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Help with finger joints!

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Forum topic by LiveEdge posted 04-06-2014 01:57 AM 2064 views 1 time favorited 26 replies Add to Favorites Watch
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LiveEdge

486 posts in 1085 days


04-06-2014 01:57 AM

I am making a few boxes for fun and decided to use finger joints on the corners. I made a little jig like you can see a million times on youtube, but things aren’t going too smoothly. I chose to use 1/4 inch fingers over about 4 inches. That makes for eight fingers and I realized the problem with the jig is that any error in your spacing is compounded over each joint. If you are off by only 1/100th of an inch you are still out about 1/12th of an inch by the end.

A few solutions came to mind. One, I could use fatter fingers. Fewer joints mean fewer compounding errors. Two. If I could cut all four pieces without moving the fence, then the errors would match on each piece and they would still fit. The problem is I can’t figure out how to do that while still offsetting two of the pieces so each finger meets the proper gap of the joining piece?

Tips? This shouldn’t be this hard…


26 replies so far

View bondogaposis's profile

bondogaposis

4032 posts in 1816 days


#1 posted 04-06-2014 02:31 AM

Show us a picture of your jig.

-- Bondo Gaposis

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LiveEdge

486 posts in 1085 days


#2 posted 04-06-2014 02:42 AM

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Paul

721 posts in 1030 days


#3 posted 04-06-2014 02:55 AM

Fewer joints don’t mean fewer errors. If your spacers and blade aren’t identical every joint will be out of place.

Measure your blade width and both the spacer on your jig and the spacer for your second joint piece with a digital caliper. They need to be within 100th of each other. I had a very hard time with cutting box/finger joints on my saw before I picked up a digital caliper and could properly measure.

Paul

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LiveEdge

486 posts in 1085 days


#4 posted 04-06-2014 02:58 AM

I think I just visualized how to start the second piece without moving the fence. That will help, I’m sure. I’ll look into some digital calipers, but I’ll probably goof around some more tomorrow.

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Paul

721 posts in 1030 days


#5 posted 04-06-2014 03:02 AM

To start the second mated piece you just put an equally wide spacer next to the spacer installed on the fence. You should not be moving the fence at all.

Paul

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Paul

721 posts in 1030 days


#6 posted 04-06-2014 03:04 AM

Other than on your miter slot I should have said. once you make the fence it’s set and should only be used for back and forth cutting of the joints.

Paul

View distrbd's profile

distrbd

2227 posts in 1911 days


#7 posted 04-06-2014 03:07 AM

If you butt the two pieces((that are supposed to join together to make an L shape) against each other when cutting them,there’s less chance of error in your spacing .this youtube video explains what I’m talking about:

https://www.youtube.com/watch?v=bTd81dlRQiE

-- Ken from Ontario, Canada

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Paul

721 posts in 1030 days


#8 posted 04-06-2014 03:15 AM

Steve’s video’s are normally great. For someone trying to get it right and tight for the first time it’s a good watch to learn the basics but as he even states it’s not perfect.

The OP is trying to get tighter joints.

Paul

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LiveEdge

486 posts in 1085 days


#9 posted 04-06-2014 03:18 AM

Well, I’m just trying to get them to fit. :). I had seen that video before and it’s the one I based my jig on. I’m going to give it a go again. I think my biggest problem was I was moving the jig to cut the second two pieces and that introduced a whole second set of errors.

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distrbd

2227 posts in 1911 days


#10 posted 04-06-2014 03:25 AM

I understand and agree 100% Paul,you wouldn’t believe how many boxes I scrapped until I finally got it right by making sure (as you mentioned) the gap between the blade and the spacer is exactly the same(as the thickness of the spacer),having a dial caliper was essential.

-- Ken from Ontario, Canada

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Paul

721 posts in 1030 days


#11 posted 04-06-2014 03:30 AM

LiveEdge,

Like Ken just stated I scrapped a TON of joints before I figured it out. Don’t get discouraged it just takes alot of practice.

Paul

PS get a caliper ;) a $20 once from HD will be enough to put your mind at ease, at least for box joint and finger joint projects.

View neverenougftackle's profile

neverenougftackle

195 posts in 1311 days


#12 posted 04-06-2014 03:31 AM

You will notice at the point of 2:11 in his video where he states that the distance between the pin and the blade,,,,,,wrong ! On this type of TS blade there is a series of teeth that is offset. these teeth protrude slightly to the outside of that blade. He at this point is pointing to the other set of teeth that is NOT off set but centered.

Now this does not seem to be that much of a difference, but as you already noticed it does not take that much for the finger joints to not mesh, and the longer the board to be joined the more the differance.. So that the outside of that first cut threw his 18” back up board,,,both of the outside cut of that openings/ the kurf cuts R&L side into that board is controlled by the off set teeth. Thus if he/you measure from the straight tooth you will be off by the width of that off set of the off set tooth.
You do not measure from the blade as he said, you measure from the off set teeth that is the closes to that pin, that equals where the distance is going to be double the size of that pin and where that side of the cut of that opening is going to be…...it is the teeth that makes that cut not that blades side.

By the way,,,making a guess in your all of your shop time how many more times would you conceive that you will be making any more Box Joints. I ask my self that same question on failing at this pin set up that you are doing some time back. I went on to make Shopnotes Magazine’s adjustable Box Joint Jig and never have regreated it. It will solve all of this ,”Blank”, back and forth guessing once an for all,,,look it up.

View Paul's profile

Paul

721 posts in 1030 days


#13 posted 04-06-2014 04:17 AM

That was the long legged lady like way of saying get a caliper and measure correctly. I thought we already covered that but maybe not.

Paul

View unbob's profile

unbob

718 posts in 1368 days


#14 posted 04-06-2014 05:10 AM

A couple of things that helped me.
The fixture pin needs to be a tight fit in the created slots. Otherwise, with a loose fit fixture pin, there can be a slight shift that adds up accumulative error. The pieces may not fit, it does not take much error.
The fixture face with pin can be attached to a back board so as it can be shifted slightly, makes things go better.

Some wood I do such as spruce, is fragile. Not good to use a mallet to knock the sides together with that wood, it chips out easily.
I use a .004” shim in the blade stack to cut the mating pieces. Since I do that, I shape the fixture pin a little like a wedge, so as both sized slots will fit the fixture pin without side movement.

Long pieces and many slots, get a little more demanding. Errors from a board cupping and not sitting flat against the fixture can make a mess of things where as the pieces may not go together.

Cutting the slots, and the parts fit now, may not fit tomorrow from stress changes in the wood “twisting and cupping. I found it best to prepare the boards, cut the slots, and get it together as quickly as possible within hours not days.
On some woods, or on very long joints, water based glue can swell the pins enough the joint will not go together or will seize part way together. There I use slow dry epoxy, doesn’t swell the pins, and acts like a lube.

Do a lot of test pieces, you will get the hang of it.

View Redoak49's profile

Redoak49

1951 posts in 1453 days


#15 posted 04-06-2014 10:44 PM

Unbob had some good advice. I have worked at building several jigs and this is the one that works the best for me. Note that there is a machine screw on one side that is used to adjust the distance between the blade(or router bit) and the notch. I have found that with the same exact setup, there are differences between making a box joint in pine versus oak versus poplar. Being able to do very fine adjustments makes a huge difference. Good Luck

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