I bit the bullet and took another stab at it. I remade the sweep for the cope with the CNC router (outside of the bell) to add draft but left the core sweep as it was because it was ok as is. There were a couple of disasters but ultimately I was successful. A couple of the photos show parts of the process. I ram my drag solid and then plunk a 9x9x7" high lump of sand rammed hard in a George Forman rice steamer onto it. During previous sweep attempts the lump was prone to shifting so I impaled it on a gutter spike. Problem solved. I also experienced a dropout of the cope in a previous attempt which required me to start over so this time I used a bunch of spikes in the mold as well. Solved that issue too. The sweeping action yields an accurate shape and leaves a solid but nappy surface, not nice and compacted and smooth like a conventionally rammed petrobond mold. As a consequence when the cope is rammed around the swept sand pattern it does not want to separate. So when ramming up to the top of the cope I set a nickel on the top of the pattern and molded in a piece of 1/2" PVC sticking out of the top of the mold. After striking off I raised the cope about 1/8" and applied a few lbs of downward force on the pipe and nickel until I felt the sand mold release. The minimal damage done to the top of sand pattern was swept away during the subsequent sweeping operation, so no harm done, but there was some damage to the cope from that nappy surface sticking to it. It pulled some sand from the interior of the cope cavity so I had some extra filing to do on the casting. I cast it in aluminum and cleaned it up on the lathe. The accuracy of the sweep gave me a good outside diameter and bevel at the base of the bell so I had an accurate machining reference. I clamped the bell onto the mill and machined 4-90 degree flats onto the stub on top of the bell and mounted it to the lathe with a 4-jaw. It centered up great and spun pretty true. I free handed the had wheels on the lathe and I think I got a pretty good result. I haven't figured out how to get a true surface on the inside top of the bell as I don't have a turning tool or boring bar long enough so I'll have to do it on the mill later. Next is bondo and lots of sanding. Also pictured is my power riddle. The last part I need to cast is the arm. The pattern is ready to go. Split pattern made on the CNC from MDF. Pete
So, a remaining issue for you, if I get your process straight, is that you sweep a male petro cope MOLD and then ram your actual petrobond pouring cope mold over the male swept petrobond mold and then, using the coin, press out the male forming mold. Adhesion of the forming mold to the pouring mold tends to foul the pouring mold surface. If you were able to solidify the cope forming mold you could achieve cleaner separation of the forming mold from the pouring mold. A few things I’d be tempted to try. 1) Shellac the forming mold after sweeping it. I don’t know if shellac might form a crust with a few spray applications, but it just might. Then you could spray on various primers or lacquer or enamel paint followed by silicone release agent, spray wax, etc. 2) If the shellac won’t coalesce on the surface, I’d try making the forming mold with clean (not petro) sand mixed with about 2% epoxy resin binder or polyester resin. For this application I’d probably use a standard 8 to 12 hour resin rather than rapid-set epoxy. Mind you I’ve never tried resin binders, but have read about it. http://jarod.eells.us/projects/FoundryEquipment/sodium-silicate.html “You can also make incredible cores with a two part epoxy binder. I would not use more than a total of 1.5 to 2.0% total epoxy binder and hardener on a 60 to 90 Grain Fineness sand. Let it harden completely before you take it out of the box. Dust the box with baby powder before you ram the sand into it. This will help it to release from the core box.“ Once set, all manner of finishes could be applied and good clean separation achieved. Same for baked linseed. Please forgive me if I have your process all screwed up. It’s still early in the morning here. On the other hand, I may be no more awake by afternoon.... Denis
Exactly correct. One of the selling points of the process is that once you've utilized your cope MOLD to make your cope you return it to the sweep rig and further sweep it down for use as the drag mold/core. In my opinion, and as you suggested in your post, a good route in this case would be to somehow solidify that element by making it out of something other than petrobond and just sweeping the drag/core part as a completely separate operation, probably with a similar material. Note that videos posted by others both in this thread and elsewhere showing bellmaking with sweeps show a building up of curing materials as opposed to cutting down from a mass. Kelly brought up a concern earlier in this thread about the surface finish left as a result of sweep edge angle/design. I saw him use a sweep in some other application before, coating something I think, so his comment was probably borne of his own experience with it. Although smoothness of the edge is imperative no matter what, the geometry of the cutting edge matters and perfecting it both in terms of cutting performance and surface finish for a given media is something I would leave to someone who plans to do it routinely. I doubt it will be me. The "best guess" geometry I used gave me an accurate and smooth cut, but that nappy surface texture was probably as good of a finish as I was going to get with petrobond. And ultimately that surface texture is what really created or at least aggravated the release issue. It made for a very rough surface finish on the casting as well, but it yielded an accurate shape and size with good core alignment. I've spent untold hours finally getting to this point using this method. I'm stubborn like that, but this is absolutely a viable molding method. Someone else driving the bus may have had different ideas and intuition or may have even abandoned the method early in the process, but it works for producing an otherwise unattainable shape. Pete
I got my new aluminum bell pattern bondo-ed and painted last week and cast the bronze today. It was a success. I also cast the support arm as well as another set of mounting hardware. I made the original set of hardware from aluminum patterns and had the dovetails machined. I misinterpreted the drawing and didn't realize the mounting plate had a through-pocket. I found out when I saw HT1's picture above. So I milled the pocket out of the milled part and used it as the new pattern. It worked quite well. Notice the shrink cavity and the cracking on the support arm. That cracking appears in various places all along the edges of the web on both sides of the casting. It's still perfectly useable but I'm kind of stumped about how to prevent it in the future. The shrinkage can obviously be overcome but how about the cracking?
