Flush Dowel Joints Using a CNC

      Edge doweling accurate enough to allow flush joining of panels edge to edge is problematic on CNC equipment. November 10, 2006

Question
I'm new to CNC and I just started working for a shop that has a HolzHer Panel saw and CNC pod machine. They want to produce cabinets, and dowel them together. They are doing all boring on the CNC, including horizontal boring, and they seem to be getting joints that don't come flush, and aren't even consistent in their degree of mismatch. Should I expect consistent, good, flush connections between gables and tops/bottoms using this method? If the answer is yes, what is the best way to diagnose the problem(s)?

I have noticed some slight inconsistency in the output of the saw in terms of final accuracy. It can cut 2 parts the same size, but when you stack them and feel the edges, it can be slightly off. When you reverse them and put them face to face, there is about 1/4 of a mm of runout between them.

I had them drill a hole on the left station one inch in each way from the corner, then flip it and drill the same hole on the right station, only halfway through. The 2 holes are offset by as much as half a mm and the result is not completely consistent.

Forum Responses
(CNC Forum)
From contributor A:
You should be able to get flush joints. First, start referencing the holes from the outside, meaning on your sides, place the vertical holes from the edge of the panel and on the bottoms, reference the horizontal hole from the bottom. This way, the variation in material thickness doesn't transfer to the joints. Same thing goes for getting the front edges of the panel. Reference the holes from the front edge so if the depth of the sides and tops and bottoms aren't the same, the error is in the back.

If that won't bring the cabs flush enough, go back to the saw, make sure the panels coming off are square and accurate, and check your pods, reference stops and drill head, in that order.

I wouldn't worry about panels being .25 MM off - that could be as good as it gets as sheet material always has tension in it and moves as it is being cut. Just keep the error where it won't be noticed.



From contributor J:
Put a sizing route on the panels. Sometimes the saw will put out what it thinks is X size, while the CNC is machining what it thinks is correct, so the CNC has the last word. You can also include an adjustment to your programming to correct minutely in X or Y, that will compensate for minor differences in panel Z.

Example: If panel size is 30 1/2 by 23 1/4, enter panel size as 30.490 X 23.240 and size it slightly small with your router. If you are trying to use dowel construction, then getting the CNC to do that consistently in the horizontal is difficult. I'd suggest instead that you go to confirmat screws, as they are much easier to deal with.



From contributor M:
It is much quicker to do the horizontal drilling with a 32mm boring machine with left and right stops. Even better to have a 32mm dowel inserter. There can be many, many reasons for a not flush condition. Reference pins out of alignment, out of square conditions on saw blanks, horizontal drills out of alignment, accuracy issues with the machine, somebody monkeyed around with the tool parameters, etc.

Start simple. Route your test pieces with the machine in question. That way, if the part didn't move during cutting, you will be reasonably sure it's square. Then, in another program, drill some vertical holes in the non-mirror field and check for accuracy from the reference point and also center to center. Make adjustments. Now do the same in the mirror field(s). I usually drill holes near the four corners and insert half-dowels. Then drill the same program in the mirror field on another piece. When you put the two pieces together, it should feel like one thick block of material. After you are accurate in your reference zones, try to tackle the horizontals.

Good luck. Did it lots of times until I changed companies and went with a router instead of a p2p. Even with a p2p, we did the horizontals on a dedicated machine after time studies showed us the penalty for doing it one hole at a time on the CNC.



From contributor C:
There are too many reasons to list, but don't ever use a CNC for horizontal boring and expect to get a flush fit when doweling to an adjacent panel. Granted, they are very accurate, but edge boring is a different animal (if you expect flush fit). As stated above, a relatively inexpensive horizontal boring machine is much more accurate and efficient for edge boring when you use dowel construction.

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