Hey Scott, no worries.  A 1-1/2" radius tool will be 3" wide.  I used a 3" plastic putty knife from Home Depot, measured 1-1/2" down on both sides and a tick mark in the center of the blade with a Sharpie.  I then used these marks to round over the square edge into a perfect half circle. 

You can do the trick that some guys use when making round spars - trim the corners off at 45°, then trim those corners off, then trim those corners off until it approaches a half circle.  Hope that makes sense.

Here's a pic of my tack weld 1" radius tool and my 1-1/2" fillet tool.

Find a scrap cut from a length of 3” PVC drain pipe, the thick-walled kind (Schedule 40) not the thinner stuff. ID’s close emough to 3” for our purposes to serve as a template for marking out cut-to line. Just center it nicely on your workpiece so opposite edges catch the maximum ID points evenly.

Thanks for the support on my math question about radius tools. A 3” putty knife marked as you suggested to achieve the 1 1/2” radius tool. To clarify, will I need a 4” putty tool to make a 2” radius, etc., etc. Is that how the math works on that?

The Forum ROCKS! 

Yep,’tis.

Radius = 1/2 diameter (or width in this instance)

   I've found taping fillets to be worth the extra time, but pull up the tape while it's still wet

Yeah but taping fillets only give you a clean edge, it doesn't establish the size of the fillet, the tool does.  I tried taping my fillets and it was a disaster.  Where do you put the tape?  The intersecting angles all change, so the distance from the jointvaries. 

If you put the tape a bit too close and you pull it up, you leave a shoulder.  If your tape is too far out, it doesn't do anything.  Even though I'm way past that point, I'd love to hear your process.  Do you place the tape with the fillet tool as a reference?

I'm with Skully on taping fillet edges. Even when it works the way it's supposed to, you end up having to sand the shoulder. It also encourages fat, heavy and expensive fillets. The rounded tool automatically adjusts the fillets' thicknesses to perfectly fit the angle made by the wood. It gives you the minimum possible thickness necessary. The only possible use I see for the technique is a classroom situation when you want decent looking fillets as soon as possible with minimum time needed to develop newbies' skills and you don't care about the sanding time because it'll be done at home.

For glass taped seams, you'd use a tool radius that matches the minimum bend radius for the weight of glass (about 1/2 inch for 6 oz. tape, more for heavier, less for lighter ). Matching the minimum bend radius prevents the glass pulling away from the fillet while not applying more putty than is needed. You get very thin fillets that way.

If the fillet ends up with a ridge of epoxy on either side, It's a simple matter to use a putty knife to scrape it off and use it for the next part of the fillet.

Laszlo