Spark Plug constructlon revisited

[Blogwitch]

What a very nice and informative post Gail, very similar to the OLD STYLE posts that were the norm on here.
Rather than just a next bit picture, a FULL explanation of what needs to be done to obtain the final result.
A real joy to read.

Keep it up

John

 

[GailInNM]

Thank you John (both of you) and Earl for the kind comments.

John1958: PEEK should work just fine. Here in the US we use Corian because it is easily available, chaeap and adequate for the application, not because it is the best material. For the type of engines that I build, small low speed cool running hit and miss, I think that even plain clear acrylic rod would work fine. It is available everywhere but it is not too pleasant to machine because the strings wind up on it when turning. For appearance it think the exposed section could be painted white with epoxy appliance touch lup paint before the electrode is glued in. I say epoxy paint because most other paints would come off. when cleaning up the excess epoxy during assembly.

Blogwich: John, you and I are both of the same generation and nearing the end of our active model building days due to physical limitations. But since we both still have active minds we can still contribute to the hobby by sharing some of the information that we have acquired during the last 50 or 60 years. I think we are of like minds on this.

Gail in NM

 

[GailInNM]

The nut used to attach the spark plug wire can take many forms or even be optional in some cases. The easiest nut is of course to use a standaard 0-80 nut. Since my fingers don't work all that well I have trouble with them sometimes so I make a knurled nut. An easy alternative to a knurled nut is to drill a tap a short length of 1/8 hex brass.

I make 0-80 knurled nuts out or either 1/8 or 5/32 brass rod. As I had enough nuts on hand I did not make up any for this run of plugs., so there are no photos of the process. But I will describe the process a bit.

I use a scissor knurling tool. When properly adjusted scissor type tool applies very low side pressure on the stock so several inches can be knurled at once. About 1-1/2 inches of 1/18 diameter brass with no support on the end. I have a small live chuck that I made many years ago and if I use it in the tailstock I can knurl 5 or 6 inches of 1/8 stock can be knurled.

The knurling increases the diameter enough that I change collets to 1/64 inch over the stock diameter. I can then drill and cut off the nuts. I then tap them in the lathe using a tap driver in the tailstock.

A simple threaded sleeve with a groove cut in it can be used with a clip on wire connection similar to what is frequently on old model aero engines. Looks nice. but I don't know of any commercial clips small enough for this size plug. Clips can be made, but they are a pain to make in this size.

One 'no nut' option is to make a straight in connector out of 3/32 OD brass tubing about 1/8 long and is split along the length. It is then swaged down to be a snug slip fit on the screw threads and the wire soldered to the side of it. As they are not very pretty the are best used if a boot is used on the plullg to cover them up. I will probably use this on the engine I am working on as I will be using a boot on the plug and will include details when I do the pllllulg boot thread.

Gail in NM

 

[JtH]

Gail,

Thank you so much for a gin clear posting on a subject which as been a mystery to me for years. I am presently in Panama, but when I eventually return to Australia I will be following in the footsteps of John to try peek..

Kind regards

John (another one..)

 

[GailInNM]

JTH: Thanks John. I am glad you are finding this thread useful.

While the nut was optional in some cases a sealing washer to bo between the plug and engine is not optional. The washer needs to be made from a malleable but stable material that is softer than either the plug or the engine (usually head). PTTFE does not work well because it creeps (not stable) under pressure. Alloy aluminum is too hard in many cases, but near pure aluminum will work. Tradition dictated annealed copper for the washer and that is what I use.

The ID of the washer needs to be a close fit on the plug threads and the OD needs to be at least equal to the body of the plug. For the plug as drawn these are 0.190 ID and 0.245 OD. The thickness should be at least the pitch of the thread, 0.025 in case of the 10-40 thread.

A quick note on plug threads. 10-40 was chosen as it is a quasi-standard for the next size down from the almost universal standard of 1/4-32 for model engines. Rimfire makes 3 excellent plugs (the Z series) with 10-40 threads. Unfortunately due to limited demand and production run size they are a bit expensive. This plud as drawn is roughly the same as their Z1 plug.

I will describe two different methods of making the washers. The first method is specific to the the #10 threads and will probably apply mostly apply to USA builders because of available materials. The second method can be used for different size threads and is a little more work if you are only making a few washers.

