Question about using pattern cutting router bits with an aluminum template

[Neal]

I have an aluminum template and (will have) a top bearing pattern cutting bit to use with the template. Assuming I set the dept correctly so the cutters are below the template surface, are their any dangers of using the top pattern bit with the aluminum? Shouln't the bearing prevent the cutters from cutting the aluminum?

Thanks

[mpc]

In theory the bearing should prevent the blades from contacting the aluminum. However...

a) slightly tilting the router accidentally during movement risks the bit hitting the aluminum and it is quite easy, and fairly common, for this to happen especially for folks just starting with pattern work. Especially because just under 50% of the router base is supported; the rest is hanging off the edge of the template. Some folks like to attach an "outrigger" block to the router base to reach down to the tabletop to reduce tipping. Others add a large "offset base" to their router with a second handle; this extra-wide piece is kept over the template to give your second hand a better shot at keeping the router flat.

b) If the edge of the aluminum template isn't 100% square then it is possible the bearing could ride on the "narrow" portion of the template which then lets the blades cut into the fat portion of the aluminum. Adjusting the bit depth so the bearing rides closer to the bottom of the template minimizes this risk.

c) starting/stopping the router sometimes leads to "jerk" motions (especially on routers without "soft start" technology), or lifting the router a tad, leads to bits eating aluminum. Many folks inadvertently lift/raise/tip the router while trying to turn it off.

Thin spacers between the workpiece and template (like several Popsickle sticks spread around) give you a little insurance room. If the cutting length of your router bit allows, a 1/8th or 1/4 inch thick spacer provides a fair bit of forgiveness space protecting the template, your workpiece, the router bit cutting edges, and you.

Plastic, wood, and aluminum are used for templates, instead of more durable metals, because most carbide router bits can cut those materials without immediate fragmenting of the bit. You'd rather have the stationary thing take the damage compared to busting a piece off something spinning at 15,000-20,000 RPM.

mpc

[Neal]

Thanks! Appreciate the insights and tips.

[capncarl]

Good answer by mcp! I've made a teardrop shaped lexan router base for this purpose, to hang back over the material. I saw somewhere a router bearing with a fender washer above and below the bearing for the purpose of keeping the beating on the template, that would work well.

[mpc]

One other thing about pattern routing that just occurred to me - something most woodworkers probably already know but may not be obvious to a rookie: pre-cut the workpiece to about 1/8th to 3/16ths of an inch away from the final shape using a band saw, jigsaw, hand saw, whatever. Just get close; the cut doesn't have to be a consistent distance from the pattern line either. Don't make the router bit make the whole/entire cut if possible. The router pattern bit should really be trimming a close and probably lumpy edge to get the final pattern shape. For shapes that can't be reasonably pre-cut by a saw set the router bit depth to not cut through the workpiece in one pass... make several passes, about 1/4 inch at a time, to work your way through the workpiece.

Heat is the enemy of any cutting surface. The more material it has to chew through the hotter it is likely to get. Ergo let the bit do finally trimming to the pattern shape or make several shallower cuts. A router bit cutting edge just trimming a pre-cut workpiece spends most of its time in air, cooling off. A bit doing the entire cut is cutting 50% of the time and ends up in a "hot pocket" with little airflow... so make a shallow cut to reduce the amount being cut and to reduce the load on the router motor to a reasonable value.

mpc

[LCHIEN]

I'm agreeing with MPC on this. Take a few skimming cuts to take off the highest points of the lumpy edge. Don't have to press the bearing against the pattern yet, but OK if it does. Speedy passes are good and lack of accuracy due to speed is OK because you can't overcut and it won't dive in. As the overall trim gets closer then concentrate on pressing the bit against the pattern but don't be overly concerned about it. After I think I'm done I'll make one more pass at moderate speed pressing the bearing firmly tot he pattern and it I hear the zzzzzz oof the bit cutting I'll make as many more until it doesn't make that noise anymore and now you'll have a clean cut edge. Should nly take one or two passes... if you keep hearing the zzz of cutting then your pattern is slipping!