# Need help designing mold



## applebuilder (May 12, 2009)

I'm making a concrete mold and I need some help with the dimensions. I don't know what the shape is called so here's a cheap picture:









Basically the base is supposed to be a perfect square with I guess a "trapezoidal pyramid" with a flat top on it. I haven't decided the dimensions for the base yet, but for reference sake let's say the bottom is 6"x6"x4", the pyramid height is 2" with the flat top measuring 3"x3", and with a total height of 6".

I need to know what angle to cut the trapezoid corners and also the proper angle cut to create miter joints. Thanks!


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## ACP (Jan 24, 2009)

Would this mold almost be like crown molding in reverse?


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## applebuilder (May 12, 2009)

Yeah I guess you could say that, but with a square base on it.


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## woodnthings (Jan 24, 2009)

*My guess is that you have a 2 part mold*

The bottom or base is 6" x 6" by 4" high.
The truncated pyramid on top of that has a base 6" x 6" sq. the top or flat measures 3" x 3". The base miters are all 45 degrees. The truncated pyramid requires compound angles and bevels. There is a link: http://search.yahoo.com/search?ei=utf-8&fr=slv8-&p=compound%20angles&type= (see the woodweb link). where a chart calls for a square with 90 degree corners, a bevel angle of 33.9 degrees and a miter angle of 31.6 degrees. I suck at math, so I like charts, but can't verify this will be correct , but I think so. What do you plan to cut the angles on? Only a compound miter saw or table saw and miter gage will work. Small pieces like these can be dangerous to hold while cutting so watchit!:yes: bill
Here's the chart:


Click here for full size image38-52 b


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## applebuilder (May 12, 2009)

I was planning on using a circular saw set to the correct bevel and going against an angled guide. I'm still a little confused about how to figure out the correct miter and bevel cuts. Which equation or chart do I need?


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## applebuilder (May 12, 2009)

Thanks for the chart


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## The Everyman Show (Jul 10, 2009)

Applebuilder this answer may not translate very well, but here goes: whenever I get myself into situations like this (finding the bevel etc.) I break it down to where I can understand what I am looking for and I use basic carpentry geometry that has been around for thousands of years. I am attaching a couple of images that show how to breakdown complex angles and determine what the bevels are. I think if you look at these, and you have an adaptable way of looking at different problems, you can generally find a way to solve your problem and come up with a solution what you are looking for. http://joe.bartok.googlepages.com/hip_tetrahedra_3d.jpg http://ca.geocities.com/web_sketches/framing_math_notes/plan_angle_formula/plan_angle_geometry.html
Like I said, it may not translate very well, but it works for me. Good luck,


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## pianoman (Jan 16, 2008)

The old fashoned drawing table with scale is what you need...you`re making it too difficult! A skill saw is not the tool!!! You need a miter saw. Just tip the slope wood like it`s crown and cut at 45 degrees. Rick


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## woodnthings (Jan 24, 2009)

*This link show a video of a cool jig*

This jig is for cutting very small parts at angles. like you may need to do. You don't hold the parts with your hands and you don't use a "skilsaw" either! Use a table saw and a jig. It's about 1/3 to 1/2 way thru the video.
http://marleyturned.com/id97.htm
:thumbsup: bill


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## snowi (Mar 2, 2008)

*tuff project*

I for one think that this is the hardest possible mould that you could make.

1. Remember it has to be done in reverse. So your design will show up on the formed concrete.
2. Concrete does not do well on shapped corners. As the size of the material ( aggarate) will make the corners brittle.
3. Working with such a small mould makes it hard to design a breakaway form.
4. The form sould be done upside down to make the curved edges less brittle.

this is my 2c worth

Ralph


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## applebuilder (May 12, 2009)

Update: I have increased my dimensions to 10x10x5 for the base and the *truncated pyramid has a 3.5x3.5 top with a height of 2. All efforts at beveling have been abandoned as the only tool I own that could handle the job has broken. What I've decided to do is replicate the exact dimensions to wood, tape the inside joints to hold the mold together and then secure it with a sealant on the outside. All the information provided has been very helpful and I'll definetly be using it again very soon, thanks. 

Ralph: Thanks pointing out the brittle nature of concrete. This is my first time working with the stuff and I'm sure you've just saved me some headaches. Would you mind elaborating a bit more as to what you meant by doing the form upside down and do you have any other tips on how to avoid the brittle corners? I'm assuming by "shapped corners" you meant shaped corners but I'm not sure which curved edges you were talking about.

*I knew somebody had to know what that thing was called, thanks woodnthings


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## woodnthings (Jan 24, 2009)

*Casting molds have side taper*

In order to remove the mold after pouring in the material, the molds must have a very slight taper to the walls, sometimes as little as 5 degrees. When you pour a liquid like concrete, a very stiff liquid,into the mold, the smallest point or tip must be at the bottom, tapering upward to the largest opening upside down, like a funnel, in other words from what it would look like as the finished product. You then turn the mold over tap it and the casting should just drop out. 
:thumbsup: bill


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## applebuilder (May 12, 2009)

See, if I didn't know that I woulda been in for a world of pain later, thanks. About the brittle corners, you think it would help if I ran a bead of non-absorbent caulk along the interior edges and smoothed it out with my finger (like in bathroom showers)?


