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How do you make positive/negative router patterns?

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Forum topic by mbs posted 04-10-2013 03:32 PM 1685 views 0 times favorited 9 replies Add to Favorites Watch
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mbs

1496 posts in 1688 days


04-10-2013 03:32 PM

I’m making breadboard ends on tables but the end of the table isn’t straight. Let’s say the ends have a very lazy M shape for discussion purposes. How would you get a perfect fit between the breadboard end and the table? I believe the proper tool for this is a pin router (or CNC) but I don’t have either one. The options I’ve considered are:

1) Brute force – make the table end using a bandsaw and spindle sander. Use it as a pattern to scribe a line on the breadboard end, bandsaw close to the line, spindle sander … Issues – really need to be perfect when spindle sanding. I need 6 of these so it wouldn’t be very efficient doing them one by one.

2) Cheat – Make the breadboard end 3/8” thicker than the table. Treat the end of the table like a tendon and the breadboard end like a mortise by making a large motise in the breadboard end. Cut the design in the breadboard end only. Issues – I started doing this and I’m concerned about the strength of the breadboard ends. The 3/16” overlap seems a bit flimsy to me. Not sure it would pass the test of time with Grand kids.

3) Make the positive and negative simultaneously. Use a scroll saw to carefully cut the pattern in 1/4” wood. Then mount the positive to the table and the negative to the breadboard end. Cut near the pattern with the bandsaw then clean it up with a router and trim bit. Issues – need to be good with the scroll saw.

Option # 3 looks the best to me right now. With your vast experience, what’s the best way to get a perfect fit?

-- Sorry the reply is so long. I didn't have time to write a short reply.


9 replies so far

View Monte Pittman's profile

Monte Pittman

15460 posts in 1086 days


#1 posted 04-10-2013 04:09 PM

No vast experience here, but i would go with #3. #1 virtually never works in the long run.

-- Mother Nature created it, I just assemble it.

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tenontim

2131 posts in 2493 days


#2 posted 04-10-2013 04:18 PM

I make a jig like this one. I set the end pieces so the opening is about 1” wider than the table top, so that I can insert a wedge from each side to hold the jig in place. Use the jig initially to square up the ends of the table top, by using your router to cut off the ends square. Do this in several passes, to prevent burning up your router or a bit. I use spiral bits. After you have it square, move the jig back the distance you need for the amount of tenon you want on the end of the table top. Set your router to the proper depth to get the thickness of tenon desired and remove the waste with the router. I use a straight, flat bottom bit for this. Make your first cut next to the jig, then move towards the edge. Make sure you support the router on the uncut section of table top next to the jig, to prevent it from tipping.

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mbs

1496 posts in 1688 days


#3 posted 04-10-2013 04:39 PM

Tenontim – I believe that would work well for straight cuts but I’m looking for a curved cut.

Rick – I want a breadboard just for appearance. The table I’m making is similar to this one. I understand your concern about the difference in growth rates. The table top is about 14” wide and is mostly wenge which shouldn’t grow much with humidity changes. The breadboard end is cherry.

-- Sorry the reply is so long. I didn't have time to write a short reply.

View REO's profile

REO

665 posts in 822 days


#4 posted 04-10-2013 11:11 PM

unless you can use the cut line and there is no kerf it will always take two patterns for anything but a straight cut. OHpin routers still need two patterns for this type of cut unless they fudge the construction as you have thought out. I have used autocad and offset the pattern for the tool diameter and sanded to the lines to make the pattern.
It looks as though the ends are proud of the top surface.
Your best bet may be to cut them with the same pattern and then use a router with a template and cut a trough in the edge of the end piece without breaking the sides and then carefully fit the end of your top as though it were a tenon to fit the trough. Any discrepancy would be covered up with the overlying edge of the end piece. dowel or biscuit the end on. The 3/16” edge would be much like the edge of a seam binder for hardwood floors.

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mbs

1496 posts in 1688 days


#5 posted 04-12-2013 04:10 AM

REO,

Your suggestion is the same as my #2. I tried it and I’m concerned about the strength of the thin lip of the large mortise.

I’d like to make the positive and negative patterns work.

-- Sorry the reply is so long. I didn't have time to write a short reply.

View Moron's profile

Moron

4724 posts in 2642 days


#6 posted 04-12-2013 04:42 AM

I’m going fishing soon : ))

try “doable”

its easier to go with a wheel that already works and would suggest cutting a floating tenon into the bread board end

-- "Good artists borrow, great artists steal”…..Picasso

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mbs

1496 posts in 1688 days


#7 posted 04-12-2013 02:32 PM

Moron – I don’t understand your advice.

-- Sorry the reply is so long. I didn't have time to write a short reply.

View WhoMe's profile

WhoMe

1127 posts in 1992 days


#8 posted 04-12-2013 02:45 PM

With how I rout things, it is positive when it looks good and negative when it looks bad.
Other than that I have no advice. Good luck with your project.

-- I'm not clumsy.. It's just the floor hates me, the tables and chairs are bullies and the wall gets in the way.. - Mike -

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a1Jim

112894 posts in 2325 days


#9 posted 04-12-2013 02:56 PM

I agree with the floating tenon approach.That way you cut you bread board and the table top to fit and use a rabbiting bit on both pieces ,use both pieces as pattern to make the loose tenon. As far as wood movement goes just glue the only the center 3” or so and use wood pegs with elongated holes in the loose tenons to allow for wood movement.

-- http://artisticwoodstudio.com Custom furniture

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