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Drilling holes in plywood

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Forum topic by MrRon posted 09-18-2021 04:44 AM 814 views 0 times favorited 33 replies Add to Favorites Watch
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MrRon

6184 posts in 4483 days


09-18-2021 04:44 AM

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.


33 replies so far

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SMP

4816 posts in 1145 days


#1 posted 09-18-2021 05:00 AM

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.

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TLewisD

25 posts in 34 days


#2 posted 09-18-2021 05:07 AM

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.

-- All I do is with blessings from God. Laus Deo

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tvrgeek

2281 posts in 2889 days


#3 posted 09-18-2021 09:47 AM

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.

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Francisco Luna

1050 posts in 4633 days


#4 posted 09-18-2021 10:16 AM

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…

-- "Menos es mas" Ludwing Mies Van Der Rohe

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TravisH

782 posts in 3175 days


#5 posted 09-18-2021 11:08 AM

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/

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Kudzupatch

297 posts in 2448 days


#6 posted 09-18-2021 11:58 AM

Backer block OR
drill through with a small bit, then drill half way from one side with your finished size bit and finish from the other.
That is only options I know of.

-- Jeff Horton * Kudzu Craft skin boats* www.kudzucraft.com

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prazbotta

42 posts in 515 days


#7 posted 09-18-2021 12:08 PM



drill through with a small bit, then drill half way from one side with your finished size bit and finish from the other.

- Kudzupatch

+1

This is what I did for my train platform years ago. However, when adding things later, it is a pain getting under it to finish the hole.

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Knockonit

970 posts in 1442 days


#8 posted 09-18-2021 12:39 PM

and that 64k question raises its ugly head again, who ever solves this one, will be angelic
rj in az

-- Living the dream

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CWWoodworking

2205 posts in 1419 days


#9 posted 09-18-2021 12:41 PM

You could try a plunge router if dimensions work.

I frequently use plunge router to recess leg levelers. It makes cleaner cuts than most drill bits.

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BlasterStumps

2127 posts in 1679 days


#10 posted 09-18-2021 12:54 PM

Plus one on the use of a plunge router bit. If your plywood is laying flat, you can use a screw jack to raise up a backer board so it it tight against the underside

-- "I build for function first, looks second. Most times I never get around to looks." MIke in CO

View SMP's profile

SMP

4816 posts in 1145 days


#11 posted 09-18-2021 01:41 PM

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.

View controlfreak's profile

controlfreak

2876 posts in 841 days


#12 posted 09-18-2021 01:58 PM

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.

View Peteybadboy's profile

Peteybadboy

3997 posts in 3189 days


#13 posted 09-18-2021 02:12 PM

My dad was a serious model railroader. I would like to see your progress.

To your questions a cone shaped counter sync bit would clean things up nice.

-- Petey

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CWWoodworking

2205 posts in 1419 days


#14 posted 09-18-2021 02:49 PM



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.

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dbw

626 posts in 2876 days


#15 posted 09-18-2021 03:30 PM



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.

-- Woodworking is like a vicious cycle. The more tools you buy the more you find to buy.

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