Why so fast?

[Windows]

Hand held power tools operate at thousands or tens of thousands of strokes or revolutions per minute. Why is that? Are there any companies that make low speed portable power tools?

 

[Trevanion]

Believe it or not, in the world of machine woodworking slow spinning tools are actually far more dangerous than fast spinning tools. There’s a sweet spot, where the tool is fast enough that you’re not getting kickback and the tool is cutting efficiently with a smooth surface, but not so fast that you’re burning the wood and shortening the life of your cutters drastically.

 

[Windows]

Yes, that’s interesting. What is it that makes the lower speed more dangerous? Is it related to hand tremor frequency? Or to sizes of wood cellular structures? Or properties of the metal doing the cutting?

I guess RPM is a weird measure anyway. The contact area of the cutting surface over time might be a measure that makes more sense to compare between categories of tool. I guess bandsaws probably have fairly low RPM.

Part of what I’m wondering is whether there is more variety in power tools than I am currently aware of.

 

[Trevanion]

Yes, that’s interesting. What is it that makes the lower speed more dangerous? Is it related to hand tremor frequency? Or to sizes of wood cellular structures? Or properties of the metal doing the cutting?

There are a lot of factors, but nothing to do with what you've mentioned in particular. Take for instance a handheld power planer, the cutter block (or drum as some people call them) spins at around 15000RPM, which sounds fast but really at that size it's only 85mph at the rim. With two knives in the cutter block it will be cutting the timber 500 times a second or thereabouts, pushing that over a piece of timber at a rate of 20M/m will result in one cut for every 0.6mm of linear travel, or 42 "cuts-to-the-inch" in old terms, leaving a very clean surface, though handheld machines are typically poorly adjusted and it may be half that number. The slower you make the cut, the coarser the "cuts-to-the-inch" will be, the harder the machine will be to push through the timber as you will be taking larger chips for the same feed rate, as well as a higher tendency for kickback to occur because of the larger chips being removed. Of course, the obvious solution is to reduce the feed rate to increase the "cuts-to-the-inch", but these machines are designed for speed and ease of use in mind.

In regards to my first post, a machine that this principle is very easy to see firsthand is with a variable-speed router using something like a 1/2" roundover bit, using the slowest speed you will find that you have to keep the router moving very slowly to prevent tear out and kickback, too fast of a speed and you will get burning and the bit will overheat causing a much-reduced lifespan of the cutting edge, and then there's a sweet spot in the middle where the bit works wonderfully with no burning, at a reasonable feed rate.

 

[Windows]

Thank you. I’m starting to see the constraints now.

I guess the design constraint for a handheld planer is that we have a user-controlled feed rate that isn’t tied to the cutting speed and we have a cutter that will continue to cut even if the feed rate drops to zero. Reducing the feed rate increases the depth of cut at the current position, assuming constant cutter speed, then the tear out happens as we move to the next position because the blade slams into the edge of the cliff we just cut.

So, apart from user education and giving people practice time to improve their power planing, we could help people using handheld planers by:

1. Giving manual control over cutter speed so people can pre-match their preferred feed rate
2. Adding a depth stop to prevent continuous cutting when the feed rate drops (I’ve seen people create jigs for this)
3. Use the same tech as in computer mice to measure feed rate and pass that info back to the motor to create a more direct, and continuously adjusting, link between feed rate and cutter speed.

I’m sure there are other ideas here.

I’ve just bought a handheld planer and intend to create a depth stop jig for it. I didn’t notice whether handheld planers offer manual or continuous cutter speed control because I was buying based on brand/price and didn’t think much about it. Do manufacturers do the feed rate detection and feed it back to the cutter speed?

 

[Doug71]

I always think the rpm that a tool needs to run at is related to the size of the cutter and achieving the correct tip speed. A small router cutter might run at 24,000rpm but a 150mm diameter spindle moulder block might only run at 3,500rpm, because of it's larger size the tip of the spindle moulder block travels a lot further in one revolution than the tip of the router cutter does so therefore it is moving a lot faster. For this reason the spindle moulder block needs to rotate slower than the router cutter to achieve that sweet spot cutting tip speed that Trevanion talked about.

Many power tools these days are full of clever electronics and constant speed under load is incorporated in a lot of them (including planers). My Festool track saw sounds terrible when it's running just free without a load, it kind of rattles and sounds like something is loose but it's just the electronic speed control constantly monitoring and adjusting the speed.

 

[Woodbloke]

I always think the rpm that a tool needs to run at is related to the size of the cutter and achieving the correct tip speed. A small router cutter might run at 24,000rpm but a 150mm diameter spindle moulder block might only run at 3,500rpm, because of it's larger size the tip of the spindle moulder block travels a lot further in one revolution than the tip of the router cutter does so therefore it is moving a lot faster. For this reason the spindle moulder block needs to rotate slower than the router cutter to achieve that sweet spot cutting tip speed that Trevanion talked about.

Agreed, it's the 'sweet spot' of one tooth or cutting edge that's critical. If memory serves, the little orange Hayward pocketbook (original edition) had a lot of useful stuff on this topic - Rob

 

[spb]

Another point that Dan touched on but didn't really elaborate is that (especially for hand-held power tools) the faster it spins, the less torque it needs to do the job, which means the less force it puts back onto the user and the easier it is to control. Hence the reason a blade binding in a cut gives a bit of a jolt before stopping instead of wrenching the saw out of your hand.

You can see this difference taken to an extreme in the comparison between a drill and an impact driver for tightening bolts. The drill, especially in a low gear, is putting out a constant high torque, and as soon as the bolt is tight and can't turn any more, all of that torque goes straight back into your wrist. The impact driver delivers lots of small hits rapidly, and won't knacker your arm just because you weren't paying attention to when it tightened up.

 

[Woodster]

It’s an interesting question. One of the places I used to buy metal from had a cold saw which ran at a very low speed.

Edit: Probably a similar speed to this one, 45/90 rpm.

https://www.thesawcentre.co.uk/multi-cu ... chine.html