Spark Plug constructlon revisited

[GailInNM]

Spark plugs always seem too be of interest to the IC crowd so here is another construction thread on my recently finished d production run of plugs. These are threaded 10-40 and are just over an inch long overall. They are similar to the plugs I used in my tinyengines which were detailed at:

https://www.homemodelenginemachinist.com/threads/tiny-i-c-engine.7397/page-13

Unfortunately many of the photos in that thread have disappeared, but the drawing remains and enough of the photos remain to get the idea. I have made about 35 of these plugs and about 25 of hem are in use withe remainder saves as spares. Only one of them failed and that was due to an eror on my part during assembly. These plugs had a long reach nose which I do not need on my current engine design..

The new plugs are of similar design, but with out the extended nose. Construction techniques have changed some what to make construction easier for me. I am legally blind so hand operations are very difficult for me. Most of the machining was done with CNC as I can not read the dials well enough to do much manual machining, but all the machining can be done without CNC. I will try to make suggestions on manual machining methods were it might be useful.

Related to these plugs will be molded boots for them, but that will be in a separate thread. For those interested here is a link to the thread on boot molding for previous plugs. Again , many of the photos are no longer avilable..

https://www.homemodelenginemachinist.com/threads/ignition-boot-molding.14539/

So to start things off here is a PDF drawing of them.


Z1GPDF-3.pdf

 

[GailInNM]

The body is the most complicated part. On most construction projects I generally start with the complicated parts first, then I can alter the simple parts to fit if I mess up the complicated part.

I started with 1/4 inch diameter 12L14 steel bar. The end was faced off and three diameters wore turned with a 0 radius righ hand turning tool . The tip was turned to 0.156 for 0.094 that will be miilled down to form the groound electrode. Next the section to be threaded was turned to 0.190 and finally the main body was turned to 0.245 diameter. The body diameter was selected so the plug would fit into a .250 counter bore if necessary.

An 1/8 hole will drilled in the end for the full length of the part. Finally the body was threaded 10-40 with a single point threading tool. The threading to a shoulder is easy with CNC. If threading on manual machines you may want to invert the threading tool and thread from left to right running the lathe in reverse. You could also use a 10-40 die but you may need to reverse the die for a finishing cut to get to the shoulder.

And finally the body is parted off.

Gail in NM

 

[GailInNM]

The body was moved to the milling machine where it is held in a 5c collet in a vertical collet closer. Since I was doing multiple parts I put a stop in the collet. The ground electrode was milled to width with an 1/8 inch 4 flute end mill.

The milling operation was done in 3 steps 1/32 deep to avoid bending the electrode tap. Normally I would clime mill with CNC, but to avooid the chance the end mill would grab and bend the electrode I used conventional milling.

The body was removed and burrs were removed using a needle file and small brass brush.

Gail in NM

 

[GailInNM]

An anvil was made form a length of 1/4 steel. The end was turned down to 1/8 diameter for a length of the body, minus the electrode, plus about 3/64 inch. The edge of the end was broken slithtlllly with a fine file.

The anvil is clamped in a vice using a vee block and the body is slid on.

I used the face of the 4 ounce hammer to IRON the electrode over the end of the anvil and then gave it one light tap with the hammer to set the bend.

And it looks like this.

Gail in NM

 

[GailInNM]

Back to the milling machine and the 3/16 AF hex is milled on the body. I used the vertical collet closer and CNC for this, but it would be easy to do with either a rotary table of a spindex on a manual machine.

Then back to the lathe to put a slight chamfer on the edge of the hex to make it easier to get a spintite on when installling the plllug..

And some finished bodies.

Gail in NM

 

[GailInNM]

EDIT:
SEE POST #44 IN THIS THREAD FOR AN UPDATE ON INSULATOR MATERIAL.

Next up is the insulator. I make my insulators out of Corian. Corian is the trade name for a filled acrylic plastic used in the USA for counter tops. There are some generic versions that work just as well. It machines very and is not fussy about cutting tool angles, although they do need to be sharp. I used the same 0 radius to make these that I used to cut the steel bodies.

I have been asked in the past about usilng PTFE (Teflon ) for the insulators on small spark plugs. It works very well, but I am not a big fan of using it. First off it is quite flexable which makes it deflect when machining so it is difficult to holding dimensions. Tools must be very sharp and have high rake angles to machine cleanly. And, it is difficult to get a good glue bond some means of mechanically increasing firctiion be resorted to.

