A new Swivel shooting bench

Discussion in 'Metal casting projects' started by Stevan, Jul 20, 2020.

  1. Stevan

    Stevan Copper

    You may remember that my wife and I were going to Wyoming on a prairie dog hunting trip and I had to cast some new handles for my gun case. Well they worked great! We had a fabulous time, the wife I think enjoyed it more than me. Se is certainly as cold-hearted and blood thirsty as am I. We killed over 100 the first day and over 50 the second day. Here is a photo of her and our equipment. We do our shooting off collapsible shooting benches but one has to shoot in about a 180 degree arc and constantly moving the chair and the bench is a pain. One can buy a swivel shooting bench as pictured in the second photo. These are like $150.00 each...but why buy one when you can spend hours and hours engineering your own.

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    I found I had a piece of 3 inch exhaust pipe for the central tube [Material that I make my oil burner devices out of.] and years ago I tore apart the kids swing set and couldn't bear to throw out the tubes, so I had some nice light, thin wall 2" tubing for the legs. I needed to cast a central part to tie the legs to the central pole. So I designed something up and realized that I couldn't bore out a 2 inch hole for the legs, except on my lathe so I had to include in the design some clamping stubs so that the lathe could grasp the part. This was definitely a part that had to be lost foam....as much as I wanted to sand cast this, as I will be making 2.....There was no way. Plus...I have had bad luck with cores. In picture #3 you see my materials. #4, you see the cardboard pattern I made to make sure the part fit my lathe. #5 I had to try it both ways. #6 you see how I make rounds out of foam. #7 the fully vented pattern...I actually added even more venting, I went a little overboard and actually had a difficult time putting in the pouring cup. The small rounds are for clamping in the lathe and the big ones for the legs, the central hole was for the center pipe.

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    I was very worried that this part was so large that my crucible wouldn't be big enough so I got a bucket, filled it with sand. Poured all the sand in another bucket, then Put the pattern in the first bucket and poured the sand back in. I was then able to see that the remaining sand would fit my crucible. I made several modifications to the pattern [note the lathe stubs are hollow among other things.] To get the part small enough. Seed pictures #8 and #9

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    Well the casting was an unmitigated disaster, my home made crucible cracked and started leaking!!!
    [​IMG] I had no place to pour the metal so I had to try pouring the mold...I ran out of metal. OMG!!! well....not to worry. This pattern was really a little too bulky, the next pattern is almost ready and much more elegant. More in the next installment.
     
  2. Al2O3

    Al2O3 Administrator Staff Member

    Good luck on the next go round Stevan.

    From one lost foam caster to another, the vent straws serve no purpose in lost foam and aren't needed. They cannot vent when blocked by foam and when the foam evaporates they immediately fill with molten metal and cease to be a vent. The entire mold surface is the vent. You might also try adding some vibration to pack the mold/flask. If you get the chance, this thread and three videos at the link document my approach to lost foam. You would not need vacuum assist to succeed.

    http://forums.thehomefoundry.org/in...-the-lost-foam-edition.1020/page-2#post-24356

    Best,
    Kelly
     
  3. Stevan

    Stevan Copper

    All great points....However...you didn't see it in this post but my molds are all coated with plaster. The vent straws are all internally connected with hot-wired galleries and are quite effective. With coated molds, of you don't have venting, more than the pouring sprue and 1 riser, you have problems with getting the metal in to the mold. I probably didn't need to coat this mold with plaster as the finish is not that important....but I like to coat them anyway.
     
  4. Rotarysmp

    Rotarysmp Silver

  5. rocco

    rocco Silver

    Kelly coats his foamies as well. If you haven't already, take a good look at his lost foam posts, what he's been able to do with foam is truly astounding.
     
    Last edited: Jul 21, 2020
  6. Al2O3

    Al2O3 Administrator Staff Member

    Sorry Stevan, but have to disagree with you on all those points and you're barking up the wrong tree if you want to get that lost foam casting sorted out.

    The plaster is porous and does vent, maybe not quite as well as some of the commercial coatings, but pretty close. I used plaster for several years and many, many, castings. It performs very similar to the commercial coating I now use, at least in regard to permeability........if applied sparingly. If I hadn't moved on to dip coating (for speed, uniformity, and repeatability) instead of brush, I'd probably still be using it. With plaster you don't need (in fact don't want) a coating any thicker than is required to cover the foam so it looks white and you cant see color through the coating. Any thicker just makes it less permeable and the perception that a thicker shell provides more support is misguided and unneeded if the loose sand is well packed and vibration is the tool of choice to achieve that. Hollowing a lost foam pattern is not good practice, can destabilize/collapse the mold media, and promotes folds and other casting defects.

