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Hi folks!

Simply put, how percise do we need to be when building fixtures and jigs??? How about compared to cabinet makers, architects or other highly percision requiring field?

To elaborate what I'm trying to get at, I have built a nice looking bench and go bar deck to build my first guitar. However in doing so I realized that unless I have the skill, tool, or both in my case, I'm just not going to get any fixitures that are "perfect" in terms of measurements, straightness, and etc.

Let me share couple things I'm experiencing now and perhaps you guys could clear things up for me in terms of level of percision recommended.

GO BAR DECK
-I bought 3/4" birch plywood to build my deck. However I notice that the plywood sheets(24 sq in) are not flat and bit warped in the center... roughly 1/8". Now my guess is that this warp will now affect the radius on my radius dish when in use. I know 1/8" warp might be small but if I ignore this warp on ply, wouldn't this defeat the purpose of even getting a particular radius on a radius dish? We can just ask for a general number like 15'-20' rather than asking for particular number. I thought that one solution might have been to use mutiple layers of ply to reduce most radius caused by the warp on ply, but really, do you guys recommend or would go this far to make it within particular degree of flatness?


WORK BENCH
I know that there are many different ways of building bench and one way of building might bring more percision/accuracy than others when it comes to flatness. I built mine using 2x4 studs and two sheets of 3/4" plywood to lay on top. After checking with straightedge, I realized that few places were lower/higher by 1/16-1/8". I thought about taking the top off and reflatening the structure using chisel and sandpaper, but I really didn't how I was going to make the whole top flat within 1/16th of an inch. Also, I notice several luthiers like Kevin Ryan and John Mayes using carpets on their bench and thought that carpets might cause greater diviation than 1/16".

SOOO Having said above, how much is too much and how much is acceptable to good when buiding guitars? Yes, subjective so I'm asking what you did or do.

After writing I realized that I have asked numerous questions that maybe not so orgainized. I apolozise in advance for writing is not one of my strengths.

Thank you, David

Hunt around and you may find a place that does millwork on conference-table-sized slabs; they'd probably run the workbench through their belt sander for a nominal fee. For a standard worktable a 1/16" or even 1/8" dip is not a deal-killer, unless you're planning to use it as the base for a router table or for sanding stuff flat & true.

FWIW Corian countertops are very flat, and dried wood glue pops right off!

I'd worry about the ply on the go-bar deck, just find another sheet that is nice & flat. If you're shopping at Home Depot or similar, you might have to pick through the stack to find one that is flat. Then get two and double them up. Or go Corian again.

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The member formerly known as erikbojerik....

Thank you very much for the comment and advice and yes, what you said can be a solution. However I'm more interested hearing opinons of others in terms of how percise do they get when buidling guitars.

It really does depend on the part, jig, tool you're talking about.

As far as the Go-Bar deck, most folks use two layers for the bottom and two for the top. Doubled up 3/4" plywood or MDF will usually render a flat enough surface. The radius of the back is not overly critical. You could have a 13'-17' and it really won't make much if any difference.

Same with your bench, you don't need to have a dead flat surface to build a guitar. It can help but it's not necessary and actually there is only one process I do that requires me to have a dead flat surface to perform the task.

Now you're tools are certainly another thing. If you're tools are not true, square, flat, level than what ever you do with them will also be NOT true, square, flat, level etc...

As far as your jigs go, if you can build most of them to a 1/64"-1/32" tolerance that would be ideal, but if you can only get a 1/16" tolerance most of the jigs will be acceptable.

Of course there are other areas that require you to have a very tight tolerance. Cutting your fret slots is one of these areas. If you can cut them to a +/-0.005" tolerance that would be ideal but honestly I think if you could get +/-0.010" that would be acceptable.

It really does depend on the task at hand. Having said this of course the higher the level of accuracy you can build to the better your results will be.

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"There's really no wrong way, as long as the results are what's desired." Charles Fox

"We have to constantly remind ourselves what we're doing....No Luthier is putting a man on the moon!" Harry Fleishman

"Generosity is always different in the eye of the person who didn't receive anything, but who wanted some." Waddy Thomson

Always shoot for the highest level of accuracy that you can achieve, and live with the knowledge that it is the best that you can do at that point in time. If you know that you can be more accurate, then be more accurate.

Errors accumulate.

If you know that your go bar is not flat, and it bothers you, then fix it because you know its not flat and it will always bother you. If you know that you can fix it, then fix it.

Its not a big deal if your bench is off true flat by that small amount. Think through your building process and see what could be affected by the bench.

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Expectation is the source of all misery; comparison the thief of joy.
http://redrivercanoe.ca/

Go bar deck…an 1/8” in not going to make any real impact as long as it is not twisted. I suppose that if you profile your braces in the dish and then induce an 1/8” bow into the dish when gluing the braces, the radius will be slightly different and there will be some stress present that could be a problem but who knows, it may end up being a good thing. For the amount a deflection that we are talking about, I would worry about it.
Your work bench…there just aren’t any benches that are flat enough to say true a neck blank or flatten a fingerboard on. A piece of well supported granite 1 ½”x3”x30” will do the job nicely. As for the bench, it should be relatively flat so when you are building jigs they won’t be racked and out of square, so I would fine someone to surface it for you if it is twisted. So for building a guitar, your jigs need to be accurate more than the surface that is keeping the jigs and fixtures off the floor. Honestly, you could build using a 3’ pole with a vice on top of it, provided to have the right jigs.

