FIRST robotics CNC router

Discussion in 'Lost foam casting' started by garyhlucas, Sep 8, 2019.

  1. garyhlucas

    garyhlucas Silver

    I am starting a new thread on this topic because I am building a 24” x 48” CNC router for the local high school Mercury 1089 FIRST robotics team.

    Yesterday I cast 5 plates for mounting casters onto the router frame and they all came out quite well. The improvement over previous efforts came from a thinner mud coating on the foam, and a longer sprue.

    All but one had porosity when I cut off the sprue. I believe it was the last one without porosity. I had a burner overheat problem and was trying to restart it but it wouldn’t fire up. So I went ahead and poured it to empty the crucible. So likely it was colder than the others that I poured at 1300F.

    The aluminum was bright and shiny under the mud. What to do about porosity?
     

    Attached Files:

    Tobho Mott likes this.
  2. garyhlucas

    garyhlucas Silver

    I am a design mentor for team and joined last season because my 15 year old grandson joined. I found out they had no idea how to use 3D modeling so I have switched them over to SolidWorks.

    These plates are for mounting casters the bottom of the frame. Next I will cast the mount for the router, another easy part. Then finally I want to cast the carriage for the Y axis, and the vertical Z axis frame.
     
  3. Al2O3

    Al2O3 Administrator Staff Member Banner Member

    1. What's the orientation of the part in the flask?
    2. What does the sprue, feed system, and pouring cup look like?
    3. How are you measuring the pour temp?
    4. What's the source metal? Did you degass?
    5. What is the furnace atmosphere/burner tune?
    6. Is it a dry or humid foundry environment?

    It all matters.

    Best,
    Kelly
     
  4. Gippeto

    Gippeto Silver

    Can't remember if you're in Canada or not Gary, but I got these degassing tablets from Inproheat in Edmonton...1 tablet does 100lbs, so it's surprisingly affordable to use even at a hobby level. Hoping to get back to casting some parts soon so I can try them out.

    Al
     

    Attached Files:

  5. garyhlucas

    garyhlucas Silver

    Kelly,
    Sorry for taking so long to reply, been busy and also researched some of what you said. Here are the answers to what you asked.

    1. The parts were oriented angled upwards at maybe 30 degrees from vertical
    2. The sprue was rectangular about 1" x 1-1/2" and about 4" tall. The pouring cup was aluminum foil tape in a rectangular slight cone shape.
    3. I have a nice thermocouple pyrometer with a stem long enough to reach the bottom of the crucible through the vent hole.
    4. The source metal was nice clean 356 alloy from the Mercedes light pole base.
    5. I did not degas. Is this something you do all the time? How do you do it? I see there are several ways. I don't have a welder so no argon or even nitrogen.
    6. Furnace is burning propane with an invisible flame coming out the exhaust, sound very smooth.
    7. The day I poured it had rained for the previous 3 days and was quite humid. The furnace is outdoors under a tarp and nighttime condensation is probably occuring too. I see that aluminum has a huge affinity for hydrogen. Started up the furnace with the crucible and some metal already in it. Maybe makes sense to bring furnace and crucible to temperature before introducing the metal? Use the rack I made to go over the exhaust to do a little preheat and drive off the moisture, or would that make things worse?

    Thanks for your advice.
     
  6. FishbonzWV

    FishbonzWV Silver Banner Member

    It sounds like you are running your burner too rich.
    While all the other items Kelly listed count, the atmosphere in the furnace affects the melt the most.
    Notice the one without the the holes? You pulled it early, it didn't have time to absorb the hydrogen while in the molten state.
    Try leaning out your burner, less gas, more air.
    And cut down the 'hang time' that it stays in the furnace, pour as soon as you can.
     
  7. oldironfarmer

    oldironfarmer Silver Banner Member

    I firmly believe you get the cleanest melt by putting the charge in the crucible before heating. Aluminum has the lowest affinity for hydrogen right at the melting temperature. Just like with air in ice, the hydrogen will escape at the melt front.

    How are you measuring the temperature. For me the only accurate way it with the burner and blower off. Trying to measure as it is still heating will give you erroneous reading.

    That being said lost foam seems to be quite unpredictable as far as porosity is concerned. The good news is that amount of porosity is not damaging to the utility of the parts, it's just aggravating that they are not clean castings.
     
