6 replies to this topic

[Pablo]

Today a Slotblog friend asked for my wire tinning method, and I decided to write it down.  If you are tired of watching your chassis rails oxidize, try this.  Philippe de Lespinay published this technique a few years ago on Slotblog.  It's an imperfect science.  If you have a better, faster method, let's hear it.  I assigned a fire watch (Bunky) and made sure he had burn cream, ice bucket, and a fan to suck the smoke out. 

 

End result:

Tools and materials needed:
-Leather gloves and eye protection.
-Weller 8200N Gun. (you can use an iron, but the Weller gun tips develop a nice groove for the wires to nest in)
-Strong acid flux that has a nice hard bite like "stay-clean" or "Chicagoland", and 60/40 solder.
-400 grit wet/dry sandpaper
-Clean cotton rags or paper towels.

Sand the wire, wipe it clean, then slather it with acid. You can't use too much - rotate the wire horizontally in your fingers to keep the acid moving.  Clean your gun tip and load it with a big load of 60/40.  Apply the tool to the work with a rapid 2 inch back and forth motion; subsequently moving up and down the length of the wire while continuing to rotate it so the flux doesn't bead up.  Make the solder flow all along the wire, then in a rapid move, throw down the gun, grab the rag, and wipe wipe wipe the flowing excess solder from the wire several times (in the direction away from your hand, Bunky LOL)  You won't get 100% coverage every time.  If some untinned spots remain, set it aside to cool, come back to it later and re-do it. After the wires cool, clean them up, I use 91% Isopropyl Alcohol.
Now you have a clean, tinned wire.  Nothing is perfect, especially when you solder dissimilar metals.  But you do the best you can.

 

Bunky, hand me that burn cream quick !!

[SlotStox#53]

Nothing like a nice stack of ready tinned wire to grab from to build chassis with   About time I actually bother to do this  

 

Got the gun ,acid and solder..... plus some chassis in need of building... Crack on !

[Bill from NH]

K&S, at one time, used to plate all their piano wire, but no more. I forget if they used nickel or chromium. I don't recall when they stopped. I have a tube of .062 Sullivan piano wire & that's plated too. If one built a lot of chassis, tinning with a wave soldering machine would come in handy. But those start at about $25 K. Pablo, do you also tin your brass parts? Not so much for preventing tarnish & corrosion from forming, but pre-tinned parts are often easier to solder together.

[A. J. Hoyt]

I think this is related closely enough to mention it here, because the topic of "dissimilar metals" was brought up.

 

When soldering your tinned steel wire or steel guide tongue to brass, it is important to note that a very hot solder joint (like with a very hot iron or mini-torch) heats up both parts to nearly identical temperatures but brass and steel (while having similar density) have very different coefficients of thermal expansion (inch/inch/degree), meaning, the parts grow and shrink at different rates along with different conductivity (cooling rates, heat flux, btu transferred over time).

 

Solder is also a dissimilar metal and has different expansion rates when solid from the brass and steel.

 

The higher the temperature, the greater the change (delta) of volume. This is not an issue when the parts expand with hot, melted solder (necessary to get good distribution and coverage) but is a potential issue when it is cooling (after the solder turns solid). Obviously, a higher temperature solder will force a necessarily hotter process to joint the parts together.

 

The effect is that, after, the solder has turned solid, the parts continue to cool - the brass slightly faster than the steel (not sure about where the expansion rate of the solder falls).

 

As the joint cools, regardless of the rate, the steel and the brass will continue to shrink (volume reduced) until both reach room temperature. There is a region of solder affixed to the steel part and a region of solder that is affixed to the brass part. Clearly, if the shrink rates are different (all three metals), there are residual stresses in the solder between the parts. The higher the temperature of the process, the higher the stresses, potentially inducing significant micro-cracks in the very ductile metal matrix lattice of the solder.

 

These cracks will not heal themselves; instead, just continue to expand during the cyclical loading of the solder joint due to normal racing. If the car receives a wall shot accident, the crack could be very severe and not visible on the outside. This can, obviously, be repaired by re-soldering.

 

For a long wire bond line to a brass pan, I would recommend letting all of the parts cool to room temperature and just barely re-flowing the solder (between the brass and steel parts, only - you created the coverage and bond to the brass and steel parts earlier at high temp) keeping the heat and delta temperature as low as possible in the bonded parts just to "relax" the solder between them.

 

Just the normal flex over time of racing can change the characteristics of a previously fast chassis. Doing a quick setup in a proper fixture and re-flowing the solder could give a second life to a once-fast chassis.

 

Just my 40% of a nickel (perhaps misguided insights from a degree holding mechanical engineer). I welcome responses and I did NOT intend to hijack the thread - I think this is inseparably related (fortunate pun).

 

Keep it in the slot,

 

AJ

[Pablo]

Hi Bill   No I don't usually tin my brass, but I know it can be done if needed.

AJ, thanks for the detailed input I only disagree with one thing you said: "I did NOT intend to hijack the thread".

You didn't hijack anything, dude, if there ever was a perfect place and time to discuss this, it's right here right now

Steube said "never use wire and rod together" - that is exactly what he meant.  I have learned my lessons the hard way.  In long lengths, brass curls up like a Lay's Potato Chip when soldered to wire.  However, if the rails are separated like this:

or like this:

No problemo

[A. J. Hoyt]

Then I'll add a couple more tidbits related to steel with other material rails.

 

The solder between the rails acts as a damper on the unrestricted sum of the individual rails' resonant frequencies. More solder, more damping. Placement is important, too.

 

I have to believe that we want to damp out any and all spring resonances and I think may be the wisdom behind adding an adjacent brass (or bronze brazing) rod to the steel rails. Brass is more ductile and, in general, has much greater damping properties as compared to steel wire. To find a way to add a dissimilar (damping purposed) rail between two other rails (as you show above) should create a "symmetric" residual stress and the net rail on each side would, at least, still go straight ahead.

 

Basically, the rules almost restrict the design to a go-cart chassis with no suspension except the tires. If you want twist (but no resonance), some clever thought with these considerations in selecting the main rails is important.

 

Well, you brought it up.

 

Now, back to your chassis build. I, for one, am really looking forward to see how you tie the front axle down into the remaining slots at the right height.

 

Keep it in the slot,

 

AJ

[Pablo]