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Rich S

"The inconvenience of poor quality will linger long after the thrill of a bargain has been forgotten"

The world's earliest go-bar decks were caves where the craftsman used sticks from the ceiling of the cave to the work piece which was positioned on a BFR (big &*^%^ rock). With this said you can see that the "utility" of the deck, since this method worked, exceeded the accuracy of the deck. For a go-bar deck a high degree of accuracy is not important. You can shim things if it bothers you. What is important is that your dishes are accurate and supported well enough so that the cumulative force of many go bars does not change the domes.

As for the bench - I built my first ten guitars on a stinkin rolling kitchen cart in a unused bathroom........ It worked for me and I had never even seen a proper work bench so I didn't know any better.

As Rod indicated some jigs and fixtures do need to be dead-nuts accurate with a good example being what you use to slot a saddle and/or how you cut your binding channels, cut rosette channels, etc. But IMHO one of the beauty of the go-bar deck is that the go-bars themselves counter inaccuracies to a great degree so long as your work piece is well supported and true.

A word about domes on guitars - no mater what degree of accuracy we build into our domes the domes on a guitar are dynamic - they will change and rise and fall with humidity, string tension, etc. I build my tops to 25' but depending on my bracing and the spring-back of the top itself I seriously doubt if my finished tops are 25'. My bridges are pre-radiused to 25' and sometimes when I trial fit a bridge it will require sanding on the guitar top in it's relative position to fit well. Sometimes the bridges will require very little if any sanding to fit well. This tells me that my domes are not always 25'.

Our neck angles are corrected with flossing the cheeks to set the neck angle correctly relative to the bridge height and position. So what I am trying to say here is that building with a high degree of accuracy is important, certainly so, but.... we also will be doing many things to custom fit the guitar's component parts for a precise level of fit and performance. And in my view this is all part of what it takes to build a guitar and expected.

So don't agonize over the accuracy of your go-bar deck or jigs if they will do the job well enough and you will find ways to increase the accuracy of how you build, many of which will be required anyway, as you get into the actual building of the guitar.

Here's a thought...the tolerances you have described are perfectly acceptable for general carpentry work. The tolerances required for acceptable instrument building are much tighter. If you're building jigs, hold yourself to the new tolerances...it'll be good practice!

If you continue to give yourself excuses for less than perfect work it will be a difficult habit to break.

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JJ
Napa, CA
http://www.DonohueGuitars.com

Without getting to in depth here I will say a complete understanding of the allowable tolerance and the expectable deviations of the given part or assembly being built is required to know the expectable tolerance for is fixture. Lets say we want to build a fixture for a part that needs to hold a +or- 1/16” tolerance. This means the finished part can deviate .063” in, X, Y and maybe Z axis. This tells me that when I build the fixture for this part I need to constrain miss shape, size and fit to allow any of the parameters within expectable tolerance range to work in the fixture but exclude and parameters outside the expectable tolerance range.

Most of the joinery we use this is easy to do as we work with a +/- 1/32”, 1/16” but on things like fretboard slotting we must work with in .001”. .0001” Also you have to concern your self with the stacking of tolerances. As a theoretical example only lets say between any two given frets a tolerance of +/-.002” is fine but between the nut and the 12th you must be no more +/-.008 then obviously the continual overage or underage of .002” eleven times would exceed the allowable tolerance of half the scale. I did not give this example to confuse but rather to show that choosing the allowable tolerance of a part is based on its function and fit, and the fixtures must take these things in to consideration.

In my mind a good fixture should be both a “go” and a “no go” indicator. A good set of construction fixtures should allow you to start with a loose tolerance at the early stages of creating components from raw materials and end with close repeatable tolerance at the finial assembly.

Let's get practical here.

Accuracy is a good thing to achieve, but is the warping on your gobar deck going to actually show up in your guitar. Only if your radius dish followed the warp, which it should not. Commercial radius dishes are thick enough to hold their shape under normal usage, so the warped plate will not make any difference.

Your workbench is another issue, but just don't use it for a reference for flatness, and you should be fine.

This is a lesson to you regarding the flatness of plywood. In other words, it seldom stays flat. MDF has a lot more stability in this regard, and you may want to get familiar with the material.

Best luck on your first guitar.

Uhhh, start by spelling "precise" with the "r" before the "e"...

Thanks for adding so much to the post Rick

I have to agree with JJ. Strive for the best and most accurate work you can. If you settle for "This is good enough" or "It won't matter for this jig or fixture" it will lead to bad habits and a lot of excuses for poor workmanship. So strive for the best you can achieve. You may surprise yourself. Most of all enjoy. The end results will be worth the effort.