Gail in NM

 

[GailInNM]

Getting staarted on the first method. Sometimes, not often, things just seem to work out our our way. I needed 0.245 minimum on the OD and 0.190 ID. It just turns out that the most common 1/4 copper tube in the USA is 0.250 OD with a wall thickness of 0.030 resulting in 0.190 ID nominal. It is available at many smaller hardware stores cut to length which is convenient since a foot is enough for more washers that you will want to make. The big box DIY stores also sell it, but most of them want to sell you a 25 foot coil. A few of them sell cut lengths.

If you buy a new piece it will probably be fully annealed and vey soft. If you have some that has been setting around the shop for a few years it will probably be work hardened by just being moved around.

I had about 10 feet in a partial coil that I had for some time so it was work hardened. After roughly straightening the end I cut off a few inches. You will get about a dozen washers er inch plus you need half inch or so to clamp on.

To anneal it I just heated it to a dull red and let cool. Some people say to quench it in water, but the results are the same as if you let it cool in air. If you have over heated it water quenching will wash off some of the fire scale that may have formed.

After it cools I clean it up a bit with abrasive paper . To straighten it I place it on a hard surface and roll it with a bit of flat metal. This will most of the bend out of it.

Gail in NM

 

[GailInNM]

While not perfectly straight thee tubine is straight enough to be usable. The tube is clamped in the lathe with about 1/2 inch protruding. If using a chuck. it may be advisable to insert a piece of 3/16 rod in the tube to prevent the chuck jaws from deforming the tube. A collet is preferable if available.

If the tube was cut using a standard tubing cutter as I used the end will have been swaged in to a smaller ID on the end. About 1/16 inch is cut off to get past this section. Use a sharp tool and very llight cuts. Copper does not play nice because ii
is soft that the tool will dig in.

Lightly deburr the ID of the tube with a countersink and the OD with a fine file.

Check to see if your spark plug will fit inside the tube. It probably will not as the tube will be slightly out of round. If it does not fit, run a #12 or #11 drill about 1/2 inch into the tube. and again check to make sure the plug fits.

Gail in NM

 

[GailInNM]

All that is left now is to part off the washers to about 0.030 thickness. I use a 0.o30 wide parting tool that has the tip ground to a 15 degree angle so side nearest the part cuts first and the parent stock side last. This reduces the burr on the part to make clean up easier. You will have to look hard in the photo to see this. but it is there.

Then part off the washers too thickness. I catch the washers on a piece of wire so I don't have to find them in the swarf tray. Lightly deburr the ID and OD of the parent stock with a countersink and fine file between each parting off operation. I get about 5 or 6 washers and then feed out some more tubing and repeat the operations.

The washers will have a burr on the cut off side. The straighter the tube the less burr. I pick off the major part of the burr with a hobby knife or tweezers and then give them a rub on abrasive paper or with a fine file. If you are making more than half a dozen or so washers you will save some time and fingernails by making a pocket about 0.020 deep in a scrap of metal to drop the washer into to hold them while filing the burr off. I just plunge a 1/4 inch end mill into the jig and wobble it plus and minus a couple of thou on each axis so thelle washers fit easily in the pocket.

That's it for method one of the washers. Method two will follow , probably in a couple of days as I will be busy with the USA Thanksgiving holiday and family gathering (today) the rest of today and recovery tomorrow. I wish all my USA friends a happy holiday.

Gail in NM

 

[GRAYHIL]

When I make small spark plug I put a small under cut in the plug body first at the end of the thread.(not very deep only a few thou)
The washers are made as you say but the bore is tapped not as quoted :

Check to see if your spark plug will fit inside the tube. It probably will not as the tube will be slightly out of round. If it does not fit, run a #12 or #11 drill about 1/2 inch into the tube. and again check to make sure the plug fits.:

The washer will the screw on the plug and settle in the undercut and not come off or be lost.

Grayhil

 

[GailInNM]

Grayhhil: Thanks for joining n. Your method has a lot of merit and I can certainly recommend it. If a #12 drill is used the washer will barely slide on the threads. When building my engines I countersink the plugl hole to the major diameter of the thread. This makes it easier to start the plug ans makes sure the plug will seat on the head. The first time the plug is installed the washer swells into this countersink then has to be unscrewed from the plug to remove itt.

