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Long finger joints in car wood.


chistech
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I have been making a lot of new wood for many different cars and most of the curved parts use a 1-1 1/4” long finger joint. Things like the arch in the rear well area of a four door or the deck rails on a coupe or roadster are where this type of joints are commonly used. Most commercially made finger joint bits only make 1/2” long joints. I realize I can cut these by hand or cut them on a bandsaw but I’m looking for a more duplicatable way to machine them. It’s my understanding that there is someone out there (possibly by the name of Jay) who has had a bit made up for his vertical mill to duplicate these joints. If anyone has any ideas or knows of some way to make these joints more accurately than by hand, your input would be appreciated. I am considering a toolmaker to make the cutters and arbor but if someone has already done this, it will save me a lot of engineering.

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Christech, do you use a shaper? I probably have the same finger joint bit that you have but have not seen one for 'long' cuts. I would contact a company that makes cutters. Last simple one I had made about a year ago was around $200 so I would bet like Restorer said, it will most likely be expensive. If you are in the business, you know it will be worth the cost. I have cut 10 miles worth of dovetail joints by hand but cant imagine attempting a finger joint. I think I would lose my narrow mind!

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If you have a mill it would be very simple to make your own tooling. My approach would be to simply make an arbor long enough to stack slitting saw blades and spacers on to make the finger joints you want. Slitting saw blades are readily available with diameters large enough for 1 1/4+ depth of cut and tooth count for a very fine cut in wood. You would, of course, have to calculate offsets and cut depth, but with a calculator and a mill dial that would be very easy. A DRO would make it seamless. Just my thoughts...

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Thanks everyone. I have been doing a ton of research and I've decided to turn an arbor on my lathe with a 3/4" shaft for my vertical mill collet out of 1" rod and then leaving a 1/4" thick flange, turn 2" down to 5/8". I'll either thread the bottom with a 1/2 die or bore the end for a 3/8" hex bolt. I will make a 1" diameter washer with a 5/8 recess to go over the end of the arbor and apply the needed pressure on the blade stack. I will stack 4.5" carbide saw blades with appropriate spacers between the blades to make box joints rather than finger joints. Todays 4.5 blades will yield 3/16" if two are sandwiched together with teeth staggered which is the average thickness of the fingers on the joints I'm duplicating. I don't see much difference in strength and will get the same splicing results as the finger joints. I started to price out tapered slitter saws along with long enough arbors to make a finger joint bit and the price shot up to over $1000. For the slight difference of joint type and the fact that all this joinery will be unseen, I'll go for function rather than exact authenticity. With the 4.5" blades, it will allow me to make even deeper joints if needed. I might post the making of the bit in the restoration forum even though it's not a restoration of a car. I'm sure others might like to make one themselves.

Edited by chistech (see edit history)
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10 hours ago, Restorer32 said:

We had a cutter made to cut long finger joints one at a time.  We use our vertical mill and can make joints accurate to within a thousandth of an inch. Our cutter cost about $400 but this was 10 or so years ago.

I was actually going to PM you as I figured if anyone had tackled this issue it was probably your shop. Thanks

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9 hours ago, TAKerry said:

Chistech, do you use a shaper? I probably have the same finger joint bit that you have but have not seen one for 'long' cuts. I would contact a company that makes cutters. Last simple one I had made about a year ago was around $200 so I would bet like Restorer said, it will most likely be expensive. If you are in the business, you know it will be worth the cost. I have cut 10 miles worth of dovetail joints by hand but cant imagine attempting a finger joint. I think I would lose my narrow mind!

The finger joint bit I purchased was for a shaper. The spec's showed the blades were 15/16" deep but when the bit arrived, the spec's were incorrect. I don't currently have a shaper and don't think I'll end up needing one. I either use my vertical mill or a hand held router with the mill doing the majority of the work. I agree, making the joints by hand might work for one or two but not for multiples plus the accuracy will never be the same as a proper bit. I've done enough stuff like that already on these old cars that I went crazy years ago!

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1 hour ago, 37_Roadmaster_C said:

If you have a mill it would be very simple to make your own tooling. My approach would be to simply make an arbor long enough to stack slitting saw blades and spacers on to make the finger joints you want. Slitting saw blades are readily available with diameters large enough for 1 1/4+ depth of cut and tooth count for a very fine cut in wood. You would, of course, have to calculate offsets and cut depth, but with a calculator and a mill dial that would be very easy. A DRO would make it seamless. Just my thoughts...

