A little about Using Jigs

Dancin' the Jig:

We received an email asking about the jigs we make and thought that not only were there great questions, but in answering the email, this “deep” subject had so many contributing factors, that the answers were not short and sweet… but rather quite lengthy.  After replying it became apparent that this info might help people with understanding the important aspects of getting a jig made, as well as using their jigs.

Below, we have copied part of the email we received along with our reply.  We hope it will be of some help!


[Original email]
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I am so darn sick of the Holy Grail search. I have been working off of a non-heat treated jig, filing my arse off.  Since I last emailed you, I have 4 weeks worth of time in 15 inserts.  I am happiest with #15.  Go figure.  My question, I have had lots of jigs made over the years. Too many, lol.  When I send this to you, how accurate is it to the insert? Secondly, why is it, I never get the same or darn close sound off jigs I get made from inserts when I love the insert sound? I figure you will have a quick answer.
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[Wade’s reply email]
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It never ends....  and once you find it, you’ll realize there is something better... and start looking for it. :D

Sorry, no quick answer.... but grab a beer and some popcorn and nestle in for the novel :D

I’m not sure how many jigs you've had made, or who made them all... but the knowledge required to build a jig correctly is pretty entailed.  To the point that, though I think I know everything that needs to be accounted for, I still miss things (and have to start over on the mapping, or have to remake a jig) and learn new things quite often....  but even considering that... there is a "hard to work with" factor that really cant be handled (replicated onto a jig) without a very expensive machine and a very time consuming and expensive process, and that is asymmetry in the board/prototype (variance from one side of the tone channel to the other.  The issue of asymmetry from side to side of the tone board from hand work is nearly impossible to avoid... whether its only .001" off from side to side, or .030" (and yeah, I’ve seen that much.... to the point I wasn’t comfortable making the jig...)  That lack of symmetry in the prototype to be mapped, REALLY can cause issues...  And even at that, if the jig could be made exactly to match the asymmetry, actually cutting the asymmetrical profile onto the part would be next to impossible because the cutting instruments are "straight", so they would "average out" the asymmetry creating a completely different profile it itself, in almost an uncontrollable fashion.  If it weren't for those darn rules of physics... this might not be a problem :D

I’m not going to divulge all of the operations I do when mapping and creating a drawing of a tone board for a jig, but I can explain some of what goes on in the process...  and yes most of it is science, but some of it is "voodoo"  :D  When I map a jig, I use a measuring device that reads to divisions of .00005".  I create a profile of the tone board's two sides (each side of the tone channel).  Then I verify those measurements, to make sure there aren't any anomalies. Once I have the plots, I apply a mix of mathematical evaluation, correction, and trending to the plots.  After that, I evaluate the board graphically, looking for minor inconsistencies cause by the material or tools used to create the board, like burrs, grain, fibers, file marks, ect... (just to give you an idea, the measuring device I use will easily pick up a scratch from 400 grit sand paper) and make any corrections (i.e. inject voodoo) that are needed to truly represent the smoothed board surface.   I then create a CAD drawing of the corrected board plots, adding in the cork notch features (sometimes this is voodoo, because I seldom see a "perfect" cork notch - so I have to base it off of what I know of the prototype).  That gets overlaid onto the jig blank and located to the bore of the jig blank.  The whole process of trying to "average" out the differences from the two sides is what creates the variation from the jig to the prototype.  The more variance side to side, the more different the jig is from the actual prototype, thus creating a difference in the parts created on that jig.  I think the least amount of variation from side to side I have ever seen was around .001-.002"...  and I know that took a lot of work to get it that way (knowing the guy that sent it to me for a jig, he is VERY particular and his attention to finite detail is amazing).

There are a lot of things that come into play on jigs, not only making them, but using them... using a jig accurately is an art of its own.   In my mind, a .001" difference in a sound board is a big thing...   and getting that kind of accuracy from a saw, file, and a jig, on a piece of hand turned material...  well it's just hard as hell to get any kind of repeatability.  You have to be SUPER anal about things, tenon diameter, tenon size (repeated accurately from part to part), tenon wall parallelism (lack of taper on the tenon), length, tone channel depth....  then once you have the parts the exact same each time...  then you have to focus on the jig....  part location in the jig the same each time, same amount of torque on the set screw, no dust in the jig bore, account for any possible introduction of inconsistency based on different wood densities....  Now that the part is mounted...  being sure to NOT cut below the jig surface with the saw,  filing the part to the jig exactly...  as in same amount of file pressure for every part, same hand placement on the file (because it will bow), same filing angles and so on... all the while watching for, and accounting for any flex in the material while its held in the jig.  It can be quite daunting just getting accurate replication from part to part.

Having said all that, I cant really say what is going on with your current jigs - not being able to see the original prototype and the jigs made to match...  If they were jigs I made, I would imagine the explanation will lie in the info above....  but, like I said, I am always learning... so if I screwed up, I need to make it right.  I am by no means infallible...  nor is my guy that does the actual profile cutting of the jig on his machine (I don’t have one, so have to farm that part out)...  Things can get screwed up going from one guy to another and one computer to another, through a different program, and then to the machine...  not to mention machine setup and blank locating...  lots of places error can occur.

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So, keep in mind.... no matter how perfect the jig is, what you get from it, likely will not be since we are all using hand tools to cut the material away.  As perfect as your soundboard may sound, if its not symmetrical, a jig will not represent it perfectly.  And finally, if you have a tenon that is for example .615" (which seems to be a very common size that I see) - the bore of the jig is .626-.627" (for clearance to fit a true 5/8" tenon) and that leaves a difference of .011" - and since the soundboard is based off of the radius of the tenon, that number is then divided by 2.  So, if youre tenon is .615" and the bore is .626, half of that is .0055" - and you can expect that your prototype will set BELOW the surface of the jig by .0055".  From there, add in the inconsistencies from asymmetrical sides, and you can easily end up with something that doesnt look quite right - and in the end, there is a pretty good chance that what you have is the best you can get without physically being here, and making decisions about the CAD file BEFORE the jig ever gets cut.

Its a very complicated situation, and hopefully we helped explain some of the "details" involved with making custom jigs, as these details have been known to cause some tension between us and our customers.

If you have any questions, feel free to contact us!

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