[Geof]

The metal is not exotic but it is a kind of composite which is why I put it here.

It is a gear from the steering mechanism of one of my lawn tractors; a broken gear now.

The first picture shows how it looks when it is together and you can see the interesting through hole. Interesting because the two straight sides are parallel while the curved sides are tapered, and the corners between the straight and curved parts are really sharp, no discernible radius at all.

How do you make such a hole?

The answer is you don't, you make all the stuff that surrounds the hole by a process called powder metallurgy also known as sintering. Steel powder is compressed at extremely high pressure and temperature in a die fusing the particles into a solid mass. Much the same as the procedure for making carbide inserts.

I should put in a caveat here: It is my personal conclusion that this part is made by sintering based on the overall appearance, the appearance of the fracture, the hole shape that would be extremely difficult to make any other way and the fact that this part is in a not very expensive consumer product.

[Einstein_1985]

The hole can be easily made with a wire EDM machine, costs next to nothing have those made in a factory in India.

[DaveKennedy]

Well, I'd drill and tap it and recess 2 socket head machine screws either side of the hole. Through the 2 halves. Then I might recess a ring with a hole saw around the hole and super glue a steel ring in. Assuming it's going to be 'sandwiched' in place. I like to think some of my 'professional repairs' are more of an improvement than a bodge.

It might have been cheap to produce (in numbers) but it clearly wasn't up to the job.

[Paulo E.]

Cool looking part.
I agree with Einstein, EDM is the way to go.

[templecorran]

I guess you've tried all the 'simple' things like searching for a replacement part from the tractor manufacturer?

[Paulo E.]

(flame2) ROFL

Maybe the part is no longer manufactured or he wants to do it as a special project but yes LOL you do bring a valid point. Having said that, Geof is one sharp dude so if there is a reason he has one:cheers:

[peterpan3]

Wire EDM is the only way to go with this, or a really good welder.

[AMCjeepCJ]

I don't think he wants to build one. He started this thread to keep his other thread on topic because myself and a few others kind of started talking about this type of technology instead of his original question, which was, "What has been the Single Biggest Advance in Machining Technology in the last 50 Years"...


See post #20, 22, 28 and possibly others but post 31 directs everyone here:
http://www.cnczone.com/forums/showth...244#post629244

I think he posted here only as a way to keep everything on track in the other thread and to show an everyday example of what we had been discussing.

Note he said, "gear made in single operation?"

You can't make that part on a wire in one operation, it has a counterbore...

[Geof]

I started a new thread because I didn't want to clutter up the other one.

Where did I say I was going to repair it?

I agree that shape could be done by EDM, I guess even the counterbore, but I seriously doubt this one was made that way. It is easy to see there are no tool marks on the sides of the gear and around the end of the hub there is a very small flash as if it came out of a die, I think it probaly came out of a multi-part die.

Nobody is seriously suggesting the OEM made this part by EDM are they?

No I haven't tried the simple things yet. Give me a break, it broke Sunday afternoon, I started the thread Sunday evening and my time is 6:36 am Monday and I am having a cup of coffee.

My title is wrong anyway because it would not have really been made in a single operation because powder metallurgy requires preforming of the powder mass.

Dave's comment about it not being up to the job is correct. The lawn mower is only a few years and I bought it the year it was introduced. It is the style with the steering connected to individual hydrostatic transmissions on each rear wheel to vary the wheel speed depending on the radius of the turn. It is fun to drive because the front wheels can turn a full 90 degrees making it pivot around the center of the rear axle.

[Geof]

What was AMC saying about the Internet and information in the other thread?

The coffee kicked in and I realised I could enter the part number impressed in the side of the broken gear into Google.

Seven hits and all of them say something like: part# 717-04282A replaces part#717-04282.

No prize for guessing which is my part number; maybe this is not the first one to break.

Maybe I should order the opposite side one while I am ordering?

[LeeWay]

Just an observation and perhaps a wrong one, but those sharp crisp corners may have contributed to the parts demise. I think it would be better to actually have a small radius in there. Don't know if the shaft is as crisp, but a radius would seem to make it stronger.

[tjwal]

The hole can be easily made with a wire EDM machine, costs next to nothing have those made in a factory in India.

