HiYa Guyz,
I thought I would split up my questions to make it easier to search in the future!
Question 2:
I have been given access (verbally at least) to 2 rare British heads that I wish to copy. These heads being rare and valuable the owners are a little reluctant to letting me have them for any lenght of time..understandably so. So once I get them I think I would need to make a rubber mould of the heads and cast a copy out of resin to buld my pattern from and ship their heads back to them ASAP! Which rubber should I use (one needs to be very flexible for the intake and exhaust ports to remove)..many different ones out there, which resin should I use or another medium? I am very interested in keeping it dimensionally as close as possible.
Part 2 of the question, I seem to recall that the shrink value of aluminum(for the scale I am using is 2% is this true? Is there any way I can use part that comes from the rubber mold for the final pattern?
And lastly where can I find really good imformation on lost foam casting...most of the stuff I have found on the net so far is buy our product its the best and very little of anything else.
Thanks in Advance again!! [img]/ubb/images/graemlins/grin.gif"%20alt="[/img]
wanderer
Gentry's Garage
gentrysgarage@yahoo.com
Precision Patternmaking question
Moderator: Harold_V
Re: Precision Patternmaking question
Gentry's Garage,
I take it from your previous post that the heads you are talking about are for a motorcyle. Well, do you realize that you will need to make a core for the intake and exhaust? Or are you doing something different?
Ron
I take it from your previous post that the heads you are talking about are for a motorcyle. Well, do you realize that you will need to make a core for the intake and exhaust? Or are you doing something different?
Ron
Re: Precision Patternmaking question
HiYa Ron,
Yes these are motorcycle heads [img]/ubb/images/graemlins/smile.gif"%20alt="[/img]! Yes again on the seperate cores [img]/ubb/images/graemlins/smile.gif"%20alt="[/img]. The problem is how to do it. The way I am thinking is to use a very flexible rubber as thick as I can make it and peal it out of the ports, then fill it with a stiffer rubber increase the size 2%ish (with a hot wax dunk???)then make a core box mould. Sound good or am I all wet?
Thanks again Ron for your reply [img]/ubb/images/graemlins/grin.gif"%20alt="[/img]
wanderer
Gentry's Garage
Yes these are motorcycle heads [img]/ubb/images/graemlins/smile.gif"%20alt="[/img]! Yes again on the seperate cores [img]/ubb/images/graemlins/smile.gif"%20alt="[/img]. The problem is how to do it. The way I am thinking is to use a very flexible rubber as thick as I can make it and peal it out of the ports, then fill it with a stiffer rubber increase the size 2%ish (with a hot wax dunk???)then make a core box mould. Sound good or am I all wet?
Thanks again Ron for your reply [img]/ubb/images/graemlins/grin.gif"%20alt="[/img]
wanderer
Gentry's Garage
Couple of questions
How big are these parts?
Aluminum shrinkage is generally 3/16" per foot. If the part is small enough, you may be able to accomodate the shrinkage without modifying your pattern.
Your best bet for accomodating shrinkage is to build up the pattern where needed by coating it with gelcoat to build up the dimensions you need. If you're going to investment cast the heads, then no coreboxes are needed. All you do is pour wax into the silicon rubber mold, strip the rubber out of the wax and embed the wax in a plaster slurry.
I've done a little lost foam casting using patterns wade from hot wire cut styrofoam inulation. It works better than I expected. I was pouring aluminum into green sand molds and saw only a very slight whiff of smoke from a vent when the metal hit the foam. Good venting is critical to getting lost foam to work well.
Aluminum shrinkage is generally 3/16" per foot. If the part is small enough, you may be able to accomodate the shrinkage without modifying your pattern.
Your best bet for accomodating shrinkage is to build up the pattern where needed by coating it with gelcoat to build up the dimensions you need. If you're going to investment cast the heads, then no coreboxes are needed. All you do is pour wax into the silicon rubber mold, strip the rubber out of the wax and embed the wax in a plaster slurry.
I've done a little lost foam casting using patterns wade from hot wire cut styrofoam inulation. It works better than I expected. I was pouring aluminum into green sand molds and saw only a very slight whiff of smoke from a vent when the metal hit the foam. Good venting is critical to getting lost foam to work well.
Re: Couple of questions
HiYa JP!
At the most they are a foot in the widest dimension.
What type of gelcoat are you suggesting is it brush on or a dunk affair? I would think it would be easy to remove but would it hurt aluminum?
In making the silicon mould would I cut it to remove the intake and exhaust tracks from the pattern piece? Investment casting would be the easiest to cast but would it hold up to tight tolerances (about er 15-20 pounds of aluminum?)
I think that the foam might work for me but I would need something that would have to made in the silicon mould do you know how it is done...Thanks for bearing with me! [img]/ubb/images/graemlins/grin.gif"%20alt="[/img]
wanderer
Gentry's Garage
At the most they are a foot in the widest dimension.
