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Table saw blade speed - a photographer's perspective

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Forum topic by jamsomito posted 11-14-2018 01:49 AM 1007 views 2 times favorited 19 replies Add to Favorites Watch
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jamsomito

311 posts in 631 days


11-14-2018 01:49 AM

Just thought I’d share something cool I looked into this last week. I’m simply a woodworking hobbyist. I’m also a photography hobbyist. I’m currently in the middle of a weekly photography challenge, and last week’s topic was shooting in manual mode. Frankly I’m terrible at guessing exposure and I was missing too many shots out in the wild, so I thought I’d set up a “studio” shot where I could take my time and control everything. My goal was to take a shot of my table saw mid-cut to see the teeth as they were chipping away at the wood. I was envisioning an awesome shot of the teeth tearing big chunks into the wood, chips and dust flying all over, just a gritty, jarring photo. Yeah that didn’t happen. The exercise proved futile but was still fun and interesting. Here’s what I found.

I couldn’t freeze the blade in any of my shots. I quickly found my camera wasn’t fast enough. Here’s a couple shots from my session. For you fellow photo buffs, these were at f/2.8, 1/8000s, and ISO 6400. I also had my garage door open with sunlight bouncing off the fresh snow on the ground, all my shop lights on, and a studio light on full blast directly above the camera. Don’t worry, I set it all up on a tripod with a 10 second delay – plenty of time to get my wits about me and make a safe cut.

It almost got there so I did some quick math. I have a Powermatic 63 and it spins the blade at 4500 RPM. For my diablo blade (10”, but not a true 10” – more like 9.5”), the teeth are traveling a little over 2,200 inches per second. My camera’s fastest shutter speed is 1/8000 of a second, or 0.000125 seconds. In that amount of time, the teeth have already traveled over a quarter inch! Crazy to think about with that many decimal places.

In relation, a router spins at 24,000 RPM, which sounds terrifying considering the above, but in reality the bit is so much smaller that the actual tooth velocity is only about half that of the table saw blade’s.

So what would it take to freeze the teeth in a photo? Well, a heck of a lot more light, and my guess is maybe 4x as fast a shutter speed, which would be about 1/32,000s, or 0.00003125 seconds.

I was curious, did a quick search, and came across these cool youtube videos of actual high-speed cameras. The first shows a couple cuts and a couple kickback scenarios at 1/21,600 second shutter speed. I can’t say I agree with his handling of the saw, but the videos are interesting nonetheless. The second shows clips at 1/44,000, 1/75,000, and 1/150,000 seconds (!). Wowzers that’s fast.

cool slo-mo video 1: https://www.youtube.com/watch?v=x9PAYltlgIs
cool slo-mo video 2: https://www.youtube.com/watch?v=33fxFXwqz-c

I guess I learned a few things:
1) table saw blades spin fast and the teeth move crazy fast
2) every pass of a blade tooth takes a minimal amount of wood a) this explains the fine dust you get from a table saw b) this explains why sharp teeth matter so much for a good quality cut

This was fun and I just wanted to share. Hope you enjoy.


19 replies so far

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Redoak49

3670 posts in 2194 days


#1 posted 11-14-2018 12:17 PM

That was really interesting.

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jonah

1928 posts in 3504 days


#2 posted 11-14-2018 02:03 PM

That first guy is lucky he didn’t kill himself handling the material like that. If he’d been using a 3HP+ cabinet saw it might have kicked that 2×4 back into his face and killed him.

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jamsomito

311 posts in 631 days


#3 posted 11-14-2018 02:25 PM


That first guy is lucky he didn t kill himself handling the material like that. If he d been using a 3HP+ cabinet saw it might have kicked that 2×4 back into his face and killed him.

- jonah

I knew that comment was coming. I agree. That old saw even looks like it’s out for blood. But the videos were interesting for the rest of us despite the creators risks.

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jbay

2895 posts in 1104 days


#4 posted 11-14-2018 02:43 PM

I thought the slo mo’s were pretty good.
Seeing the gussets removing the sawdust was very cool.

Thanks for taking the time.

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Brawler

10 posts in 36 days


#5 posted 11-14-2018 02:52 PM

I’m not sure if this would work, but did you ever think about a strobe light?

-- Daniel

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jamsomito

311 posts in 631 days


#6 posted 11-14-2018 03:05 PM


I m not sure if this would work, but did you ever think about a strobe light?

- Brawler

I do have a flash, but it won’t work at fast shutter speeds. It has to do with the way the DSLR’s shutter mechanism works. There are essentially 2 curtains that cover the sensor – one moves down to expose the sensor and the other moves behind it to cover it again, and the time between the two is the exposure time, or shutter speed. The problem is these shutter curtains, being material objects, take time to move across the sensor, and at high speeds, the top curtain is so close behind the bottom one that the whole sensor is not exposed at any one given time. The flash is much faster than that, so all you would see in the final image is a stripe of the image.

