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Thread: Calculating Tablesaw blade speed

  1. #1
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    Default Calculating Tablesaw blade speed

    The motor for my tablesaw burned out and I am in the process of replacing it. It is a 9 inch craftsman/rockwell saw, older one with cast iron table.

    After some looking for an affordable alternative, I happened on a nice cast iron framed 1.5 HP 1720 RPM induction motor.

    The origional motor is long gone but I assume it was 3600 RPM

    I wrapped the 1/2 inch V belt around the pulley on the saw and measured the outside diameter of the belt 2.125 inches. I am not sure if this is the correct way to measure pulley size.

    I assume about a 3/12 or 4 inch pulley should get me close,but it is just a guess. I don't know what the surface speed of the blade should be. (or the suggested RPM's for a 9 inch blade)

    The saw will be used in my basement on weekends for home projects, it isn't in a production setting.

    Can anyone point me in the right direction as far as calculating the correct pulley size for the motor?

    Phil

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    Default Re: Calculating Tablesaw blade speed

    See:
    Question frequently asked how to get speed X from drive Y (or whatever...

    http://www.culvermotor.com/index.php...alculator.html

    ALSO: www.gizmology.net/pulleysbelts.htm
    __________________
    Steve
    http://forum.canadianwoodworking.com/image.php?type=sigpic&userid=12450&dateline=127309  6828 Wood Wreck - Structural framing specialist.

  3. #3

    Default Re: Calculating Tablesaw blade speed

    Hi Steve
    Have you got another address this is what I get:

    Forbidden

    You don't have permission to access /index.php...alculator.html on this server.

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    Default Re: Calculating Tablesaw blade speed


  5. #5
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    Default Re: Calculating Tablesaw blade speed

    Here are my thoughts (sure hope my math is right, I am bit tired today) on a few issues you raise.

    A search for "9" rockwell beaver tablesaw RPM" leads to few listings quoting around 3400 RPM. A quick review through Grizzly, Delta, etc and you will see saws as low as 3000 RPM (like my 10" hybrid), more in the 3400 range and some as high as 4600. As long as you don't exceed the RPM posted on the blade then you will be safe. Most I've seen are above 5400 RPM so ...

    You can "multiply" the speed of the motor to the blade by using the pulley circumference. Circumference = PI x pulley diameter.

    so, if your motor is 1720 and you would like a blade speed of 3440 then your arbor pulley need to be 1/2 the diameter (hence circumference) of your motor pulley.

    That leas to a bit of confusion ... you measured the table saw pulley's outside "diameter" (which I'm reading as the circumference) as 2.125. The would be a real diameter (across the face of the pulley) of .67. Is this really the case? That doesn't lead much room for the arbor shaft. I would suggest just measuring right across the "face" of the pulley, don't worry about the circumference as its linear with PI ... but if that is really the case, you need a motor pulley twice the size.
    Last edited by CheapScotsman; 09-04-2008 at 02:25 PM.

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    Default Re: Calculating Tablesaw blade speed

    2.125 was what I measured at the outside of the belt , by wrapping the belt around the arbor pulley and measuring with a vernier caliper to the outside of the belt.
    This is not the "pitch diameter" , the pitch diameter would be somewhat smaller. I need to guess at how much smaller the pitch diameter is so for ease in calculation I can assume 2 inches pitch diameter to be pretty close.

  7. #7
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    Default Re: Calculating Tablesaw blade speed

    .. so figuring it out with this calculator, speed of motor is 1720 , arbor pulley is assumed to be 2 inches:

    4 inch pulley gives 3440 RPM
    4 1/2 inch pulley gives me 3870 RPM
    5 inch gives me 4300 RPM

    I went through my blades and the only one I could read a speed rating on was a 10 inch carbide blade and that was rated at 5500RPM.

    .. so I guess I could aim for the 4 1/2 inch and see how that works ?