  8. Al2O3

    Al2O3 Administrator Staff Member Banner Member

    Gary,

    In no particular order:

    1. Use clean metal - Sounds like you had good stock. Oils and paints are source of H2. THey usually volatilize before the melt unless you are adding dirty stock to a crucible that contains molten metal.
    2. Pour as cold as possible - If you are really pouring at 1300F, you are doing well with lost foam. I'm usually 100-150F hotter than that, and in general, I think lost foam requires higher pour temps than conventional sand casting to melt and evaporate the foam. Most of my parts are 1/4" wall with high surface area to volume ratio so will tend to need a little more heat.
    3. Minimize exposure time in molten state - I don't think there is a huge difference between charging the crucible cold vs hot. I do think preheating any stock that you will add to a partially filled melt is advisable because adding cold stock extends the molten metal exposure. Don't skim dross until just before you're ready to pour. H2 is soluble I AL but not in Al2O3 and dross cana ct as a diffusion barrier to H2. Some cover fluxes can also be beneficial in this manner but more useful for separating dross and impurities from the melt.
    4. Minimize exposure/sources of H2 - I agree with Bonz that furnace tune can be very significant and maybe mostly so in your case, but I would not discount the contribution of atmospheric humidity. You've probably driven out residual humidity on the surfaces of your furnace by the time the metal melts but you are introducing a very large volume of water laden air with your combustion gas. Not much you can do about that other choose your days wisely, use a crucible cap and blanket with inert gas (impractical for most home/hobby casters), or degas.
    5. Gate in manner that promotes direction solidification. - This sort of happens naturally in top fed lost foam casting with vertically oriented castings. Barring massive sections, the bottom of the part gets the coldest metal, freezes first and the portion near the sprue last. It's also why if you have porosity, it tends to be in the sprue and where it's attached. Have you section one of those parts in different locations top to bottom? It's not certain that the porosity is H2 but probable.
    6. Degas - I must confess, I rarely degas. But, I use a resistive electric furnace so have no fuel, no combustion air, and even believe the furnace atmosphere to be (somewhat) depleted in O2. In any case, I'm not introducing large (in fact not any other than what can diffuse into the furnace) volumes of H20 via combustion air. I also use virgin ingot for most things that matter. When I was using mostly scrap casting stock with multiple melt cycles and sprue stock, I did see some porosity, especially on metal that had many re-melt cycles. I built a degassing lance and used Argon because I have welding gas on hand. It works well but is a pain in the ass to use and I suspect even more so with a fuel fired furnace. Link to that is below. I almost never see signs of visible porosity when I degate. One exception was when I cast the Tail Light Housings for Jack. Those patterns had a lot of material other than foam in them which may have been a factor and I poured at very high temps because Jack had a lot of time invested in the patterns and I had one shot. They had some surface porosity.
    Since you don't have welding gas, tablets may be more practical. I thought Budget Casting Supply used to sell Chlorinated degassing tabs but I don't see them on their site. They do list cover flux.

    https://www.budgetcastingsupply.com/Aluminum-Flux-and-Degasser-s/1827.htm

    Some folks claim to have used pool shock with success. I've never used it.

    Here's the link to my degassing lance build:

    http://forums.thehomefoundry.org/index.php?threads/degassing-lance.204/

    Best,
    Kelly
     
  9. Al2O3

    Al2O3 Administrator Staff Member Banner Member

    Why is that OIF?

    Best,
    Kelly
     
  10. oldironfarmer

    oldironfarmer Silver Banner Member

    Because you are not submerging stock which has contaminates on the surface into the melt. I have some very clean pours from very porosity laden ingots by melting the ingots in a dry environment. I only add clean ingots to a pool of metal.

    I did some tests where I melted known porosity filled ingots and poured very clean castings. The tubes for pressure testing were some of those. My standard procedure now is all scrap makes ingots and those ingots only go into a clean crucible. Makeup after melt is from remelted ingots which are clean. I do not get progressive porosity with multiple remelts. Verified by testing.

    In my mind I have proven that the melt front will release hydrogen as an ingot melts if it is not submerged in liquid. When you look at the curves the solubility is very low at 1,250F. If you warm from there with a cover like Al2O3 you don't gain porosity. Dunk in a dirty ingot and the hydrogen has no place to go. Dunk in a clean ingot and the surface oil and porosity in it still has no place to go but you still get very good results.

    This is all sand casting but it is the same for lost foam unless the porosity in the lost foam is coming from the lost foam process due to elevated temperature, contaminants, etc.
     
