium blade enclosure according to the present invention.

Referring now to fig. 1, step Fl provides the adoption of particular expedients in die design, that is, in respect of a like die for the manufacture of a stainless steel blade enclosure, it will necessary to use greater bending radii R _Ti since, for example, for a titanium slab with a thickness of about 2 mm— reference size for this technical field— there is no opportunity to obtain bending radii smaller than about 2,5 mm.

In the step F2, as shown in fig. 1 , there is provided a cold drawing operation for the titanium slab: a choice of such machining is suggested by the high ductility offered by titanium— see the percent elongation in the previous mechanical characteristic table— on condition that the drawing pressure values within the press greater than in a like macliining of a steel blade enclosure. Again with reference to the step F2, it will be necessary to carefully adjust the combined action of the main press piston and the blank holder in order to recover, even partially, the drawing action loss which can reach even 20% to 30% of the bending radius.

The step F3, finally, already considered the high spring back of the titanium— see the Young's module in the previous mechanical characteristic table— leads to the complete dimensional recover of the die by means of welding operation or, alternatively, a riveting operation. Particularly, said welding operation, in order to obtain joints with the same mechanical and corrosion proofing characteristics of the rest of the die, is a MIG welding carried out according to the following expedients:

^■ removal of impurities from joint edges with titanium or stainless steel brushes;

^■ degreasing, for example, with acetone;

^■ short-arc welding with torch tilt angle ranging from 45° to 90°;

^■ shielding the welding bead with inert gas— for example Argon— even during cooling down to a temperature of about 200 °C;

^■ at least doubling time for exposure to torch in respect of a like steel welding;

^■ removal of the wire end— necessarily made of the same material— before starting the next welding. It is clear from the just provided detailed description of the manufacturing process for lawnmower titanium blade enclosures according to the present invention that a lawnmower titanium blade enclosure can be obtained thus giving the lawnmower in the whole light characteristics, strength against corrosion, wear, (structural and aesthetic) ageing besides the complete biocompatibility of the product.