Pattern repair

Questions about tools and jigs you want to buy/build/modify.

Pattern repair

Postby Steve Sawyer » Sat Mar 16, 2019 3:52 pm

So, if you're like me, you don't learn from your mistakes. I don't know how many times I've done some pattern routing, and not wanting to take a huge bite with the router, I set the stop on the router's plunge for the maximum depth, then back the bit off some to do the first pass, but what I fail to notice, is that I've retracted the bit so that the bearing is NOT hitting the pattern, and is instead just above it. Needless to say, as soon as I get close to the pattern, I start routing into it, ruining both the pattern and the stock.

I did this (again!!) the other day (when will I ever learn?), but fortunately I was using the pattern to route a second pattern (this wasn't for a guitar but for another project). So I didn't destroy the work, just took a chunk out of the two patterns.

I ended up re-making the pattern, as it wasn't that complex, but I did an experiment for that time down the road when a pattern gets dropped, or I lift the router off before the arbor has spun to a stop and nick the pattern, or do something else stupid. (I have no shortage of the stupids - enough to last me until I start assuming room temperature).

I had this stuff sitting in the cabinet that I purchased some time ago, thinking "this might come in handy some day". It's called Plasty-Craft, and it's a jar of plastic beads that become gooey and pliable when heated in a hot (150°-160°) water bath. I just set a pan on the stove with a fine sieve in the top, heated it up to the require temp (used an infrared thermometer to check this) put in about a teaspoon of the beads, and per the instructions, waited for them to become transparent. I fished them out, kneaded them into a short "snake" and pressed it into the gouge on the pattern and let it set up.

I didn't expect the plastic to stick to the MDF of the pattern (it doesn't stick to your hands, or metal spoons, or the sieve that I was using) and it didn't, so the next question was how to stick it in the hole that I'd shaped it to fit?

I had a little glob left over, so I sliced it up into several pieces to experiment with. The plastic when set is quite hard and waxy, very similar to UHMW or HDPE, and cuts like cheese with sharp tools, but I knew this might be a challenge to glue onto the pattern. I tried yellow wood glue, CA and Bind-All.

As I expected, the wood glue and CA would not bond more than superficially to this stuff, but the Bind-All did the trick very nicely. I was able to get it off of the scrap of MDF to which I glued it, but it appeared to be a failure of the MDF fibers, not the glue.

If you try this stuff for this application, try to get it as flush with the intended finished shape as possible. Even though it cuts easily, shaving a ton of this stuff off with a chisel leaves too many opportunities to further damage the pattern, and if you push too hard on the chisel, you will cause that MDF fiber failure I spoke of above.

I don't think this would have any problem surviving a pattern-bit bearing at all. It's very tough stuff, but you might want to watch your bit temp - you don't want to end up melting this repair! :D

I think this stuff has other possibilities, including "dams" for fret board repairs, forming neck profiles etc. Very easy to work with and capable of duplicating complex shapes and detail.

Molding Plastic.JPG
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Re: Pattern repair

Postby Peter Wilcox » Sat Mar 16, 2019 4:17 pm

I use plastic wood. I have template repairs for the same reasons, mostly when I lift the router before it has stopped. I've also used it on small defects in guitar bodies with an opaque finish.
Maybe I can't fix it, but I can fix it so no one can fix it
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Re: Pattern repair

Postby Randy Roberts » Sat Mar 16, 2019 5:11 pm

I've used similar stuff called Friendly Plastic and have found a lot of uses for it over the years, and with it being reusable, a jar lasts forever.
Makes great cauls.
If you have someone that just loves the profile of the neck on some guitar, you can duplicate it exactly.
Just cut an arc profile, larger than the arc of the neck itself in two scraps of 1/8 or 1/4 inch plywood. wrap the neck in Saran Wrap so the plastic doesn't stick to or mar the neck. Stick a "worm" of the warm soft plastic onto the arc in the plywood. Press the plywood against the neck at the level of the first and 9th (or 11th) fret. Now you have a perfect template for reproducing the profile in the new neck.

Also makes a good "donut" for routing binding mortices:

http://www.mimf.com/library/Randy_Rober ... -2007.html

Oh, and please take the time to let your router stop. It's not just your templates you're protecting.... I'm missing the tip of my fourth finger because my time was oh so valuable that I couldn't wait 5 seconds for the bit to stop.
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Re: Pattern repair

Postby Steve Sawyer » Sun Mar 17, 2019 10:21 am

Randy - great suggestion re cauls!!

And your "donut" too - because this stuff is slippery like UHMW it would be perfect for any kind of guiding application. Good point about using saran-wrap. This stuff when warm doesn't stick to much, but I'd hate to find out the hard way that it has an affinity for shellac or nitro!!

I have found when using any slippery plastic like UHMW/HDPE that the really aggressive double-sided carpet tape - the kind that uses a fiberglass substrate - is good for securing it. I've made fences and stuff faced with it and screwed and taped with great success. Should work just as well with this stuff.
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