Anyone Knows Much About Slab Saw Motors?

[TheFossilHunter]

Hi

Does anyone know much about slab saw motors? I have a 20 inch slab saw and the motor broke. I am looking to replace it but I have several concerns.

1) Should i look for a standard motor or specialty motor for power equipment?

2) I currently have 0.75 HP motor but looking to imcrease the power to 1HP or preferrably 1.25 or 1.5hp. However, with my current motor, the surge protector used to constantly trip when the friction on the blade increased. I practially couldn run the saw. apparently there was a power surge at those time. I am afraid that going to more horse power will increase the amperage; on the other hand, having a more powerful motor may, in my opinion, minimie the power surges as the blade should handle the rock easier. Ive looked up some motors at Grainger and some of their power equipment motors show in their specs the full load as the ratio of two amperages, for example 20Amp/10 amp. what does it mean? under load 20 amp and without load 10 amps? I do want the motor to have anough capacity in case the blade gets stuck but at the same time i dont want to trip the breaker.

3) should i look at motors with two or more capacitors? i understand this would allow to get uo to full power more gradually?

4)what rpm is preferrable for my situation? i dont have my current motor with me now and cant look up the rpms on it. my saw also steps down the rpms by about 6 times.

5) should i look at constant duty motors? what about protection? thermal protection? should a motr has some kind of fuse or reset button?

5) any other advice?

Thanks a lot,. Any info would be much appreciated

Mike

[jkfoam]

Mike,

I'll try to help with some of your questions. I have a 18 inch slab saw that I have used for over 30 years.

1.) I think a standard motor is all that is necessary.

2.) I use a 3/4 HP motor on my 18 inch saw and this has always been adequate. If you want to increase your HP then I think you shouldnt have to go more than 1.0 hp. More HP will just require more aperage and cost more to run. With regard to that 20 amp/10 amp spec in the Grainger catalog, that is the current reqirement(amps) at full load at 110 volts versus the current requirement at full load at 220 volts. Many motors can be wired for operation at 110 volts or 220 volts.

3.) I think a single capacitor motor would be more than adequate for your operation.

4.) I suspect your current motor is a 1725 RPM motor and I recomend you stay with that RPM. I've seen recomended blade speeds for standard notched diamond blades stated as 3000-3500 sfpm (Surface feet per minute) which translates for a 20 diameter blade to be 570-670 RPM.

5.) I would think that thermal protection reset button would be a good idea.

6.) As for other advice, I recomend cutting slow to save blade wear and extend blade life. Also, cut lots of rocks. For me, every time I cut a rock its like opening a Christmas present and discovering whats inside.

JKFoam

[TheFossilHunter]

Mike,

I'll try to help with some of your questions. I have a 18 inch slab saw that I have used for over 30 years.

1.) I think a standard motor is all that is necessary.

2.) I use a 3/4 HP motor on my 18 inch saw and this has always been adequate. If you want to increase your HP then I think you shouldnt have to go more than 1.0 hp. More HP will just require more aperage and cost more to run. With regard to that 20 amp/10 amp spec in the Grainger catalog, that is the current reqirement(amps) at full load at 110 volts versus the current requirement at full load at 220 volts. Many motors can be wired for operation at 110 volts or 220 volts.

3.) I think a single capacitor motor would be more than adequate for your operation.

4.) I suspect your current motor is a 1725 RPM motor and I recomend you stay with that RPM. I've seen recomended blade speeds for standard notched diamond blades stated as 3000-3500 sfpm (Surface feet per minute) which translates for a 20 diameter blade to be 570-670 RPM.

5.) I would think that thermal protection reset button would be a good idea.

6.) As for other advice, I recomend cutting slow to save blade wear and extend blade life. Also, cut lots of rocks. For me, every time I cut a rock its like opening a Christmas present and discovering whats inside.

JKFoam

Thank you JKFoam.

1) will go with a standard motor as long as it is continuous duty, right?

2)I agree that 1hp should be enough but the rpoblem is that there is a lot of friction when the maximum blade circumference is engaged, there is also friction on the sides, i tried to adjust the blade angle to be more straight, and the blade itself is pretty straight but it still get's stuck...or at least cuts with great difficulty. when that happens my surge protector trips..every single time. this is why i was planning to increas the power to help the blade cut a bit. i dont care about the cost to run that much, much concern is not to increase the power as much as to trip the breaker from just normal running mode.

