THE HEAT – CONTACT

HEAT - CONTACTFor soldering of carbide inserts on tool holders turning tools, local hardening tool, soldering brazing small parts using heating by high frequency currents or by the flame of a gas burner. However, HDTV equipment bulky and expensive, it is not for the home workshop or school group. Use the gas burner all the rules can only welder qualifications.
But there is another, almost forgotten in our time, the method of heating — contact machines. It is most acceptable in the home, school, collective farm or state farm workshop, in the technical circle.
 
The essence of the thermal action of electric current on the conductor. Allocated the amount of heat depends on the amount of current of time by its action on the conductor and the electrical resistance of the latter.
 
Taking into account this dependency we have developed a device, which is a step-down transformer whose primary winding is designed for 220 V, secondary — 2 V. the cross-sectional Area of magnetic core of about 50 cm2. The transformer is mounted on the base, and the contact of the tyres of the secondary winding — gasket-insulator.
 
The base unit is made of steel sheet of thickness 5 mm. from the Bottom it screwed legs. At the base are cut two Windows: less ventilation, more — for bolts contact tires on textolite an insulator thickness of 10 mm. Holes at the edges of the insulator serve to its attachment to the base.
 
The ends of the secondary transformer winding is wound into the holes of the clamps and fixed by bolts (grooves of the clamps allow them to deform when tightened and thereby ensure a reliable electrical contact).
 
The transformer is covered with a protective casing and attached to the base corners 25X25 mm To one of the upper corners screwed insulating pad for wire connection of the primary wrap. with feeding cable and switch apparatus.
 
Since Amateur designers do not often have all the necessary to repeat the described handmade, we recommend you to start manufacturing unit with the selection of the package of plates of transformer iron (preferably W-shaped), and only then to deal with the calculation of the windings.
 
Assume that the cross-sectional area of your magnetic core (Q=a*b) 36.8 cm2. Then the power transformer secondary winding P2 = 36,8*36,8 = 1354,2 W, and the primary P1 = 1354,2/0,95 = 1425 watts. The current I1 = 1425 watts/220 V = 6,48; I2 = 1354,2 W/2 = 677,6 A. Find the area of the cross section of the primary winding:
 
S1 = 6,48 A:2 A/mm2 = 3.24 mm2.
 
The wire diameter here
 
d1 = 4*3,24/3,14 = 2 mm.
 
The cross-sectional area of the secondary winding
 
S2 = 677,6 A:2 A/mm2 = 338,8 mm2,
 
and wire diameter
 
d2 = 4*338,8/3,14 = of 20.77 mm.
Accordingly, the designated number of turns:
 
n1 = 220 000/222/36,8 = 270 turns
 
n2 = 270*2/220 = 2,5 round.
 
Fig. 1. Machine terminarea.
Fig. 1. Machine terminarea:
1 — guard 2 — main switch 3 — power cord 4 — base, 5 — isolator, 6 — pin tires, 7 — clamps 8 — bolt clamps, 9 — leg (4 PCs.).
 
Fig. 2. The clamping lever.
Fig. 2. The clamping lever (installed in optional drilled in the insulator hole with M6 thread):
1 — front, 2 — arm, 3 — clamp.
 
Fig. 3. Contact area of the device.
Fig. 3. Contact area of the device:
1 — pin tires (red copper), 2 — pressure lever 3 — carbide insert, 4 — solder 5 — tool holder cutter, 6 — insulator of the base; A and B — zone heating.
 
The efficiency and economy of operation of the machine largely depend on the size of the plane of contact of the body of the cutter from contact tires. The amount of heat generated by the passage of electric current depends on the resistance of the conductor at the points of contact. When a large plane of contact there are little heat in significant energy consumption. Conversely, when a small plane generates a lot of heat, which leads to instant heating of the contacting layer. To avoid melting metal and violations of the contact, the contact area is chosen empirically.
 
When the location of the body of the cutter on the tire, as shown in figure 3, the center of emergence of heat will be located in area A; in area B visually observed heat center does not occur due to the large surface of contact.
Before soldering (including carbide inserts to tool holders cutters), you must perform several preparatory operations to ensure the good spreadability of the solder and the wettability them of the parts. The support surface plates sanded and degreased. Tan also preparing the surface under the plate on the body of cutter: it should be straightforward, without ledges and debris around the edges. Protect surfaces from oxidation during soldering is flux (brown).
 
Bonding occurs in this sequence. The cutter holder is placed on the tire machine. Between connecting surfaces are placed with tweezers solder (a scrap of sheet brass). For a more reliable clamping of the heated parts to each other and to contact the tire lever is mounted on the plate-insulator. In the groove of the lever is inserted a stop, which is pressing carbide inserts to tool holders cutters.
 
When you turn the device the contact zone is rapidly heated, the metal melts, the contact is broken and the process is terminated. This can be avoided in two ways: by working in an intermittent mode and gradually applying voltage to the windings. In the first case, the machine starts 1.5—2 seconds, then turns off. At the moment of shutdown, heat is transferred from the contact points on the body of the cutter, not aplasia metal.
 
A smooth supply voltage to provide Latrom. Increasing the voltage to achieve the same result as in the first case: heat waves spreading across the holder from the place of contact, providing heating to the melting temperature of the solder. Control over the soldering process is carried out visually.
 
These modes give the heating rate of the tool holder in the range 80 — 100 deg/s. This reduces internal stress and prevents cracking in the cemented carbide. To avoid the appearance of cracks in the soldered seam, it is necessary to slow cooling.
 
High quality brazed joint must not be thicker than 0,1 mm. the Length neprodanih places should not exceed 10%.
 
In the absence of hard alloys as cutting plates, you can use the wreckage of mills, drills and other tools. The debris is brought into the desired form on the grinding machine, or they heat up and otkalyvalis to obtain a rod of rectangular cross section, which when re-heating razrushaetsya chisel on a separate plate.
 
Our apparatus is multi-purpose. In addition to making tools, it can be used for local hardening of tool (drill, chisels, screwdrivers and so on). Just touch the contact tire the part of the tool you want to temper, and hold it for a few seconds. The heating temperature may be controlled visually, by the color of the metal. It is necessary to observe a few precautions: work gloves and safety glasses on a grounded apparatus.
 
A. BOBROVNIKOV, V. ZINYUK, Murmansk

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