    I am thinking if it spins faster the feed rate should also be faster. I probably tend to feed my stock more slowly than ideal ,sometimes burning it because I am trying to be too slow and cautious. I assume if you were figuring all this out for a CNC operation with a constant feed rate you would tend to go towards a higher blade speed.



    Thanks for the help, I really appreciate it !

    Phil

  8. #8

    Default Re: Calculating Tablesaw blade speed

    I stand to be corrected
    but I would not be to concerned about getting to maximum speed as I bbelieve that the faster the blade turns the less power and therefore a 2 to 1 ratio pulley size motor to arbor should be ideal.

    J Poirier

  9. #9

    Default Re: Calculating Tablesaw blade speed

    I was looking at Tablesaw blades today and noticed that Freud rates their 10 inch blades to be good for 7200 RPM's. Some of the other less expensive blades were rated at 5200 RPM.

    Somebody should correct me for this but seems to me that the RPM on most motors is given as a no load or with nothing on the arbor. A motor shaft with a pulley on it and a belt attached to another pulley probably ain't moving anywhere near the rated motor speed.

    Ed
    edh

  10. #10
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    Default Re: Calculating Tablesaw blade speed

    I have asked at a few places what the optimal rim speed of a blade is and have yet to get an answer. I suppose each blade has a different optimal rim speed.
    In any case... You assumed the speed of the original motor to be 3600. That's possible but it's more likely 3450. The motor you're putting on runs at half the speed of the motor you took off so doubling the size of the original pulley will give you the same speed you had before. It will also cut your torque in half so in essence you end up with a 3/4 HP motor.
    All of the above is very roughly speaking. There are far to many variables to give accurate numbers but in cases like this, getting close is usually good enough.
    J.P. Rap Mount Hope Ont.
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  11. #11
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    Default Re: Calculating Tablesaw blade speed

    I installed a 4/12 inch pulley and it seems like a good speed.

    Below I sort of rambled on about all the stuff I have done to this saw to get it working half decently, read on only if it interests you ..

    the motor had 5 inch hole spacing so I made up an adaptor to fit the bracket on the saw. for a moment it seemed to have no starting torque, it would need a spin to start, even with no belt installed.

    Then I found the switch wasn't adjusted right , the contacts needed to be open when it is stopped and closed when it is running and the centrifugal weights weren't up close enough to the switch to make it start properly.

    The driven pulley raises and lowers with the blade,so the belt tension is heavier when the blade is down because that slackens the belt so the motor swings out away from the pivot...and gets "heavier"... this isn't exactly perfect engineering, but an economical trade off that's been tried and proven to work.

    Now I have a heavier motor and an adapter bracket thickening up the mounting it is perhaps keeping the belt a bit too tight when the blade is at the hieght of the table. I might be able to fit a slightly shorter belt and that will help. I got the next size shorter belt but just need to fit bolts with thinner heads, as I found they were binding the bracket from pivoting all the way closed.


    when I got the saw I got a cast iron back table given to me that had come from the factory without ever being ground to fit against the saw. The mating edge on the bed of the saw wasn't ground square either. I had to do a lot of work to get the edges ground to mate up the back table to the table on the saw. It was an excercise in fitting to get it to sit flat and level and to drill all the bolt holes, and bolt on the pipes for the fence.

    I also got a heavier cam lock fence along with the saw but it wasn't original to this saw. It was wider so I shortened it to fit the distance for the tubing, which fits into a V grove along the edges of the table.
    while I was at it I went and got some nice long tubing.

    I got a sheet metal back table along with the saw so I mounted that behind the cast iron table, and drilled it out so I can mount my router up under the table. Now I can move the fence to the back of the table to use along with the router.

    The stand wasn't too good so my dad made up a 3/4 plywood box for the saw to sit on. it doubles as a waste bin under the saw,catches a lot of the dust. I screwed some nylon pads to the bottom of the plywood box so I can drag the whole thing around a bit in the shop, and it stays fairly stable.