What type of gelcoat are you suggesting is it brush on or a dunk affair? I would think it would be easy to remove but would it hurt aluminum?
In making the silicon mould would I cut it to remove the intake and exhaust tracks from the pattern piece? Investment casting would be the easiest to cast but would it hold up to tight tolerances (about er 15-20 pounds of aluminum?)
I think that the foam might work for me but I would need something that would have to made in the silicon mould do you know how it is done...Thanks for bearing with me! [img]/ubb/images/graemlins/grin.gif"%20alt="[/img]
wanderer
Gentry's Garage
Re: Precision Patternmaking question
Wanderer, Hold on a minute, Your thinking wrong.
Imagine that you want to cast an aluminum rectangle, say, 12 x 6 inches by 1 inch deep. In the center of the rectangle you wish there to be a 5 inch hole. The overall pattern will have to be larger to accomodate for shrinkage, however, the hole will have to be smaller for as the ingot cools and shrinks so too will the hole shrink by getting LARGER. The aluminum shrinks towards the center but the aluminum around the hole will shrink towards the sides making the hole grow larger.
Whenever a pattern is made, it is built oversize to accomodate for the shrinkage, if any ports, openings, holes are needed in the casting that will require a core, the opening will get bigger thus the core will be smaller than the finished hole in the casting. That means that the core has to be smaller than the desired opening you want. Sometimes you can ignore it, sometimes you can't.
Ron
Imagine that you want to cast an aluminum rectangle, say, 12 x 6 inches by 1 inch deep. In the center of the rectangle you wish there to be a 5 inch hole. The overall pattern will have to be larger to accomodate for shrinkage, however, the hole will have to be smaller for as the ingot cools and shrinks so too will the hole shrink by getting LARGER. The aluminum shrinks towards the center but the aluminum around the hole will shrink towards the sides making the hole grow larger.
Whenever a pattern is made, it is built oversize to accomodate for the shrinkage, if any ports, openings, holes are needed in the casting that will require a core, the opening will get bigger thus the core will be smaller than the finished hole in the casting. That means that the core has to be smaller than the desired opening you want. Sometimes you can ignore it, sometimes you can't.
Ron
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Doug E.
Re: Precision Patternmaking question
Ron,
Is this one of those phenomenoms like that a vortex turns a different direction south
of the equator than is does north of it? Because when I work with shrink fits like
tires on loco wheels, I heat the tire to expand it and enlarge the ID of the tire to enable
it to fit on the wheel before cooling, when the ID shrinks and gets smaller. Or does this
only work this way on the west coast, and on the east the holes get smaller when
heated? [img]/ubb/images/graemlins/smile.gif"%20alt="[/img] The foundries I work with like me to add shrink to the cast in holes. We
make cores smaller only to leave machining stock.
Regards,
Doug
Is this one of those phenomenoms like that a vortex turns a different direction south
of the equator than is does north of it? Because when I work with shrink fits like
tires on loco wheels, I heat the tire to expand it and enlarge the ID of the tire to enable
it to fit on the wheel before cooling, when the ID shrinks and gets smaller. Or does this
only work this way on the west coast, and on the east the holes get smaller when
heated? [img]/ubb/images/graemlins/smile.gif"%20alt="[/img] The foundries I work with like me to add shrink to the cast in holes. We
make cores smaller only to leave machining stock.
Regards,
Doug
Re: Precision Patternmaking question
Doug,
I appreciate your humor [img]/ubb/images/graemlins/grin.gif"%20alt="[/img], but you are not talking apples to apples here. We're trying to give an educated answer in a short paragraph. The solution to prevent the hole from enlarging requires a well to be added to the gate. That will provide a source of metal to flow into the casting as it cools (shrinks) and prevent a cooling dimple from forming and reduce the enlargement of the hole that I discussed previously. Your example of heating a wheel is not the same as a casting. As the casting cools in the mold it will pull away from the form, in all directions. If you take that same casting and heat it up, the metal will expand uniformly and the ID will increase, but so will the OD. See the difference?
Ron
I appreciate your humor [img]/ubb/images/graemlins/grin.gif"%20alt="[/img], but you are not talking apples to apples here. We're trying to give an educated answer in a short paragraph. The solution to prevent the hole from enlarging requires a well to be added to the gate. That will provide a source of metal to flow into the casting as it cools (shrinks) and prevent a cooling dimple from forming and reduce the enlargement of the hole that I discussed previously. Your example of heating a wheel is not the same as a casting. As the casting cools in the mold it will pull away from the form, in all directions. If you take that same casting and heat it up, the metal will expand uniformly and the ID will increase, but so will the OD. See the difference?
Ron
-
Stephen Thomas
Re: Precision Patternmaking question
.Ron said " If you take that same casting and heat it up, the metal will expand uniformly and the ID will increase, but so will the OD. See the difference?"