If you do want to use PTFE, here are a couple of tips. Fir, after facing of the end, drill a hole the size of the electrode as deep as practical. Insert a length of your electrode material iln the hhole and griip it in your tail stock chuck. The PTFE will act as its own bearing and act as a center and stiffen the PTFE for machining. Second, cut some shallow grooves in the area to be glued for the glue to flow into to increase the frictiion to resist pulling out.

I purchased my Corian on eBay ome time ago. I purchased it as blanks for making custoom pen bodies. The blanks were about 12mm square. or a little under 1/2 square and not to consistant in size. They were about 5-1/2 inches long. I found a round collet that they would just fit into and gripped on the corners of the blank with about 2-1/2 inches sticking out of the collet. I tuned this end to about 3/8 inch diameter. There was a little bit of taper due to the material deflecting while turning. Changed to a 3/ 8 diameter collet and turned the other end to 3/8 so I ended up with a 3/8 diameter rod. Since there was done a long time ago there are no photos of this.
Gail in NM

 

[doc1955]

I get my Corian at Menards. I buy a cutting board and the cut it up into pieces. Gail I like your method of bending the tabs. I will keep that in mind on my next run of plug making. I did a few videos on making plugs.

 

[GailInNM]

Hi Doc -- I watched your excellent videos when you first posted them and wanted to reference them when I started this thread. I was having trouble finding them because of problems on my end. Thanks for chiming in and saving me the trouble of finding soI could llink to them.Our methods are a bit different , but the important think is they both result in the same thing --good pllugs.

On all my previous plugs I hand bent the ground electrode. but I can no longer see well enough to do that. In retrospect, I should have come up with the anvil method for them as it is much faster and more accurate and repeatable.

On Corian. I cn no longer safely use a table saw so I bought the pre cut blanks. The seller on eBay offered assortments of different colors so I asked if I gould just get a lot of white ones. As the pen makers wanted colors he had a excess of white ones. Said he would send me 50 blanks for US$10 with free shipping. When the package arraved he had stuffed about 70 of them in a flat rate box along with several usful small sheets. So overall hey cost me a lot less than I could cut them myself. For my size plugs that's enough material for over 300 plugs, or about $03 per plug.

GAIL IN NM

 

[GailInNM]

Back to making insulators. With the 3/8 diameter held in a collet the end is faced off with the 0 radius right hand cutter and the Z axis 0 is established.

To mimimize then cut the 1/8 diameter about 1/2 of its length using several passes. I then cut the remainder of the 1/8 dimiter, again using several passes. The same procedure was used for the 5/32 diameer, but cutting it longer than the finished part to allow room for a parting tool. Before cutting off i used a small center drill to remove any bump left on the end from the facing operatiion and put a small dimple on the end for a later drilling operation. Notice that about half of the length of the 5 /32 is tapered to the end. This taper can be cut at this time or as separate operation after the part is cut off. The un-tapered section is to allow chucking on it during later drilling operations.

Gail in NM

 

[GailInNM]

The drilling of the insulator is done on a manual lathe. One end could have ben done while in the CNC lathe before cut off but for me it was easier to just do in on the manual lathe.

The insulator is held in an 1/8 inch collet. No collet stop is needed as the part will stop when the 5/32 shoulder hits the collet face. I had turned the taper on the 5/32 diameter while turning on the CNC. If I had turned the part on a manual lathe I would have just turned the 5/32 end as a cylinder and turned the taper at this time. If turning the taper now, it should be done before drilling as the drilling will make the part more delicate. It is not necessary to face off the end as any variation in the overall length will be compensated for by the grinding operation on the center electrode during assembly.

Gail in NM

 

[GailInNM]

Center drill and drill the 0.047 hole in the 5/52 end. This hold does NOT go through the insulator. About 1/16 inch remains undrilled. The 1/16 inch is not critical and can vary +/-0.025 is no problem. It is necessary to peck drill this hole to to keep the drill flutes from clogging.

Changing collets to a 5/32 inch, the part is reversed to drill the other end. As this hole will break into the 0.047 hole, therre is no need for a collet stop as I can feel it break through. I used a #65 drill bit which makes a easy slide fit on the center electrode. This allows enough clearance for epoxy to be worked in to secure the enc of the center electrode during assembly.

