Having had a couple of free hours to myself over the last day or two, I got into my shop for a little R&R.
There were a couple of jobs that needed sorting, one for a member on here, who thought that it would make a simple little article, and another from a local mate who wanted the clutch modifying on his 1/4 scale model car. I will be doing both bits in this same post.
John Somers is part way through his Upshur engine build which has a pair of gears inside of it, one on the crankshaft, and the other on the end of the camshaft. When in mesh, the smaller crankshaft one drives the larger (double the number of teeth) at exactly half crankshaft speed.
Depending where you obtain the gears from also affects what the centre hole size is. Some will come with a mounting flange, or like these, with nothing at all, just a hole through the centre, and as usual in cases like this, neither is of the correct size.
So John suggested that I might be able to make up a picture ridden post, just to show how I would put things right, and end up with the right sized hole in the correct sized gear.
This is John's posting on his own site.
http://start-model-engineering.co.uk/2011/05/upshur-vertical-single/upshur-4-stroke/~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
These are the gears that John sent to me for modification. Both are made of stainless steel and cannot be any thicker than the original thickness, so I can’t put a converter hub on there to get around the problem. So it requires a slightly different method than normal.
Just to keep my mind focussed when doing a job like this, I make up a rough layout page showing what I have and what is required.
I converted my lathe over to accept Myford chucks (they are much smaller than my normal ones) and duly fitted a four jaw self centring one. I prefer these as they give a much better gripping power over a 3 jaw when handling small parts.
Because the gears are stainless, I decided to use stainless for the modification, so I chucked up a piece of 11mm diameter.
I then turned a spigot on the end that was 0.002” (0.05mm) smaller than the hole through the gears, and about 0.010” (0.25mm) longer than the thickness of the gears.
This shows what it looked like.
The reason for being so much smaller than the hole is to allow for clearance for the silver solder (silver BRAZE for our US cousins) to penetrate the joint by capillary action.
The filler piece was then really rough parted off, just leaving a very thin flange to stop it falling through the holes in the gears.
I soon had the two required done.
This next picture shows what some people class as a black magic art, but nothing could be further from the truth.
Silver soldering just requires a few basic rules to be adhered to to get good results every time.
The parts should be clean, chemically if possible (a wipe over with cellulose thinners), good quality silver solder and flux, plus a little flame technique.
I am a real tight a**e when it comes to solder, purely because it is so expensive nowadays. I use only just enough to get the job done. Why use too much and then have to clean it off afterwards? So this picture shows just what I used.
For stainless, Tenacity flux is a must. But I use it all the time on everything, mixed up with a tiny amount of water until it becomes a ‘runny’ paste. When it dries up in the mixing tub over time, I just add a bit more water to get it runny again and carry on with it. This batch must have been on the go for well over 18 months.
So, assemble the bits. The ring of solder goes under the flange, flux painted into the hole and around the solder, the slug is then pushed down into the hole.
The reason for holding these parts like shown is that, as everyone knows, the solder will always flow towards the heat, so because I want the solder to penetrate right through the hole, the parts get heated from underneath.
Just like this.
Once the parts reach cherry red, you will easily see the silver solder starting to run. When that happened, still keeping the heat on the part, I pushed down on the flange, just to make sure the slug was right through the hole.
After letting the assembly cool down until things had solidified, it was turned over.
As you can see, the flux and solder have penetrated perfectly through the joint.
No black magic, poking about or feeding extra solder in. Just the correct amount of everything, and a bit of heat.
After both were given the same heat treatment, they were just cleaned up with a wire brush.
Tenacity flux comes off very easily.
Now comes the magic bit.
I love using soft jaws, they are about the cheapest method you can get to really lift your accuracy while using a lathe.
So here, I have swapped the jaws over in my chuck, locked the jaws down onto a thin bit of bar and have just finished gently boring them out so they are a very close fit to the small gear size. Everything has to be completed on the small gear before boring out again to take the larger gear.
Notice the back recess, to allow for the sticky out bits on the soldered up gear.
The gear was bedded into its recess, and very gently faced off until I had a perfectly clean face.
This did mean I took a minute amount off the original gear.
Having turned the gear around, the same was done on the other side.
Without touching the gear, using a spotting drill, I put in a nice centre for the drills to follow.
By climbing up in size, I eventually ended up at 7.8mm.
Followed up with an 8mm machine reamer, to give the correct size for the shaft it is to be fitted onto.
The same procedure was followed, boring out jaws etc to fit the larger gear, eventually ending up with a 6mm reamed hole.
Job done and dusted.
I hope you enjoyed this little escapade about getting yourself out of a sticky situation.
Now onto the next, getting the clutch done.
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Please don't ask about what engine it is, all that arrived here were these two pieces with a note saying that it was off a quarter scale model car and the clutch, rather than screwing onto the threads shown, he wanted the clutch backplate taper boring so that it could be fitted onto the other end of the crankshaft.
In this case, I'm not one for asking why, I just get the job done. It's his problem afterwards.
So after stripping the clutch, I found that the backplate fitted perfectly well into the previously bored soft jaws, so it was just a matter of getting the right taper.
I invariably always have my tool post set perfectly square to the topslide, so this method works just fine for me. With a 1/4" thick parallel resting along the tool post face with the end of the para resting along the taper, I tightened up the loose topslide at that angle.
I will be using the topslide for cutting the taper, so I mounted up a tiny carbide boring tool, and marked it up showing the depth I needed to bore it to.
I just followed the general procedure of boring to about 2/3rds size, checking with engineers blue and slightly adjusting the angle if needed. I only needed to move it a tiny amount just once to get the angle perfect.
Boring finished.
And it fitted the the crank perfectly.
I hope you enjoyed following me along in the shop for a few hours.
Hopefully, I will have two very satisfied mates.
Bogs
