# Box Joint Jig - Accuracy question



## gbarnas (Sep 25, 2021)

I was looking at making a new box-joint jig for my router table. I have a project coming up later this year where this would be useful. I currently have 3 separate jigs, one each for .25, .375, and .5 spacing. I came up with a design for a single jig with interchangeable guides that I could 3-D print. One guide, one baseplate, and 3 inserts to set the cut size and spacing for each size.

The problem is that 3-D printing has a .2 to .4 mm tolerance, which is roughly 7 to 15 thousandths of an inch. I think that's far too much, but figured I'd ask before investing in a test print. I'm also thinking that I could print the parts oversize and then cut the keyway that interlocks the parts on the router table with tighter accuracy than printing could obtain. Has anyone machined 3-D printed parts?

Anyone here with 3-D design/printing experience interested in looking at my design?


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## Sigung (Nov 20, 2013)

I have no experience with what you're trying to do, but I do have a lot of experience making box joints. They have to be within .003 to .004 to fit right, if that helps…


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## gbarnas (Sep 25, 2021)

Thanks - that's helpful. I'll try milling it from a hunk of plastic cutting board to test the idea before proceeding further.


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## MadMark (Jun 3, 2014)

.005 is tight or loose - remember the error doubles and is cumulative. When my 3/8 router bits wore .003 the joints got tight & time for a new cutter.

I cut my box joints on an Incra that holds +-.002", non cumulative.

3D printing has an inherent +-1 pixel at whatever rez your printer is. Mine is 12.5nm or about .005. This means a couple thou tweak will have no effect until it jumps .005. Additionally, the "squish" is significant. Your holes will not fit if you make them exact size. I have to make small dia holes .015 over.

Yesterday I was making an LED mount. T1-3/4 led mikes at .192 but I had to bump to ,218 before I got a clearance fit. At .203 it was no-go.









YMMV


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## Lazyman (Aug 8, 2014)

I think that it may depend upon the design. I think that we might need to see a rendered image to provide real feedback, if you can show us one. If you build in some adjustability, that can obviously help. You might study the Incra box joint jig and see if that provides any inspiration for removing inaccuracy. The key is a design that doesn't multiply the error or at least has adjustability to remove the error.

If I was trying to design one, I think that first step would be a test by setting up your dado or box blade and see if you can print a key that fits a cutout in a test cut. It should fit with light friction. If you have to force it in or it wobbles, then the tolerance is not good enough. I printed some dogs for the spoil board on my CNC machine and was able to get what I consider a perfect fit. I use the dogs to get perfect alignment with the axes during setup. The dogs are tight with no slop but are also easy to slide in and out. It took 2 or 3 tries to get that perfect fit.


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## jarheaddoc (May 22, 2018)

I am not a fan of making box joints on a table saw, mostly because of the set up for the dado blade. And there is also not always getting a flat bottom to the joint.

You are going to get tear out regardless of what method you use.

I can't speak to anything more sophisticated than being about to start my computer and point and click my way to bankruptcy by shopping on line. I can, however, speak some about the two jigs I've used.

The first is an old woodsmith brand jig that I've used on both the router table and table saw. As with anything, set up is crucial, and this jig is easier (IMO) to use on the router table due to the consistency of a router bit vs. stacking a dado.

The next one that I've used with fair success is the Porter Cable jig for 1/4" box joints. Set up is a pain, but the jig makes really nice pins and tails with close to no tearout. Yup, there's a cost to it, and it limits how wide a board you can use.

I'd certainly want to make a jig like this if I was able to, though there is a lot to be said for not reinventing the wheel. with that said, I hope you're successful in doing it, and I have to go dig myself deeper into debt on Amazon!


