Grill Element #2:

Physical characteristics:

Each grill element is made of:
1 - A 5.5 cm length of 3/32" x 3/32" brass channel stock (P/N BCM-015-36-01 from Small Parts Inc.).
2 - A 5.5 cm length of 3/32" x 3/32" brass square tubing (P/N 1503 from K &S available at Hungate hobby stores)
3 - Some length of K type thermocouple wire soldered in a hole drilled at the center of the piece of brass channel and threaded through the square tubing.
4 - A heater element made of 30 AWG nickel chromium (Nichrome) wire by Consolidated Wire & Associated Companies of Chicago Illinois apparently no longer in business.
5 - The Tefzel jacket stripped off  28 AWG copper wire by Vector (W28-6AS) that is apparently no more available, but seems to be very similar to the Vector W28-6A available from Mouser as P/N 574-W28-6A (Stripping tip).
6 - A short piece (~1") of  copper wire by W.L.Gore &associates of Newark Delaware that looks like 30AWG , item #5 in the picture below.
7 - A short piece (~1") of Teflon jacket from Belden 83001 wire (item #4 in the picture below).
8 - A piece of Belden 83001 wire long enough to reach the heater power source (shown  in the picture below as item #1).

Connecting the heater wire:

- Nickel Chromium does not solder easily. It requires an acid flux that can cause corrosion and premature failure if not properly washed off.  However it is easily welded because of its high resistivity.
- Copper, on the other hand, is difficult to weld because it has a very low resistivity.
To facilitate the welding of two wires together we normally flatten them out so they do not slip off each other as they are welded. But in this case, because the copper wire needs more current to heat up, but too much current melts away the Nickel Chromium, flattening out the copper wire and leaving the Nickel-Chromium round seemed to help create a good weld.  This single procedure needs more accuracy in weld timing and wire flattening and positioning, but, with practice, seems to be the answer.
Note added the 28th of January, 2008:
We were not successful in welding 28AWG Vector wire to the Nichel-Chromium heater wire, but did OK with wire from an old spool of wire manufactured by W.L.Gore & Associates of Newark Delaware that looks like 30AWG. These magnified photos of the tiny weld will give an idea of the difficulties encountered (photo#1 , photo#2)

Connection Procedure:


The heater wire (#7) is insulated by a Tefzel jacket (#8), and a piece of TFE Teflon jacket (#4) covers the weld joints (#6).
The stranded Belden wire(#1) is not welded directly to the heater wire (#7) for the following reasons:
1 - Once insulated it would make the two joints  too bulky to fit inside the brass 3/32" channel.
2 - Welding a single conductor wire to the heater wire is much easier than welding a stranded wire.
Item #5 is an approximately 1" long, piece of solid copper wire. We use an old spool from W.L.Gore & associates of Newark Delaware that looks like 30AWG.

    Heater Connection Step by Step Instructions:
A

Flatten the ends of a piece of small gage conductor wire(#5)  by squeezing it with a pair of pliers.
Electrically weld it to the Nickel-Chromium heater wire (#7).
B
Strip 1" off a piece of Belden 83001 wire.
Spread the opening by means of a needle or any sharp object.
C
Slide it onto the assembly all the way to and over the Tefzel jacket.
D
Solder a 6" piece of Belden 83001 wire (#1) to the 30AWG conductor (#5).
E
Cover the joint with a piece of shrink tubing.
F


Repeat steps A through E at the other end of the heater wire.


Heater wire shaping instructions Soldering the Thermocouple


This welding procedure will present a problem if the extra piece of Nickel-Chromium wire is not properly  physicaly connected to the heat sink.
Grill Element Final Assembly.

During curing of the epoxy, the two parts of the element, the channel and the tubing, are held together by means of 3 binder clips.  One 1"1/4 clip holds the two parts tightly together, the other two clips (3/4") keep the parts in alignment.

    Photo of clamped element.


Stripping Tip:

Cut a piece of Vector wire about 1cm longer than required. With a wire stripper strip 5mm off  one end. with a sharp pair of scissors trim a couple millimeters off the other end (jacket and wire, in order to leave a clean hole through which to thread the heater wire) . Use exposed wire end to pull the wire out the jacket taking care not to stretch the jacket. Cut to needed size at stripped uneven end.

Supplemental Heater Wire Heat Sinking:

It became evident that the ends of the heater wire were not totally inside neither brass stock parts and that overheating would most probably result. Extra thermo conductive epoxy was painted onto the heater wires as they enter the brass in order to sink off some of the heat.
A 1/4" long 1/8 brass channel caps was also added on the end of the element, as was done for prototype #3.

Inter Element Insulation:

Material:
1 1/32 thick Ceramic Tape such as Cotronics Corp. Cat.No. 390-21 that comes in 1 wide rolls ( www.cotronics.com) or equivalent.
2 Double sided adhesive tape of negligible thickness. We use a 1 wide roll of unknown origin (the only identification on the roll is the number:1558)

Procedure:
1 Cut a length of the insulating tape slightly longer than needed (57mm).
2 Stick its less fussy side to the double sided adhesive tape and cut to same length.
3 Trace a line 3/16 from one of the long edges of the tapes, and cut with a pair of sharp scissors. Better cut a hair narrower than 3/16 to make sure the insulation is not wider than the element is high.
4 Cut to the right length (55mm).
5 Carefully peel off the double sided adhesive tape backing.
6 Stick it to one side of the thermode element.
7 Repeat for the other side of the element.

2007 August 28
Last update: 2008 April 7