Pin Routing and Plaster of Paris


OK, yet another pin routing thread. I figured I wouldn't hijack woodworkers thread again.
So I had a chance to use my pin router this weekend, and it worked great. I used it to make a relief for a house-number sign. It took almost no time at all (carving the molds took some time, but once I had those the sign itself was pretty fast). I switched out the router bit at the end to a half-round to create some nice bevels, and used a half-round chisel to distress the backdrop.
Check out the pics! ( http://www.ulvr.com/cgi-bin/yabb2/YaBB.pl?num 83824903 )
That worked so well I decided to try it out for inlay, and that's where I ran into issues. The problem was/is in creating a inverse mold. I want to be able create accurate inverse molds quickly and easily. So what I tried was plaster of paris. I created a shallow dish, and poured the plaster around my original mold (OK, it was a copy of my original mold, as I didn't want to wreck the original). The hope was to remove the original, and have an absolute perfect inverse. The problem was that the plaster of paris cracked when I was lifting the original. I had sprayed my original with WD-40 before trying this, but I don't think that was good enough. Does anyone have any ideas on how to make this work? If I can get past this hurdle then I should be able to do very accurate, and very nice looking inlay, in very little time.
As far as the pin router goes, if anyone is interested, I created it with MDF which is glued together (I tried putting some screws in, but it seems that screws and MDF don't mix very well...). I used an arm configuration as that would allow me to use bigger pieces. As you can see from the pictures, it's simply clamped to the router table... No bolts required. After playing with it, it seems that that's good enough for what I was doing (cutting 1/4" cedar). It will shift if you push it to hard though, but using the pin router doesn't seem to do that. I have a detachable pin-holder, which holds either a 1/2" or 1/4" pin. I can make another one if required. In the picture you see the 1/2" pin. I cut the 1/2" bin a bit short so I tapped the end and added a bolt, to give me a grip when raising/lowering the pin. The pin can slide up or down if I loosen the wing nuts on the pin holder. I have about 1/4mm play in any direction with a pound of force, which I think is awesome. The whole thing took about an hour plus glue-drying time to put together (but some of that time was spent designing).
If I can figure out how to make inverse molds, I'm thinking of making a bridge configuration for the pin router. That would likely have even less play, and would not have the misalignment risks. But I still need to figure out how to make the inverse molds.
John
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In article

Use vaseline. I'd also suggest some plaster impregnated gauze for the first few layers around your original before pouring plaster.
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wrote:

I am wondering if part of the problem is that the mold members need to have some relief angle built into them.
I wonder if you could use 1/2" bit, and do some grinding on it so that the numbers and such have (perhaps) a 1/16" angle on the sides. I think trying to pull straight sides out of a mold is asking a bit much of any lubricant that you put on it.
I really think that is the key, even though it would take a bit of work to get to work right.
--
Jim in NC



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Thanks for the suggestions. If possible, I'd like to avoid the relief as it complicates the inlay (pin depth becomes more of a factor, as does wear of the molds, etc). I will resort to it though if all else fails.
I got a few more suggestions from friends. One that I found interesting was that I was told was to drill small holes below the mold to get rid of the vacuum effect when pulling the number up. I was also told to use a thicker board that would not warp when I was pulling the numbers up.
So... my next experiment will be: color the sides of the mold with crayon, heat, and use a cloth to smooth, then add the Vaseline. Use a thicker board that will not warp underneath. Let the Plaster of Paris set for at least 24hrs. Drill a few small holes through the setup. Be very, very careful when pulling the mold up.
I will try to avoid the relief angle but may resort to that later. For the actual inlay I'm going to use a 17/32 pin and a 1/2" bit, so I have just the tiniest bit of wiggle room. I'll try it this weekend and let you know how it goes.
Wish me luck!
John
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On Wed, 8 Sep 2010 07:38:07 -0700 (PDT), John wrote:

Use the vaseline like someone mentioned and in place of plaster use bondo, will withstand much more abuse getting it free.
basilisk
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"basilisk" wrote:

---------------------------- If you use Bondo, use mold release available from any fiberglass supplier.
Lew
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On Wed, 8 Sep 2010 09:40:02 -0700, Lew Hodgett wrote:

Good point, and another- if bondo is too thick, you can thin it with polyester resin or just use the resin by itself.
basilisk
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So... my next experiment will be: color the sides of the mold with crayon, heat, and use a cloth to smooth, then add the Vaseline. Use a thicker board that will not warp underneath. Let the Plaster of Paris set for at least 24hrs. Drill a few small holes through the setup. Be very, very careful when pulling the mold up.
I will try to avoid the relief angle but may resort to that later. For the actual inlay I'm going to use a 17/32 pin and a 1/2" bit, so I have just the tiniest bit of wiggle room. I'll try it this weekend and let you know how it goes.
Wish me luck! ****************************************** Another think I just remembered from pulling fiberglass parts. Use those holes to put a few blasts of compressed air between the mold and casting. That can really help to break them apart!
--
Jim in NC



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