Drilling accurate router sub-base mounting holes

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Hi all,
I got Spielman's "Router Jigs and Techniques" and a sheet of 1/2" acrylic then sat down to make a few of the custom sub-bases he shows in his book.
For acrylic I figured I needed to get the mounting holes pretty close to exact in relation to each other and waste as little acrylic as I could in the process. The problem is that the mounting holes on the Porter Cable 690 series (and possibly other makes and models - I don't know) are arranged in an equilateral triangle with the centers 4" apart from each other. That's great and all, but I needed to know how far those screws are from the center so I could lay it all out.
I broke open the absolutely most useful book on the planet (Pocket Ref - Thomas J. Glover) and fired up MS Excel then sprained a few brain cells going through some trigonometry to get the answer.
That answer is 2.309".
If you have a router with a 4" triangular mounting hole configuration and want to make a new sub-base, here are the steps I used to lay out the sub-base:
Mark off a square slightly larger than the radius of your router base on the corner of your base material. Draw an X to find the center. Set your compass to a radius of 2.309". Draw a circle with the pivot point at the center of your square. Move the pivot point to some point on the circle and use the compass as a divider to walk off and mark 6 points on the perimeter of the circle. The seventh mark should fall exactly on the first mark. If it doesn't, make a small adjustment and start over. Three of those marks will intersect the circle exactly where you need to drill for the mounting screws. Drill and tap (if necessary), then mount the router to the sub-base material. Now would be a good time to trace the outline of your base and cut it out. Insert a center cutting bit (V bit), fire up your router, then lower the bit until it just scores the material. This mark is exactly where to cut/drill to make the center hole. If you wanted to be real spiffy, you could drill a small hole at the center mark and put it on a pivot pin the rotate it against a sander. This will give you a perfectly round base with a perfectly centered bit and eliminate the concentricity offsets that may be encountered by using the factory base.
The same general technique would work for making a base for a router table.
For different router bases, here are the magic numbers based on the distance between mounting holes:
2"    1.155" 2.5"    1.443" 3"    1.732" 3.5"    2.021" 4"    2.309" (PC 690 series) 4.5"    2.598" 5"    2.887" 5.5"    3.175" 6"    3.464"
Here are the radiuses you would use for locating the mounting holes on routers that use four mounting screws arranged in a square. The column on the left is the distance between two adjacent mounting holes:
2"    1.414" 2.5"    1.768" 3"    2.121" 3.5"    2.475" 4"    2.828" 4.5"    3.182" 5"    3.536" 5.5"    3.889" 6"    4.243" (PC 7518)
I thought I'd post this info on for the archives. Hopefully it will come in handy to someone someday.
Good luck! -Rick
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Um, maybe this is just a dumb question. Why not drill a hole. Drill another 4" away. Drill a third 4" from both Bisect the lines which connect them Draw 3 (you could even use two) lines which connect the holes with the centers of the line opposite them. Bore the center hole using the intersection of those lines as the center. Trim rest of base as required. (using any of your methods below - trace, rotate about center ...)
Fewer measurements (less error) No math required.
-Jack

table.
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Too damn easy, Jack. Where's the fun in that????
:)

out.
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Jack wrote:

Even if your layout is perfect, you are still assuming that the collet is exactly centered to the mounting holes of the router base. Until Mitutoyo or Starret starts making router bases, I'll stick with the point-plunge method to determine where the center of the bit falls.
No measurements (no error) No math required.
-Rick
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Well IMHO you are on the right track but you should also consider the following: When the base is removed and then reinstalled will it return to the exact position it was in when the center hole was drilled? If you mark the base so the base lines up with the same holes in the router casting you will probably be fairly close but not necessarily dead on. Even the sequence of tightening the screws may cause the base to shift. If you are using a plunge router, is its axis of rotation absolutely perpendicular to the base? If not, the center of the base can only lie on the axis of rotation at one plunge depth? Earl Creel

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Earl Creel wrote:

You bring up some excellent points. Unfortunatly, there doesn't seem to be a whole lot we can do about any of these things. For the things I can control, I try to be as accurate as I can. For everything else, I use a belt sander <G>.
-Rick
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Some inaccuracies in router bases can be fixed but most just are not worth the trouble. Perhaps the most important thing is to realize that some router castings and/or bases are off by quite a bit and take that into account if you are trying to make a cut where accuracy is important. Pat Warner strongly recommends pattern type bits for cuts requiring accuracy because with pattern bits the base only controls depth and does not control where the cut is made. Earl Creel
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Want to say that there are pointed pins that you can use with a centering pin to accurately mark the screw locations. You center onto the acrylic/plastic plate, then clamp and tap the pointed pins with a hammer to mark the screw hole location centers
John
wrote:

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You're kidding, right? .
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What's the problem?
wrote in message

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Umm take the base off your router, EYE ball it over the new base" or measure center the base over the new base and use a marker or punch through the router base attachment holes to mark the location on the new base holes .
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measure
...or uuuhh take the base off your router and make a photocopy. Glue, tape or affix the photocopy to your plastic. Drill, cut or punch as req'd.
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Lawrence L'Hote responds:

You guys stole BOTH my secret methods! :)
Charlie Self "It is even harder for the average ape to believe that he has descended from man." H. L. Mencken
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I was going to explain how I used a 'kit' that had 'pointed' headless screws sized for the base-plate {and a 'center point' to fit the chuck}. Put them in, put on plastic, push down - screw holes are marked. Later, chuck up the 'center point' and mark for either a 'guide' cut-out, Zero Clearance, or just a big hole.
However, a photo copy sounds even easier. One caveat . . . I'd include a 6 inch steel ruler when making the copy. Some of the machines don't make exact 1:1 copies. Even a 1/10 off may give a headache with screwheads winding up at an angle.
Regards & Good Luck, Ron Magen Backyard Boatshop

use a marker or punch through the router base attachment holes to mark the location on the new base holes

tape
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to be sure, use two reference measurements, at right angles to each other....
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tape
Actually I bet a Vix bit would work great to drill the holes through the router base.
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You hit it Leon. That's my method. I use a transfer punch.

measure
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Leon wrote:

Use the original base and a "Vix" bit for the mounting holes. Chuck a "V" bit in the router, mount the base and use the "V" bit to locate the center hole.
-- Jack Novak Buffalo, NY - USA (Remove "SPAM" from email address to reply)
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measure
bit
hole.
yep VIX bit my method also plus I use carpet tape to "clamp" the pieces together as well. I only recently discovered carpet tepe for WW its a miracle
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Opus, Jack, I just now read this hint about 1 minute after I posted this suggestion also. I give you all the credit. ;~)

measure
bit
hole.
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