I want to drill 48 dog holes through 3/4" MDF.
I tried Forstner and spade bits. The Forstner did fine but heated up and became dull. The spade bit left a jagged edge on hole top.
What bit would be the best for this application?
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A lot of people will answer this with Forstner all the way, but you pay for quality. Spade or flat bits are perfectly up to this job but most inexpensive ones (and a few that aren't that cheap!) could do with a bit of a sharpen before first use so they give a good clean entry hole. Regardless of the bit type you pick it would be good to get HSS, since this can run hot without being noticeably weakened.... however you should still take it easy, for chip clearance and to prevent scorching of the holes.– GraphusCommented Jan 31, 2020 at 14:59
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4You might want to edit your Question to include whether a clean exit hole is important for what you're doing.– GraphusCommented Jan 31, 2020 at 15:00
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If the forstner bits heated up enough to change colour, you might also be losing temper. So maybe give it more time to cool and unclog.– user5572Commented Jan 31, 2020 at 16:19
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You can always just dip your bit in a cup of water every few holes...– SaSSafraS1232Commented Jan 31, 2020 at 17:00
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Yup, and practice re-honing forstner bits. Easier that twist bits! Though spade bits are easy to hone as well.– user5572Commented Jan 31, 2020 at 17:52
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6 Answers
Drill a pilot hole for your Forstner bits before drilling the main hole.
The problem here is that MDF isn't nearly as compressible as wood. If you look at the center of a Forstner bit you'll see that the center spur doesn't actually cut. Material isn't mechanically moved away from the center, it's just compressed to the side and later cleared by the cutting edges. This works fine in softwood, plywood, and even hardwoods, but MDF simply doesn't compress. This prevents the bit from moving down and engaging the material fast enough.
If you drill a pilot hole slightly smaller than the center spur on the Forstner bit it will allow the face cutting edges to engage properly and it should cut through the MDF like butter.
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Nice. I have a project coming up that requires lots of clean holes in plywood, and I'm definitely going to try this.– user5572Commented Jan 31, 2020 at 17:53
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Plywood has slightly different issues...the big one is that the layers will delaminate after the spurs cut the top layer free and then the round chip blocks the face cutting edges. Only thing I know of to fix that is to pull the bit out and clear the chip... Commented Jan 31, 2020 at 18:34
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Also, I've noticed that pilots for Forstners in hardwoods actually make the situation worse because they allow the bit to feed too fast, which causes chip-out at the start of the cut and generally makes it hard to control the feed rate if you're not using a drill press or lifts your work if it's not clamped down. So pilots for MDF, none for wood. Commented Feb 1, 2020 at 1:04
I used once short auger bits with pretty good results. Be aware that they have a lot of 'grip' so to say.
I started first with a pilot hole, just half size of the needed hole, and then 'finished' with the bit as needed/wanted.
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I used an auger bit and a bit and brace to drill something like 20 holes into a 3" thick laminated MDF top. It does work, and the holes are clean, although you can break out the far end if you aren't careful. If you use an electric drill probably use one with a side handle because it will want to torque your wrist off. My main issue was with getting accurate 90 degree bores -- I used a bushing & a jig but there was enough relief on the bit that it still had play. Not as big a problem in practice as I thought since my dogs have a slight relief angle on the flats. Commented Feb 3, 2020 at 3:43
An alternative to using a Forstner bit is to first drill a pilot hole, then use a counterbore tool. See examples. They're typically used on steel, but they make quick work of MDF too.
As a young`un working in a high school, I remember making many many holes in MDF for building lab equipment.
The main weapon was a pillar drill and a suitably-sized hole saw, and a jig. The tricks are to get the correct size saw, and to find a good speed that doesn't burn the wood. MDF having a high glue content likes to char in the hole if you overheat it.
The center drill bit goes through first, and the saw edge cuts into the outside. For a good-enough finish you flip the part and come in from both sides to meet in the middle.
At the end, you'll have a puck up the saw looking like this:
They make great wagon wheels for the local kindergarten to hammer on, or string +glue a lot of them together on threadded rod to make handles.
A last trick is to clear the chips periodically by raising the saw out, and you can also drill small through-holes in the foot of the slot to help the saw breathe.
I've had good luck using "speedbor" flat spade bits. They are boring normal flat bits but there's a cutting tang on the outer edge of both flats, and they work to cut the circumference loose before the main cutting edge lifts the swarf out.
Not a brand recommendation, there are plenty of examples at your average bigbox store.
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1Just make sure to avoid the ones with the snails ("Speedbor Max"). You want the bit spinning fast and feeding slow at the start for a clean cut. Commented Feb 1, 2020 at 1:00
Using the tools at hand without too much mucking around, etc: Do the first 1/8" or so with the Forstner bit, then change to you spade bit to finish out.