Ya, well, let freedom or is it success ring. Waiting for the video of the bell ringing. Looks great. Way to go. Denis
that cracking looks like a design issue, thin the inner web and make the transition smoother, this image gets you an Idea of the cross-section your looking for , there are some very specific references for the size of fillets, sadly I cannot find one in my reference materials when I have to do something in an I beam shape, this is my go to example , that's right a pipe wrench , good on the eye, good in the hand , take I look at one it is actually a more complex shape then it ppears at first glance V/r HT1 P.S. round things off well, I know that in CAD and 3D work, straights and squares are easier, but in casting it is worth the extra time on the pattern to make things more... "organic" then make many copies from the pattern. And well done !!!
Thanks for the feedback. It makes sense. Well, except for some final sanding or maybe a few file strokes this job is done. I'm going to give it to my sister for her birthday. I'm getting the U.S. letters made as a rubber stamp for the next one that I'll give to my veteran friend. Pro tip: Bronze is REAL grabby. Clamp a stop to your drillpress table!
Petee716: I'm currently looking at making F clamps and G clamps, my foundry mentor/guru has been through this before and has a whole stack of oddball deep throat G clamps in aluminium bronze which shrinks a lot. The I beam cross section of G clamps has a similar shrinkage and cracking problem that even forms holes dues to shrinkage on the inside of the two 90 degree corners. He was able to solve the problem by using metal heatsink blocks embedded in the sand moulds to act as "chill plates" and force fast solidification where the shrinkage was occurring. As far as drilling bronze and brass goes, some people keep a set of drills specifically for brass with the tip "Dubbed" with a pocket oilstone to remove the clearance/rake behind the cutting edges to zero for a small part of the tip. This stops the digging in of the drill bit in grabby materials like brass, bronze and phenolic. This means you need to keep a set of sharp drills for pattern making, a set for brass and bronze, a set for metalworking and an optional sacrificial blunt set for people who wander into the shop and want to borrow a drill for their trailer and then try to drill concrete or plasterboard with it too.
I think the first plan of attack will be to thin that web. It is far more massive than it needs to be, nearly 7/8" thick and the cracks are present in varying degrees over the whole length of the arm. I am also going to incorporate the dovetailed mount insert right onto the arm pattern instead a casting it separately and bolting it onto the arm as I've done here. (It happens that the everdur material I just bought from Lou consists of 5/16-18 bolts so I got off my purist high-horse and used them instead of making them from raw material). But in any event, those bolts become the weak link so I'll cast that part already joined to the arm. This will set up some different feeder requirements anyway, but I'll keep your suggestion in the toolbox! Thanks. Pete
Yes it's everdur 95:4:1. I know that bronze is its own animal when it comes to drilling and I'm starting to learn. And I'm lucky I didn't break it and I'm lucky I got away with a flesh wound during that episode. I have had a rash of broken drills lately though, 3/8" and up. It's usually been while hand drilling poorly clamped steel, generally near the end of the hole.
Seems a mite gummy, then. I'm looking for less drill-grabby-behavior in a copper-based alloy. Come to think of it, 660 bronze seemed a bit grabby, too, and it had ample lead!
Brass, bronze, and some plastics, e.g. polycarbonate/lexan. I took an oilstone and stoned a small flat on the flutes of the drill bits I used for drilling holes in the last item.
The angle and rake on the point can make a big difference. I couldn't see having multiple sets of drills for different materials but I recently inherited a large cache of tooling, including three drawers full of drill bits, so apparently my Father and Grandfather were of a different mind, so now I have choices, but it took me two days to sort them all. Still, if you were going to be doing a lot of drilling it might be worth grinding a drill for purpose. Best, Kelly
When I cast a silicon bronze ball to roll down a drainpipe pulling a string with it, I had no trouble drilling a hole through it... Until I got to the very center. That last 0.5 mm or so murdered no less than 3 drill bits. I think I went to a sharp punch of some kind to poke through it before I could finish drilling the hole. Not sure what happened there. Jeff
you probably had something like a geode ( Google Search it) going on, with some very hard "alloy", or would it be a mixture crystalizing out in the center making HARD HARD spots Does anyone have a equalibrium diagram of Silicon Bronze??? V/r Ht1
I've seen rock geodes and had similar thoughts. This was a lost foam casting, a solid heavy ball cast from an uncoated foam pattern with a fat sprue gated into the top and no other provisions to offset for shrink, so there very well could/should have been any number of weird folds and shrink voids and other things going on inside it. But I don't think there were any voids that I encountered while drilling, unless it was super tiny and at the center of the tiny hard center. In any case, aside from that bit at the center it drilled easily enough. Jeff
Blunting the edge with a stone helps my bits from grabbing and breaking off in bronze. I had the hole from hell on a bronze tree lamp. (sorry, won't publicly post a photo here) pm if you wanna see it It went up a crooked 20" section and needed a 12" long 3/8" drill bit. Several times it tried to break my arm while boring it out! I had to come in from the side of it to drill and then weld up/repair the surgery. Taught me next time to cast in sections! Don't know what I was thinking.