Thanks for clarifying the amperage ratio. I will look at motors where the higher number for 110 volts is under 20ampers.

3) reason i wanted to go with mor ethan 1 capacitor again is that when i turn the motor on, the lights dim for a second, there is probably a power surge going at that time. wanted to get up to that ampearge more gradually.

4)will stay with the same rpm as i currently have, since mpost motors are 1725

5) i dont know if thermal protection and the button are the same thing. Some motors say they have thermal protection, but no button. I will find out.

6) will cut slow. My electric feed motor moves pretty fast for rocks that are too big, but i also have a pneumatic drive which i am trying to adjust now so i will try to go slower. in fact when i dont go at the slowest speed, my surge protector trips.

Thanks for all the advice.

[Auspex]

Most of the trouble you list describes a worn-out motor and/or inadequate wiring. Some of it could also come from a worn out blade and/or too high a material feed rate. A gutsier motor might help some of the problems, but not all.

[TheFossilHunter]

Most of the trouble you list describes a worn-out motor and/or inadequate wiring. Some of it could also come from a worn out blade and/or too high a material feed rate. A gutsier motor might help some of the problems, but not all.

Yes, well the blade is new, althought it's a cheaper kind. the wiring is new too, i just have to see if its the right size wires, otherwise i guess overheating would cause additional resistance.

[jkfoam]

Mike,

Let me make a couple of points.

First, asurge protector protects electronic equipment from transient spikes of high voltage that can damage sensitive electronic stuff like computers etc. I don't think those spikey transients can do much to a 3/4 hp motor. Also is that surge protector rated for the current draw of a motor. It may be undersized for the current dreaw of the motor and that is why it is tripping out when you get a load on it.

Second, Your blade may be binding in the rock you are cutting because you are feeding it too fast into the cut. I feed my rock into the blade at about 1 inch per 5 minutes. I can change my feed rate on the feed motor by changing the pully size. I go even slower for certain rocks like psilomelane (a manganese oxide jasper).

Third, If you have a fairly recent W.W. Grainger catalog they have a pretty good discussion of motor thermal protection in the motor terminology section at the front of the "Motor Section". Personally I like a motor with a manual reset button you have to push after the motor has cooled to restart it.

Fourth, Lights dimming when you first turn the motor on. The reason the lights dim when you first turn the motor on is because the motor is drawing a large amount of current when it first starts because it is not UP to speed. When it gets to its designed RPM the current drops to its designed current draw. A capacitor motor allows a motor to start with more torque thus reaching its designed RPM faster thus a shorter light dimming time period if at all. The slower a motor starts and comes to design speed the longer the period of excessive current draw. . A motor designed for a fan for example does not have a capacitor because there is little torque required to start the fan. Now a motor used to run a compressor will have a capacitor because the compressor will reqiure a lot of torque to get started. If you put a fan duty motor on a compressor it may have to struggle to get up to operating RPM if it can even get there at all all the while drawing too much current.

I hope this helps. I may have over simplified some things out of my own ignorance.

JKFoam

[TheFossilHunter]

Mike,

Let me make a couple of points.

First, asurge protector protects electronic equipment from transient spikes of high voltage that can damage sensitive electronic stuff like computers etc. I don't think those spikey transients can do much to a 3/4 hp motor. Also is that surge protector rated for the current draw of a motor. It may be undersized for the current dreaw of the motor and that is why it is tripping out when you get a load on it.

Second, Your blade may be binding in the rock you are cutting because you are feeding it too fast into the cut. I feed my rock into the blade at about 1 inch per 5 minutes. I can change my feed rate on the feed motor by changing the pully size. I go even slower for certain rocks like psilomelane (a manganese oxide jasper).

Third, If you have a fairly recent W.W. Grainger catalog they have a pretty good discussion of motor thermal protection in the motor terminology section at the front of the "Motor Section". Personally I like a motor with a manual reset button you have to push after the motor has cooled to restart it.

Fourth, Lights dimming when you first turn the motor on. The reason the lights dim when you first turn the motor on is because the motor is drawing a large amount of current when it first starts because it is not UP to speed. When it gets to its designed RPM the current drops to its designed current draw. A capacitor motor allows a motor to start with more torque thus reaching its designed RPM faster thus a shorter light dimming time period if at all. The slower a motor starts and comes to design speed the longer the period of excessive current draw. . A motor designed for a fan for example does not have a capacitor because there is little torque required to start the fan. Now a motor used to run a compressor will have a capacitor because the compressor will reqiure a lot of torque to get started. If you put a fan duty motor on a compressor it may have to struggle to get up to operating RPM if it can even get there at all all the while drawing too much current.

