Smith-Kettlewell Soldering Series: Vinther Fingertip Soldering Iron

Table of Contents

Soldering I
Soldering II
Soldering III: Tinning Stranded Wire
 Soldering IV: Popular RF Connectors
Soldering V: RCA and Motorola Plugs
Soldering VI: Resistance Soldering
Soldering VII
JA3TBW Solder Guide
Soldering Kinks
Vinther Fingertip Soldering Iron

THE VINTHER FINGERTIP SOLDERING IRON

by Bernie Vinther

Abstract

Using the soldering tip of a battery "cordless soldering iron," this instrument is of the quick-heating fast-cooling type; it can be guided into place while it is still cool, and then energized by pressing a foot pedal. Moreover, an external transformer has been substituted for the large handle and heavy battery of the cordless unit. The particular tip assembly chosen happens to have an extension arrangement which provides a ready-made handle. The result is that, even with its supply cable, the finished soldering iron is of feather weight, it can be put into position directly by feel (before it is energized), and it extends very little beyond the fingertips--hence the descriptive term, "fingertip soldering iron." [While seeming simple enough, it is cleverly implemented, and the right tip had to be found, and the editor considers this to be a major advance in soldering tools for the blind.]

"All right, gentlemen, push the pedal to the metal and let the soldering begin!" Formerly being a sighted electronics technician, now as a blind person I've been frustrated by whatever approach at soldering I tried; I always seemed to leave something melted or burned, especially my fingers. In trying to use various methods (described in Smith-Kettlewell reports), locating the parts to be soldered with a hot iron and applying the solder, the materials would accidentally be jostled out of position, or things would become overheated while I was trying to feed the solder. Therefore, I most often chose to use a "cordless soldering iron" (first made by the Wahl Clipper Corporation, and then also marketed by Radio Shack). This iron has the following advantages:

First, the tip is cool to start with, and heats up quickly when you're ready to solder. Therefore, the tip can be guided into position with fingers of your free hand, after which the iron is energized when you are out of the way. You no longer need a system of landmarks to guide the iron to the connection. After the connection is made, the button is released, and the iron cools down by the time you are ready to make another.

Because it is only energized when you need it, there is no danger of leaving a hot iron lying around where it could burn something or someone, or present a fire hazard. There is no chance of leaving it on to oxidize badly overnight, which would give you a lot of cleaning work to do on it.

An important advantage over regular irons is the distance from your fingers to the soldering point. The tips of standard irons are normally 3 inches or more beyond the tips of your fingers; this makes for a "wand"--which is also hot--that is hard to control. The cordless iron at least reduces this distance to about 2 inches, and the proposed iron described here further reduces this length to 1-1/4 inches.

Also, the cost of a temperature-controlled iron is outrageous in comparison. The ones used by the Smith-Kettlewell group cost over $120.

I found that I could overcome the frustrations of feeding the solder by wrapping a small length of it around the tip of the cordless iron when it is cold. The length of solder needed depends somewhat on the size of the connection and the diameter of the solder. However, I usually use only 3/8 of an inch--perhaps as much as 1/2 inch--of 23-gauge solder; this is fine for most applications. This eliminates the need for intricate manipulations or complex solder-feeding systems that can plug up or move the parts out of position.

Yet the cordless iron is not without disadvantages. First, it is not very rugged, and its tip can be damaged by mishandling (a problem accentuated by the fact that the battery and handle assembly are heavy). Next, its primary way of dissipating heat is in the connection you are soldering; therefore, it can overheat if left on too long. Pushing down on the button in the handle often caused extra undesirable movement of the hand--my hand would tend to move away from the connection at times. Finally, it seemed that the battery would always run down, just when I was almost finished with a project I was dying to try out. These many frustrations were, however, more than overcome by the quick-heating fast-cooling feature, and that the solder was applied without complicated techniques or feeding systems.

All this led me to devise the following improvements, which even offset some of the disadvantages.

