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Making a finger joint cutter, an experiment!


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As many know, I do a lot of body wood work and the need for a long finger joint cutter is becoming more apparent. In the past making one or two joints by hand has worked but now I need to make many more joints. I need a tool bit that will make deep joints, 1 1/8” or deeper, as commercially available bits have a maximum depth of 5/8”. Inquiries in the general forum yielded what I expected, any long finger joint bit has to be custom made.

     Looking at tapered slitting saw blades, their required arbors and spacers quickly shot any estimated prices easily over a thousand dollars for a bit. Thinking about my needs and my capabilities I decided to attempt to make a multi blade box joint cutter. The difference between a finger joint and box joint is the tenons and mortises are straight on a box joint rather than the tapered one of a finger joint. With all the wood I’m making being covered either by metal or upholstery, the integrity of the joint matters over authenticity. 
      My bit design will consist of an arbor with a 3/4” diameter for my mill collets, a 5/8” shaft for 4.5” saw blades, and a 1/2-20 threaded end with a counter bored washer to apply pressure to the blade stack. I’ll be using 4.5” circular saw blades, with a 1/16” kerf, grouped three together, with spacers in between each group equal to the width of the blade group kerf. I have eighteen blades coming for a total of 6 slots being cut in each pass which so cover the majority of the wood thickness I have to joint. 
     While I realize this short thread posting will not be a car restoration, it will be about an important tool than many have seemed to want or need in the past. If the moderator seems fit to move it, he will.

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Made up my arbor last night on the lathe. Came out very nice, true, and with very close tolerance so they’ll be no blade slop. The 3/4” section is 2” long, a 1/4” blade flange left the 1” diameter of the stock, the blade area is  5/8” diameter, 2” long, then the end was turned to 1/2” and threaded 1/2-20. Made up a recessed end cup to apply pressure on the blade stack when the nut is tightened. Bored the cap with a 1/2” drill, used a 5/8” end mill to make the recess, then using a cutoff tool, separated the cup washer from the material. I will make my spacers the same way using .002 shims where necessary to make all spacing equal. I put a dado set on my arbor to check my shaft tolerances as this dado set is pretty high quality and fits tightly on my saw. The set just slid on like the arbor was made for it. We’ll, I guess it kind of was.😁 Now, waiting on the blades to come in from Amazon. I wasn’t going to invest big dollars in blades until I tried out my design. 

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Edited by chistech (see edit history)
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I am far from a JP for sure but my machining is getting better. I was a little hesitant on making the arbor and asked my buddy with a machine shop if he would make it. He assured me that I was capable and to just do it. Sometimes we all just need a kick in the butt. If I screwed it up all it was is a 5-6" piece of 1" round so what the hell I thought. Pleased I attempted it. I feel more confident on the vertical mill than the lathe but now I have a standoff mount for a landau arm to make for my current job I have in the shop. I'm completely re-wooding and restoring all the rotted sheetmetal in another 32 Olds roadster that was sent to me from Syracuse NY. It's one of 16 ever made in CN and probably the only one left in existence. The sheet metal is really bad so there will be a lot of panel making, shaping, cutting, tin knocking, and mig welding. I'm just making all the wood, including the doors, putting the doors with glass and latches back together, installing the body on the wood with the doors. So basically a body restoration ready for fine body work when it leaves. It's a big job though, probably 20 times more than my own car as far as the body goes. I might start another thread on it if I have time. 

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Mike, I’ll be using this in my vertical mill so the blade speed will be around 2500 and the feed is controlled by how fast I turn the crank. The pieces with be held by the vice and the pieces will all be square. Some of the ends will be angled so the vise will be turned to match the angle. Slow, steady, and hopefully as safe as they can be done.

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I'm going to second Mikes advice about going slow. A slow spindle speed will allow a slow feed rate without burning the wood. Also, your mild steel arbor isn't going to be as rigid as a hardened one would, so pushing the feed too much will cause a lot of chatter. In my experience the saw blades don't give as nice a finish as milling cutters. I would also recommend going to Mcmaster or someplace like that for your spacers. They have almost any size you could want, and it will avoid the stacked error you are going to get with lathe made spacers. Work holding is going to be a different set up for every piece, a whatever it takes type of thing. Here are a couple pictures of one of the many parts I have cut on my mill. I do a lot of work using a vise too, but not for this piece.

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Don't kid yourself Ted...Your work is every bit as good as mine.

A question though...table saw arbors have a LH thread. I'm wondering if that will make a difference the way you are using this. If it does, the simple solution is to flip the blades over and reverse spindle.