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## northwoodsman (Feb 22, 2008)

How big is the bed on your 3D printer? Are you going to have to mate sections together? As Madmark stated you are going to have to account for squish, e-steps, flowrate, etc. What filament are you going to use? You will have to dial this all in for each filament you test also. I would go with ABS for durability. What type of jig are you wanting to build? A miter gauge for your router table with a fixed key where you advance the stock with each pass? Or one with guide fingers (similar to a Leigh)?


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## gbarnas (Sep 25, 2021)

After speaking with a 3D designer, I abandoned this project. It seemed like a simple project - a female part that set into a fence (TS or Router Jig) and a few Male parts that had the right size pin the right distance from the cutter. My primary use was on a router table. Anyway, it wasn't possible to get tight-enough mating tolerances at a reasonable price. It would work, but the printer type & material to obtain that accuracy would cost more than a typical commercial jig. I went back to the "one jig per size" setup.

If anyone has 3D print capabilities and wants to experiment, DM me.


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## MadMark (Jun 3, 2014)

I gots a 3d printer. What do you need? Do you have:


An .STL file?
A CAD drawing?
A dimensioned sketch?
A physical part?


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## gbarnas (Sep 25, 2021)

@MadMark2,

No, No, Yes, Maybe..

Total computer geek (in IT 44 years) with training in drafting and - would you believe - don't have a CAD program? I can build the part out of wood pretty easily, and even not use the female part. My original design used a small cutoff of T-slot on the portable part and a T-nut on the jig. Made everything thicker than I wanted but I could put that together for a POC and send pics.


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## Lazyman (Aug 8, 2014)

One thing to note is that if I need an exact size, I design and print it slightly oversized with slightly thicker walls and just sand it to size.

BTW, if you are looking for a good CAD program that works well with 3D printing, I highly recommend OnShape.com. It is a parametric design program that runs in your browser (there are IOS and Android apps as well) and best of all it is totally free as long as you do not mind your designs being public. They have excellent and free online tutorials and it seems to be much easier to learn than Fusion 360, IMO. I was immediately able to design and print bandsaw throat inserts after the tutorials. I have just started using it again (did the initial tutorials last fall) and last week, I drew a modified version of the the Dakota belt grinder addon for a bench grinder as a way to learn the package. Just for grins, I am actually now printing a 1:3 scale model-just because I can.


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## MadMark (Jun 3, 2014)

NanoCAD is a free AutoCAD work alike.
To design 3D parts use TinkerCAD (Autodesk), also free. TinkerCAD outputs .STL files that the printer's slicer program uses to create the final .gcode file that prints the physical part.
Send me what'cha got and we can go from there.


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## MadMark (Jun 3, 2014)




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## gbarnas (Sep 25, 2021)

Those look awesome! Exactly what I had envisioned. Looking forward to trying them!


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## Lazyman (Aug 8, 2014)

When I have used a box joint jig, the biggest issue for fit is not the thickness of the pin on the jig but setting the spacing between the pin and the dado blade or router bit. In fact, you can use the smallest one for all sizes as long as you set the spacing correctly and always keep pressure against the same side for all cuts.

When using a router, I prefer a jig that works like this Woodsmith one. The beauty of this approach is that the fingers guarantee a perfect fit. The fingers on the jig sets the width of the both the finger and the gap and gives you a perfect fit. It does not even require a bunch of test cuts to dial it in and you can even have different finger sizes for a more interesting joint. I used a one off version of this jig here to make giant finger joints on a cabinet.


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## MadMark (Jun 3, 2014)

Lazy man: does that jig come in 1/4" fingers?


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## Lazyman (Aug 8, 2014)

You can make the fingers any size you want as long as the router bit will fit between them and they can vary in size on the same joint. It may not be obvious in my picture above but the fingers are different sizes. The middle 3 are 3" wide on one side but 2" on the other and the outside fingers. Obviously with small sized fingers, you have to deal with small screws to attach it to whatever horizontal backer is but as long as they are solidly attached and square, it will work. The great thing about this design is you that don't have to make test cuts to dial in for a good fit and there is no risk of a cumulative error either. One other thing that may not be obvious in the picture is that there is a piece on one end that you align the piece against to make setup quicker.