Unrelated to building of plugs butt related to plugs I just conducted an interesting experiment with supprising results. I took one of my test hIt and miss engines with a 3/8 bore and 1/2 stroke and fired it up and adjusted the fuel mixture to get it running smoothly. Had to readjust it a little as I had last run it in June at about 6600 feet elevation. I am about 5300 feet elevation so the mixture changes a little bit. The head on this engine is thick so it had an extended nose plug in it. I replaced to plug with one of the short reach pllugs from this run and installed. It is about 0.280 inch shat the plug I was using. I just wanted to see if it would run at all. It started right up and without adjusting anything I could not tell any difference in performance from the original plug. This ment that the spark was about 0.28 inch up an 1/8 diamater hole from the combusttion chamber. It was just s happy at full load as it was at idle. This result was totally unexpected. Photo shows the two plugs. I will clean the long reach plug before re-installing it. It has about 250 hours on it since clleaniing.

Gail in NM

 

[GailInNM]

OK, back to making washers. This time out of flat sheet copper. The first problem is acquiring some copper sheet the right thickness. You can buy copper sheet but most of the time they want to sell you far more than you need.

For washers this size What I use is copper pipe as used for water plumbing . Looking at the specifications for copper water pipe in the USA. The wall thickness for "1/2" pipe is right in the middle of the thickness range I want. I put "1/2" in quotes because there is no dimension on the is pipe that is 1/2 inch. Going back in history, they started of with 1/2 inch usable ID. Then they added an allowance for scale and mineral build up and then round up to to the nearest neat fraction for the ODn. The result is that "1/2" pipe measures 5/8 inch OD. You don't need much. A 3 inch length is enough for 30 to 30 washers. If you have a friend in the plumbing business they will probably have a short cut off that they will give you. If not, the big box DIY stores usually have short length in the 1 to 3 foot length at reasonable prices.

I first cut the pipe into about 3/4 long sections.

Then split the sections length wise. I put them in a small drill press vice tightening them only enough to hold them lightly and cut them on the band saw.

Gail in NM

 

[GailInNM]

The copper in the split part is in full hard state from work hardening during the manufacturing process. It will be annealed to a soft state before unrolling it. If you have not worked with copper I suggest that you try to open the piece up a bit before annealing using the method in the second photo of this post so can get a feel for the difference between dead soft and work hardened.

To anneal just heat it up to a dull red and then let it cool. I use a DIY style propane torch for this.

It is soft enough to open with you fingers at this point but it is easier if you open it up most of the way by inserting a pair of needle nose pliers in it and then opening up the pliers.

Then finish opening it using your fingers. After it is mostly open turn it over and press it down on a hard surface. It still will not be flat but it will be close.

There will be fire scale on both sides of the copper. In the above photo the copper had been over heated so there is lots of fire scale . Brush off the fire scale with a brass or stiff fiber brush. This is to keep the scale from being pressed into the copper during the final flattening operatioon.

To finish flattening the copper blank I insert it into a mill vice with flat jaws and clamp it. To make it easy I do it is several stages so I have something to hold on to.

Gail in NM

 

[GailInNM]

With some flat blanks in hand it is time to make the center 0.190 diameter hole.s I have used 3 different methods at different times so will describe all three.

The first way is to use a CNC milll. The blank is supported on a pair of thin parallels in the mill vice. Only a moderate clamping pressure can be used as the copper is soft and will buckle if to mulch pressure ils applied. Check that the vice griiips both ends of the blank bp llightly clamping and then checking that both ends are being gripped. If the pipe was cut squaare then it normally OK. but if not a few strokes of a file will square it up. I plunge in slowly with a 3/16 4 flute center cutting end mill and then open the hole up with circular i orbit with a radius of about 0.0015. Repeat spacing the holes at least 3/8 apart. Note in the photo that I am holding the blank down with the handle of a chip brush. Cop[per grabs when the mill breaks through and may try to pull out of the vice. Don't even think about using a finger for this!!!

Gail in NM

 

[GailInNM]

The second method is the same as the first as far as setup but done on a manual mill. Put 3/16 holes in the blank with an end mill then finish the hole to size using a 0.180 drill. This will leave mmore of a burr than the first method but it is not too bad to remove. If you try to drill the hole with out using an end mill first the blank will probably pull out of the vice and you will have a large burr to remove.

Gail in NM

 

[GailInNM]

Just to show how out of date I am with the technology of on line shopping I looked on Amazon with a search for copper sheet and found many listing for 23 and 24 gauge copper sheet. 23 gauge is 0.025 inch thick and 23 gauge is 0.021 inch. Either would work OK but I would prefer the 23 gauge. Prices range from US$ 12 to US$ 20 for a 12 inch square sheet. Smaller sheets are also available.

