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Discussion Starter #1
I was going to ask why (besides certain commercial jigs simply requiring specific-sized template guides) templates come in so many sizes, generally as large as the cutter diameter requires and as small as can be to maximize the concave-corner "resolution" of the template.

The I realized the general answer to the above is "plunging".

But that leaves the question whether anyone ever uses template guides that are the same diameter as the cutter diameter, thus allowing a 1:1 template? Obviously, the bit would have to neck down for the guide, or rather, to keep bit/guide installation sane, have a cutter diameter somewhat larger than the shaft.
 

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Discussion Starter #2
Oh never mind. Having written the stupid question, I now realize that's called a flush trim bit. I guess any possibly disadvantage from using a bit with a bearing (in long-term use, huge templates, multiple pieces, etc.) is insignificant compared to the inconvenience and possible risks of a template guide captive between bit and collet?
 

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But that leaves the question whether anyone ever uses template guides that are the same diameter as the cutter diameter, thus allowing a 1:1 template? Obviously, the bit would have to neck down for the guide, or rather, to keep bit/guide installation sane, have a cutter diameter somewhat larger than the shaft.
Having written the stupid question, I now realize that's called a flush trim bit. I guess any possibly disadvantage from using a bit with a bearing (in long-term use, huge templates, multiple pieces, etc.) is insignificant compared to the inconvenience and possible risks of a template guide captive between bit and collet?
Hi Benedikt

Because a cutter is inserted through the guide bush that would not really be advisible..... At least not when plunging. Even if not plunging the cutter shank or the vast majority of cutters would not be long enough to do this and you'd loose the ability to adjust a cutter, say 1in long, to make a 1/8in cut in the material (as you require when recessing for hinges, etc).

There are several dangers when using a top bearing template trimmer - firstly they are problematical to plunge without damaging the template, secondly if you tilt the router away from the work whilst making a cut the edge will be damaged (it happens!).

There are some useful tricks you can do with guide bushes and cutters; for example I regularly use a hinge mortise template with a plunge router. The standard guide bush/cutter combo is 16mm GB/12mm cutter and this is available for my current router, however when I first bought this jig my small site router was a Festool which uses incompatible guide bushes. The solution was to utilise a standard Festool GB (17mm) in conjunction with a 13mm cutter. In most cases the corners are squared out with a corner chisel, however I sometimes have to deal with hinges having radiused corners, 10mm radius. For those I employ a 24mm GB and a 20mm cutter to do the recessing

Regards

Phil
 

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Benedikt, the reason for the many sizes of guide bushings is to allow for different offsets. This is useful for many applications, for example you can use two different sized bushings with a plunge router to make the cut out for a mounting plate in your router table. The small bushing lets you cut the lip the mounting plate rests on; a larger bushing will let you cut the opening without disturbing the lip. Rousseau has a special guide bushing with a collar designed just for this purpose so one bushing does both jobs.
 

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Discussion Starter #5
I saw the Bosch inlay routing kit which similarly contains a thin cutting bit, a small-diameter bushing (and also the white Bosch-base bushing holder), and a collar that expands the bushing radius by the bit diameter, to switch from outside cutting to inside cutting with the same template.

It appears this sort of thing ceases to work seemlessly with corners so sharp that the collared bushing is pushed out more than the collar thickness. This is true for inner corners less than 90 degrees, right?

Actually, even with square or less-than-bushing/collar-radius corners, don't you always have the problem that the "frame" piece corners have the bit radius, while the "inset" piece corners are square?
 
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