Repairing Mistakes

[craigyboy]

I'm Building a Stuart number 8 horizontal engine. Just working on a second attempt at the conrod, bent the first. I was just working on milling out the fork, it slipped and the mill has taken a lump out of the other side of the fork.

Any advice on what I can do to save several days/evenings work on this? I guess I need to fill the hole with something, not sure there is enough to braze.

 

[Swifty]

You could machine the damaged area until you have a flat surface, then silver solder a washer of the same material on to it, then remachine. I know that it is probably very thin, but with care it should be ok.

Paul.

 

[Mechanicboy]

Do not take so much material, use file and make it flat enough to lay a piece material + silver brazing so strength is not weakened before machined- again to the right size.

 

[gbritnell]

By looking at the picture I assume you were trying to hold the rod by the big end and mill out the inside of the fork. With this there is no support and the cutter grabbed the rod and sprung it into the cutter. You need more support near the forked end.
Here's what I would do. Find some way to hold the shaft of the rod. I know it's tapered so here's how to hold it. Draw 2 lines on the tapered shaft as far apart as you can get them. Now using micrometers measure the diameter of the shaft at both lines. Subtract the small diameter from the large diameter and divide the answer by 2. This will be the side opposite (trigonometry term) Now divide this number by the distance of the 2 scribed lines. This will be the hypotenuse. Divide the side opposite by the hypotenuse and this will give you the sine of the angle. There are online trigonometry calculators that you can use. Now that you have the angle take a square piece of stock, aluminum if you have it, and put it in the lathe. With a small boring bar set at the calculated angle bore a tapered hole in the block. Now split the block as close to the center as you can using a bandsaw or slitting saw.
A note here: the block doesn't need to be as long as your layout on the shaft of the rod.
Now clamp the rod into the tapered block and set it vertical by using the side or bottom of the rod (big end) Using a ball nose mill cut the round bosses completely off of the fork. Now turn up a piece of stock the same diameter as the bosses and a little longer than needed. Silver solder this piece to the forks. Using the same tapered block clamp the rod horizontally and drill and ream the holes, or whatever needs to be done. Now cut out the center section of the rod with a bandsaw, hacksaw or whatever. Now stand the rod up vertically again like when you cut the notch for the round stock and mill the excess from the sides and center of the forks. .
If all this seems a little too complicated then just saw the bosses off and using a round file, file the notches for the rod but you will still need to make something up to hold the rod closer to the forked end.
gbritnell

 

[portlandron]

Been there - done that!

I mill of the end of the forks using a cutter the same diam. as the built up area on the end of the forks. Then silver soldered a piece of brass rod across both forks.
Then drilled it and cut out the center section and brought the outside end down to the proper dimension.

 

[craigyboy]

I had the big end in the vice and the flat part of the fork on a piece of sacrificial flat bar on a 3-2-1 block. I had a clam pressing down on the flat section. I think it moved when I nicked the flat bar. You live and learn...

This is my second project after a Stuart Oscillating Engine. I can look at it as I may have bitten off more than I can chew or a chance to learn silver soldering.

Thanks for the advise.

 

[Mechanicboy]

Here is my method as i did with triple expansion steam engine. Drill with right drill size showed as in drawings, then cut with saw to remove the material and then mill with small milling cutter, take light cut each time.

 

[bazmak]

With brass and bronzes its easy to trim and silver solder pieces in psn before remaching,apart from colour change you cant tell the difference.When i was a boy it was standard practice to repair expensive bronze casting only if it was economical.With model engineers labour costs does not come into it

 

[craigyboy]

Thought I would show the result. One new brazing hearth, silver solder gear and new propane torch, just to save having to do all the same work again.

Light cuts this time better secured.

View attachment ImageUploadedByModel Engines1431278751.166838.jpgView attachment ImageUploadedByModel Engines1431278767.569262.jpgView attachment ImageUploadedByModel Engines1431278781.809340.jpg

Needs a little cleaning up, but I'm quite proud of my first bit of silver soldering.

Lots of thanks for the good advice.

Craig

 

[Mechanicboy]

The next job is polishing the connecting rod and the bad job will be unvisible.

 

[cwelkie]

Good recovery Craig!
That brazing hearth will serve you well over time. Sure beats trying to stack an odd assortment of bricks and such when you need one.

 

[bazmak]

Its a hard way to learn and expensive but a good way to learn
you wont make the same mistake,but if you do you can fix it