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Mike P
03-20-2009, 10:54 AM
Hi All,

I'm making up a 12" x 18" (ish) mounting plate for a router table out of 1/2" aluminum plate and wonder the best way to machine a recess (or step) around the center hole for the inserts that fit around the router bit.

The recess/step will be about 0.20" deep, and about 0.50" wide, and will surround the 3" diameter center hole. Again, this lets the plastic clearance inserts sit flush with the top of the plate.

I have an M-head Bridgeport, rotary table and a boring head. I have not had much luck with getting clean, pretty cuts from the boring head, and wonder if the surface finish of the step would suffer from multiple plunges with the boring head to create the step.

What about grabbing the plate with the rotary table using the center hole and simply milling around the hole? I think there's room behind the head for the plate to clear the column, and if not, I can slide the ram out some more.

Is this the easiest way to do it, short of CNC'ing it?

TGTool
03-20-2009, 11:26 AM
Well, one approach that occurs to me with the equipment that you have is to mark out the outer edge of the recess with a visible line. Then just use the mill to machine close to the line using the normal X-Y movements. Finally set the boring head to just clean up the outer edge. If esthetics are important, this won't leave sweet looking machining marks, but it will give a uniform recess for the inserts.

The rotary table is also a reasonable proposition, provided swinging the large plate is possible. Clamping the plate to the table presents a challenge, but not an insurmountable one.

Using the boring head should also work. Most tools will have a clearance angle on the end so stepping out to size will leave a slightly sawtooth cross section, but it's a small amount and if the steps are uniform the high points should also be uniform.

winchman
03-20-2009, 11:38 AM
Put the plate on the rotary table. If your RT has a tapped hole in the center, use it to hold the plate. If not, you'll have to come up with another way to hold it.

Machine the recess with an endmill, which will give you a good finish. Make the ID of the recess slightly smaller than the center hole you'll bore later. You may need to drill a starting hole close to the center if your endmill won't make a plunge cut to start.

After the recess is done, change the setup, and bore the center hole. I'd probably just finish up with the endmill, being very careful not to cut all the way through. Then I'd break out the center, and file the edge to remove the burr.

Roger

Evan
03-20-2009, 01:01 PM
Cut the hole to the OD of the recess. On the lathe make an insert out of steel to fit the hole to provide the recess step, fastened on the bottom. Buy some small rod magnets and insert them in holes in the edge of the plastic inserts to hold them in place.

1/8" by 1/2" neo rod magnets are 50 for $17 at K&J Magnetics.

Your Old Dog
03-20-2009, 01:07 PM
.........I have an M-head Bridgeport, rotary table and a boring head. I have not had much luck with getting clean, pretty cuts from the boring head, and wonder if the surface finish of the step would suffer from multiple plunges with the boring head to create the step. .............

You aren't blessed with a real long boring bar are you? :D That and a loose table gibs will screw it up in a hurry. I use the shortes bar I have and lock the gibs down.

FatWheels
03-20-2009, 01:33 PM
A first thought on making this sub-plate is that you must maintain concentricity with the router arbor above all. I'd first drill and ream a .5000 " hole and mount a suitable shaft in the router collet. Now you can drill mounting screw holes and be sure after you're done with the machining that it won't be all for naught.

Whatever method you choose can indicate off this initial bore until you're ready to enlarge it for your throat insert. I don't have a mill so I'd make a temporary mount for the router and use it, an index pin, and a suitable carbide bit to mill the aluminum carefully by hand. Also, a big carbide cutter, slow feed and speed, and suitable lube makes the prettiest shower of light fluffy aluminum 'snow' you've ever seen. Set up a shop vac first!

regards,

Jim

Duffy
03-20-2009, 01:41 PM
If the ram will move out enough to allow the plate to clear, why not drill two holes on the diagonal and bolt the plate through a piece of 1/4" plywood to the table. Plunge and cut the circle, change to a 1/2' bit and cut the recess. if the holes bother you, tap and plug them after.

tdkkart
03-20-2009, 01:50 PM
A Kearney and Trecker with a rotary head would bang this out in a heartbeat. Too bad I've got access to one and you don't........

If you're careful, stick it to the rotary table with double sided tape. Take light cuts.

Jpfalt
03-20-2009, 03:23 PM
Using a template, a router guide bushing and a carbide bit you can cut the table insert recess in aluminum using the router just like you would cut wood.

derekm
03-20-2009, 03:53 PM
If the ram will move out enough to allow the plate to clear, why not drill two holes on the diagonal and bolt the plate through a piece of 1/4" plywood to the table. Plunge and cut the circle, change to a 1/2' bit and cut the recess. if the holes bother you, tap and plug them after.
its possible to do this without any extra holes in the finished workpiece with conventional clamping from the centre using a rotary table with radial t slots...

GKman
03-20-2009, 04:17 PM
its possible to do this without any extra holes in the finished workpiece with conventional clamping from the centre using a rotary table with radial t slots...

Extra holes always come in handy later. We just don't know yet, what for.:D

Mike P
03-20-2009, 04:20 PM
These are all great ideas, THANKS!!

I'm going to print out the thread and take it home and figure out which one will be the winner based on how much space i've got on the mill.

FatWheels - I will ABSOLUTELY drill the mounting holes with the router indexed into a reamed hole. That's brilliant!

Evan - Holding the insert with magnets is awesome.

Machining the recess with the router did occur to me, and is looking more popular than ever. Very quick, and I've been machining aluminum with a router for years, long before I had the mill.

The weekend could be super fun!

Thanks again.