box joint jig: advice sought

[Sully]

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[ptt49er]

Looks good to me Jay.

What are you having them make the template out of?

 

[Sully]

Thanks. I'm going to use 1/4" 6061 aluminum.
J

 

[Bigdog72]

Is this going to be a "for sale" item?

 

[Sully]

Is this going to be a "for sale" item?

I'm just making them for myself to use on an older model 24" Porter Cable omnijig. PC never made a 1/4" or 3/8" box joint template for it. PC did make a 1/2" box joint template but it has been discontinued and is rare (and very expensive at the places that do claim to still have them in stock).

I'm also tweaking a design for through dovetails. I have the adjustable thru template but want something that is more plug and play with less set up.

J

 

[striker]

Jay,

I'm not fully understanding your math issue but I'm sure others will comment. Generally speaking, 1/2" means something different than .5 or .500 to a machine shop. You may want to be clear on your tolerances - cumulative error is not your friend when it comes to box joints.

Stephen

 

[Sully]

Jay,

I'm not fully understanding your math issue but I'm sure others will comment. Generally speaking, 1/2" means something different than .5 or .500 to a machine shop. You may want to be clear on your tolerances - cumulative error is not your friend when it comes to box joints.

Stephen

I'm not too worried about the measurement specifications since I will be working closely with the machinist. We will convert all the line drawings to the appropriate scales and discuss the need for accuracy and precision. Thanks for the reminder.

The math issue was a request for someone to double check the geometry of the box joint profile that would be produced using the bit size, bushing, and template dimensions described in the table and figure. For example, the first row of the table is supposed to give a 1/2" box joint profile when cut on the end of a board. The bit size is 1/2" and the bushing size is 1/2". Here's a sketch of that arrangement (click for larger version):



The geometry checks out to give 1/2" cut spaced every 1/2". I probably wasn't very clean in my original post.
J

 

[striker]

Jay,

You get my vote for correctness. 1/8 on the finger sounds a little small- is it gonna bend/brake?

 

[Alan in Little Washington]

The only comment I would have is the fit of the bushing. You may not want the bushing to be the same size as template slot- could be too tight of a fit for smooth operation and maybe too much wear on the template or the bushing.

You can use a smaller bushing/bearing and bit, or a wider template opening. When you make a cut you would need to make sure you run the bushing against both sides of the template. Adjust the size of the fingers/slots as needed for these changes- requires a little more math. That is what I would do and that is how some DT jigs work. Let me know if you want a diagram size chart to further explain.

 

[Sully]

Yep, I was worried about that 1/8" finger also. :dontknow:

The smallest finger I've ever seen on a jig like this is 1/4" so I probably need to rethink that one.....

Thanks,
J

 

[Alan in Little Washington]

I use my tablesaw for 1/8" fingers (actually for most box joints.)

 

[Sully]

The only comment I would have is the fit of the bushing. You may not want the bushing to be the same size as template slot- could be too tight of a fit for smooth operation and maybe too much wear on the template or the bushing.

You can use a smaller bushing/bearing and bit, or a wider template opening. When you make a cut you would need to make sure you run the bushing against both sides of the template. Adjust the size of the fingers/slots as needed for these changes- requires a little more math. That is what I would do and that is how some DT jigs work. Let me know if you want a diagram size chart to further explain.

I follow you. Row 2 and 3 in my table are this type of scenario (gap is larger than the bushing).

The PC Omnijig often uses a gap that is matched to the bushing size, in particular for 1/2" half-blind dovetails and the 2 inch "hand cut" pins the bushing is 5/8" and the gap size is nominally 0.625". In actuality the variation in both of these is at the thousands level. The brass bushing should be softer than 6061 aluminum so any wear would be predominantly on an easily replaceable bushing. Brass is also somewhat self-healing so I think I'll be okay here.

J

 

[Sully]

I use my tablesaw for 1/8" fingers (actually for most box joints.)

The "fingers" in the case I'm talking about is the 1/8" fingers of the router template. The fingers of the box joint for that template come out to be 1/4".

I do have a work-around to avoid a 1/8" aluminum finger on the router template. In this case I would setup to route every other cut and then have a gauge block to offset the workpiece by the appropriate amount to make the rest of the cuts.

Thanks to everybody for the feedback. It is very helpful! :icon_thum
J

 

[Alan in Little Washington]

The "fingers" in the case I'm talking about is the 1/8" fingers of the router template. The fingers of the box joint for that template come out to be 1/4".

I do have a work-around to avoid a 1/8" aluminum finger on the router template. In this case I would setup to route every other cut and then have a gauge block to offset the workpiece by the appropriate amount to make the rest of the cuts.

Thanks to everybody for the feedback. It is very helpful! :icon_thum
J

I understand. But, still I can do box joints with 1/8" fingers using a jig on my tablesaw. You can't do that with a router and template. I just like the tablesaw more than the router for cutting through stock, especially if you want something like 1/2" fingers in stock thicker than 3/4". Routers are so noisy and messy, and router bits do not really like to cut when they are "captured" by wood on both sides. That is why most recommend cutting router dados the width of the bit, using multiple passes and a plunge router. For me it is just easier and quicker to do with a table saw. I built a jig with replaceable inserts that have 1/8", 1//4", 3/8", 1/2", and 3/4" wide guide pins that works well with my TS and dado blade, but will eventually replace it however, with one that uses an adjustable width guide pin. I saw it in one of the WW mags a few years ago and I believe Monty has one posted on the site here somewhere. The only negative with the table saw is more blow out unless you use a sharp blade and a good backer.

 

[Gofor]

Not going into pros and cons of methods, but with the close tolerances, you may want to be sure you have an accurate centering tool for installing the bushing. Even a few thousandths off will result in fingers/slots that won't match. Also, if you get your bits sharpened, the reduction in diameter could have a major impact.

Go

 

[Alan in Little Washington]

Not going into pros and cons of methods, but with the close tolerances, you may want to be sure you have an accurate centering tool for installing the bushing. Even a few thousandths off will result in fingers/slots that won't match. Also, if you get your bits sharpened, the reduction in diameter could have a major impact.

Go

A similar situation happens on the TS with a dado blade. That is why I am going to change to a jig with an adjustable instead of a fixed guide pin. It will be easier to adjust the pin to match the dado than visa versa.

 

[Sully]

A good centering pin is a must for any template operation. This particular application is no different in that regard.

Actually it is possible to cut 1/8" box joints with a router, it just requires a 1/8" bit and the appropriate template. The setup is a little trickier in this case since it requires two passes on the template, but it is possible. That being said I don't really see myself making 1/8" box joints.

There are advantages to both methods of box joints (table saw versus router).

J