  11. Al2O3

    Al2O3 Administrator Staff Member Banner Member

    I took the question as to whether charging the crucible cold or hot affects porosity rather than composition and cleanliness of the charge. Without regard to the latter I was merely commenting there was probably only a small difference in exposure time from the point the charge in either case became liquid until it reached pouring temp. Cleanliness would have been under my reference to minimizing exposure to sources of H2.

    That nasty SS one that was half absorbed into 2oo+ melts with all the crud in the bottom stopping up the leak? or a new one of those?......just a playful jab there OIF. Pokin' the bear :). I start every melt with a clean crucible too.

    I think that first part is true, except I wouldn't qualify it with ingot submersion bit. Solubility works both ways.......raise the temp and get higher H2 solubility; lower the temp and lower the amount of H2 in solution at equilibrium.

    As the true Bifilm Theory disciple that you are, I see where you're coming from, and is where I was going with the directional solidification bit, but in real life you rarely get perfect directional solidification.....but we try. Solubility is also a function partial pressure of available H2.....and you're green sand guy right OIF? So you've got steam in contact with your molten metal in the mold. Not so in lost foam and or V-Process and their claims on the benefits of no binder.

    Well low isn't none. Two types of H2 porosity which are inter-dendritic porosity, which is what we're talking about at the metal front and optically visible, and secondary porosity which is sub micron and you wont see optically but I think for most hobbyists and this thread we're primarily concerned with visible defects. Al2O3 forms instantly on the melt surface but I don't think it's a perfect diffusion barrier and if it is, why doesn't it keep the H2 in as well?

    It's a pretty bold claim considering I don't think you actually measure hydrogen content, do you? -Just optical observation of visible defects and sometimes under magnification, correct? H2 porosity isn't the only form and cause of defect. There can be many forms of oxides produced in repeated melts that are not easily removed, especially in scrap metal, but most people just lump everything into porosity and I think most foundry men would class what we call porosity as severe porosity.

    The die casters have some advantage because they can rapidly cool the die. For sand shell, and investment we take what we get. All the reading on lost foam, there is discussion about what polystyrene decomposition and some even list those by products which they state as being insoluble in Al. After all, it's plastic.....just a hydrocarbon chain....C8H8. So if H2O dissociates upon contact with the molten metal surface, why not C8H8? That hydrogen bond in H2O is a damned strong one. Maybe it does....or maybe it never gets hot enough to do so because there is supposed to be a liquid layer of PS between the melt and gas phase stuff and maybe the gas phase stuff diffuses out before it dissociates. Dunno, but doesn't seem to bother me.

    Seems to me like Gary just doesn't want castings with pock marks. Furnace tune and temp may get him there. I'm just feeling a bit contrary and professorial today :D

    Best,
    Kelly
     
  12. FishbonzWV

    FishbonzWV Silver Banner Member

    You guys get carried away...turn your burner down Gary.
     
  13. garyhlucas

    garyhlucas Silver

    I think you guys are missing an important point with regards to our furnace. Unlike most homebuilt furnaces this one has the crucible sitting with the top of the crucible flush or even a little higher than the bottom of the large lid you open for pouring and there is a second lid for charging with that opening almost completely blocked by the top of the crucible when it's open. There is about a 1" hole in the center best described as an observation port because that is not where the exhaust comes out! The exhaust port is on the side of the furnace about 3" down from top of the furnace body and the burner is tangentially mounted so the flame whirls around the crucible and goes out. Very little heat comes out the top hole.

    I have been running with the air full blast and I turn down the gas until there are no colored flames indicating a rich burn. So I really don't think the burner is contributing hydrogen to the melt. All the other things, humidity, damp metal, dirty metal etc. Yeah I can see those things as something I need to improve. I am thinking the electric toaster oven I have that goes to 450 F as being a good clean preheat for the metal to dry it before it goes in the crucible.

    One problem I have is I still haven't built an ingot mold for the excess metal and for cleaning scrap as a first step. Trying to find some cheap angle iron and haven't managed that yet. I might make it from some 2" SS pipe that is available and I can split in halves. I have access to a Marvel series 8 tilting head bandsaw that makes easy work of cutting it up.

    So one question that may be really important. How fast do you pour? Do you want to get it all in quickly as possible? Or do you do a slow steady pour to minimize turbulence and to allow the foam to burn out making room for the metal? This seems like it might be important.