"

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I'm a believer in "appropriate workmanship". As for workbenches, I treat them about the same as a sawhorse. I like it level enough that my tools won't roll off on the floor, and about however flat the piece of OSB ends up when it's screwed down to a 2x4 frame is generally fine. A carpenter's square and level, tape measure and a bag of deck screws, and Git'R'Done. When I need a tool that's precise to .0005", I'll make it that tight. If it needs to be within .01", I'll do that. Then my bench only needs to be good enough to hold these jigs and fixtures so I don't have to work on the floor. Other than that, I've got work waiting to be done to pay the rent, and can't afford to spend time on the romance of a pretty perfect bench beyond what tolerances will impact on my work.

Of course some day I'd love to make a dream bench, perfect butcher block surface, classic tail vises, dovetailed drawers and all the bells and whistles that can make the aesthetics of a shop so much more pleasant. As for now though, if I need any tool to be precise I don't want it married to my glorified sawhorse. I'd rather be able to shelve it or hang it, because as I said, I think of benches as disposable at this point in the development of my shop (and I do dispose of them regularly).

As for precision - think through every item's intentions and what criteria is required to perform it's tasks. Then make everything as precise as it needs to be. Obviously if a half inch tolerance is all you need for a bench but it doesn't take much to get it within 1/16", don't be sloppy. But if it would take ten minutes more work to bring the tolerances from 1/2" down to 1/4", vs four hours extra to bring it to within 1/16", I'll gladly settle for the 1/4".

Appropriate workmanship. It's a constant choice that has to be made by the individual craftsperson in just about everything you do.

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Eschew obfuscation, espouse elucidation.

On the subject of benches, as someone who spent a number of years as a cabinet maker, I just don't see it being any harder to hold workbench parts to + & - 1/32" than to + & - 1/4", and if you cut properly, then assembly of parts is much easier. That kicks over to guitar parts as well. It's just not that hard to make parts that are very precise if you know how machines work and how to design jigs and fixtures. That's where the precision really counts...jigs and fixtures. Then with near perfect parts, it's pretty easy to assemble near perfect instruments. It's when you have to compensate for funky early work that you start drifting in to constantly fixing screw-ups.

The other thing is not to attempt aesthetic details that are beyond you. Don't try to get away with patched purflings or funky miters (as I've seen recently on more than one highly regarded luthier's guitars in for finish with Addam...); that stuff sticks out like the proverbial sore thumb and makes the rest of the guitar suspect even if it's fine. Better to go understated and really clean than all zoot and bling and sloppy.

And if you ever have to apologize for something other than normal wear and tear when you show or demo an instrument to a prospective client, you've lost the game.

The accuracy with which you build these shop fixtures should be directly commensurate with the bearing that their accuracy has on the outcome of the guitar's construction.

The go bar deck being warped will really only cause trouble if the radius dish is under sufficient pressure to cause it to follow the warping....which is unlikely if you're using a dish of reasonable quality.

As far as workbenches go, I'm not running a workbench showroom, but am building guitars. All of my tops are 2 1/2" thick HDF and measure 3 feet wide by 6 feet long. The benches are heavy enough to stay stable under aggressive pushing, sanding, filing while I work, but the tops are nothing that I need to polish at the end of the day.

If you need a reference surface, pick up a 24 inch square granite reference palette from Grizzly or another vendor. You need to be able to beat on your benches though. The beauty of using MDF or HDF tops on them is that they can be replaced every few years with little trouble or expense, they are very durable and easy to maintain.

When considering precision in guitar building, similar accuracy to what a good cabinet guy would shoot for, more accurate than and architect needs to observe since the builder's accuracy will stray from it in the real world every time and not necessarily as accurate as machine or tool work. I've heard guys talk about holding tolerances of less than .005" on their guitars, but give it 15 minutes and it will change to be outside of their tolerance....even in the most tightly controlled environment. A good fit is a good fit.

Fixtures that will be used to achieve dimensions and shapes that have a bearing on the accurate assembly and geometry of the guitar when it's done should be fashioned within a few thousandths of an inch to ensure consistent repeatability from one guitar to the next or they're worthless and will cause you more trouble. They need to be made from materials that are stable, rigid and strong.

Take your time to get them right when you make them and the time and work that they save you over the years that you build will make them valuable tools. I usually prototype a jig or fixture a few time before I settle in to make my final version of it.

Regards,
Kevin Gallagher/Omega Guitars

Just checked my 4'x6' assembly table, and it's flat to within 1/64" per foot in all directions (better than that in some areas). That wasn't hard to do, and the table is six years old and has been partially disassembled and moved once. Melamine faced particle board on top of 3/4" ply on a 2x4 frame. All did was measure, saw, and assemble the frame with a bit of care. Less flat may work OK, but I find it helpful to be able to trust my assembly surface to be flat enough to align parts that are getting glued up. We all build differently, and our needs vary. I don't use spherical radius dishes, and I need a pretty flat surface for my go-bar deck. A double thickness of 3/4" hardwood ply has served well.

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Howard Klepper
http://www.klepperguitars.com

When all else fails, clean the shop.