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## gbarnas (Sep 25, 2021)

Mark - we had hosted an event for the family this past week, so haven't had enough shop time to put these to the test. Heck - I haven't visited LJs for over a week! I have a project - a cabinet with 7 map drawers this week so I'll be putting this to use and will report back the results.

My only concern is the 1/4" insert and making sure it can be 1/4" from the bit. It seems like it will just shave the edge of the receiver and I want to be sure it doesn't affect operation.

Once I know this will work, I'll make this available from my website either as a jig kit or printable download.


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## MadMark (Jun 3, 2014)

The lower corner will get clipped, but the damage shouldn't extend far enough to impact operation. Go slow and hope the plastic doesn't turn into fluff! I computed that factor in, but I've never run a router into a 3D print so I honestly don't have a clue!


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## gbarnas (Sep 25, 2021)

Yeah - there's 3/8 between the pin and the edge of the jig, so 1/8" will be trimmed up to the depth of the cut - usually 1/2 to 3/4" It won't cut into the receiver guide slots, which is good. The larger (3/8 and 1/2") have plenty of space to work.

I'll try and get something together in the next day or so and post the results.


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## gtrgeo (Mar 22, 2017)

Not to detract from the 3d printed solution as I am always looking for ways to incorporate my printer in my woodworking activities, but I thought I would throw this out as another maybe less obvious tool for creating box joints. I bought the Rockler cross lap jig for a project and noticed the locating 'pin' mechanism was similar to the Incra I-box. Just out of curiosity, I tried the jig to make some 1/4" box joints. It worked fantastic and has become my go-to jig for box joints. I had a self-made jig previously based on a Woodsmith project which worked but could be finnicky. The thing I like most about the adjustable width 'pin' on the Rockler jig is that I do not need to fuss with shims on the dado set unless I am trying to meet a specific pitch on the box joints.

Anyway, here is a review I did on the jig Rockler Cross Lap Jig


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## gbarnas (Sep 25, 2021)

Yeah - let me go click that link posted by someone that made 6 posts in an hour. with links.
Any way to ban this kind of abuse? Can mods delete these posts to prevent accidental clicks?


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## MadMark (Jun 3, 2014)

Cricket will as soon as she can. I've flagged and blocked, she'll dele the msgs shortly.

Just got a woodcutting laser working 16×16 with a 5W laser. cuts 1/4" pine like buttah.


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## therealSteveN (Oct 29, 2016)

I think for the OP's use he has a solution in sight using the plastic pins Mark is showing. For anyone else reading along just wanting to make a box joint jig, I'll offer my thoughts.

For a TS jig, I recently made one and out of the box it cut a tight fitting joint. It took me a very short time to make, EXCEPT the time I allowed to glue up, (laminate 2 pieces of plywood for the back fence), but it could have been done single thickness, and finished in less than an hour easily enough.

I loosely based my jig on this one Tommy made. I had made several of them in the past though, so I did a few things my way. I think I explained my rationale, but send me a message if you get stuck on a point, and I'll try to talk you through it. I think making good useable jigs is the quick path to doing better woodwork, and also gives you that "atta boy" motivation when you do make a workable jig, that makes an otherwise harder job easier to do, so I am a jig fan. Both for using them myself, and for helping noobs better do, and understand this hobby.

For my money of I was making one for the router, I would also make the one Nathan referred to here. There are a few videos on U tube for it, but to get the most understandable info for making one I just bought the plan.

WoodSmith video

WoodSmith plan for their router jig


Using it, you can make it as a fixed size and only make one set of "fingers" to let's say make a 1/2" spacing, or you could make several sets of fingers to have several spacing widths. I haven't done it, but I think you could also insert different sized spacers along the joint, to make a staggered looking box joint. IOW eye candy.


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