Describing the Vinther Fingertip Iron

The Wahl Clipper Corporation makes a variety of tips for their instrument. One of these is known as the "Tuner Extension Tip" (apparently designed for making long reaches into TV tuners). Bearing the Wahl Part No. 7556, it is the same as the standard-length "fine" tip No. 7545, except that it is mounted on long stiff leads--about 3-1/2 inches long. Moreover, being held apart by an insulator, the conductors are encased in heat-shrinkable tubing. The resultant lead assembly resembles a piece of flat 300-ohm "twin-lead" TV lead-in wire.

You might ask, "But what good is a 4-inch-long extension to a soldering iron tip? Weren't you just complaining about length a little while ago?" The 4-inch long extension serves as the handle. The fact that it is insulated means that it doesn't heat up very much, and you can easily get to within 1-1/4 inch from the soldering point. Voila, we have a fingertip version of the quick-heating fast-cooling iron.

Instead of running this thing with a heavy and expensive nicad battery pack, why not choose an appropriate transformer? This would solve another problem; the on-off button can be in the primary circuit of the transformer, and I chose to make this a foot control on the floor. No longer does the battery run down. No longer does pushing a button cause accidental movement of your hand. Finally, being of such light weight, it is much easier to be gentle with this fragile tip construction, and the tips will last longer.

The battery in the Wahl cordless iron is a 2-cell unit, supplying 2.5 volts. I simply went down to my local supplier and got a 5-volt transformer whose secondary centertapped. (My transformer is only good for about 3 amps. Though the tip draws about 7.5 amps when cold and settles to about 6 amps as it reaches soldering temperature, its intermittent use has caused me no problems. Nevertheless, a 6-amp transformer is specified here.)

The two-wire cord that supplies the tip should be as flexible as possible. In hardware stores, rubber-covered "heater cord" is sometimes available which is more flexible than standard plastic-covered "zip-cord." Belden Wire Company does not regularly distribute non-shielded rubber-covered two-wire cable. For now, the Belden zip-cord listed here is 22-gauge. While they rate this at only 5 amps, intermittent use with this iron should not cause a problem. [If you have any better suggestions, the editor is interested in announcing them--be specific, with catalog numbers, please!]

Also still somewhat in doubt is the best way to attach the 7556 tip to the cable. The pins provided are steel, and do not take solder readily (although, if you wrap them with several turns of flux-core solder and heat them before making any connection, soldering them will be much easier). These pins are 0.037 inches in diameter, and they are spaced at 1/4-inch centers. A socket such as the inside portion of the Amphenol 6175 two-lead "UHF" antenna connector might work. I found some automotive push-on connectors which I was able to modify to do the job. As will be seen in the section "Tip Construction," these male pins on the back are actually crimped into copper tubing, which you can get at by removing a small portion of the heat-shrinkable tubing of the tip assembly. This tubing can easily be soldered to, and the wires of the cable could even be fitted into the tube ends after the pins are extracted with healthy pliers.

A straight SPST foot switch in series with the primary will work. However, as mentioned, this type of iron can get very hot if left on too long where the work pieces are small--the heat capacity of the work not being enough to dissipate the iron's energy. I thought, "Wouldn't it be nice if there were a way of operating the iron at reduced power?" I tried using a sewing machine foot control which has a variable resistor inside it; however, I soon found that the resistance of this control wasn't high enough to make an appreciable difference.

Another way of reducing the power might be to build a double-ended foot-control switch, one with a button at either end. Thus, by pushing down on the "toe end" of the control, full power would be applied; by rocking back on the "heel end" of it, it would be run at about half power, which would be fine for maintaining the tip temperature. This could be done by having a pushbutton operate the primary circuit directly at the toe end, and by putting the pushbutton at the heel end in series with a 15-watt 120-volt lamp, or perhaps a 500-ohm 20-watt wire-wound resistor.

At present, if I need to maintain the heat for a while, but not supply full power, I merely push up and down on the switch about once a second, and this seems to work fairly well.