    I re-wired it and fitted a switched plug , so the motor plugs in to a switched outlet, I can easily unplug the motor when changing the blade,or when not using the saw. and I have an outlet there that isn't switched which is handy as it inevitably gets used as a worktable for other jobs too.

    I still have a couple of bugs to work out, and if I did it all again I thing I would spring for something better to begin with but I think I can get some good use out of it now. This one has a sheet metal box shaped piece under the cabinet and it needs a bit of strengthening. Older models used cast iron for the frame and that was a little sturdier. I will stiffen it up with some metal braces. There are some nylon parts in the raising lowering mechanism that are maybe a bit worn. I couldn't feel any real noticable play in the arbor. I wonder if all the parts are still available through sears. I had good luck getting bits for my radial arm saw which is about the same age.

    one bug that I couldn't easily fix was that I found when I installed the fence , the cast iron locking part sits a bit above the table , so it got in the way, I added a 4x4 as a back fence and that got the locking part of the fence out of the way. a 4x4 is kind of clunky for a back fence, I noticed the cheapo tool store has some 4' framing levels that are made of alunimum I beam material. I am thinking of picking up two to use as a backer for the fence. I think they were on for about 5 bucks apiece. The motor I picked up from KMS tools, It is used but they had it on for 5 bucks, not bad for a 1.5 horse motor. The most expensive part was the long tubing for the fence.

    all in all I probably have 3-400 in it so that would pretty much pay for a newish rigid saw or something like that, but I always liked the heavier feel of these older ones.


    Phil

  12. #12

    Default Re: Calculating Tablesaw blade speed

    I stand to be corrected
    but I would not be to concerned about getting to maximum speed as I bbelieve that the faster the blade turns the less power and therefore a 2 to 1 ratio pulley size motor to arbor should be ideal.
    It will also cut your torque in half so in essence you end up with a 3/4 HP motor.
    Remember: Power = Torque * Angular Velocity

    So power remains constant. Torque goes down by a half, but angular velocity is doubled, so the net effect is no change in power output.

    Cheers,

    Michael

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    Default Re: Calculating Tablesaw blade speed

    Quote Originally Posted by mbowler View Post
    Remember: Power = Torque * Angular Velocity

    So power remains constant. Torque goes down by a half, but angular velocity is doubled, so the net effect is no change in power output.

    Cheers,

    Michael
    Interesting. I don't think I have ever heard it put quite that way. I'm going to ponder what you said while I peruse the forum and I'll get back to you with my thoughts.
    J.P. Rap Mount Hope Ont.
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    "In this world, you must be oh so smart, or oh so pleasant." Well, for years I was smart. I recommend pleasant. Elwood P. Dowd

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    Default Re: Calculating Tablesaw blade speed

    not sure of the math, but the motor is rated to be 1.5 horse, the physical size is larger than any 3/4 horse I have seen. It has a cast iron housing, which adds to the weight, but it also seems to have a lot less vibration than a lot of the motors I have found. I think the reduced motor speed helped with the vibration.

    usually on motors the stamped label says HP: 1 or HP: 1/4 or whatever.
    this motor says "power" 1.5 , I assume it means the same.

    changing the pulley and using the slower motor worked fine. I did some ripping with it and couldn't slow it down at all so it definitely isn't 3/4 horse.

    Phil

  15. #15

    Default Re: Calculating Tablesaw blade speed

    Quote Originally Posted by J.P. Rap View Post
    Interesting. I don't think I have ever heard it put quite that way. I'm going to ponder what you said while I peruse the forum and I'll get back to you with my thoughts.
    Think of it this way. At twice the blade speed, there is half the torque, so each tooth can cut half as much wood. But there are twice as many teeth cutting per second, so the feed rate can remain the same as at the slower blade speed. Same amount of work is accomplished in each case.