I held the drill in a spring loaded small drill sliding chuck so the drill operation was very quick as I did not have to wind the tail stock wheel. This drill chuck can in the next post being used to dill 0-80 screws.

be seen

Clean out any swarf in the part an the insulator is done.
Gail in NM

 

[GailInNM]

The 0-80 screw for attaching the spark plug wire to the plug is next. It is held in a 1/16 collet in the lathe and a #65 hole is drilled through it. I use a solid carbide printed circuit board drill bit for this. The point on these drill bits are very good about self centering. If the screws have a fairly flat head there is no neeed to center drill the screw head. If the screw head is not flat or if you are using a HSS drill bit it may be necessary to center drill the screw to get the drill to start on center.

In this photo you can see the sliding small hole drill chuck mentioned in the previous post.

And a little closer photo.

Gail in NM

 

[GailInNM]

The center electrode is made of 1/32 hobby shop music wire. Before starting to cut them I wipe the wire down with a paper towel saturated with acetone to remove oil from the surface. It's just easier to do the whole length than the individual pieces. Revoving the oil makes the soldering job and improves the epoxy bond during assemblly later.

The cut lengths should be about 1-1/4 inches ;ong. They will be cut down to final length after being installed in the insulator.. The extra length makes it easier to work the epoxy into the insulator during assembly. Since the length is not critical they can be cut by hand using an abrasive cut off wheel. I used a Dremel cut off disc mounted in a Proxxoon IB/E rotory tool. I like the Proxxon as it has a nice metal bushing at the nose that makes mmountin it easy for use in fixtures and as a tool post grinder.

The cut off disc was adjusted so there was 1-1/4 inch from the tool mount. The wire was butted against the mount and the wire rotated with thumb and for fingers as it was fed into the wheel. Took about 2 minutes to cut a 36 inch length into 28 electrodes.

The cut off operation leaves a small burr on the ends of the electrode which must be removed to make assemblly easier. I use a non-woven light deburring wheel in a bench grinder. I can hold about 5 wired between my fingers and rotate tem with thumb and forfinger whille pressing them against the whee. Very quick but the same result can accomplished by rotating the electrode in a folded piece of 400 grit abrasive paper.

Here are a few of the finished electrode blanks.

Gail in NM

 

[GailInNM]

Time to do some assembly. The 0-80 screw needs to be soldered to the end of the center electrode. I start by making some solder preforms from 0.015 diameter electronic soft solder. Don't worry if you don't have any sine solder like ti as other methods work as well and I will talk about that later i.

I start by wrapping a length of the solder around a center electrode. Then using a #11 hobby blade I cut off one turn sections to make the preform.. It iis hepfull to use a new blade that still has a sharp tip so it will fit into the coil of solder. I can generally cut two preforms with each cut. Do the cutting on a piece for wood or plastic to avod dulling the blade.

Flatten the preform with a scrap of metal to make the end even with each other.

Dip the end of a center electrode into some paste flux. If I can see that I have some on it that is enouh. Insert the end of the electrode into the solder preform. The flux will hold the preform in place. Slide the 0-80 screw on the electrode with a twisting motion to distribute the fllux a bit.

Clamp the other end of the electrode in a small bice leaving a generous amount sticking out from the vice. This assures that the vice does not soak too much heat from the electrode when soldering. Slide the preform and screw so the end of the screw and electrode are even.

I heat the screw and electrode with a small butane torch until the solder flows. Most often the screw will jump away from the end of the electrode. So, before the solder solidifies I hld a scrap of metal on the end of the electrode and then push the screw back to the end with another scrap.

Clean the joint up with a bit of alcohol or acetone to rremove any excess flux.

If you dont have any fine solder, you can solder the screw in place with thicker solder using a soldering iron.This will probably result in a fillet between the electrode and the screw. You may have to file off some of this fillet so the screw seats on the insulator when the electrode in inserted into the insulator. It's also fail to counter sind the end of the insulator a bil to help clear the fillet.

Gail in NM

 

[GailInNM]

Now to gllue the electrode assembly into the insulator. I use 5 minute epoxy for this. The epoxy needs to be reasonably thin so it can be worked into the insulator. My epoxy of choice is IS made by Bob Smith Industries. and is packaged as two 4 ounce bottles in a package. It is a 1:1 mix and is very non critical about the ratio making it easy to mix small batches. Most hobby shops have it with the larger shops having it under their own name. Amazon and d eBay also have numerous listings for it. It also has good shelf life. I have used some that was over5 years old and it acted just the same as wen it was new..

