Thick section casting (CNC Router Drop Plate)

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Harry
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Thick section casting (CNC Router Drop Plate)

Post by Harry »

I have been fighting a fairly simple casting because it is so thick. The part is basically a 3/4" thick plate 6" x 8" and more often than not I get some pretty terrible shrink in the thickness. I am almost certain it is because I have been using only one feed to the part, it barely fits into my 9" flask so I have been trying to make it work and sometimes it does but more often it doesnt.

Thinking two things to try will be using a larger flask and a second riser/bob on it and pouring a lot cooler. Pouring cooler might do it on its own. I have been also playing with tilting the flask on a few different parts to control how the mold fills and I can see some promise in this but it hasnt helped with this particular part.

Will get some pictures of the part and the failures along with posting the solution once I have something that is consistent.
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dallen
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Re: Thick section casting

Post by dallen »

now I'm now old timer at this stuff, actually pretty new at it, but I poured a backing plate our of aluminum a couple times to see where it was going to shrink, this was a round 7" disk with a 3/4" X 3" hub on the back side where the spindle would screw in at. both times in the aluminum the shrink was in the middle. The first time I poured it I only used one gate and no bob, or riser, the second time I used a bob with two gates and no riser. Both of those castings had big shrinkage in the middle.

When I did it in Cast Iron I used two gates, with a really big sprue, and a hugh riser in the middle. that one poured with almost no shrinkage, I also pour it in a flask that was too small, so I couldn't gate all the way around it with bobs for it to feed from, so the shrinkage was off center.

I think that you have answered your own question when you said that your part is 6X8 and you flask is only 9X9 you need a couple more inches of room so you can get some hot metal all the way around the part, and it will more than likely have to be hot at the top of the pouring range for the alloy your using, and if you can you may want to pour it using a horn pour from the bottom filling from the center outwards towards your bobs and pour fast.

Damn shame a person can't by horn gate forms, nice thing about pouring with one of them is that the metal comes in from the bottom and you don't have a wash out like it would be apt to due with pouring stright down from the top in the center.

How flat does this part have to be, or I guess I should say how close to flat do you have to get it?.
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Harry
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Re: Thick section casting

Post by Harry »

I will be trying to avoid a riser in the middle of the part. They are just an extra pain in cutting them off. It will be milled flat so no big issue with being perfectly flat when casting, just a consistent thickness.

Since I have gotten a couple of good ones out of half a dozen pours I think it really doesnt need much to hit every time, the second riser and bob should do the trick.

I am curious about your suggestion to pour hot. That sounds counter intuitive. I would think the cooler the metal is the less there is to shrink.
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Nudge
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Re: Thick section casting

Post by Nudge »

I would think the cooler the metal is the less there is to shrink.
I would be thinking the same thing, I was on the understanding the cooler the metal the better :?
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_FL_
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Re: Thick section casting

Post by _FL_ »

Go for foundry101 setup. Bigger flask and huge runner bar. Let me know if you need more information.
Rocco
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Re: Thick section casting

Post by Rocco »

I have zero experience with this sort of thing so take this for what it worth, how about a large chill? Embed a large steel or aluminum plate in the sand close to the main cavity of the mold. I think this might cause the thick section of the casting to freeze before the riser does thus allowing the riser to continue feeding molten metal as the thick section freezes hopefully filling the shrink void.
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Harry
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Re: Thick section casting

Post by Harry »

Rocco, I had thought about that too and think it is a good idea. The thing I keep going back to is that about 30% of these came out fine so it could just be a temperature thing as I wasnt paying close attention. Next time I am casting I will try the second riser and use a 12" flask.

This is good practice since tooling plate is one of things on my near horizon, too expensive to buy when all it is is cast aluminum milled flat anyhow. Looking at 3/8 to 1/2" plate for making matchplates on.
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4cylndrfury
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Re: Thick section casting

Post by 4cylndrfury »

Here is my take, just my .02 worth. How deep is the shrink? Lets assume its 1/4". if youre milling it anyway, just make the pattern an inch thick, or maybe an inch and 1/8th. Mill the resulting casting down to 3/4, and melt the chips back into ingots. It may produce a lot of dross, so that might not be the best idea, but its just something that popped into mind.
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Harry
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Re: Thick section casting

Post by Harry »

Most times it is not that severe 4cf. Milling away that much material though would add a lot to the machine time so I am trying to get things as near net as possible with just a small machining allowance.

I shipped the first two of these parts today machined from the good castings I had gotten. In the next few days I will be casting several more and working out the shrink issue. I will be sure to take my camera out to the shop and get some pictures. Pretty certain all this is going to take is a second riser and Bob.

One thing I noticed was I had some very minor gas porosity that showed after I did the mill work. It might be time to start using degasser.
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Harry
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Re: Thick section casting

Post by Harry »

I did not get much time in the foundry yesterday but I did pour one of these in a different flask with a shrink bob at each end. As I looked over the prior failed castings I am more convinced that the problem was only partly shrink and more mold collapse. The small flask I had them in had very little left around the edges to support the sand and using the wooden blocks behind the sand presses on the sand when weighting the mold rather than holding the flask down so I think it just squeezed the cavity closed.

Here are two that had failed on me before.
100_5515.jpg

100_5516.jpg
A shot of the most recent one in front of the other two, you can see the two Bobs on the ends, I poured this in a 7 x 17 flask, it was close along the sides but plenty of room on the ends.
100_5518.jpg
I molded in the cope because it is easier to cut the bobs knowing where the part is but I did not cut them in as close as I could have nor were they as tall as they should have been with the tops barely above the top of the part. When I mount this pattern on a matchplate I will put in locating pins and holes on the plate so I can locate the bobs in the cope in close relation to the part in the drag without overlapping them.
100_5519.jpg
The casting came out useable though. There is some light shrink/wrinkling on what was the top of the part but the thickness is there for machining. I take 0.080" off these and it can be 0.020" of one side and 0.060" off the other, any way it works out so both sides are cleanly milled and final thickness is 0.670" I do these surfacing passes with a 3/8" bit at 300 IPM taking 0.010" off each pass, it takes about a minute for each of these passes. I also mill the outside with an ever shrinking profile starting at 0.020" per pass down to 0.003" for the final passes. It takes 23 trips around to mill the edges and runs about 10 minutes.

This is going to be a drop plate used to mount a router lower on the Z slide. I have to cut the pattern for the actual mount that will bolt to this and also some patterns for fixturing jigs so I can run a lot of these parts through the cnc without spending a ton of time in setup on each one.

Here is a rendering of what the finished product will look like.
Rendering.jpg
Looking to get the total time spent on these down to just over an hour so I can price them at $90 complete or $40 for the drop plate on its own with all the holes drilled and ready to mount on the customers Z slide and use their router mount. Using a larger bit for the surfacing and faster feed speeds all around along with better fixturing and running a number of these at a time to amortize bit changes over many parts I think I can get total machining time down to less that 20 minutes for each finished with holes spotted. It is faster to drill small holes on the drill press and less costly in terms of tooling than milling them out on the cnc so I use a center drill on the cnc to give me a pilot starter hole so the layout is done and the drill centers on its own on the press by leaving the part loose rather than clamping it down. I do clamp a block on the drill press table so if the bit catches and tries to spin the part it is against the block and cannot start spinning so all I have to do is hold it down by hand. Works well for getting accurately placed holes and I have not had a part get loose and start spinning around on me yet.
I'd rather laugh with the sinners than cry with the saints the sinners are much more fun...
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