I built some shaker doors using a tongue and grove router bit set. My stock was 3/4”, the bit is 3/4” with the groove being 1/4”. For panels I used 1/4” plywood but the panels did not fit tight and it cause some issues while painting them. Is it typical for them to not fit tight? If not, what can be done to fix it? Do I need a different bit?
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My guess is your plywood isn't actually 1/4 inch. The birch plywood at my Home Depot that's labeled 1/4" is actually only 5mm.
So the easy fix is to find a plywood supplier that sells true 1/4 inch sheets.
Most plywood is slightly thinner than advertised. I usually use a drop of glue on the stile. This still allows the panel to float. There also is space balls. You place a one or two on each side. This holds the panel in place and centered. You could use pieces of veneer to shim the panel tight to the front.
As mentioned earlier undersized plywood is probably the culprit. Plywood is notoriously inconsistent in thickness which is why you see so many solutions to the problem, from special router bits, dadoo shims to many different jigs. Unfortunately when making cope and stick doors with a router bit set you don't get an option to adjust the groove size. Possible solutions include using compressible inserts, usually soft rubber balls placed inside the groove during assembly. If the panel is properly sized these will exert just enough pressure to keep the panel from moving. If, as I believe your doors are already assembled you could try injecting some gorilla glue into the backside of the panel and groove seam with a hyperdermic style injector, since Gorilla Glue expands as it drys it may tighten up the joint but be prepared for it to effervesce and keep wiping it with a damp rag. A better solution may be to pin the panel in place from the back side with ½"brads. One centered in the top and bottom should surfice but since it's plywood you could use more.
Standard rail/stile bit are intended for full thickness raised panels. But if you have a panel raising bit with a back cutter, I often use 3/8 MDF for the panels which gives a perfect fit. I find the added weight of the thicker panel gives a nice sturdy feel to the door.
There are rail/stile bit sets with adjustable groove cutters Freud (https://www.amazon.com/Freud-Premier-Adjustable-Shaker-99-762/dp/B0062I1JGI) and also CMT (https://www.cmtorangetools.com/na-en/router-bit-sets/adjustable-tongue-groove-bit-sets-for-mission-style-cabinet-doors.)
No need for space balls with sheet good panels, cut 1/16" narrower & glue them in.
Lots of good info here. You have picked up on the fact that plywood thicknesses, like 2x4's, use nominal measurements. Undersize router bits used to take care of the problem pretty well but, the variations in sheetgoods today make those hit or miss.
An adjustable bit like RobertEJr links to are a good bet. Cutting the slots with multiple passes on the tablesaw is another answer in some cases. I have stacking slot cutters so the router table works for me. Where things get hinky is when a specific profile is mating up as with rail and stile frames. Here the adjustable bit set will triumph.
For your current rattle problem if you have not glued up yet; three or four blobs of silicone rubber in the groove will cure to custom made 'space balls'. I use this method for large framed tops and it has worked out well.
Good Baltic Birch plywood will fit the groove well. Good plywood with an mdf core fits well too. I use the Baltic Birch if I'm painting, and the mdf core if I'm not.
But most likely, anything from the home center will not. Bring some calipers with you to measure.
Just a couple of things to add to this thread:
1. Panel buddies are more forgiving than space balls. Panel buddies are soft rectangular foam and compress more than the harder rubber space balls. If your panels are off by 1/16" or so, the panel buddies will still do their job without making glue up difficult.
2. When making flat panel doors, I use 1/2" ply for the panels and then back cut it to get a snug (not tight) fit in the rails and stiles. It's a guaranteed way to keep the panels from rattling around and it makes the doors more substantial. Just remember set your router bits so the panel isn't proud of the rails and stiles on the backside.
It’s my understanding that plywood doesn’t move a whole lot. Is there anything wrong with just glueing it up?
When I use plywood in a panel, I always glue it.
Yes, its perfectly fine to glue the ply panel in. In fact, it strengthens the door.
The panel is fit tighter when gluing so spacers are not used.
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