We think on the same lines! I'm going with simple circular saw blades because they want about $150 each for slitter saw blades and most need a 1" keyed shaft. 

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@chistech, where are you located? I can find slitting saw blades from less than $10 to over $500 depending on size, material and brand. For wood you do not need name brand carbide blades.  Simple circular saw blades will work, but a higher tooth count will make a smoother cut. Making a keyed arbor is a simple matter if you have a lathe and a mill.

   Another idea to save on blade cost would be to make angled spacers for the blades. You would need to draw it out and do the math, but the basic idea is to mount the blade at a slight angle to the spindle of the mill. This effectively makes the blade "wobble" vertically. While this would be an absolute NO GO for metal, but for wood, with a slower feed it would allow one blade to cut a full slot width. If you designed the spacers to evenly distribute the tilted blades around the spindle it would even be in balance. For example, with two blades offset 180 deg you could cut two slots making a three finger male joint with minimum vibration. This tool design effectively grinds out the slots while making square cuts.  By making differently angled spacers and using various diameter blades you could easily have a selection of tooling for various width and depth cuts.

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I had these finger joint cutters made about 20 years ago, don't remember who made them or exactly what they cost but they were expensive. Still using them today. I made a sled type jig for my shaper that I use to hold and feed the wood. Nice to be able to cut multiple fingers at one setting.

 

The second picture are cutters that I use in my mill. The biggest one is a 8" x 3/16". the middle sized one is 6x5/16", the smallest is a homemade arbor with a 4-1/4" saw blade on it. The smaller one works well but I wouldn't use soft steel on anything bigger than that, the two bigger ones are hardened ground arbors.

IMG_2737.JPG

IMG_5176.JPG

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3 hours ago, chistech said:

I might post the making of the bit in the restoration forum even though it's not a restoration of a car.

Please do.

 

1 hour ago, NewOldWood said:

I had these finger joint cutters made about 20 years ago, don't remember who made them or exactly what they cost but they were expensive.

I looked all over for something like that. I'm not surprised you had to have them made.

 

 

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2 hours ago, 37_Roadmaster_C said:

@chistech, where are you located? I can find slitting saw blades from less than $10 to over $500 depending on size, material and brand. For wood you do not need name brand carbide blades.  Simple circular saw blades will work, but a higher tooth count will make a smoother cut. Making a keyed arbor is a simple matter if you have a lathe and a mill.

   Another idea to save on blade cost would be to make angled spacers for the blades. You would need to draw it out and do the math, but the basic idea is to mount the blade at a slight angle to the spindle of the mill. This effectively makes the blade "wobble" vertically. While this would be an absolute NO GO for metal, but for wood, with a slower feed it would allow one blade to cut a full slot width. If you designed the spacers to evenly distribute the tilted blades around the spindle it would even be in balance. For example, with two blades offset 180 deg you could cut two slots making a three finger male joint with minimum vibration. This tool design effectively grinds out the slots while making square cuts.  By making differently angled spacers and using various diameter blades you could easily have a selection of tooling for various width and depth cuts.

I’m located in MA and looked in quite a few places like eBay and McMaster Carr. My buddy owns a machine shop and he also looked for me including calling his tool and bit maker. Those finger blades New Old Wood shows are now quoting at over $200 each. I just purchased on Amazon 6 sets of 3, 4.5 x24 tooth carbide blades for $16 ea set.  They are only 1.8mm wide so I will probably end up stacking in groups of three, that’s why I purchased 6 sets. Many 4.5” blades only have 3/8” arbors and the finger joint bit I purchased might be a 3/8” mandrel but I think the mandrel would be too flexible with a stack of blades on it so I’m sticking with a heavier arbor to prevent that. I will start machining the arbor tomorrow while I wait for the blades to arrive. Got to order some 5/8 shims also. 

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I am posting in two different threads as different people are following each thread. Ended up getting a satisfactory result of my shimming so the tool is ready to use. Here’s a picture of the joint made after the recent adjustments. One groove has a tiny space but nothing to speak of or lessen the joints integrity. The two pieces went together with a light tapping of a rubber mallet.

803FDC65-0147-414E-B43B-FA3F4CDA4BB6.jpeg

BD4BDF6B-E3FC-48C6-A138-828DA9612239.jpeg

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