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View Roadster280's profile

Slow down table saw blade for ripping?

by Roadster280
posted 04-20-2017 05:34 PM


22 replies so far

View ErichK's profile

ErichK

76 posts in 384 days


#1 posted 04-20-2017 05:58 PM

Thats an interesting idea! I have no idea, but I’d love to see the answer.

View pintodeluxe's profile (online now)

pintodeluxe

5331 posts in 2533 days


#2 posted 04-20-2017 06:12 PM

Part of this makes sense to me… effective teeth per second hitting the wood. We normally adjust this with number of teeth on the blade, and the motor speed stays constant. Reducing speed with the same blade would yield less teeth per second.

The part of the equation that doesn’t make sense to me is blade geometry. Blade gullet on a crosscutting blade or high tooth count combination blade often isn’t large enough to remove the waste. So even if you run the saw at a lower RPM, you may see burning. Interesting discussion. It certainly helps to have variable speed on routers, so I would be interested to see more research on this for tablesaws.

-- Willie, Washington "If You Choose Not To Decide, You Still Have Made a Choice" - Rush

View Loren's profile (online now)

Loren

9146 posts in 3368 days


#3 posted 04-20-2017 06:16 PM

Some sliding table saws have variable speed. I
think it’s mainly used for cutting laminates.

View Rick M's profile

Rick M

9925 posts in 2100 days


#4 posted 04-20-2017 06:25 PM

Seems like a lot of workaround to avoid buying a rip blade. If I didn’t want to switch blades I’d buy a combination blade which has those 5 large gullets to help clear chips. A general purpose blade like the Fusion is a crosscutting blade that can do light ripping.

-- http://thewoodknack.blogspot.com/

View bonesbr549's profile

bonesbr549

1419 posts in 2787 days


#5 posted 04-20-2017 06:57 PM

I do not see that as wise IMO. Most saws would not work long time at a frq other than enginnered. Why not just use the proper plade and feed of material through that blade.

I think thats undue long term stress.

-- Sooner or later Liberals run out of other people's money.

View William Shelley's profile

William Shelley

362 posts in 1189 days


#6 posted 04-20-2017 07:01 PM



I do not see that as wise IMO. Most saws would not work long time at a frq other than enginnered. Why not just use the proper plade and feed of material through that blade.

I think thats undue long term stress.

- bonesbr549

Not true. Most 3ph motors sold in the last … 15 to 20 years or so are more than happy to be driven by a VFD. Exceeding the rated frequency will shorten life, or running the motor at a drastically lower frequency (like 10hz), might also cause some small issues. But 40hz shouldn’t be a problem.

That being said, I think that what pintodeluxe said about blade geometry is what really matters here. The issue isn’t how fast the teeth are moving, it’s that when ripping, there’s a lot more saw dust that is generated and has to go somewhere (the gullets between the teeth). Ripping blades have bigger gullets.

-- Woodworking from an engineer's perspective

View MrUnix's profile

MrUnix

5593 posts in 1919 days


#7 posted 04-20-2017 08:09 PM

The difference between a ripping and cross-cutting blade is not the “teeth per second” hitting the wood, but the number of teeth engaging the wood.

Here is what Freud recommends:

The number of teeth cutting the wood simultaneously must be between 3 or 4 for ripping and ideally 5 to 7 for crosscutting. With less than 3 teeth cutting the sawblade begins to vibrate leading to an uneven cut. If you want to cut work pieces with increased thicknesses, but wish to maintain the same diameter saw blade, then use a blade with less teeth. If instead you want to cut work pieces with a reduced thickness, but also maintain the same diameter saw blade, then use a blade with more teeth.

Slowing the blade down does work well for stuff like non-ferrous metals (ie: sheet aluminium) and things like plexiglass, where it tends to melt when run at full speed. I just wouldn’t run it at too much of a reduced speed for long periiods of time though, as it effects the motors fan and it’s ability to move enough air to keep it cool (assuming you have a TEFC motor).

Cheers,
Brad

-- Brad in FL - In Dog I trust... everything else is questionable

View Roadster280's profile

Roadster280

31 posts in 1839 days


#8 posted 04-20-2017 10:31 PM

Thanks all. The saw is a PM 72, and changing blades is a PITA (having had one before), so I was idly pondering that option. Sounds like it isn’t going to be perfect. Nothing to lose by trying it. The saw’s in the garage, the blade arrived today and the new VFD arrives on Monday. I have a 5hp VFD, so I might have a play over the weekend.

View teejk02's profile

teejk02

469 posts in 845 days


#9 posted 04-20-2017 11:14 PM

Excuse my ignorance on such stuff but IMHO ripping a board is a combination of cutting and jointing the edge at the same time…faster would seem to be preferable for the latter.