    I adapted my approach from studying commercial lost foam operations and the companies that have pioneered the process and sell the equipment for the same. You won't see anything that resembles straw vents or vent holes in foam patterns in commercial lost foam operations. It's just one of those things that green sand casters intuitively think is needed........but it isn't. The other thing that doesn't translate well between the two processes are gating systems. Green sand is controlled by gravity, hydrostatic pressure, and feed system dimensions. Lost foam is almost always controlled by the rate at which foam is evaporated and coating permeability.

    Use a light coat of plaster, and beyond that success will depend on how you position/pack it in the flask, sprue, and feed it......and also pouring temp.

    Best,
    Kelly
     
  7. Stevan

    Stevan Copper

    The Prairie dogs in that part of Montana often have fleas that carry the Bubonic Plague.....so you don't even touch them. It is generally safe to be in the field with them as if the plague was there it goes through the town pretty quick and their ain't nothing to shoot at....but you want to be careful. The bunny hunt looks fun.....but i would not be sanguine about hauling all my firearms to a place where there where so many gun grabbers. I assume NZ is like Australia...Where they take the guns from the law abiding and let the criminals have free reign.
     
  8. Stevan

    Stevan Copper

    Very interesting. I will try lighter coatings and see how it goes.....but I have in fact had good results so far. My crucible failed me this time. One point I will make is that I only stick a hot wire down every vent and connect everything together. I do not try to hollow the pattern. I thought about it but with wall plaster it is just not possible.
     
    Last edited: Jul 21, 2020
  9. Al2O3

    Al2O3 Administrator Staff Member

    Some times things work but not always for the reasons we think they do.......

    Did you use a pouring cup of some kind? or just try to pour onto the cross section of the sprue? Tough to offer much without seeing the fully prepped pattern in the mold and the failed result.

    Best,
    Kelly
     
  10. Stevan

    Stevan Copper

    Here is an earlier pattern that had no straws and was sucessful/... but it was a very simple part.

    [​IMG] Here it is before the pour, [​IMG]

    And here is the picture of my part that failed. There is a vet sprue in here somewhere but it is not obvious, I think I uncovered it after the picture was taken. The can is the pouring cup. It had to be bent because I was not careful with the design of the vents. This cast would have worked....if the crucible had not leaked. [​IMG]
     
  11. Al2O3

    Al2O3 Administrator Staff Member

    Do you have a couple pictures of the failed casting?

    Best,
    Kelly
     
  12. Stevan

    Stevan Copper

    Got the new pattern finished and plastered. AI203....you can see the failed casting in the background of photo #1. I sawed it up and the metal was at least good.....meaning without significant porosity. Robert on Alloy Avenue was kind enough to give me some tips that really helped in reducing porosity...specifically keeping the air fuel mixture lean.

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  13. Al2O3

    Al2O3 Administrator Staff Member

    Everyone has there go-to's for first pass success. I'd sprue and position that part in the flask as you have except I might split the sprue so it feeds opposing sides of the top cylinder for a little more symmetry. I'd use a fairly small sprue, say no larger than 1" diameter or square (this is mostly so it has a enough strength to handle and dip coat it not because it needs 1" of sprue). I'd make sure the part was buried >6" deep and vibrate the flask for good mold compaction. I would not hollow the sprue nor use a riser or vents. I don't think the vents hurt. I just don't think the help. I'd put a pouring basin on top the sprue and then fill the mold to nearly the top of the pouring basin to prevent run outs.

    Making foam patterns is a whole separate subject but looks like you've done a good job to me. Not sure what the wall thickness is on your part but I'd thin them up to about 1/4". I've found it to be much easier to get high quality castings with parts that have high surface area to volume ratios than bulky thick walled parts. A 1/4" wall is plenty strong for a part like that and you have much less foam to evaporate and gas to displace through the same surface area of the coated pattern. I used 2 1/2"D round "kush" cups and a conical foil transition to the sprue diameter for the first two years of lost foam casting but not any more because they tend aspirate and entrain air while pouring. I now use a reusable offset pouring basin and it was a very noticeable improvement.

    The biggest evidence of the efficacy of all this is my pours are comparatively so tranquil now and in most cases no flame, black smoke, or turbulence in the pouring basin at all. The basin fills, followed by a very pronounced pause while the sprue is evaporated, and then more rapidly takes metal once the metal propagation front reaches the pattern features and fans out. A part like yours would have an elapsed pour time of about ~10 seconds. Since I started this practice my first pass yield is nearly 100% and flaw populations near zero. You'd probably have to watch those videos I linked in the saucer cup and spoon thread for all this to make much sense.

