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Here comes the Newbie again!

752 Views 3 Replies 4 Participants Last post by  MEBCWD
Since I am awaiting delivery of my new machine, I have too much time to think about things.

Question, When drilling, let's say a 1/4" hole, does one use a 1/4" upcut, downcut, compression or do you use a smaller, say 1/8 bit and let the machine wallow the hole to 1/4?

I've read that downcut can overheat due to the chips being pushed down, and upcut makes the surface jagged, I just don't know.

I guess it depends on the material being cut, MDF on a spoilboard is one thing, hickory or maple maybe another. I want to drill in MDF and HDPE for the bolt holes to start with, but if I get good enough and want to do something that can be double sided, I'd need a few dowel points.

I know I am questioning, but just thinking ahead.
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Ideally, you'd use a 1/4" drill bit. A drill bit will always be better than "drilling" with a router bit.

Couple issues with using drill bits, though.
Ideally, you'd spin them slower than the spindle should be run. The workaround is to just feed faster.
The other issue is that spindles aren't really made for drilling, and high thrust loads are not good for bearings. But if you are just drilling wood, the forces aren't too bad.

A downcut won't just overheat when plunging, it will burn the wood almost instantly.
An upcut would be my choice. It won't chip anything if you are just plunging.
On shallow holes, a compression bit will act like an upcut. But on deep holes, the chips won't come out, unless you peck drill with retracts to clear the chips.

Interpolating with a smaller bit is very common, but smaller bits tend to be limited in length, which can limit your hole depth.

If you are through drilling and need a good finish on both sides, a clean spoilboard will usually prevent chipping on the back side. Or, use a carbide tipped "V" boring bit. But those need to go pretty deep into your spoilboard.
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I cut Longworth chucks out of 1/2" BB with a 1/4" compression bit and cut the full depth in one pass. For the center mounting hole that is 0.260", though, I bore down only 0.400" and then run a screw into the spoilboard. When I take the chuck off I finish the center hole with a 1/4" Forstner bit against a clean surface so I don't get any tearout or chipping or splinters.

Like Gerry said a clean spoilboard will give you a clean back side but because I cut a lot of Longworth chucks I can't always guarantee the center hole has a clean section of spoilboard under the BB. Boring 4/5 of the way through and finishing with a Forstner only takes a minute and gives me clean results.

I don't know if you'll be able to do that with your projects, though.

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Another thing to consider is that every machine is different. Support table, tightness, backlash, spindle HP, maximum feed rate, and other things about your setup, all contribute to how each bit works on that machine.

To get used to using your machine it will help to experiment with different bits and different stratagies. Start with the information you have for the bit from the manufacturer and user comments. If they say it could cause problems then shy away from those bits or strategies when doing your testing. You will be the best judge of how the bit works on your machine.

Doing this will give you some very simple design jobs to do for cutting the test items and give you some very simple quick jobs to cut on the new machine.

Play with the software, play with the CNC, have fun, and get creative!
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