Hi all, well, this is my first post on this forum!
I have recently purchased a couple of CNC machines - the first was a Smithy Granite 1324 with the Flashcut CNC package. All is working well with the Smithy for the very few small projects I have made on it so far.
I have also recently purchased a 96" x 48" x 12" Z Shopbot PRT Alpha (servo-based CNC router). I have spent the last week leveling the machine to within 0.01 degrees using a digital level at 32 points within the XY space. Took quite a bit of work to get it within these limts but due to the type of work and results I wanted to achieve with it, felt it fairly important to spend the time on it.
Now to the general questions I have. My intent is to machine either positive or negative forms of RC sailplane wings (large scale aircraft with multi-wing panels up to about 35' total wingspan). The intent is to create hollow molded flying surfaces using fairly well known techniques but my quandry is whether I should machine positive forms which I would then use to make negative fiberglass molds OR machine the negative mold directly. The two materials of choice are either MDF or Extira and my preference is to machine negative molds of the top and bottom surfaces rather than positive forms of each followed by full fiberglass female molds. This would save quite a significant amount of work and expense.
I will use Rhino to create the models and Rhino CAM to create the toolpaths. My intent is to slightly "overmachine" the negative molds, then seal the surface just machined, spray a surface coat into the mold cavity (Duratek or the like) and then re-machine to actual size and then polish.
Obviously indexing is going to be very important as I am not sure I would want to leave the machined part on the router bed and spray the surface coat in situ! Also, since a number of these wing panels will be in excess of 8 feet in length, indexing is a must!
I understand the basic needs for setting up the toolpaths for "overmachining" and the general requirements for indexing etc. What concerns me the most is the success I will have machining the negative mold in the first place. I am not looking for molds for 100's of parts - at least not initially. If the model becomes a marketable item, I would probably redo the molds using machined positive forms to create a fiberglass mold with tooling gecoat as a surfacing agent.
Has anyone here created such molds using these techniques and is there any advice you can provide regarding the best machining strategies, bits to use, material preparation, material finishing that I haven't thought of and re-inforcement/stiffening methods that would be needed for the long (10 to 11 foot) mold forms?
We have made molds for aircraft fuselages and wings in the sizes you are looking at. We machine everything as negative molds directly, sand them lightly, wax them and use regular mold release products and procedures before laying up the glass or carbon fiber.
We use a material called Remboard which is a plastic material that cuts like wood but does not warp or change dimensions with temperature or humidity. Unlike MDF, Remboard comes in thicker blocks or sheets and it can be glued together easily with Epoxy if even thicker sections are needed.
A Remboard mold can be used for production and will last a long time if properly waxed and cared for.
We machine the molds on a large 5 axis wood router with regular and exstra long ball endmills, typically 1/2" or smaller, but for smaller molds a smaller 3 axis machine will do fine too.
You can produce the toolpath with whichever SW you have that will make a negative mold from your model.
With regards to indexing, then it is always a good idea to start off drilling index holes if you have sections which later needs to be bolted together before layup. Good SW meant for mold making will have features to get the indexing holes in the model.
Doing it this way, you save the time and hassle of overmachining, spraying and remachining your molds. They are done in one go of roughing and finish. Remboard is very stabile and, if thick enough, will easily support it's own weight without you having to add support structures.
Steen, many thanks for the information. A one step process would definately be easier and with the ommission of several other steps and materials, possibly on par cost wise.
I googled RemBoard and saw numerous references to machined molds from Remboard but could not find any supplier details. Is Remboard the same as Freemans Supply RenShape? If not, do you know of a supplier I could view for pricing etc?
My intent of using MDF or Extira was for cost reasons. Since this is a hobby for me and I have no solid plans to expand to a business, many of the comments I have seen related to machinable plastics and their cost, ruled plastics out for me but maybe that was an evaluation on my part based on a false economy. Since wings and flying structures are fairly thin structures, I may look at combining a product like RemBoard as a surface media and build the stiffening structure below it with cheaper materials.
Let me know if you know of some suppliers so I can evaluate cost. I like the idea of a simplified mold making process!!
I will see if I can get a supplier name and contact info for you.
Remboard is defenitely more expensive than MDF would be but it is a lot more stabile and it had has the same density all the way through which makes for much nicer molds.
You probably dont need to add any support structure to it. You will need to clamp it while machining it, or for instance build a vacuum box to hold it down to the table. Then when you do the layup I assume you will do vacuum bagging on a stabile table so the vacuum and table will hold the mold flat during that time. It is really only when moving the mold around that you will have to make sure it is stiff enough to not break and it takes a lot to break this material.
When you get going on this, I would like to see some pictures of your airplane! Also interested in hearing more about this or other projects like that you might have going on.
I'm just wondering, how do you guys join negative molds when fiberglassing? I made a positive mold and just fiberglassed the outside (for testing purposes) which worked, but I think using a denser material and a negative mold would produce better results.
I can make the top and bottom megative molds of the wing, then what, do you glass them seperatly? join them after? The wings i'll be making won't be on the scale of what you guys are doing, much smaller. Nothing tiny but like a chord of 20 inches, 15' wingspan (just rough figures).
ScaleSoar, sorry if this is offtopic to your original question, but I don't see these discussions often, just wanted to get some advice.
A negative mold will defenitely give you a better result. If you prep the mold well, you can even paint it inside before doing the layup and then have a pretty much finished part, paint and all, when you pop it later.
We lay up the upper and lover molds separately. Then after you pop both of them you put one back in the mold again for support. Now you can place and epoxy in any support structures and hard points for servos and other hardware you need inside the wing, or whatever it is you are making. When that has set, you add the other shell, either just on it's own if it is strong enough, or supported by it's own mold. The joints can be made in different ways but needs to be on the inside of the structure which can make it a little difficult to secure a good seam.
Steen, so I have been searching for RemBoard and have contacted about 5 different Poly board distributors. None of them have heard of RemBoard but almost all of them had heard of reNboard or, in the case of Freeman, marketed as RenShape modeling/styling/tooling and high temp boards.
These are all Polyurethane foam boards with densities ranging from 10lb to 48lb in the standard boards and up to 112lb/cuft for the really hard stuff for foundrys.
All of these boards are expensive compared to MDF as I suspected from other peoples comments.
Just out of interest, what density board have you used for your negative molds assuming it is a polyurethane board you are using?
After discussing the project with freeman supply, they advised either a 15lb foam with duratec surface coat or MDF with the same - full circle you might say!!!!
A piece of board in the size I am looking for in a denisty applicable, in their opinion, for my needs, would be about $600. Probably a factor of ten times more expensive than the MDF/Duratec route - and that would be for one half of the mold. All told for the project I have I would be looking at $2000 to $2500 for all board stock. Obviously the difference between the two material choices is the time spent making the mold!
Am I on the right track with the material research I have done? Is it really that much more expensive?
Thanks again for your help.
Oh, BTW, the attached pictures are of one of several large scale aircraft I have scratch built - this is a 1:3 scale, 34' wingspan model. To date, I have used more traditional techniques for the flying surfaces - foam core with carbon/hardwood skins. I am now considering hollow-molded flying surface structures for future aircraft of which I have 4 more in the planning process.
MDF/Extira Molds for RC aircraft flying surfaces
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