Clamshell Cope and Drag Hinge (No Pins) Al parts to be cast.

I was approached by a guy who sells heavy duty sewing machines to leather crafters to make 50 parts in aluminum. Originally they were cast iron, but customers ended up breaking them. He wanted them cast in Al.

(The machines have been in production almost unchanged for 100 years. On change made is the addition of a motor as they were originally designed to be line-shaft driven!)

He sent me two "repaired" parts to serve as patterns. The parts themselves are not critical dimensionally as they are covers for a portion of the inner works of the machines. They were "a little rough." ;-)







Here they are after some die grinding, bondo, sanding, bondo, sanding and filing, bondo, paint, sanding, paint, bondo, sanding----you get the picture.

Draft had to be added to the "ears" of the part as it was machined off after the casting was made many years ago. Bondo to the rescue.



The beginnings of the follow board---basswood.


Here is a video of the clamshell cope and drag closure method I dreamed up. I suppose it has been done before, but it is a new idea to me.



Time will tell how it works out.

Denis

 

I rammed up a mold today. I was using petrobond that I got from a guy. It is several years old and I do not know how it is in terms of proper workability and adhesion. It seemed to work OK but may be a little dry. I should probably by some fresh stuff to compare to it. I am a petro nubie.

The system worked just fine. The first time through I probably did not ram the sand quite enough and I just was reassembling the mold when the cope collapsed. Partly I was clumsy and partly not tightly rammed I think.

The second time through all went well.

Pics:
Splitting cope from drag


Patterns pulled using magnets
Pouring basin cut
Splash basin and runners


Next comes the pour. This mold will only need about 2 pounds of Al as each cover weighs only 6 or 7 ounces in Al.

Denis

 

You can freshen the Petro bond with a little bit of alcohol. I add a capful or two to 20lbs in the muller. Use either 90% rubbing alcohol, or Denatured alcohol from the hardware store. It doesn't take very much to improve the greenstrength.

 

Thanks. I thought I remembered people talking about adding alcohol and was hoping someone would freshen my memory so I could freshen my Petro. I also ordered 50 pounds for comparison and needed some more anyway.

Denis

 

Warming the PB to the touch will surprise you on how good it returns it to all it's glory.
After a year of use, I started coating my muller with oil just before adding the burnt crust.
My theory is the oil smoke from the crust permeates the flask and adds the oil to the bulk of the sand. Coating the muller and adding only the crust to mull should replace the oil there. I could be way off base but that's what I see in my mind's eye.
That is the process I've been using for the last two years and the PB is doing good.
If you have fairly new PB then warming should help a lot.

 

I tried adding a small amount (a tablespoon or two) of demnatured alcohol to some of my old petrobond. What a difference such a small addition made. The post-alcohol sand was amazingly moister feeling and had much more green strength. I made a pre and post video that is not of the highest definition but still shows the difference.

Here is how it ran in the muller prior to adding the alcohol:


And here is after:


Thanks for the information, Bonz and Tobho.

Denis

 

And I did pour the clamshell mold today. For the first time to pour the mold, I was surprsed to have two usable peces result. I anticipated poor or incomplete filling or some other issue. But the mold filled fine even with no vents and the surface finish was OK. I poured pretty hot at 1350 or so and that may have tended to give not as good a finish as if I'd been 50 or 100 degrees cooler. But filling was more importatn than finish, so, I opted for hotter. I did get some shrinkage in the squarish foot section of the castings. This I will weld up this time and next time I will make side risers to hopefully prevent shrinks.

I will also make another clamshell or two so that I can pour 4 or six parts per go. I also poured flask sides this time as melting only a couple pounds of metal seemed like a waste. I need to make a half dozen flask sides anyway so that I have two full sets of the cope, cheek, and drag flasks I have been making. They are pouring very nicely.

Here are pics of the sewing machine parts poured today.


Note shrinkage


Denis

 

Excellent work MS.
Regarding the shrinkage, do you think it's possible to increase the size of the pattern in those areas to makeup the shrinkage?
I have a pattern that was giving me trouble in a thick spot, I changed the gating but cleanup tripled. I'm now thinking about going back to the original gating and just adding more meat to the pattern in the offending area. I know...more metal equals more shrink but there must be a balance point where it could work.

 

How fine/coarse was the sand portion in that Petrobond?

 

Thanks, bonz. Adding meat makes sense. But, I’ve had limited success trying that in the past. I added a big bunch of metal to an area of shrink shown in this photo.


So, I added about an inch to the height of the round part and recast it. Well, no joy as it shrank worse and more toward the bottom (the hottest area) than the top making the distortion worse! So, I should have added a necked-down riser on that area if I wanted to riser it. In the end, I jsut went back to the original design and reduced the thickness with better results and made up the difference with a washer. If I get ambitious, I might make a silicate necked-down riser. But that is a lot of trouble.

I do not know as it is "second-hand" petro. The sand is pretty fine though. Finer than my 100-mesh olivine from OCL in Portland.

Denis

 

I hope to get some soon...

 

Doing great!

I might suggest a deeper gate so it freezes last. No more cleanup but it can be sized to freeze last and avoid all shrinkage. Are you going to make a pattern for your gates?

 

Andy,

I do plan to make pouring basin and runner/gate pattern as part of the follow board. That was the plan from the beginning. But, before getting to invested in the pattern board etc, I wanted to be sure the pattern filled properly etc. So, that is what I will be doing today.

I did think I would add a 1" diameter by 1" high hot riser at each gate with a gate that is about 1/3 to 1/2" thick connecting the riser to the cavity. But, you think I could just make a thick gate and that would be enough? I'm listening...

I just now tigged in the shrinks with aluminum. But, I sure don't want to do that 50 x 2.

Denis

Added later: this is the added riser I tried. But it caused shrinkage at its juncture with the part itself.

 

Put the sprue where the riser is and even though you will have two sprues in the cope if you don't spill down the one you are not filling, the hottest metal will be directly above your shrink area and thus feed it. Single point cleanup by cutting the sprue off. An alternate method would be to move the gates to the center so they feed directly into that area and feed from a large runner to give the problem area hot metal from the sprue.

 

The J86 I use which is a Petrobond clone was made with 120 mesh sand. Now all I can get for core sand is Okie 90. The finer sands are being used in the Fracking of oil shale so there is a shortage of the finer sands according to my supplier.

 

I think you can easily solve your problem using Chvorinov's Rule. It is incredibly simple to use. You just have to calculate the modulus of the part that shrank and the modulus of your feeder, and the feeder has to have a greater modulus. Modulus is volume of a part of the casting divided by the area of that part which is exposed to sand to cool. The lower the area for a given volume the greater the modulus.

The part to which you added a big riser was just making a Tee joint. I'm sure if you calculate the modulus of each piece you'll find the riser and square under it have the highest modulus and will certainly freeze last and shrink at both ends.

You've won most of the battle since you know the blocks on the ends freeze last, the rest of the casting has a lower modulus.

So measure the blocks and calculate the volume. Then calculate the area exposed to sand. Divide and get the modulus. Then pick a gate which covers the same area of the casting as your hand cut runners but make it thicker and calculate the volume and the area which is exposed to sand, leaving out the area contacting the casting and the area contacting the sprue. Calculate the modulus and if it is not a larger number than the casting block, add thickness and calculate again. Note it is more metal to be poured but no larger saw cut off of the finished casting.

I've got a 30 minute video showing the process step by step but I don't think you need it, just bring a gate in which has a larger modulus and it will freeze last and therefore take all the shrinkage. It works like magic. Forgive the self promotion.