View Roadster280's profile

Roadster280

31 posts in 1839 days


#10 posted 04-21-2017 12:54 AM


Excuse my ignorance on such stuff but IMHO ripping a board is a combination of cutting and jointing the edge at the same time…faster would seem to be preferable for the latter.

- teejk02

Well that was my thought, “teeth per second”; but as others have pointed out, the blade geometry may well be the arbiter of success.

View Rick M's profile

Rick M

9925 posts in 2100 days


#11 posted 04-21-2017 01:00 AM

You never mentioned what problem you hope to solve. The Fusion will rip, just not efficiently for the reasons I already gave you.

-- http://thewoodknack.blogspot.com/

View Roadster280's profile

Roadster280

31 posts in 1839 days


#12 posted 04-21-2017 01:14 AM



You never mentioned what problem you hope to solve. The Fusion will rip, just not efficiently for the reasons I already gave you.

- Rick M

Fair point! I’ve had a 72 before, some years ago, and been away from woodworking for a while.

Big picture is that we are building a house in the next year. Although we will have a builder build the house structure, I may well build the kitchen cabs, vanities, doors etc. Also have a Woodmaster molder, so for a lot of effort, we might end up cost-neutral, but with a better-outfitted house.

In preparation for that, I found a PM72 for (comparatively) not much money, and having had one before, the PITA is changing the blade on it. It uses a single wrench (which the new one doesn’t have, but no big deal) and a block of wood. I would rather just keep a single blade on it, except for “special” cuts * yet to be defined!

No problem to buy a rip blade, I just wondered whether I might save a few minutes by turning the dial down a tad. It seems not.

View Rick M's profile

Rick M

9925 posts in 2100 days


#13 posted 04-21-2017 01:40 AM

I thought maybe you had tried it and it wasn’t ripping well.

-- http://thewoodknack.blogspot.com/

View jbay's profile

jbay

1572 posts in 619 days


#14 posted 04-21-2017 02:24 AM

Perhaps just figure a better way to change blades instead of a block of wood.
Everything we do is a process, no matter how many steps it takes. It’s just another thing.
Maybe a wooden screw clamp would work better.
I’m just thinking if you found a better way it wouldn’t be such a PITA
Personally I haven’t heard of the need to change speeds on table saws. Different blades for different cuts is all.
If changing speeds was all it took, than table saws would all be made with variable speed control.

Just giving my thoughts, don’t mean a thing… ;)

-- If anyone would like to see my Portfolio, PM me and I would be glad to send you the link.

View Roadster280's profile

Roadster280

31 posts in 1839 days


#15 posted 04-21-2017 02:30 AM

I get it, for sure. I just have the opportunity with using a VFD to change the speed. I would imagine that the cost of variable speed comes into play for new OEM saws. But for me, using a VFD to get 3phase out of a single phase supply, the speed control facility is a bonus.

We’ll see!

View MrUnix's profile

MrUnix

5593 posts in 1919 days


#16 posted 04-21-2017 02:54 AM

But for me, using a VFD to get 3phase out of a single phase supply, the speed control facility is a bonus.
- Roadster280

The variable speed aspect on something like a table saw is less useful than on something like a drill press or lathe, but it doesn’t hurt to have that ability. On most, there will be some additional inputs (ie: SP1/SP2 on the Teco’s) that let you quickly switch to an alternative speed. In instances where a lot of work is done that can benefit from a slower speed, a simple normal/slow switch can be wired easily so you don’t have to constantly fiddle with the speed buttons or dial.

The other features available (soft start, controlled breaking, programmable overload protection, constant torque and over-torquing, trivial emergency stop wiring, etc…) are just icing on the cake :)

Cheers,
Brad

-- Brad in FL - In Dog I trust... everything else is questionable

View Roadster280's profile

Roadster280

31 posts in 1839 days


#17 posted 04-21-2017 11:30 AM

Thanks, Brad. I have swapped the motor on my drill press for a small 3ph, and that has a Teco on it. I did a similar thing for my lathe, but didn’t like the specific motor, so I’ve sold it, pending replacement. There’s another small Teco waiting for it. But the lathe will have to wait until the house is built.

I didn’t realize there was an external fixed speed input. I knew you could remote the speed potentiometer. Thinking about it, any VFD that has a remote variable speed input could be set up with a resistor network and simple switch to provide fixed speed increments. Even overspeed.

I might look for a multi-position rotary switch to set the various speeds for the drill press. That works really well, especially at higher speeds, but I need a crib sheet to show the effective rpm at whatever frequency I have set the VFD to. A rotary switch with fixed positions would eliminate that.