Like this: https://northrup.photo/gear-basics/camera-body-features/x-sync-speed/

And another slo-mo video of a DSLR shutter mechanism: https://www.youtube.com/watch?v=usfEGOwamJ8

Because of this, I’m limited to shutter speeds of 1/250th of a second on my camera, which this demonstration showed is way too slow to capture a table saw’s teeth while it’s running. That’s the shutter speed at which the curtains can move completely over the sensor, leaving it fully exposed, prior to the 2nd curtain covering it again.

To fix this, you basically need a high speed video camera at that point ($$$$$$) which has a totally different “shutter” mechanism, or a camera with high-speed sync on the flash, which essentially fires the flash in a strobe-like effect so you get equal “stripes” on your exposure as the shutter curtains move across the sensor to fill out the whole image. This is much more obtainable, but on more expensive professional DSLR’s. But again, you’re limited to the camera’s 1/8000s maximum shutter speed. So you’re back to the crazy expensive high speed video cameras again.

Interesting problems with high speed imaging.

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bondogaposis

5096 posts in 2556 days


#7 posted 11-14-2018 03:21 PM

Try it with low light and use your flash. You might be able to show more.

-- Bondo Gaposis

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GR8HUNTER

5155 posts in 918 days


#8 posted 11-14-2018 03:30 PM

VERY NEATO :<))

-- Tony---- Reinholds,Pa.------ REMEMBER TO ALWAYS HAVE FUN

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theart

62 posts in 760 days


#9 posted 11-14-2018 03:38 PM


I m not sure if this would work, but did you ever think about a strobe light?

- Brawler

I do have a flash, but it won t work at fast shutter speeds. It has to do with the way the DSLR s shutter mechanism works. There are essentially 2 curtains that cover the sensor – one moves down to expose the sensor and the other moves behind it to cover it again, and the time between the two is the exposure time, or shutter speed. The problem is these shutter curtains, being material objects, take time to move across the sensor, and at high speeds, the top curtain is so close behind the bottom one that the whole sensor is not exposed at any one given time. The flash is much faster than that, so all you would see in the final image is a stripe of the image.

It sounds backwards, but the key to shooting faster than the max shutter speed is using a longer exposure so the whole sensor is open at once. The strobe is then used to produce a a very short pulse of light mid-exposure. This does, however, require low ambient light and a very bright strobe, so it’s not something I would try with a table saw.

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jamsomito

311 posts in 631 days


#10 posted 11-14-2018 03:43 PM



Try it with less natural light and use your flash. You might be able to show more.

- bondogaposis

Hmm now that I think about it I might be able to do a longer exposure with a single flash. That would give a blur for the blade but possibly superimpose the teeth. I don’t know my flash fire time, I’ll have to look it up. Could be a cool effect.

I also have some concerns blasting myself with a flash while I’m trying to make a cut on the table saw. I might need to leave that to someone else to try.

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jamsomito

311 posts in 631 days


#11 posted 11-14-2018 03:44 PM


It sounds backwards, but the key to shooting faster than the max shutter speed is using a longer exposure so the whole sensor is open at once. The strobe is then used to produce a a very short pulse of light mid-exposure. This does, however, require low ambient light and a very bright strobe, so it s not something I would try with a table saw.

- theart

Yup, my thoughts exactly.

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Andybb

1524 posts in 809 days


#12 posted 11-14-2018 05:29 PM

Kool! Just curious what blade was used in the 2nd video. Wondering if the teeth are offset as it looks like every other tooth is shaving wood and it is alternating to the inside and outside of the center of the blade.

-- Andy - Seattle USA

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jamsomito

311 posts in 631 days


#13 posted 11-14-2018 05:33 PM

Yeah, it looked like ATB rip blade to me… on cross cuts. It’s really hacking those fibers on the last clip and leaving a rough cut.

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oldnovice

7337 posts in 3573 days


#14 posted 11-14-2018 06:25 PM

High speed photography makes the real world very interesting.
The videos also show that the sides of the saw blade teeth are as important as the tips!

While I was an intern in the lab of a very large business machine company, the engineers did high speed potography on computer card reader mechanism with a film camera (before digital as this was over 50 years ago) at somewhere around 4000 frames per second. An 24 inch diameter film feed reel was depleted of film in a matter of seconds and the results of the card reader moving mechanism, made of stainless steel, looked as if it was made of rubber. Of course we had to wait until the film was developed, not an instantaneous view for feedback of the test._

-- "I never met a board I didn't like!"

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Andybb

1524 posts in 809 days


#15 posted 11-16-2018 12:01 AM


The videos also show that the sides of the saw blade teeth are as important as the tips!
- oldnovice

Yeah. I saw that. Interesting tidbit.

-- Andy - Seattle USA

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