First insert the electrode all way through the insulator to make sure there is no swar in the insulator blocking the transition of the 0.034 hole ad the 0.047 hole. It should be a free easy fit. I thn lightly clamp the smaller end of the insulator in a small drill press vice vee groove. Doon't clamp so tightly that you might distort or crack the insulator.

Mix a small amount of epoxy. You don't need much. Using a piece of of wire or stirring stick apply small amoount of epoxy somey near the end of the electrode and start the electrode in the insulator. Continue to apply epoxy to the electrode as you press in into the insuylator while rotating the electrode and oscillate it back and forth a small distance to work the epoxy into the jjoint.. Easier to do than describe. When the electrode is almost fully inserted clean off excess epoxy with a paper towel trying to keep epoxy off the screw threads. Seat the screw on the end of the insulator. Then clean the screw and joint with an acid brush saturated with acetone and then clean the other end of the insulator with the same brush.

Check that the screw is still in contact with the insulator and set aside for epoxy to cure.

Gail in NM

 

[GailInNM]

With the epoxy cured, it is time to bring the center electrode to length. At this point there is about 3/16 of an inch sticking out from the insulator. This needs to be reduced to 0.016 inch. If I try to just grind this to length it is probable that the heat from the grinding will damage the insulator.

I did this in two stepsl First I cut off the center electrode sot here was about 0.030 to 0.040 inch protruding from the insulator using a Dremel cut off wheel. Then I ground the electrode to the final length with a grinding wheel. Because I am old. nearly blind and my hands shake I made two simple jigs. If you don't have any problems doing fine hand work you can do this without the jigs, but if you are making more than a couple of plugs it is well worth m spending the 10 minutes or so to make up the jigs.

The jig for the cutoff operation is just a piece of 1/4 X 3/8 metal that has an 1/8 hole drilled through the 3/8 dimension. the 3/8 dimension around the hole is milled down to a point that when the insulator in inserted into the hole the end of the insulator is recessed about0.030 to 0.040 inch. In operation the insulator - center electrode is inserted in the hole and the center electrode is cut off with a cut off wheel. All the jig does is prevents cutting the electrode too short. The jig was held in a small drill press vice. If the part is rotated when feeding in the cutoff wheel it only takes a few seconds to cut off the excess length.

Gail in NM

 

[GailInNM]

The jig for the final grinding operation is the same as that used for the cut oo except that more materil is milled off so about 1/16 inch of the insulator is sticking out. I mounted the jig in a drill press vice and mounted that on my tool grinder.I could then adjust the position of the jig close to the grinding wheel until when i put the insulator in the jig and twirled it against the grinding wheel the electrode was reduced to 0.016 of the center electrode protruding.

The jig could also be mounted in a lathe tool post and a grinding wheel on an arbor be used in the spindle. In either case a 46 grit grinding wheel works well, but this is not critical.

Unless the burrs from grinding are laarge they will not have to be remove, but a folded piece of 400 grit abrasive paper will remmovve them quickly if desired. Clean the end of the electrode and insulator as any matalic particals here may short out the plug.

Gail in NM

 

[GailInNM]

A moment of truth approaches. Slip te insulator all the way into the body. If all the machining dimensions have been held the center electrode should be about 0.025 inch. from the ground electrode. Minor variances can be taken care when when setting the gap later by bending the ground electrode. If there is a major variance then it may be necessary to alter a part.

If the gap is OK, then the insulator assembly is glued into the body with epoxy n the same manner that the electrode was glued intoo insulator.

Any excess epoxy is removed with paper towel and acetone before the epoxy starts to cure.

The join and the electrode end are cleaned with an acid brush saturated with acetone. To function properly the electrodes must not have any epoxy left on them.

This completes the spark plug assembly I set my gap at 0.025 inch by bending the ground electrode. Hint: Most 6 inch flexable machine rules make in the USA 0are .25 thick. some import rules are 0.5mm thick.

Now all that is left are a the nut to secure the spark plug wire and the washer to seal the plug to the engine.

Gail in NM

 

[Johno1958]

Thank you very much.
Sooner or later I'll have to have a go at making some long reach plugs and maybe sooner than i think. The little engine engine I have been making (stalled at the moment) have plugs that are buried very deep inside the head and will benefit from a longer reach option.
I have been on Ebay to find Corian but here in Australia without paying massive postage from the US a bit hard to find but I have bought some Peek from China which I can say is a very hard and firm
type of plastic by to feel of it, which is suppose to be heat resistant. I guess I'll find out.
Cheers
John

 

[ruralearl]

Hi Gail, Thank you for this post. Regards, earl