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Box Joint Jig - Accuracy question

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Forum topic by gbarnas posted 04-14-2022 09:46 PM 928 views 0 times favorited 22 replies Add to Favorites Watch
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gbarnas

123 posts in 276 days


04-14-2022 09:46 PM

Topic tags/keywords: boxjoint jig

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?

-- Glenn, Jersey Shore, NJ


22 replies so far

View Jerry's profile

Jerry

3591 posts in 3142 days


#1 posted 04-14-2022 10:27 PM

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…

-- There are good ships and there are wood ships, the ships that sail the sea, but the best ships are friendships and may they always be. http://www.geraldlhunsucker.com/

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gbarnas

123 posts in 276 days


#2 posted 04-14-2022 10:58 PM

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

-- Glenn, Jersey Shore, NJ

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Madmark2

3485 posts in 2082 days


#3 posted 04-15-2022 05:01 AM

.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

-- The hump with the stump and the pump!

View Lazyman's profile

Lazyman

9779 posts in 2881 days


#4 posted 04-15-2022 12:11 PM

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.

-- Nathan, TX -- Hire the lazy man. He may not do as much work but that's because he will find a better way.

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jarheaddoc

10 posts in 1498 days


#5 posted 05-16-2022 07:35 PM

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

1148 posts in 5240 days


#6 posted 05-16-2022 10:11 PM

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)?

-- NorthWoodsMan

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gbarnas

123 posts in 276 days


#7 posted 05-18-2022 01:10 AM

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.

-- Glenn, Jersey Shore, NJ

View Madmark2's profile

Madmark2

3485 posts in 2082 days


#8 posted 05-18-2022 01:25 AM

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

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

-- The hump with the stump and the pump!

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gbarnas

123 posts in 276 days


#9 posted 05-18-2022 11:46 AM

@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.

-- Glenn, Jersey Shore, NJ

View Lazyman's profile

Lazyman

9779 posts in 2881 days


#10 posted 05-18-2022 12:30 PM

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.

-- Nathan, TX -- Hire the lazy man. He may not do as much work but that's because he will find a better way.

View Madmark2's profile

Madmark2

3485 posts in 2082 days


#11 posted 05-18-2022 04:42 PM

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.

-- The hump with the stump and the pump!

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Madmark2

3485 posts in 2082 days


#12 posted 05-19-2022 03:30 AM

-- The hump with the stump and the pump!

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gbarnas

123 posts in 276 days


#13 posted 05-19-2022 11:33 AM

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

-- Glenn, Jersey Shore, NJ

View Lazyman's profile

Lazyman

9779 posts in 2881 days


#14 posted 05-19-2022 12:59 PM

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.

-- Nathan, TX -- Hire the lazy man. He may not do as much work but that's because he will find a better way.

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Madmark2

3485 posts in 2082 days


#15 posted 05-19-2022 02:01 PM

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

-- The hump with the stump and the pump!

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