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Seeing since I was on a roll with the lathe, I decided to tackle the standoff for the landau arm. Thought I would get it finished and couldn’t believe I didn’t have a 5/8-11 die in my box. Damn near every other but that one. Will borrow one from my neighbor tomorrow and finish it up.  I’ll put the 3/4” collet on the lathe to hold the piece, put it in back gears, and run the die up by hand using the tail stock to keep it straight. (A little trick I learned from JP’s thread!😁)Just using the tools we have a little more helps make us more proficient. 

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1 hour ago, chistech said:

Machining eats up time quickly for sure.

Yes, it does, but you now have the part much more quickly than if you had farmed it out to a machine shop, and with less out-of-pocket cost.  Plus, you found a use for the Dartmouth Chronicle beyond lining the bottom of bird cages!

 

If you want to get it plated for rust protection, Star Plating in New Bedford does bright zinc and I think they now do nickel, also, but no chrome.  Turnaround is usually a few days, pretty small lot charge.  I've been using D&S Plating in Holyoke for chrome on smallish parts, i.e. not bumpers.

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Gary, that's a good bit of information One of the issues I have is that I don't like painted machined parts and I'm not sure I want to add plating to my repertoire. Bright zinc may be a solution...

 

Ted...it sure does but you get exactly what you want.

Edited by JV Puleo (see edit history)
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  • Nice work indeed. I'm a retired cabinetmaker and had to read this thread.  I really think that the Dartmouth Chronical is a superlative periodical to the Nevada Appeal (appall?) at least there are more pages of useless than I get from our twice weekly (weakly?), full of the latest news from the wine bar openings and sent in high school news. Still no mention of the Queen's passing. 
  • But, what I actually want to say is that if you don't want your address all over the place, you may want to watch the photos that you pose parts on for things like that. Stay safe and keep the good work going. 😎
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The experiment continues. Got in my 18 blades today so I assembled them on the shaft. Did some math and figured I needed an average shim distance of .240 between each blade stack. Then for some reason I let my brain argue with my measurements and came up .197 as what I should make the shims up as. Of course they’re not perfect but they are all within a few thousands of each other. Assembled the whole thing and it required another .250 shim on the bottom so the nut would apply enough pressure to keep the stacks tight. Once tightened up, I realized I should have gone with the .240 shims I originally came up with but decided to test the operation as it. It ended working better than I expected. I ran my mill at 2000rpms and made 1/4” deep passes. The blades cut extremely smooth and pretty damn even. The picture of the long grooved piece was the first piece and the depth of grooves are slightly over 1 1/4”. On the two pieces that are joined together, I crudely shimmed the blades using whatever washers and dado set shims I could cobble together to get as close to .240. You can see where there are gaps but also where the mortise and tenon are snug. I could only go so deep because the dado shims I used were at least 2.5” in diameter and wouldn’t allow for any more depth. Knowing that my design is very workable, I will order some real shims to get my cutter as close to precise as I can make it. The arbor ran very true and the blades while snug, still slid on the shaft without any issues. More to come. So far the cost is just about $100 for the blades and my time to make up the arbor. A lot better than $1,500-$2,000 for a custom cutter.

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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.

    I used some 5/8” heavy washers with some bronze 5/8” washer’s to get the proper spacing. Just needed to lightly dress the bronze washers to get proper blade spacing. To adjust the height of the ut from one piece to the mating piece I set the quill stop equal to the width of one blade. I cut the first joint, then drop the quill to the  stop and cut the second joint. This allows the pieces to fit together with the surfaces even. Now I can make the joints very quickly and accurately.

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Ted, not being a machinist or wood worker, I have to admit that I have the same reaction every time I look through the thread topics on this forum.  I see this topic and think "Ouch, that's going to hurt!"  😄  Seeing the photos of the wood pieces has helped me understand what kind of "fingers" you're talking about!  Beautiful work, as usual.

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  • 3 weeks later...
On 9/19/2022 at 12:43 PM, neil morse said:

Ted, not being a machinist or wood worker, I have to admit that I have the same reaction every time I look through the thread topics on this forum.  I see this topic and think "Ouch, that's going to hurt!"  😄  Seeing the photos of the wood pieces has helped me understand what kind of "fingers" you're talking about!  Beautiful work, as usual.

Same here, I cringed and thought, “why doesn’t he just use a band saw to cut the knuckles on his fingers like the rest of us do…”

 

But then I realized I had the topic quite wrong. 

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