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## applebuilder (May 12, 2009)

Here's what I got knocked up today: <a href="http://s556.photobucket.com/albums/ss3/pictureeasy/?action=view&current=DSC00918.jpg" target="_blank"><img src="http://i556.photobucket.com/albums/ss3/pictureeasy/DSC00918.jpg" border="0" alt="Photobucket"></a> <a href="http://s556.photobucket.com/albums/ss3/pictureeasy/?action=view&current=DSC00919.jpg" target="_blank"><img src="http://i556.photobucket.com/albums/ss3/pictureeasy/DSC00919.jpg" border="0" alt="Photobucket"></a>

Still need to cut the base shorter and do some fine tuning. To finish it I'm going to caulk the outside and secure it with more tape and probably run some caulk on the inside edges and corners. Stop me if I'm heading towards disaster


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## snowi (Mar 2, 2008)

*so sorry applebuilder*

I have spent the last 2 days on SketchUp trying to draw the plans for this mold. 

If your going to be making 5-10 blocks, 1 mold should do the trick ( personally I would build 2 for that)

To get started: 

Start with a piece of plywood larger then the size of you mold, now attach your beveled top pieces. ( remember that your building in reverse and upside down) with the center hole flush with the bottom plywood.

Cut your sides to fit ( free standing ) around the beveled top pieces. Now attach a 2x2 to the bottom plywood leaving 1/4 to 1/2 in of space outside of the side pieces. Center the sides and insert wedges between the 2x2 and the sides, ( this should be touching the beveled top pieces all the way around. Build a 2x2 frame to fit tight around the top of the mold. The 2x2's are to stop the form (mold) from pushing out). Use silicone to round over all of the inside corners.

When using wooden forms for concrette, you will have to apply a "release agent" so the wet concrette will not stick to the plywood, we usta use diesel fuel, just lightly sprayed on all exposed wood.

Fill with concrette, be sure to tamp the Heck out of it, and smooth off the top ( witch is the bottm of the block). Then gently tap the outside of the sides to help bring "cream" to the edge of the form. Set aside, for 24 hrs. Remove the wedges, top 2x2's ( that were just placed over the top), remove the 4 sides, and pick up the newly formed block. Repeat as required.

I hope this makes sense 

ralph


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## woodnthings (Jan 24, 2009)

*Nice post ralph*

I forgot to mention that I've poured about 100 bags of hand mixed "concrete" with only sand and Portland cement, no stones or aggregate. It can be mixed on the slurpy side and will still set up hard. I use a rich mixture, 4 sand to 1 Portland, if I recall. FYI, bill
Not any sort of expert on concrete myself, but there is a self leveling product made from gypsum or something similiar.


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## applebuilder (May 12, 2009)

Thanks for the plans, I finished the mold today. I should've clarified, the conrete shape is actually goin to be the base of a fence post which will be set inside the mold. This makes it impossible to do it upside down but I think it will come out alright as long as I do as ralph suggested and tamp well. I ended up using some asphalt crack sealer for the inside instead of caulk because I had some lying around and I was planning on using vegetable oil for the release agent. First piece is going down tomorrow and I'll post a pic of the results in case anyone is interested. Thanks for all the help!


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## snowi (Mar 2, 2008)

*pic's*

I am interested, for sure ... 
I just wish I could have found my old drawing program,

Good Luck

ralph


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## applebuilder (May 12, 2009)

Well as you can see I made the first batch too thick, but overall I think it came out pretty nice. I'll make sure to thin out today's mix and patch up the first one while I'm at it.










Just wondering, will concrete hold it's shape if you remove the mold immediatley after pouring? I'm hesitant about trying it because I have to make the mix a little thin and also because of the piece's size. Maybe let it harden for an hour or two then remove? It'd really speed things up since I only have one mold so I'd appreciate any advice you guys can give.


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## woodnthings (Jan 24, 2009)

*Hey Guy,*

Now I understand what this whole project was about! It's a cast in place base for a post. The word mold thru me off, that's why I was suggesting upside down pours and all...Anyway, I don't know how many of these you are going to need? But before I made another one I would seriously reconsider the proportions. It looks like this casting would hold up an 8"x8" post easily. The 4"x4" looks a little spindly compared to the huge base. Unless you need it for strucutural requirements as in load distribution, scale everything down about 1/2 size and it will look better. Now, who am I to say this? Former architectural student, Industrial Design Instructor MFA and 30 yrs in the automotive design studios of GM designing and scultpting cars and trucks.:yes: That's just my opinion tho and feel free to ignore me, that's OK. :laughing: bill


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## snowi (Mar 2, 2008)

*Nice pic*

Now.. I see what your doing... 

Lets just do it this way.. make a box.. center the post in side the box. Fill with concrette, tamp it.. and finish it with concrette that is added and sloped to any angle you want. 

ralph


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## applebuilder (May 12, 2009)

Can't argue with those credentials, and here's the shocker: those posts are 1 3/8" x 1 3/8". Had to make them that small to shape them correctly (I'm using all scrap pallet wood) and they're actually a lot sturdier than I thought they were going to be. The base would ideally be smaller but I didn't know how heavy the concrete would be and I'd thought I'd play it safe because I wanted this temporary fence to be sturdy (windy over here). Now that it's been cast I can safely say that thing is pretty darn sturdy, but I think I'm past the point of no return. I'm actually quite satisfied with how it came out, but then again this is my first conrete project. Once the fence is done and the first timer's buzz wears off, I may very well eat my own words.


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## applebuilder (May 12, 2009)

yeah, sorry I wasn't clear about what the whole thing was for, haha. I thought about freeshaping like you suggested but after my attempt to patch the first peice I found I was now good with working concrete. All the aggregate makes it too hard for me to shape it, maybe it'd be easier if it was cement. I poured my second piece tonight and I think it filled the mold completely this time, fingers crossed...


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