    Also something I failed to mention. I have been using a bottom pour crucible which has a pipe up the side so feeds from the bottom. You can't scoop dross out of the pipe so I just pour a small amount off at the start to get rid of any dross. Could this crucible be a problem? It seems like a good concept. My grab & pour tongs hold the crucible really well so after all the metal is gone I flip it completely upside down and tap the tongs against the bricks to knock any remaining material out.

    Finally I came up with a neat idea over the weekend. I am going to cast a bracket for a 4-1/2" grinder to mount vertically in the router bracket on the CNC mill, and the Router I am building for the FIRST team. On the router this will allow cross cutting and ripping plywood using a 6-1/2 carbide blade. It should also allow us to do accurate cross cuts on aluminum tubing and bar stock. Would be expensive and time consuming to do from a solid block of aluminum on my tiny CNC. About 10 minutes machining the foam though, and I'll knock out 2 or three in case I bungle the first one. I really get a lot of use out of my little CNC. I happened to look at the run time counter the other day and I see I have way over 1000 hours on it. I model up all this stuff in SolidWorks which I use at my job, and programming the CNC for parts like this adds maybe 10 minutes before I am cutting.
     
  14. FishbonzWV

    FishbonzWV Silver Banner Member

    Don't second guess everything you're doing. It's you burner.
    How do I know? I produced castings with holes in them for two years. Ever try to polish metal with holes, it sucks.
    One day I needed extra time to fiddle with a mould after I had started the furnace. I turned my gas down and no more holes.
    It's a fine line between rich and lean, once you find it, you'll know what I mean.
     
  15. Al2O3

    Al2O3 Administrator Staff Member Banner Member

    Remind me Bonz, you're propane for fuel and is it a naturally aspirated or forced air burner? When you say turn my gas down, is it just reducing the propane supply pressure to your gas orifice with constant air flow or are you reducing combustion air volumetric flow as well?

    Best,
    Kelly
     
  16. garyhlucas

    garyhlucas Silver

    I just looked at the left over aluminum from this same pour that went into a stainless pan making a disk about 1" thick. Cut it in half so it will go back in the crucible and it has NO porosity! I am wondering if my question above about pouring speed is relevent.
     
  17. Al2O3

    Al2O3 Administrator Staff Member Banner Member

    That's common. Last pour is coldest metal and it freezes very quickly in the ingot tray so what gas is left and coming out of solution doesn't have as long to migrate. Are you going to use all plates? If one is a spare you might section somewhere other than the sprue. It's a very large sprue for the part and the rest of the casting may look much better. I never use larger than a 1" diameter sprue and am pouring 15lb+ parts. I'd use even smaller but they are fragile and I use them to hang the part to dry the coating.

    Your furnace does have some interesting features. I was reading at the MIFCO sight and it talks about it being a combo pot and crucible furnace and it uses a cast chrome iron melting pot in the lid for aluminum, lead, lower melt point alloys. I can see the utility of the smaller sub-lid for adding charge, taking melt temp etc.

    Although having the crucible near the lid with the furnace gas exiting out the side may provide some utility, I'd suspect the gas atmosphere above the crucible/melt is essentially the same as that exiting gas furnace.

    A couple thoughts on furnace atmosphere and tune. Propane is C8H8. The air/fuel ratio for stoichiometric burn is ~24:1 by volumetric flow rate. You can get about 2500BTUs out of a cubic foot of propane. If you go to the psychrometric charts, at 95F and near 100% relative humidity you'll put about 2.3lbs (16oz of liquid water) through your furnace every hour for every 100kbtus of propane you perfectly burn. That's a lot of H2O. If you are only at 50% RH only half that much water and so on. The amount of water in air roughly halves for every 20F drop in ambient temp.......so half as much at 75F and one forth as much at 55F. The amount of water in air at/below freezing becomes very small.

    When your burner tune is rich there isn't enough oxygen to make all the Carbon CO2 and Hydrogen H20. There's not as much hydrogen in the unspent fuel as there is combustion air water but the partially spent fuel probably exists as H2 in the absence of oxygen whereas the H20 in combustion air must be dissociated to liberate the H2 which makes the rich atmosphere undesirable.

    That's all great but solubility of H2 in aluminum is a strong function of temperature. So lean out your burner tune, pour cold as possible, and on cool dry days. Beyond that degas.

    ...but Lost Foam will always like hotter metal.

    Best,
    Kelly
     

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