  16. #16
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    Default Re: Calculating Tablesaw blade speed

    Quote Originally Posted by mbowler View Post
    Think of it this way. At twice the blade speed, there is half the torque, so each tooth can cut half as much wood. But there are twice as many teeth cutting per second, so the feed rate can remain the same as at the slower blade speed. Same amount of work is accomplished in each case.
    Yes, I understand that.
    Admittedly "in essence you end up with a 3/4 HP motor." was a bad choice of words.
    I don't know enough about motors to get into the specifics of it all but I know this much. You can not make a gain in one area without a loss in another...in this case speed V tourque..."at the blade".
    The cost of higher speed is torque. The cost of higher torque is speed. You can't have it both ways.
    That said, I really don't know how torque relates to HP. I guess what I was trying to say was the torque at the blade would be similar to that of a 3/4 HP 1725 RPM motor. Again, that may be a poor choice of words but when the blade speed is doubled there has to be an equal sacrifice elsewhere.

    If I'm not mistaken, a 1 1/2 HP 3450 RPM motor has less torque then a 1 1/2 HP 1725 RPM motor.
    The OP made several changes. He went from a low HP high RPM motor to a higher HP low RPM motor. Then he changed the speed. Even after factoring in those things that cancel each other out, he still comes out ahead.
    In all honesty, I'm far to hung over to think about this logically so I'm just going to leave it at that.
    J.P. Rap Mount Hope Ont.
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    "In this world, you must be oh so smart, or oh so pleasant." Well, for years I was smart. I recommend pleasant. Elwood P. Dowd

  17. #17
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    Default Re: Calculating Tablesaw blade speed

    Hopefully this may make it more clear.
    Torque (in. lbs.) = 63025 x HP/RPM
    This means that a 1725 rpm motor delivers twice the torque of a 3450 motor (ignoring efficiencies).
    However, since you would have to double the output (motor) pulley diameter the resultant load on the belt (Torque/radius = lbs) would be the same.
    This would result in the same torque at the blade with the same speed and hence the same H.P.
    Does that make sense?
    Dave

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    Default Re: Calculating Tablesaw blade speed

    In meatlworking , eg when cutting on a lathe all of this is can be calculated. diameter , rotational speed and feed rate can be set up for a particular cutter and metal type.

    In woodworking it seems these things are figured out more by experience. If the motor speed is increased, I would assume one could then increase feed rate, as long as you aren't reaching a power limit of the motor, or burning the blade.

    the shape or thickness of the chip might be something one could look at. a higher feed rate would result in a thinner chip. at some point you may be making a rough cut by too fast a feed rate, or producing smoke and dust at an extremely slow feed rate.

    On a lathe you can examine the chips and adjust accordingly, if the chips are coming of smoking hot and turning blue or straw colored you need to reduce speed or feed or take a lighter cut. On a tablesaw I assume chip size could also similarily be an indicator. if it were a hand plane or a jointer one would probably look at the chips and adjust accordingly without putting much thought into it.

    It is interesting but I guess most wouldn't do calculations like this in a home environment. In production I think these things would be studied in depth as to maintain the fastest production rates , without burning up blades etc. I don't think I have ever really thought to look at the chips under my saw to get an indication in this manor.

    With a 3/4 horse motor , I would often hear the motor slowing down and reduce my feed rate accordingly. Now with a larger motor I am a bit scared of the thing kicking back.

    phil

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    Default Re: Calculating Tablesaw blade speed

    Quote Originally Posted by DaveN View Post
    Hopefully this may make it more clear.
    Torque (in. lbs.) = 63025 x HP/RPM
    This means that a 1725 rpm motor delivers twice the torque of a 3450 motor (ignoring efficiencies).
    However, since you would have to double the output (motor) pulley diameter the resultant load on the belt (Torque/radius = lbs) would be the same.
    This would result in the same torque at the blade with the same speed and hence the same H.P.
    Does that make sense?
    Dave
    Yes, that makes perfect sense. Thank you for spelling it out. I now see where I was confused.
    Cheers.
    J.P. Rap Mount Hope Ont.
    Carpe Ductum (Seize The Tape)


    "In this world, you must be oh so smart, or oh so pleasant." Well, for years I was smart. I recommend pleasant. Elwood P. Dowd

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