Adjustable-width hole boring bit for drill press?

[Taylor Cornington]

Does anyone have experience with adjustable-diameter drill bits? I'd like to drill thru-holes in 1/4" hardwood that are non-standard sizes, ideally in a few sizes between 15 and 30 mm to hang kitchen tools, test tubes, shop tools, etc. I can already do this on small workpieces (up to 100 x 125 mm) on a desktop CNC router, but it's a ton of work and I'd prefer to just use a drill bit in a drill press, if the current offerings can produce decent results.

There are several styles I can choose from, and I'm curious if anyone can recommend any of them. I'm not sure what the policy is here on linking specific products, but the styles I'm looking at are:

- The kind that look like forstner bits
- The kind that look like spade bits
- The kind with a pilot bit in the center and an independently adjustable cutting edge on either side

Are any of these worth spending money on?

[Jim Becker]

Because you will be using a drill press, you can use a fly cutter safely if everything is clamped down. That's number three on your bullet list. It's basically infinitely variable size between its limits and doesn't favor any particular measuring system, either...you set it using the rule and system of your choice.

[Doug Garson]

+1 on the fly cutter, haven't used either of the other two. The fly cutter is a bit of a PITA to set the exact diameter but once set it cuts accurate holes. Just make sure the workpiece is well clamped down and keep your knuckles clear, DAMHIK.

[Taylor Cornington]

Thanks for the rec, Jim. I'll pick one up. And thanks for the safety tips. I'm pretty green with woodworking but familiar with metalworking, so going ham on workholding and keeping non-sacrificial digits away is my wheelhouse.

[John Ziebron]

I have a very old adjustable drill bit but ones are still made that would work for you like this: https://www.leevalley.com/en-us/shop/tools/hand-tools/drills/67385-adjustable-drill-bit?item=72J0320 Amazon also has a large selection.

A fly cutter is really surface finishing tool mostly used for metal. What Jim and Doug are referring to is an adjustable hole cutter for wood but they are typically used to make holes much larger than the 30mm you mentioned.

[Ray Newman]

Back in the Last Century, I utilized fly cutters on a drill press. Have not seen them used or for sale in years, but it does work. Keep the speed low and securely clamp down the stock. For small holes, I had a single cutter and for larger holes I had a two cutter unit, like the one linked below:

https://www.harborfreight.com/carbide-tip-adjustable-circle-cutter-68117.html

I never bothered to try to re-sharpen the cutter as they were inexpensive enough to buy new instead of resharpening. I also found that the dual cutters worked better. To bore a test piece I marked the drill point centerpoint, then drew circumference with a compass. By slowing move the cutter around by hand, it was far easier to alter the positions of the cutter for the required hole size.

[Bill Dufour]

A boring head will clean up a hole to the needed diameter. I have an adjustable auger bit for use in a hand brace. If you have some common sizes make a setting jig.
Once you have the bit set to the correct diameter make. Cut into scrap so the pilot hole is good and the cutter just touches the wood and leaves a slight groove.
Bill D.

https://www.amazon.com/45002-Lockhead-Adjustable-8-Inch-3-Inch/dp/B00004YO6Y/ref=asc_df_B00004YO6Y/?tag=hyprod-20&linkCode=df0&hvadid=198090983914&hvpos=&hvnetw=g&hvrand=12461383546438071381&hvpone=&hvptwo=&hvqmt=&hvdev=c&hvdvcmdl=&hvlocint=&hvlocphy=9032277&hvtargid=pla-421696479524&psc=1

[Doug Garson]

I have a very old adjustable drill bit but ones are still made that would work for you like this: https://www.leevalley.com/en-us/shop/tools/hand-tools/drills/67385-adjustable-drill-bit?item=72J0320 Amazon also has a large selection.

A fly cutter is really surface finishing tool mostly used for metal. What Jim and Doug are referring to is an adjustable hole cutter for wood but they are typically used to make holes much larger than the 30mm you mentioned.
Good point on the diameter, I agree the adjustable spade bit style (although I have no experience with them) would be more appropriate.

[Warren Lake]

fly cutters work well. past they came in two sizes I can look at dimensions but lee likely had two sizes or atlas machinery in Toronto. sharpening the bit is simple. Takes more time to order one. Ive cut all kinds of materials baltic birch was fine. You need some care and lower speed when cutting larger size holes.

