Replies

1. 
   _mj_ | Dec 12, 2022 03:36pm | #1

   Compression bit:

   https://www.whitesiderouterbits.com/collections/up-down-cut-spiral-bits-two-flute

2. 
   bilyo | Dec 12, 2022 04:42pm | #2

   While the combination bits will do the job, keep in mind that you will be using only 3/8" of the tip end. No need to pay extra (if there is any) for the combo. Most any sharp straight bit will do the job.

   Wait. What do you mean by "through groove"? Are you cutting all the way through either 1/4" and 3/8"? If so, I can't help but wonder if there is a better way. Need more info.

1. 
   john_c2 | Dec 12, 2022 06:54pm | #4

   Sorry. Yes, a slot all the way through the plywood. On a piece about 8 inches long, a 1/4 inch slot 4" long. Picture something like a slot to drop a letter in a box. But both the top and bottom sides need to be tearout free.

1. 
   oxmach | Dec 12, 2022 07:26pm | #5

   My latest project (Klipsch Lascala clone speakers) has me cutting quite a few routed openings in 3/4 Maple veneer ply. 1/4 and 3/8 up cut spiral bit, no issue with tear out.

3. 
   hotdogman | Dec 12, 2022 05:28pm | #3

   Your description is not too clear about what kind of groove you want to make. However, if you are working with veneered cherry faces I would recommend a down cut spiral bit. There is less likely hood of getting tear out of the cherry veneered faces with a down cut bit. You may want to consider using a dado blade on a table saw if you are cutting grooves that clear both ends. Whatever you use make sure it is sharp.

4. 
   bilyo | Dec 12, 2022 09:21pm | #6

   You might consider making start and finish holes with a good brad point bit first. Then tape the path of the cut (slot) on both sides. Then, probably the combo bit that mj suggested would work best.

   Also, for what it's worth, you are probably more likely to get chip-out if your slot is cross grain rather than with the grain of the veneer. If you have a choice, do the latter.

1. 
   john_c2 | Dec 13, 2022 06:10am | #7

   Thanks. I thought about start and stop holes, but I'm worried about them matching up exactly. I'll definitely go with the grain.

   I think the veneers adhere better on an mdf core than on a plywood core, so this might be a little easier. We'll see.

   I'll be doing test runs, regardless.

5. 
   DRBWWing | Dec 13, 2022 11:48am | #8

   I would make a skim cut just through the veneer on both sides, then plunge and make the final cut. Fence will ensure the groove matches both side.

   I would also do the holes like bilyo mentioned.

6. 
   eddo234 | Dec 13, 2022 12:13pm | #9

   I immediately thought compression bit too, but I have one question; if the 1st part of the cut is done by the upward spiral, pulling chips up, followed by the downward spiral, pushing chips down, what keeps the top veneer from possibly tearing right away from the upward cut, before the downward spiral part even gets into the ply? As for the through cut I can see how the 1st upward spiral will prevent tearout on the bottom veneer. I actually have a compression bit, but it's in my cold shop at the moment, I'd rather not test it myself. :-) I've used it for mortises, but not any through cuts on plywood.

7. 
   _mj_ | Dec 13, 2022 02:16pm | #10

   The center of the bit is centered in the stock, the lower half upcutting and the upper half downcutting. The plunge is where most of the danger lies for the through cut.

8. 
   eddo234 | Dec 13, 2022 04:35pm | #11

   Another option is to clamp a sacrificial piece to the bottom of the ply and use a downward cutting spiral. No tear out on the top at the beginning of the plunge because of the downward spiral, and no tear out on the bottom because of the clamped piece. Cheaper than buying a compression bit too, they're pricey.

9. 
   stevieray | Dec 14, 2022 02:36am | #12

   Greg Paolini has a method of cutting slots without tearout on the downward face. Use a bit to rout halfway through the plywood. Then drill as large a hole as will fit through to the other face. Flip the board and insert a flush trim bit through the drilled hole so that the bearing references off the slot on what is now the bottom part of the board. I've used it and it works easily. You need a very thin flush trim bit for this to work--1/4 inch or so.
   BTW, I too am a fan of the Whiteside compression bits. They are fantastic. But the Paolini method is better.

10. 
    cl810 | Dec 14, 2022 10:10am | #13

    A question: What is going in the grooves? Are they serving as a slot for a piece of plywood to be inserted in to make a shelf? If so you might want to check the dimensions of the plywood vs. the router bit to avoid loose fitting shelves.

1. 
   john_c2 | Dec 14, 2022 12:20pm | #14

   Tissue box.