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:

125159

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)

125217

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.

125218

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...

125219

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?