Experiments with Finger Joint Fixture

[msimpson99]

I was in the need of some finger joints, and decided to see if I could make a fixture for my CNCCS builds. I have done something similar with the KRMx02 in the past with a removable section in the table, but could not use it as, the KRMx02 is currently undergoing some conversions.

Here is what I came up with:


It consists of a piece of 3" x 3" x 17" 15 series thick walled extrusion that is attached to a birch plywood table top. There is also a piece of 2" x 2" x 20", 1/4" steel angle attached to both the extrusion and the plywood.

The thing I am going for here is rigidity. I thought about using aluminum plate for the base instead of the plywood, but it would have been too heavy to place on and off the machine.

A note about the table top (base)
I have several removable table tops, that I use for various operations. The base used for this project was based on those designs so that it could be easily attached and removed as needed. The only real difference is that instead of 20-1/2" wide it is 20-1/4" wide. The reason for this is that I have added some dust guards and they need an extra 1/8" clearance on each side.

You can see my table top write-up here:
http://www.kronosrobotics.com/cnccstables1

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[msimpson99]

While this fixture is still undergoing changes, I will take you through some of the iterations.

This was the first pass at the fixture:


It worked, but had some issues. The first was the stock faced away from me, so it was difficult to get the stock secured properly. Also, since it sits on my plasma slats, its a little finicky to get setup. I knew I was going to have to make a table that sits on top of the frame.

[MARV]

What you're trying to cut is more commonly referred to as a box joint. A finger joint typically has tapered fingers and is quite often used to extend the length of stock (as in residential door jambs as seen at Lower and Homedepot).

I applaud your efforts as these and dovetail joints lend themselves to being cut on CNC provided you can orient the stock correctly.

[msimpson99]

I remember finger joints. I used to have one of those crazy Craftsman molding heads that had some finger cutters. Unfortunatly, most folks on the web, are using the term "Finger Joint" for the "Box Joint", oh well potatoes....

[msimpson99]

Please note that I used supplies that I had on hand so some of the parts may have some extra holes.

Lets start with the table top:


Its a piece of scrap 3/4" birtch plywood cut to 20-1/4" x 18". The base sits on the sides of the frame and is secured by four studs previously installed on the frame.

The 2" steel angle:
The 1/4" steel angle was salvaged from an old KRmx01 build, so it already had holes I used to attach it to the table. Note that any holes will work as long as you have at least 4 holes to secure it.

Originally this angle was going to be the full support of the extrusion. Now it helps secure the extrusion and stiffens the table top.

Stock Guide:

The stock guide is cut from a piece of 1" x 1/4" x 16" aluminum bar stock. It does not have to be this long, but I wanted to start off long before deciding on a final length.

The stock guide is used to both set the edge of the stock, and to set the proper angle of the stock. IE it allows you to make sure your stock is nice and square to the router bit.

[msimpson99]

Original Clamps

The lower clamp is a twist type Festool clamp. It fits the 15 series extrusion perfectly.


The lower clamp is only meant to hold the stock loosely so that the spacer block can help you set the stock to 1" above the extrusion, as shown here.


I am using a Festool heavy duty clamp for the upper clamp, as shown here.


I cut parts for a couple frames and it worked well enough, but was not without its issues, that I will go into next.

[wizard]

Nice little retro fit. I like that”I used what I had on hand” mentality, sometimes it is required thinking.

If might ask though how bad is tear out? I did not see a backer board unless of course that board is the backer. The other thing here is this, have you considered a circular saw head? Sawing just seems like the most efficient way to do finger joints.

By the way I do believe finger joints is a correct term here. Box joint is simply a more specific usage.

[msimpson99]

The tear out is not as bad as I thought it would be. I do a rough pass at 50IPM and 20,000 RPM with a 1/8" two flute bit. I am only doing about a 1/16" depth of cut. I do it about .01 shy of final.

The machine then does a full depth pass at 25IPM and 24,000 RPM (Climb Cut). For the project I am doing a little tear out is OK. I can see myself adding a backer board where needed.

[nlancaster]

You can fix tearout by placing sacraficial material on the side that tears out.

[RCaffin]

I have all my finger joints, thank you.
A replacement thumb joint could however be useful.

Cheers
Roger

[msimpson99]

New Lower Clamp

The problem with the lower Festool clamp is that it was hard to reach and was finicky to get the correct tension on the stock. I decided to go with a toggle clamp attached to the end of a piece of 1-1/2" x 1/4" x 8-1/2" aluminum bar stock. Two holes were added to attach some 5/16" carriage bolts so that the whole assembly can be adjusted.


The toggle clamp is on or off, so once tuned it works great for repeatably.


I replaced the rubber pad with this wooden one. Now, it can hold the stock firmly against the extrusion, and allow it to be adjusted.


I simply sit the spacer on top of the stock and push down.

[msimpson99]

New Upper Clamp
The upper clamp works well enough, but it is a pain to have to slide it out of the way when setting up the stock.

I cut a piece of 1-1/2" x 1/4" x 4-1/4" aluminum bar and mounted an even larger toggle clamp to it.


I left the rubber pad on the clamp and adjusted it so that it places a great deal of pressure against the stock.

[msimpson99]

How Well Does it Work?

I have already made several frames, and the fixture works perfect. I had an issue where two of my sides were slightly thicker than the original stock I started with. The meant that cuts on the the originals had to be made about .03" deeper.


I placed the cut part back on the fixture, and it aligned perfectly. It cut them a little deeper and helped me save that box.
The repeatability is great. I already have a couple probe scripts to help me do the initial setup.

[Biggs427]

Great job!

How do you produce de G-Code?

[msimpson99]

I layout the end (top down view) in CorelDraw, and use Vcarve pro to create the Gcode. I have also used SheetCam to create the Gcode for these. But you need to start with a drawing.

Here is a image of the drawing in Vcarve:
Attachment 428532

[msimpson99]

Here is a chess box I made using the finger fixture.