One more improvement completes the design. The diameter of the soldering point on this tip is 0.070 inches (before tinning). The spacing between pins of IC sockets is often 0.075 inches; thus, the tip can sometimes contact two pins at once, causing a solder bridge. The surface of these tips cannot be ground down or harmed with abrasives in any way; they are clad with iron and tinned at the factory for long life, and this cladding must not be harmed. While visiting at Smith-Kettlewell, we found that you can flatten the last eighth inch gently in a vise without cracking the iron-clad coating. Don't mash any farther back than this eighth inch, or you will damage the heating coil inside. This gives the tip a screwdriver-shaped end whose thickness is perhaps 0.050 inches, which can easily pass between pins of an IC socket. When you flatten the tip, orient it in the vise so that the flat sides coincide with the flat faces of the "handle," as we now shall call the lead extension assembly.

Tips on Using the Fingertip Iron

The way I use it, this iron permits soldering to be basically a one-handed operation. As mentioned earlier, I wrap a little piece of solder around the tip before I put it into position; pieces 3/8 or 1/2 inches long will do nicely for IC sockets, while longer pieces could be used for larger terminals. To save time when I am doing a lot of connections (one after another), I prepare precut lengths, form them into little rings, and stack them on a nail; they can quickly be picked off the nail and installed on the iron's tip as needed.

If you find short pieces of solder difficult to deal with, you may prefer just wrapping a turn and a half around the tip, then nipping this assembly free of the spool. The amount you use will actually depend somewhat on the diameter of solder you have, as well as the size of connection. The amounts advocated here are based on my experience with 23-gauge solder.

Caution! It is important to hold onto the metal body of the tip when wrapping it with solder, not the handle or the ceramic insulator that the leads go through to reach the tiny heating element inside. If you do not hold on to the metal portion, mechanical damage to the heating element can occur.

Being cool to the touch (most of the time), I place the iron on the target using an index finger of either hand. [Editor's Warning: As with the old soldering gun, cooling of the instrument takes at least more than half a minute. If you are doing a series of connections, don't let your zeal get you in a hurry; you can still get a serious burn if the tip has not been allowed to cool. The iron, once a trusted friend, can turn on you.] A wet cleaning sponge should be kept on hand to wipe off excess solder after every few connections (this is not necessary each and every time, as it is with a constantly hot iron which is always building up oxides.) If you have a connection to do right away, wiping the tip on the cleaning sponge will help cool it faster.

(At Smith-Kettlewell, we tried using the sponge as a storage place for the soldering iron. This got us into trouble in two ways. First, the tip is apparently not watertight, and I got a serious burn one time as steam came out of it. Second, the frequent drastic heat cycling was hard on the tips; we had repeated failures due to the metal portion of the tip breaking loose from its ceramic insulator.)

Because soldering is now a one-handed operation, you can use fingers of your free hand to hold parts in position. In fact, I often do not use a vise or board holder; oftentimes, I just hold the board still with the heels or palms of my hands.

Once everything is in place, I step on the foot pedal, and in five seconds or so, soldering takes place. You know when soldering occurs by two indications. One is that, when the flux has done its cleaning job, a typical "squeakiness" can be felt as you make small motions with the iron. Since you have free fingers on your other hand, the other can be when you feel the quick rise in temperature of a component which happens when "wetting" of the metals has occurred.

Viola! What a breeze! Soldering is no longer a complicated process. No more scorched parts or burned fingers. Because it's so easy to hold things while you're using this system, it makes experimenting with projects much easier; I can just "tack" parts temporarily in place without formally mounting them in some way.

If you don't like the idea of applying the solder to the iron, you can still use your favorite system for feeding solder with your free hand. [The editor actually prefers feeding solder to the work for two reasons: First, by letting it run off the iron, you stand some chance of using up the flux before it has a chance to clean the work pieces--although I've seen this system work like crazy for ol' Bernie. Second, however, I like to use the solder as my "pyrometer," as you might call it; the solder's melting tells me when the parts are hot enough to accept it. However, the squeakiness and rapid heat transfer described above are good indications of success as well, so do as you like.]

When working on old hand-wired equipment (such as an old ham receiver I had), I found this soldering iron to be a real boon for getting into tight places. The fattest part of its hot portion is less than a quarter inch in diameter, and since it is cool to start with, it is easy to find free room for this before turning it on. With a standard pencil iron, it is easy to burn nearby components with its hot barrel.

