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Slow down table saw blade for ripping?

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Forum topic by Roadster280 posted 04-20-2017 05:34 PM 578 views 0 times favorited 22 replies Add to Favorites Watch
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Roadster280

31 posts in 1812 days


04-20-2017 05:34 PM

I just acquired a three phase saw, and will use it with a VFD, so I can control the motor speed.

I bought a combination/general purpose blade for it (Freud P412), 48T. It occurred to me that although the hook angle is not going to be ideal for ripping, as a GP blade, it should be somewhat ok.

Has anyone any experience/comments about the thought of dropping the frequency to say 40Hz instead of 60Hz, and thus having fewer “teeth per second”? 40/60×48=32. On a 12” blade, 32T seems about right (maybe a little high) for ripping.

I don’t think I’ll be short on power, it’s a 7.5HP motor, van if the slower frequency causes a non-linear loss of torque, I should still have at least 3.5HP.

Just a thought that occurred to me while I wait for the VFD to arrive, to save changing blades often.


22 replies so far

View ErichK's profile (online now)

ErichK

76 posts in 357 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.

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pintodeluxe

5304 posts in 2506 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

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Loren

8981 posts in 3341 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

9796 posts in 2073 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

1400 posts in 2760 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.

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William Shelley

348 posts in 1162 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

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MrUnix

5427 posts in 1892 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

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Roadster280

31 posts in 1812 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

466 posts in 818 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.

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Roadster280

31 posts in 1812 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

9796 posts in 2073 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/

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Roadster280

31 posts in 1812 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.

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Rick M

9796 posts in 2073 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 (online now)

jbay

1500 posts in 592 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.

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Roadster280

31 posts in 1812 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!

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