View Carloz's profile

Carloz

653 posts in 312 days


#18 posted 04-21-2017 12:41 PM

I do not see that as wise IMO. Most saws would not work long time at a frq other than enginnered. Why not just use the proper plade and feed of material through that blade.

I think thats undue long term stress.

- bonesbr549

Not true. Most 3ph motors sold in the last … 15 to 20 years or so are more than happy to be driven by a VFD. Exceeding the rated frequency will shorten life, or running the motor at a drastically lower frequency (like 10hz), might also cause some small issues. But 40hz shouldn t be a problem.

That being said, I think that what pintodeluxe said about blade geometry is what really matters here. The issue isn t how fast the teeth are moving, it s that when ripping, there s a lot more saw dust that is generated and has to go somewhere (the gullets between the teeth). Ripping blades have bigger gullets.

- William Shelley


The prevois post is true, your’ s is softly speaking naive. Induction motor has a very simple construction but the math involved in designing it is very complex. It involves solving around 800 equations to calculate all necessary parameters.
If you lower the frequency from 60 to 40 Hz it will throw everything off. The power ( which is torque multiplied by rotation speed) will drop. The induction resistence of windings will drop too increasing the current. The induced current in the secondary winding ( aluminum cage in the rotor) will drop too reducing the power further.
All this will lead to overheating. I would not expect it to last any significant period of time.
Plus the idea of lowering the rotation speed for ripping has no merit at all. It will only slow down the speed rate, thats it. A ripping blade has fewer teeth to increase the space between speed so the more sawdust could fit in, not to decrease the “teeth per second rate”

View MrUnix's profile

MrUnix

5593 posts in 1919 days


#19 posted 04-21-2017 12:56 PM


Motor Horsepower & Torque Versus VFD Frequency
(Hint: A VFD does more than just change frequency)

-- Brad in FL - In Dog I trust... everything else is questionable

View Roadster280's profile

Roadster280

31 posts in 1839 days


#20 posted 04-21-2017 12:56 PM


I do not see that as wise IMO. Most saws would not work long time at a frq other than enginnered. Why not just use the proper plade and feed of material through that blade.

I think thats undue long term stress.

- bonesbr549

Not true. Most 3ph motors sold in the last … 15 to 20 years or so are more than happy to be driven by a VFD. Exceeding the rated frequency will shorten life, or running the motor at a drastically lower frequency (like 10hz), might also cause some small issues. But 40hz shouldn t be a problem.

That being said, I think that what pintodeluxe said about blade geometry is what really matters here. The issue isn t how fast the teeth are moving, it s that when ripping, there s a lot more saw dust that is generated and has to go somewhere (the gullets between the teeth). Ripping blades have bigger gullets.

- William Shelley

The prevois post is true, your s is softly speaking naive. Induction motor has a very simple construction but the math involved in designing it is very complex. It involves solving around 800 equations to calculate all necessary parameters.
If you lower the frequency from 60 to 40 Hz it will throw everything off. The power ( which is torque multiplied by rotation speed) will drop. The induction resistence of windings will drop too increasing the current. The induced current in the secondary winding ( aluminum cage in the rotor) will drop too reducing the power further.
All this will lead to overheating. I would not expect it to last any significant period of time.
Plus the idea of lowering the rotation speed for ripping has no merit at all. It will only slow down the speed rate, thats it. A ripping blade has fewer teeth to increase the space between speed so the more sawdust could fit in, not to decrease the “teeth per second rate”

- Carloz

Seems fair enough. But what is the point of a Variable Frequency Drive then?

View Fred Hargis's profile

Fred Hargis

4548 posts in 2213 days


#21 posted 04-21-2017 01:55 PM

What Carloz said about reducing the life of the motor is true….we did that with a lot of machines where I used to work. The thing is, in an industrial setting (running the motors 24/7 for the most part) the shorter life was still inconsequential. We used Reliance motors and talked to them a great deal about our applications, and we did have heavier duty motors, but it we still would have one fail in less than 2 years (+/-). I think in a hobbyist shop it would be meaningless. So, the use of a VFD has a lot of applications in industry for controlling motor speed. In a home shop you see them a lot of them on lathes for the same reason. But generally the motor is sized to overcome the lower torque incurred at lower frequencies. Then, of course, there’s the other use of the VFD in a home shop: running 3 phase motors even when the variable function isn’t all that important.

-- Our village hasn't lost it's idiot, he was elected to congress.

View EricTwice's profile

EricTwice

192 posts in 253 days


#22 posted 04-21-2017 02:20 PM

We have a large horizontal sliding panel saw that has a variable speed. It got turned slowed down for some reason and was bogging down. when we slowed the infeed rate it burned the cut. We have it set at optimum speed for the blade and do not change it.

-- nice recovery, They should pay extra for that mistake, Eric E.

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