Hmm this must be the CNC group, what is really needed is some old technology, think blacksmithing. Make up a mandrel that matches the hole. Heat up a gear blank in your forge and drive the mandrel into the hole. Clean the blank up in the lathe and cut the teeth.

Then with the $15 you've saved by not ording a new one buy a six pack and enjoy the fruits of your labor.

JohnW

[fuelsguy]

If you have the broken part it would be easy to get someone local to cast one in aluminum or brass. Your local community college or tech college would love to do it as a project.
Or you could just buy a replacement over the web.

Ivor

[garfieldsimons]

Machine and saw what you can and file the rest. One can replace all sorts of fancy equipment with contour milling, good files ... and time.
You also have the pattern so would not be a big deal to get it casted if you could find someone with a small foundry.

[Geof]

Just an observation and perhaps a wrong one, but those sharp crisp corners may have contributed to the parts demise. I think it would be better to actually have a small radius in there. Don't know if the shaft is as crisp, but a radius would seem to make it stronger.

I am sure this is correct. A small radius should always be put on corners like this or on steps on shafts to prevent crack initiation. But for making things cheaply I think often these are omitted because it means the mating shaft has to have a matching radius which in this case would have added a manufacturing step. Consider the shape of the shaft, a cone with two flats: This is trivial to do on a lathe with live tooling but adding the steps needed to radius the corner between the cone and flat would not be easy.

Replacement parts are available I just have to get off my duff and order it.

Actually regarding cutting the teeth did you notice they are not on a constant radius? This would be a simple operation with wire EDM but doing it on a gearcutting machine or in a mill could pose a challenge.

[cadman@teluspla]

This would probably be a good part for the lost wax method.
(investment casting)
They can get great detail and cost is very low.
You also don't neet large #s far a run.

[gmac]

looks like it was made with a taper for alignment onto a shaft or as a positioning stop. if the item was pressed too hard onto the taper or vibrated under pressure a nonductile material could easily crack there. probly be possible to fit both sides with steel plates and electric weld these small plates to the parts ( if they are weldable) , then drill thru and bolt the plates together for further stability. the plates will prevent some further expansion but the material may be quite hard to drill. think abrasive drills and some handwork before going into heavy-pressure machine-powered effort with HSS or cobalt drills. if that snapped before from expansion tension then punching a hole into it could cause more tension stress also. welding to it would tend to put it back into compression as HRS plate would contract when cooling-make sure its very well clamped and then weld both sides at one heat so plate doesnt warp it uncontrollably.
skip the trial and error repair work and buy a new one if its under $25. its going to be at least 2 hrs work on it plus re-aseembly of the machine and you have lots of mowing to do. use some headwork to make sure the new one doesnt suffer the same fate- cut and polish a radius into those inside corners with a small abrasive bit

[John_B]

Definitely investment cast. MTD, now America's largest lawn equipment manufacturer, is also one of the largest consumer companies of investment castings outside the automotive industry. My employer has been trying to make inroads to this market for some time, it is VERY VERY cost competitive in this sector - just like the automotive world.
They is a way of making a rubber mold of the old part (you can super glue it together, weld it, whatever) pour in your wax, then make your investment shell to cast one or a few.
Check out Polytek.com for the rubber, or google for rapid mold rubber or rapid prototype rubber.
Rgds,
John B

[Geof]

looks like it was made with a taper for alignment onto a shaft or as a positioning stop.......

It bolts onto the top of the 'king pin' on the front wheel and is what takes the steering drive.

Replacement parts are available and I do not intedn to fix it, I think if I did try drilling holes in it I would find it very hard as you suggest. In the past I have tried machining powder metal parts and they where very hard.

On the top od welding I have no idea if this is possible with sintered parts. If I was going to repair it I would make a collar with a couple of thou clearance to go around the boss and a disc as large as possible to fit with the root of the teeth at their smallest radius and then silver solder everything together.

[SORCHEROR]

that gear was made from cintered powerded metal,thats why it apears to be a weird alloy,but you can cut a new one out of steel and edm or wire the hole or just buy from the factory,alot cheaper,unless its old and dont have them anymore