Still, though, the constantly hot pencil iron has its place. As I mentioned before, these "cordless" tips are very fragile in their construction. A little fingertip tool simply will not do heavy jobs--on large items. For unsoldering, these fragile tips cannot withstand prying, twisting, and other such procedures for separating parts.

When working with CMOS circuits, I've tried grounding the centertap of the transformer. So far, I have had no problem, neither due to static charges nor due to the AC supply. (I have ruined CMOS devices by using the battery iron only, since it wasn't grounded.) Grounding the centertap does not actually ground the iron's tip; it is insulated from its heating element. However, not enough static charge apparently builds up to damage the CMOS devices.

As listed in the next section, there are other tips available which, with a little mechanical engineering, could also be used. The smallest unit has an almost needle-like soldering point on it. I don't like it, though, because its tip is so fine that I can't put enough solder on it, and it slips off the connection more easily. One use for this tip is for cleaning out plugged-up holes in PC boards; it is so fine that it can actually reach right through them.

Tip Construction and Availability

One problem with these "Tuner Extension Tips" is their scarcity, even among dealers of Wahl products. A good source for them is Fordham Radio (see "Address List"), where they can be ordered under their Wahl number 7556.

Another disadvantage is their cost, about $7. In dissecting one, it was found that they are actually made by adding the extension assembly to the "fine" tip, Wahl No. 7545. If you have the right tools--a small crimping tool and a heat gun to shrink tubing around the assembly--you could precisely duplicate the 7556 from the 7545 (a cheaper tip which costs from $3.25 to $4.00 and which can even be gotten from Jameco). With some mechanical engineering, one could machine a lightweight handle with screw-type binding posts so that the more common short tips could be used instead of this rare one.

The pins on the short-legged tip are fitted into 2-3/4-inch lengths of copper tubing, which is then crimped down tightly onto them. Likewise, steel pins are fitted into the far ends of the tubes and crimped in place, leaving about 7/16 inches protruding. (The tubing used is 1/16 inch o.d., and 0.040 inch i.d.) A strip of fiber insulation--often called "fish paper"--is placed between these conductors, whereupon they are wrapped with "transformer tape" (which is fairly resistant to heat). (This fish paper insulator may be the hardest thing to duplicate; it is 1/16 inch thick, which is unusual. Bakelite would work, as would the fish paper salvaged from burned-out units. Just about any tape would do; just apply it an inch back from the forward end so that it won't get warm.) With the fish paper strip being 3/16 inches wide, the conductors are then spaced at 1/4-inch centers. Finally, heat-shrinkable tubing (capable of a "recovered size" of 5/8 inches) is slid up to within 1/2 inch of the soldering tip and secured, leaving 3/8-inch-long steel pins to protrude at the back end.

An exact replacement for the tubing has not been found in small quantities. "Small Parts" has 1/16-inch o.d. copper tubing, but its inner diameter is 0.035 inches. (The pins of the tips are 0.037 inches.) However, their 3/32-inch tubing (being 0.066 inches i.d.) would probably work. This is Small Parts number TTRC-2, and comes in 12-inch or 36-inch lengths.)

Other Wahl Tips

Vinther Iron Circuit

Belden No. 9712 two-wire zip-cord is used to power the Wahl 7556 tip. One side of the tip goes to the centertap of the transformer's secondary; this centertap is also grounded to the anti-static pad on your bench and/or a wrist strap, where necessary. The other side of the tip goes through a 15-amp fuse to one end of the secondary. (This transformer should be capable of delivering 2.5 volts at an intermittent current of 7.5 amps. A suitable unit is the Stancor 6455, which has a 6-amp 5-volt centertapped secondary, and a 107- or 117-volt primary.) One side of the primary goes through an on-off toggle switch to one side of the mains. The other lead of the primary goes through the foot switch to the other side of the mains. (The foot switch should be normally open, and can be one such as the Line Master 491S. Also, foot switches with a standard three-prong male/female end on them could be used, such as the Line Master 491SC360; a three-prong electrician plug from the transformer can just be plugged into the latter type of foot switch.)

Address List