The adjustable spade works fine but rather use a drill press tool for the built in square it is.

For spade bits you can take any spade bit and change the size with a file. You either file them parrallel or some ive done a taper on the sides. Cutting edges are super easy to sharpen with a file so even if you dont alter the size easy to keep sharp. Work well for how simple they are

[Doug Garson]

fly cutters work well. past they came in two sizes I can look at dimensions but lee likely had two sizes or atlas machinery in Toronto. sharpening the bit is simple. Takes more time to order one. Ive cut all kinds of materials baltic birch was fine. You need some care and lower speed when cutting larger size holes.

The adjustable spade works fine but rather use a drill press tool for the built in square it is.

For spade bits you can take any spade bit and change the size with a file. You either file them parrallel or some ive done a taper on the sides. Cutting edges are super easy to sharpen with a file so even if you dont alter the size easy to keep sharp. Work well for how simple they are

That should work too and spade bits are cheap. Just take an even number of file strokes from each side so the spur stays centered.

[Warren Lake]

the center will keep it centered. I doubt i put enough care to make them exact even if same number of strokes each side. Make sure there is wood below it tight and the center just follows into that Ive changed sizes from the start simple and easy dont have 5/8 then make it out of a 3/4 etc

[Doug Garson]

Yes the spur will keep it centered but, exaggerated to make it easy to picture, if one side was filed to say 0.25 in and the other to 0.50 in, it will drill a 1.00 in hole even though the bit is only 0.75 in wide. The 0.50 in side will do all the cutting and the 0.25 in side will do nothing.

[Richard Coers]

Can you get a fly cutter that will drill 15mm? I sure don't have one that will.

[Warren Lake]

so what if it does didnt stop me from doing any jobs over the years. If one knife is higher in a jointer it does more work

I dont even know what 15Mm is I never work metric there are two sizes of fly cutters and sure more, I have two and will have to go into the shop to look to see what the smaller one does. They work well enough and are simple and easy to adjust. Never had an issues other than once in the beginning got a hand a bit too close.

[Warren Lake]

so no fly cutter cant go that small that without me looking but looking up 15 mm. not sure of the smallest hole from the small fly cutter but its larger than that for sure

[Bill Dufour]

15mm = .59". so a 1/2" boring bar in the middle hole would fit 15mm with room to spare.
Bill D.

https://www.amazon.com/ALL-CARB-Boring-Shank-Carbide-Milling/dp/B095K34627/ref=sr_1_4_sspa?keywords=boring+head&qid=1693948417&sr=8-4-spons&ufe=app_do%3Aamzn1.fos.006c50ae-5d4c-4777-9bc0-4513d670b6bc&sp_csd=d2lkZ2V0TmFtZT1zcF9hdGY&psc=1

[Jim Becker]

Can you get a fly cutter that will drill 15mm? I sure don't have one that will.
That's a good point...that may be below the lower limit, but it may be close. I didn't measure, of course.

[Bill Dufour]

They make 3/8 boring bar heads. So a little less then 3/8 with off the shelf bars. easy enough to grind one down for smaller holes.
Bill D

[John Ziebron]

Can someone explain to me why some of you guys keep calling a tool to make holes a fly cutter. A fly cutter is a tool used to produce a finish on the surface of a part.

[Warren Lake]

I use wrong terms for stuff all the time. odd thing is most of my friends understand me even so.

[Warren Lake]

this

507267

[Taylor Cornington]

Yeah I took a look, seems like they only go down to 1". That's fine, thankfully forstner bits are cheap for small sizes.

[John Ziebron]

this

507267

I know that this is the tool being reffered to but it is not a fly cutter.

This is a fly cutter:
https://littlemachineshop.com/images/products/480/4663.480.jpg

[Bill Dufour]

Here is a boring head. the boring bar with cutter can be mounted vertical or horizontal depending on diameter desired. Normally they come with a tapered shank but straight shanks one are made or many use screw on shank mounts.
Bill D.

[Bradley Gray]

+1on spade bits.

[Curt Harms]

this

507267

That's what I have called a fly cutter as well, incorrectly it seems.