    On molten metal prep, yes, lean/oxidizing flame for aluminum. I use a resistive electric furnace so naturally have a good furnace atmosphere. Pouring temps for lost foam aluminum require 150F-200F higher temps than conventional green sand casting. I typically pour at 1450F.

    Sometimes it's amazing how forgiving the process can be, and sometimes it will just kick you in the shins. I've put some effort into my equipment to reduce total process time but for me it is a very fast and efficient way to make prototypic/small quantities of castings with complex features.

    Best,
    Kelly
     
  14. Stevan

    Stevan Copper

    Got a ton of work done but haven't had time to post anything. Got the center casting of my bench made. It came out nice....not as nice as I would have hoped but a usable casting. First pic is the unfettled casting, #2, fettled, #3 on the lathe. #4, with the center post and the legs installed.
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  15. Stevan

    Stevan Copper

    The center section, while a usable casting was a little on the Rough Side. It worked because I gave myself a lot of thickness to the casting walls to allow for the inaccuracy off my pattern. I looked with great interest at the pattern machining setup that Al 203 had and also his good advice and I decided to try to duplicate some of what he did so I got a routing cutter for my drill press and made a table to go on top of my drill pres. I wanted a table that would have a pin on it like his. I then made pattern for creating items like he had. Not near as good as his but good enough to improve the quality of my patterns like 500%. Here's some pictures of the setup. The first one shows a platform with a ring screwed to the bottom of it and a few additional rings that I will use for other projects. When I want to change the pattern I'm making I will unscrew one ring and screw in a different one. They are sitting on top of a wooden table which I clamp to the surface of my drill press and you can see the small pin sticking up that registers to the various rings that I would screw to the board.v The pattern holder is shown upside down. I then take a piece of foam and spray contact cement it to the top of the Pattern holder. This is not near as elegant as that made by al203 but good enough to see if it works for me. Picture number 2 shows the table clamped onto my drill press and a pattern being machined. Photo number 3 shows a close up of a piece of pattern being machined. Photo number 4 shows all the pattern parts for the rest of my shooting bench project. You've got two seat holders, 2 table holders and two safety rings. These parts are all much thicker than they need to be. It turned out that they came out really accurately. If I was to do these again they would be about half the thickness.

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    Tobho Mott likes this.
  16. Stevan

    Stevan Copper

    I decided to go Whole Hog on AL 203's ideas and I only gave the pattern one coat of plaster. I thined my plaster down so that I could dip the patterns in it I diped them in the one-time pulled them out let them dry. It was a lot easier than spending a week building the coat out to 1/4 inch thick. I then put them in the sand and did the cast. I decided to use just regular tin cans for pouring cups because I didn't have an insulated one like al203 had. I took regular tin cans and cut one inch holes in the bottoms. notice that two of the buckets have an extra can below the pouring can. This is because the sprues stuck up above the sand. This turned out later to be a big mistake. You want your sand to go all the way to the bottom of the pouring cup. You have to have sand to weigh down the cast otherwise you can have a disaster as the weight of the aluminum in the pouring cup will cause the sand to rise. I knew immediately when I poured one of these that I had a problem because the sand moved. I did discover that al-205 advice was absolutely correct. I had the least stressful castings that I have ever had with lost foam. I did not have all the burping and Flame that I have had in the past note the picture number 2 which shows the seat casting and two ring castings that came out perfectly. The problem I had was with the tabletop casting which was a little larger and was not weighted down with sufficient sand so it moved. As you can see the center looks kind of wacky looking and you can see where metal extruded out from the shell coating. In a situation like this you're faced with a dilemma do I machine it and correct it or do I make another one. I decided I'd go ahead and try to clean this up. The next picture shows the bad one the next picture shows the bad casting with the top sawn off and then the following photo shows the casting clamped onto the faceplate of my lathe. It machined out okay but I still had a section inside that had moved and needed to be filled so if you look at the final photograph you can see that I fixed it with Bondo. Bondo machines quite nicely just like aluminum so you just slather Bondo in there and then machine it away. Worked good enough I now have a usable casting. Although it took quite a bit of time and effort on the lathe to fix it, it may have been easier just to make another one. The final picture shows all my finished castings all machined and ready to go. [you can see the bondo inside the bad one. I still need to make 1 more center casting but I will wait till I see how the project comes out.

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    FishbonzWV and Tobho Mott like this.
  17. Al2O3

    Al2O3 Administrator Staff Member

    Those look good Stevan. You need to get yourself some deeper flasks so you can fill the sand up to the top of the pouring cup.........it will help prevent run-outs or sand float. Cant remember, did you make yourself a vibrator to pack those molds? Keep up the good work.

    Best,
    Kelly
     
  18. Stevan

    Stevan Copper

    I just jiggle the heck out of them on the concrete.
     

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