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Drilling holes in plywood

5K views 33 replies 25 participants last post by  northwoodsman 
#1 · (Edited by Moderator)
I am building a model railroad and need to drill holes through the plywood top for wires. I've tried straight twist bits, forstener bits, spade bits and brad point bits, but they all leave tear out on the exit side of the plywood. This interferes with wires and plugs being fed through the hole. Anyone know a bit that leaves a clean hole?
I know I could start the hole from the top using a brad point and finish drilling from the bottom up. This is a bit awkward for me to work underneath the layout.
 
#3 ·
Hi MrRon,
Due to the construction of plywood the back ply being thin will almost always splinter without something to back it were the drill exits. Sometimes putting tape as a backing were the drill will exit will reduce the tear out. When ever I drill plywood I clamp a scrap of wood as backing were the drill will exit.
Hope.This helps.
 
#5 ·
As a finish carpenter, I find the same problem when doing holes on kitchen and custom cabinets doors and drawers fronts to install handles….If you can't hold a small block of wood at the other side, applying some pressure (most common method), you can try this:
1. Use a sharp, brand new brad point drill bit
2. Use the high speed on your drill….some drill are not fast enough
3. Mark with tape the ply thicknesses on the drill bit
4, Start the hole with moderate pressure, slowly, but when the tape mark is getting closer to the ply face, reduce the pressure and leave the tool do the rest of the hole…
 
#6 ·
For this type of application I end up either just drill through then go back with a countersink to clean up if the plywood is of good quality and wont continue to chip or use small desk grommets. Both end up making pulling cable smooth and give a clean finish.

You can make grommets quickly from various plastic pieces meant for different applications. I have used the tube supports from McMaster before for example when passing thin wires.

https://www.mcmaster.com/tube-connectors/tube-supports-for-plastic-compression-tube-fittings-for-air/
 
#12 ·
another trick that can help is with a brad point bit, drill the hole until the brad point just pops through. then drop some THIN CA glue into the hole.the thin CA will wick into the fibers. let dry a minute,Then either fininsh with drill with barely any pressure or a hand crank drill.
 
#13 ·
I am not getting why a little blowout on the bottom (where it is hard to get to) is a problem. Drill from the top and use lighter pressure to end the cut and flake of the bits that fray. If a larger hole a hole saw from both sides but that gets back to the hard to get to underside again. As long as the trains run on time I wouldn't worry about the bottom.
 
#15 ·
I am not getting why a little blowout on the bottom (where it is hard to get to) is a problem. Drill from the top and use lighter pressure to end the cut and flake of the bits that fray. If a larger hole a hole saw from both sides but that gets back to the hard to get to underside again. As long as the trains run on time I wouldn t worry about the bottom.

- controlfreak
For me, I have to patch blow out at work so anything that keeps me from having to mix up bondo is worth the extra effort.

It also Drives me crazy, even on the bottom. One of my pet peeves.
 
#16 ·
I only know one splinter free way without the above SOP s : Laser. :)

The Fisch Forsner are said to be a little cleaner. I don t have them yet, so can t say.

- tvrgeek
Fisch are the best Fortnser bits I've ever used and even then there will be splintering on ply. A backer board will solve the problem. Drilling until the tip comes through and then drilling from the other side is also a solution.
 
#18 ·
If I can't do the flip over trick, then I just use a backing board out of a piece of pine etc and move it so each hole has solid wood behind.

- SMP
I prefer to do the flip once that lead point gets through, and I always back up anything I drill, unless it's something that is buried, and never seen or felt, then I still back it up. Pet peeve about hole blow out.
 
#20 ·
I am not getting why a little blowout on the bottom (where it is hard to get to) is a problem. Drill from the top and use lighter pressure to end the cut and flake of the bits that fray. If a larger hole a hole saw from both sides but that gets back to the hard to get to underside again. As long as the trains run on time I wouldn t worry about the bottom.

- controlfreak
+1 seems a little too much about nothing ?
 
#21 · (Edited by Moderator)
What about using a gimlet which act as a reamer once through?
example:
https://www.leevalley.com/en-ca/shop/tools/hand-tools/drills/32206-set-of-auger-gimlets?item=33J2001
interesting info:
https://brfinewoodworking.com/sharpening-gimlet-bits/

Or, as the holes don't need to be cylindrical, why not use a tapered reamer to finish the holes.

"This is a bit awkward for me to work underneath the layout."
Anyway there is always one end of the wire for which you might need to crawl under the table to pass it through the hole (unless you can use a cable puller inserted from above?).
 
#22 ·
If you think drilling holes for wires can be frustrating, wait until you mount underside turnout controls!

I use a SHARP 1/16" bit from the top down, close to the inside of the rails, This produces minimal tearout on the bottom. It also will show you exactly where the hole is. I hold a piece of scrap wood over the hole from the bottom and then drill again using a 3/32" bit. That's plenty for a 22 gauge wire feeder.

I use the same method for under-table turnout controls, except the second hole is usually 1/4". Set the turnout and mark the center position of the points on both sides of the throwbar/ties. Remove the turnout, drill, then set the turnout back.
 
#26 ·
Some are missing the point, the OP doesn't want to "get under the table" so a backer or drilling from both side is out. A good bit with minimal pressure at the blowout point will work. Any blowout wont be seen and will have no adverse effect on the wire. In the event the OP needs to go under there to staple or secure the wire he can use a conical bit to ream the holes if it makes him feel better to soften the edges.
 
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