Sorry Ron, I'm with Doug. I don't see the difference. Cores, as far as my experience, come out crushed. You don't want to make a core too strong, or metal shrinking around it will tear, as the hole shrinks. smt
Sorry Ron, I'm with Doug. I don't see the difference. Cores, as far as my experience, come out crushed. You don't want to make a core too strong, or metal shrinking around it will tear, as the hole shrinks. smt
Re: Precision Patternmaking question
Thanks for all the replies! It is leading me in the right direction, I am interested in lost foam casting but it has to be a castable foam as the parts to be cast are pretty intricate. I was also wondering is there a way to know exactly what the shrinkage would be (as to minimulize the needed machining at the end?
Thanks Again
wanderer
Gentry's Garage
Thanks Again
wanderer
Gentry's Garage
Re: Couple of questions
With a 12" dimension, you would have to accomodate the 3/16" shrinkage.
Gelcoat is just a resin used to put a smooth surface on fiberglass parts. You paint or spray it on to a smooth surface and it copies the texture of the surface you put it on. In this case you are building up selected surfaces to make the part 3/16" larger.
To make one or two parts, it's hard to justify the amount of work needed to get to the first casting.
If I were doing this, I would get some two part silicon rubber molding compound from either a pottery supply or a jewelry making supply. Spray your starting casting with silicon lubricant as a mold release. then bed the part in modelling clay up to the parting line on the casting, set up a containing frame around the part and pour in the silicon rubber and let it cure.
Then turn it over, remove the clay, respray the cured silicon rubber and casting with the silicon lubricant and pour more silicon rubber to make the second half of the mold.
Once the silicon rubber is cured, peel it off of the casting. You can cut a channel into the silicon rubber cavity. If you can accepta part 3/16" smaller across than the original, you can pour wax into the mold to make a lost wax casting.
If you can get two part styrene foam mix, you could pour that into the mold for a lost foam casting. I have seen spray styrene foam, but don't know where to get the resin mix.
Otherwise the problem with a poured lost foam pattern is that you have to make a mold to pour the foam resin into.
If I were doing this and planned to make several parts, I would take measurements of the part and either make a pattern or else create a solid model and makea rapid prototype to use as a pattern. Then make the silicon rubber mold to make wax patterns. The hard part is the software to create the solid model is not cheap, Alibre starts at $450. The rapid prototype will most likely cost about $1000, but you can make it exactly the size you need.
The project is certainly doable, but either you will need to put in a lot of sweat equity making patterns or molds or be willing to sink in some serious bucks.
AAS an aside, I have a project I am currently working on to make a rotating part that will run about 10,000 rpm. The design for the part was done in Solidworks and I purchased three rapid prototypes, one for show, one to run and one to use as a pattern to make aluminum castings. The parts are about 4 1/2" diameter and will weigh about 8 ounces in aluminum. the rapid prototypes ran about $225 each, but match the solid model file within .010. I will probably cast 6 of the things in zinc-aluminum alloy for run testing.
Gelcoat is just a resin used to put a smooth surface on fiberglass parts. You paint or spray it on to a smooth surface and it copies the texture of the surface you put it on. In this case you are building up selected surfaces to make the part 3/16" larger.
To make one or two parts, it's hard to justify the amount of work needed to get to the first casting.
If I were doing this, I would get some two part silicon rubber molding compound from either a pottery supply or a jewelry making supply. Spray your starting casting with silicon lubricant as a mold release. then bed the part in modelling clay up to the parting line on the casting, set up a containing frame around the part and pour in the silicon rubber and let it cure.
Then turn it over, remove the clay, respray the cured silicon rubber and casting with the silicon lubricant and pour more silicon rubber to make the second half of the mold.
Once the silicon rubber is cured, peel it off of the casting. You can cut a channel into the silicon rubber cavity. If you can accepta part 3/16" smaller across than the original, you can pour wax into the mold to make a lost wax casting.
If you can get two part styrene foam mix, you could pour that into the mold for a lost foam casting. I have seen spray styrene foam, but don't know where to get the resin mix.
Otherwise the problem with a poured lost foam pattern is that you have to make a mold to pour the foam resin into.
If I were doing this and planned to make several parts, I would take measurements of the part and either make a pattern or else create a solid model and makea rapid prototype to use as a pattern. Then make the silicon rubber mold to make wax patterns. The hard part is the software to create the solid model is not cheap, Alibre starts at $450. The rapid prototype will most likely cost about $1000, but you can make it exactly the size you need.
The project is certainly doable, but either you will need to put in a lot of sweat equity making patterns or molds or be willing to sink in some serious bucks.
AAS an aside, I have a project I am currently working on to make a rotating part that will run about 10,000 rpm. The design for the part was done in Solidworks and I purchased three rapid prototypes, one for show, one to run and one to use as a pattern to make aluminum castings. The parts are about 4 1/2" diameter and will weigh about 8 ounces in aluminum. the rapid prototypes ran about $225 each, but match the solid model file within .010. I will probably cast 6 of the things in zinc-aluminum alloy for run testing.
Re: Precision Patternmaking question
3/16" per foot.