How to cool hole saw

Question

I am currently trying to work on a project that involves piercing a lot of large-diameter (~38mm) holes in rather thick (~5cm) bamboo board sheets.

For that, I am using a hole saw. However, I ran into the problem of the wood starting to burn and the bit getting stuck (due to the thermal expansion?).

If I was drilling or routing metal, I would be cooling with a mix of oil and water at the ambient temperature.

What can I use to perform similar cooling in the context of working on wood? Would water be sufficient? Should I just make frequent pauses and let the drill bit cool in the air?

Answer 1

This was mentioned in SaSSafraS1232's answer, but it works so well that I think it deserves an answer all on its own.

If you're using a hole saw and planning on cutting all the way through the work piece, then I strongly suggest you start by drilling one or more relief holes for sawdust before you use the hole saw.

Drill the relief hole on the kerf of the planned cut with the hole saw, such that the relief hole is mostly on the waste side of the hole that will be cut with the hole saw, but still overlaps the kerf of the hole saw. Here's a picture to help illustrate the technique:

Image Credit: Family Handyman (original article)

The problem with heat and burning is that sawdust builds up and has nowhere to go. One way to fix this is to slow down and clean out the kerf often. Another way to fix it is to drill holes as above. It gives the sawdust somewhere to go as you cut.

You'll still have to go slow. Check the relief holes periodically to see if they've gotten clogged. If they do, the heat will start building up and you'll get smoking and burning again. But with the relief holes, you can drill with the hole saw longer before you need to stop and check things.

The difference in quality of the resulting hole saw cut with a relief hole vs. without is just huge.

Answer 2

The problem with hole saws is that there is no way to eject the waste. If the waste sawdust is properly dealt with the saw will not heat up to the point of scorching.

In a normal twist bit the waste is pulled out by the grooves in the bit. With no grooves the waste from a hole saw clogs the gullets and packs in around the sides of the bit, heating it up and increasing friction. This is actually the cause of the binding you're feeling, not thermal expansion (or possibly alignment issues, see jdv's answer for more info). There's usually plenty of "set" on a holesaw (i.e. the teeth project well outside the thickness of the tube) to prevent binding on the work piece itself.

The best way around this is to keep a compressed air blow gun handy and stop, pull the bit out, and blow the hole out every so often (every 1/8" of depth or so). At the same time you should also pick or blow off any sawdust packed in the gullets of the saw.

You can also drill holes in the waste that overlap with the kerf the saw is cutting to give the waste somewhere to go. Just be careful not to let them wander into the "keeper" side of the cut. This is easier with brad point or forstner bits.

Answer 3

To answer your question: the DIY or home wood shop doesn't generally use any sort of bit lubrication or cooling liquid. Maybe some applications would use some sort of wax or teflon coating on the bit, but I wouldn't bother.

Use sharp appropriate bits for the material and adjust speed and feed accordingly. It sounds like you are using a hole saw, which has a few problems:

1. They don't clear chips well, clogging the hole and adding to the overall work load
2. They often have a greater amount of finer teeth, with little kerf and not much gullet necessary for clearing chips
3. The large rotating part that holds the teeth is almost always going to rub against the work no matter what you do, holding all that heat and causing burning

Hole saws are really only appropriate for specific applications.

The typical use is to go slow and straight -- try not to rotate the bit out of round around its axis or you will wallow out the hole and cause more friction. Nice even and straight pressure is best, with plenty of breaks to clear chips. A source of compressed air is handy in this case.

Alternatively, you can get a few Forstner bits that are the right size. Get one with the centre a bit proud so you don't have to drill pilots holes. Again, adjust your speed and feed, and allow the chips to clear. Forstner bits have less trouble with clogging, so you might be able to just blow the chips away as you work with puffs of air.

This is also a place where keeping square and true to the hole is important. Two much pressure to one side is where the friction and burning comes from. A drill press or guide is handy in this case. Though, I've had luck with careful hand placement and a centre spirit bubble attached to the base of the drill, in line with the tool bit.