Drill bit for plywood

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Forum topic by MrRon posted 09-21-2021 05:23 AM 445 views 0 times favorited 23 replies Add to Favorites Watch
View MrRon's profile


6184 posts in 4483 days

09-21-2021 05:23 AM

I need to drill 5/16 dia holes through 1/2” plywood. I need to drill the holes from the top only. I’ve tried regular twist drill bits, forstner bits, brad point bits and spade bits, but they all leave a jagged tear out at the exit of the hole. I need to route electric wires through the holes. This is for a model railroad platform. Finish drilling from the bottom would give me a clean hole, but at 86, working at the bottom of a layout is not possible. The platform is 36” tall. Can you recommend a drill bit I haven’t heard of that can drill a clean hole. I haven’t tried auger bits; I wonder if they would work. How about a router bit?

23 replies so far

View Bill_Steele's profile


776 posts in 2971 days

#1 posted 09-21-2021 12:02 PM

Are you able to access the exit point of the hole and cover it with layers of tape or maybe use the superglue + painter’s tape approach to attach a scrap of wood as a backer? If not, another thought I had was to start with a smaller diameter bit and progressively work up to 5/16”—that might minimize tear-out.

I think a router bit would also work—if you are careful when you plunge through.

View Lancefromvt's profile


5 posts in 1125 days

#2 posted 09-21-2021 12:17 PM

You need a backer block supporting the cut at the exit of the hole, none of those bits will exit a hole without tear out unless there is a backer block. Simply hold, clamp or tape the block to the other side.

View GaryCK's profile


134 posts in 1289 days

#3 posted 09-21-2021 12:18 PM

I’m not sure a different bit will help you avoid tearout. A backing board is really what you need. Is the bottom of the table clear from side to side? Maybe you could clamp a 2×4 across the plywood under where you want to drill the hole and draw it tight to the bottom of the plywood with one or two small screws next to the hole location. I’m not sure what else to suggest when accessing the bottom of the plywood is not possible.

-- Gary, Wisconsin

View prazbotta's profile


42 posts in 515 days

#4 posted 09-21-2021 12:26 PM

Not sure what more you are going to learn from this thread MrRon. Your previous thread had several very detailed ideas and methods. Most of which are outlined here.

Good luck! I know I’ll be watching for new ideas.

View brtech's profile


1166 posts in 4162 days

#5 posted 09-21-2021 01:03 PM

How about a 3/32” router bit in a guide bushing around the appropriate sized template from the top.
Start with an 1/16” hole, enlarge to 3/32 with twist drill bits. Drop the router bit in the hoie and route.

View LeeRoyMan's profile


2150 posts in 966 days

#6 posted 09-21-2021 01:06 PM

I couldn’t tell you how well these would work in plywood, but the acrylic bits have a long tapered tip that keeps blowout from happening in acrylic. It may be worth a try if it’s that important for a train set.
Here is an example, I’m sure if you looked for acrylic bits, you could find a separate 5/16” bit.

View SMP's profile


4816 posts in 1145 days

#7 posted 09-21-2021 01:07 PM

Ok so here is the dilemma. I think your BEST option would be Woodowl Ultrasmooth Ship Auger bits. These are the ones Chris Schwarz uses on his chairmaking, and he is able to get virtually no blowout. HOWEVER, the smallest bit they make is 3/8”

View Fred Hargis's profile

Fred Hargis

7227 posts in 3733 days

#8 posted 09-21-2021 01:11 PM

Ron, mostly since I was just killing time this morning I drilled a couple of 5/26” holes into plywood (this is 1/2” cabinet grade birch). I have a set of LV lipped brad point bits, and I ws thinking that if anything would leave a clean exit it would be them. They have a longer cutter than most brad points. Anyway, I drilled 2 straight through with the bit, for the second I put a piece of blue masking tape over the exit area. Here’s a pic of what happened. The top hole is just the bit, the lower one is with the blue tape. I suspect (but do not know) that a more gentle exit (less force on the drill) with just the bit may have left a cleaner hole…but this was the extent of my testing. Maybe it helps, maybe not.

-- Our village hasn't lost it's idiot, he was elected to congress.

View northwoodsman's profile


672 posts in 4986 days

#9 posted 09-21-2021 04:46 PM

Why do you need such a clean hole on the bottom of your train platform? Will it be visible? No matter what you use you will need a backer board to prevent tearout.

-- NorthWoodsMan

View MrRon's profile


6184 posts in 4483 days

#10 posted 09-21-2021 06:53 PM

There will be wires passing through the holes.. I will try the router bit way and look into the ship auger bit and the LV bits. I know I’m starting to get viral with this topic so my final attempts will either work or I will just accept the final outcome as “the best that can be done under the circumstances”.

View Orvile Baker's profile

Orvile Baker

281 posts in 1918 days

#11 posted 09-21-2021 11:34 PM

I use the backer board and sometimes I use the smallest counter sink bit that comes closest to the bit size just to make sure there is no sharp edge to cut the wire. I have N-scale trains and lots of fine wires, but most of the time If I have room the backer board works best for me.

-- Bud Baker , Ojibwa, WI.

View Chip201's profile


11 posts in 28 days

#12 posted 09-22-2021 03:47 AM

Try Fisch brad point bits. They are top shelf bits. I keep them near but not where I can just grab them when a lesser quality hole is adequate. Save them for fine holes in fine wood.
They are available at several sites

View pottz's profile


20623 posts in 2224 days

#13 posted 09-22-2021 03:56 AM

i dont understand why it’s so important for the holes be perfectly smooth just to run wires trough ? am i missing something or are you being too picky ?

-- working with my hands is a joy,it gives me a sense of fulfillment,somthing so many seek and so few find.-SAM MALOOF.

View MrRon's profile


6184 posts in 4483 days

#14 posted 09-22-2021 06:21 AM

i dont understand why it s so important for the holes be perfectly smooth just to run wires trough ? am i missing something or are you being too picky ?

- pottz

The wires have a plug on the end so the hole is slightly larger than the plug diameter. There are 3 wires and 3 plugs to go through the hole, one at a time. If I were 40 years younger, going under the layout would not be a problem.

View Sylvain's profile


1388 posts in 3739 days

#15 posted 09-22-2021 08:16 AM

about long backer board (2X4) clamped at each end.
To be sure to have good contact between the backer board and the table put a small sacrificial board on the 2X4 where the hole will be bored.

-- Sylvain, Brussels, Belgium, Europe - The more I learn, the more there is to learn

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