Bearings, collars, and bits... oh my!

[Andy Sowers]

As I posted in a separate thread, I'm trying to create some router templates to exactly match the inside perimeter of a clock frame I'm working on. For reference, the parts I'm trying to fit are in blue below:

temp.gif

After sitting down with paper/pencil I figured out a way to do it, but it took several steps, I figure there's got to be a more efficient way to do this.

Here's the steps I took.

1) Using a flush trim bit, I create an copy of the workpiece to a piece of 1/4" MDF.

2) Then I placed this template on top of another piece of MDF, and ran a 3/8" rabbet bit around the template. This created a second template that was 3/8" smaller than the original workpiece.

3) Finally, I used this second template along with a 1/4" bit inside a 1/2" collar. This moved the edge back out 3/8".

This created a third template which fits nicely inside the original workpiece. I plan on using this third template and a flush trim bit to route out the final pieces.


Is there a way to do this in fewer steps?

Thanks,
Andy

[Charles McCracken]

I'm not sure I follow what the end result is, but if I understand the collar/bit combinations correctly a 1/4" bit in a 1/2" collar will yield a 1/8" step, not 3/8". You'd have to use a 1" collar to get 3/8".

[Tom Veatch]

I'm also not sure I follow the OP's process, but using a 1/2 collar 1/4 bit combination can give a 3/8 offset if using a "negative" template to produce a "positive" result or vice-versa.

To visualize it, think of a 2" diameter solid template affixed to the center of a 4" square stock. Running the 1/2-1/4 collar bit combo around the outside of the solid template will produce a 2 3/4" hole in the stock. "Positive" template with a 3/8" step to a "negative" result.

IOW, cutting on the opposite side of the bit from where the collar rides the template will produce a step equal to 1/2 the sum of collar and bit diameters (sum of their radii). Cutting on the side of the bit adjacent to the collar contact point produces a step equal to 1/2 the difference in collar and bit diameters (difference of their radii).

[Andy Sowers]

Charles and Tom

Yes, what I was trying to do is using the existing workpiece as a "positive" template... I wanted to generate the "negative" template such that the new "negative" template would fit exactly inside the original workpiece. What's the best way to create the negative template?

I assume there should some combination of router collar diameter/bit diameters to accomplish this in fewer steps than I did.

Not that it really matters, but I'll try to draw up some schematics of what I did. But it will take me some time tonight to do that.

Thanks

Andy

[Andy Sowers]

Ok... here's a pictorial sequence of how I created the templates...

1) First, I used a flush trim bit to create a 1/4" MDF copy of the workpiece (the MDF template is on the left)

Picture 003.jpg

2) Next, using double stick tape I attached this MDF template to another piece of 1/4" MDF. Then I mounted a 3/8" rabbeting bit in my router table and ran the piece through. This created a second template where the each edge was 3/8" smaller than the original. In the picture below, I flipped the piece over after milling so that the original template is now on the bottom.

Picture 005.jpg

3) Then I separated these two templates and attached this smaller template to yet another piece of 1/4" MDF. This time I used a 1/4" straight bit with a 1/2" router collar to create the two templates I was after. Here's the final result after running the router around the rabbetted template showing the final two templates which fit nicely inside the original workpiece with no gaps...

Picture 008.jpg

I hope that explains things a bit better.

I was just wondering if there was a is there an easier way to generate these two templates? I suppose I could have scribed the inside of the original piece, then cut them out and sanded them until they were close, but I was after a really tight joint without any gaps...

Thanks
Andy

[Jamie Buxton]

When I'm doing things like this, I don't trust the router bit to be perfectly concentric with the template guide. (My router is a plunge, and they never seem to be perfectly concentric. Worse, the offset changes from plunge to plunge.) So when I'm trying to perfectly follow a template, I use a bit with a pilot bearing.

But you're right, it takes a lot of steps. Today in the shop I'll be making a part to edge a desk top. The part is 1 1/2" wide, turning a ninety degree corner, around a 4" radius. It looks simple when you draw it on the plan. But it is going to require three templates to machine the edges, and four more to make those templates.

[Andy Sowers]

Agreed. I used bits with bearings on the first two steps... I just didnt have an oversized 3/4" bearing that would fit on my 1/4" bit, so I was stuck with going the collar route for the last cut. I did try my best to center the collar using a centering bit beforehand.

Incidentally, I did buy a 3/4" OD x 1/4" ID bearing for this specifiic purpose, but it didn't fit on any of my 1/4" bits. I suppose that either 1/4" shank bits are slightly overisized, or the 1/4" ID bearing I bought doesnt allow enough clearance to slip on a standard shank (I'm guessing the latter).

--andy

[John Schreiber]

When I look at the objective, I don't think of templates and routers, I think of a scroll saw or a coping saw. Just a different idea.

[Bill White]

That's the reason it is called wood WORKING. H@!!. If it was easy, there would be no way the tool companies could keep up. I think that you're doing just fine.
Bill

[Chaz Alexopoulos]

Andy...couldn't you skip the flush trim bit and go directly to the 3/8 rabbet?