This would save you some time and effort from cutting from a piece of pipe as I did.

Gail in NM

 

[GailInNM]

Moving on to the third method of putting in the 0.190 holes. The holes are punched with a very simple punch and die set. I have a foot press, AKA Kick press. that I normally use. As these are not common in most home shops I set the punch and die up on my small manual mill which has a 6x24 inch table with a a R8 spindle. Smaller mills than this would struggle with punching this hole.

For those who have never done any press work here are some very brief comments on punch and dies. I say very brief as whole books are devoted to the subject.

Clearance is needed between the punch and die. If punching a hole, the punch is made to the desired hole size and the clearance is applied to the die. If punching out a a disc, or part, the die is made to the desired size of the part and the clearance is applied to the punch. This clearance is specified between each side of the punch and side of the die As a rule of thumb the clearance is from 6 to 10 percent of the material thickness. The greater the clearance the the larger the burrr on the part or hole. As the clearance is reduced a smaller burr will be formed but the pressure required to punch the hole will increase.

The punch will stick in the parent material so it is a good idea to polish the sides of the punch to reduce the friction and resulting effort to remove the punch. The slug will also stick in the die so the area below the cutting edge of the die is relieved so the slugs can fall free without having to be pressed through the thickness of the die.

Gail in NM

 

[GailInNM]

Since the washers are being made of soft copper the punch and die can be made of any mild steel and does not need hardening of any kind. Unhardened it would be good for at least 500 holes and then a touch of the die on a belt sander would make it good for another 500.

I made the punch by first milling a concave on the end of a 1/4 inch diameter rod and then turning about 1/2 of it down to 0.190. The concave makes the punch shear the hole and reduces the pressure required to punch the hole to about 1/4 the pressure that is that would required if the end is made flat. This reduces the wear and tear on the milling machine quill rack and pinion and also on the arm muscles. I mill the concave first as the milling operation distorts the end of the punch making it slightly larger. The turning then removes this distortion. Making the punch long enough so a couple of inches protrudes from the chuck that holds it makes it easier tor align the punch and die.

I made the die from a piece of 1/4 x 3/4 cold rolled mild steel. I drilled the larger diameter (0.201) hole by touching off the point of the drill and drilling to a depth of of the material so the tip of the drill just touches the bottom side. Then I drilled through with the 0.194 drill. This difference of 0.004 on the diameters of the punch and die gives 0.002 per side clearance.

The die was removed and a flat file used to remove the burr caused by drilling. This gives a sharp cutting edge on the die.

The die was clamped with the cutting edge up in the milling vice on parallels. The punch was held in the drill chuck and aligned so it would enter the hole in the die. The Z axis stop was set so the notch in the punch would enter the die about one material thickness. It can enter more but that makes it more difficult to remove the punch from int the stock .

Gail in NM

 

[GailInNM]

A couple of holes are punched.

And then a bunch. Notice n the photo that I got some of the holes too close to each other. This will make cutting the parts apart with snips or a saw difficult while leaving enough material for the OD. This will only affect one of the methods that I will describe for producing the OD in a following post.

Also notice the stripper mounted above the punch. This is optional but lit will save a lot of time and irritation in removing the punch from the stock if making a quanity of washers.

And an overall photo of the setup showing the temporary llash up for the stripper .

Gail in NM

 

[Rocket Man]

I buy small spark plugs at O'Riely auto parts store. After using them in my rockets engines they get 1400 degrees. Copper gaskets soon become loose fit. I can tighten the spark plug several times soon the copper gasket needs to be replaced. I buy flat sheet roofing copper at Lowe's I use a punch set to punch out blanks. I had to make my own punch set for the diameters that I need. I punch out center hole first then use another punch with a guide pin on the tip to punch out the finished rings. If I want copper rings to be softer metal I heat them with my propane torch. Work hardened copper gets soft again if you heat it to about 1000 degrees.

 

[nel2lar]

I had the need for some copper washers some time ago and made them from a piece of tubing. Copper tubing comes in several OD's. I took the tubing slipped it onto a mandrel that was a snug fit then used a tubing cutter to cut my washers to the desired thickness. Worked a charm for me just try not to cut into the mandrel.
Thanks for sharing
Nelson