I hope this helps. I may have over simplified some things out of my own ignorance.

JKFoam

Thanks JKF,

yes I have to see what the surge protector is rated for, and may have to get a higher rated one. i am not worried about the spikes damaging the motor. the only concern is for the motor drawing too much current and tripping the breaker in a house i am connected to. I do usually go faster than 5 minutes per inch, but the rocks I am cutting are mostly sedimentary rocks, not hard minerals like you do. I only have one size pully so i cant change the speed this way, but I have a pneumatic drive where i can regulate the air pressure and I also have a manual feed and can go as slow as i want if operated manually.

I will look for a motor with the manual reset button then. How do you when when the motor needs to coll off? does it turn itself off when too hot? or do you allow it to rest after a certain period of time?

thanks

[jkfoam]

Thanks JKF,

yes I have to see what the surge protector is rated for, and may have to get a higher rated one. i am not worried about the spikes damaging the motor. the only concern is for the motor drawing too much current and tripping the breaker in a house i am connected to. I do usually go faster than 5 minutes per inch, but the rocks I am cutting are mostly sedimentary rocks, not hard minerals like you do. I only have one size pully so i cant change the speed this way, but I have a pneumatic drive where i can regulate the air pressure and I also have a manual feed and can go as slow as i want if operated manually.

I will look for a motor with the manual reset button then. How do you when when the motor needs to coll off? does it turn itself off when too hot? or do you allow it to rest after a certain period of time?

thanks

Mike,

When the motor overheats the thermal protector will turn the motor off, and if memory serves me the thermal protector "red reset button or switch will pop out. The motor will be hot to the touch. When the motor has cooled (could take several hours) you push the switch in to reset it. If the switch doesn't stay in the motor has'nt cooled enough.

Mike, its not necessarily the hardness of a rock that dictates how fast you can cut it. One of the fastest cutting rocks is Brazilian agate and sometimes it is used to sharpen a diamond blade. A good crystaline rock may cut easily. While a softer stone may smear and coat the diamond faces in a saw blade blinding them and preventing further grinding/cutting action. A sedimentary rock with a high clay content may present a problem.

JKFoam

[Terry Dactyll]

Have any of you guys made your own slab saw by buying seperate components?.... Its something Im considering as I might need a big blade and they are very expensive to buy complete.... any tips would be greatly appreciated....

[jkfoam]

Terry,

How big of a saw are you contemplating. I assembled an 18 inch saw from a kit back in 1971. Unfoutunately the company that made and sold the kit disappeared about 20 years ago. I recently saw that Covington is selling slab saw kits. You can see what they offer. Also, I would try to buy a used saw from an old rock hound or rock shop. There is also a guy on E-Bay selling plans for building a slab saw, the plans are for sale for about $50.00 and the finished saw looks very similar to my saw.

If you like I can e-mail you pics of the internals of my saw if that would be of help.

JKFoam

[Terry Dactyll]

jkfoam.... 18" sounds about right which would give me 8 1/2" thickness of cut, if I needed that presumably...I am a fabricator/welder anyway by trade so I can make a frame up etc without any difficulty.... I will google 'Covington' and have a look and check out ebay plans.... Im in the UK so I presume its a company over your side of the pond....some photos would be great.... I will PM you my email address for when you get chance to do some photos....

Many thanks for your kindness and time....

Steve

[jkfoam]

Steve,

It sounds like you have the right skills to make a slab saw. Also, note that an 18 inch saw will only allow a 7 1/2" depth of cut. Generally there is a 3 inch collar on the arbor that the blade fits on that results in a 7 1/2' cut. Some people use a 4-5 inch stiffener/washers on each side of the blade that would reduce the depth of cut even more. Personally I have never seen the need for those stiffeners.

Also, After your post and my reply I got on line and Googled the "Royal Slab Saw". To my amazement I found that the Royal Slab Saw was still being sold. The website is http://www.cascadelapidary.biz/royalsaw.htm. The site mentions a kit but does'nt show a kit price, just a price for the complete saw, blade and motor included. From looking at the site there have been some changes in the saw from mine but I can't be surprised after all I bought my kit over 30 years ago. You may want to e-mail them and get there broshure on the saw. Their photos may provide you with some information and will probably be better than my photos.

I'll e-mail you photos of my saw. Good luck on your project.