Advise on driving an 18" circular saw blade

Am contemplating constructing a wood cutting chop saw for cutting up
fireplace logs. 18" circular blades are readily available here and would
seem about the right size for the size of logs I'd be working with.
Question is, what HP electric motor would be required (about 3?) and, what
would be best RPM for the blade?
All suggestions appreciated.....
Laurie Forbes
Reply to
Laurie Forbes
Loading thread data ...
I find it reprehensible, and perhaps even illegal, to cut down habitat that squirrels need to have in order to survive. You cut down that habitat, baby squirrels fall to the ground, which enables people to make pets out of them. And we all know that's just cruel.
Save the whales, collect the whole set?
Snarl
Reply to
snarl
7.5hp MINIMUM motor, and that's electric. If gas, make it 18hp.
Blade speed 3000 sfm. 1.5' diameter, 4.7' circumference, rpm = 636.
Figure 1760 rpm / 3 = 586 rpm, close enough, gear a 1760 motor down 3:1-ish.
This sounds *extremely* unsafe. Better type all your questions now while you still have hands.
GWE
Reply to
Grant Erwin
I have a wood cutting saw that I mostly use to cut up scrap lumber for my wood stoves. Mine is just a 10 inch blade and powered by a 1.5 hp motor. The blade turns at about 4000 rpm. Works good, but not really safe.
I would definately drive an 18 inch blade much slower. The 4000 rpm is somewhat dictated by wanting a small pulley on the saw blade mandrel so I can cut 4 by 4's. The saw blade that I have has a fairly thin kerf. An 18 inch blade would probably have a kerf two or three times as wide, and therefore would require two or three times the hp for the same cutting speed. My intuitive guess is that hp required is proportional to the surface area of the sawdust created per minute. I think you would be happy with a three hp motor. I would rather stall the motor when the blade binds that have so much hp that it never stalls. And instead throws wood at the operator.
Dan
Reply to
dcaster
I've run a 36" circular saw blade on a 7.5HP 3 phase motor. There's enough power to get the job done. That was for ripping, not cross cutting.
PDW
Reply to
Peter Wiley
Seems a bit large but I would be interested in how those values are calculated. Keep in mind I won't be ripping with it and don't require particularly fast cutting speed.
I'm not sure where the 3000 ft/min derrives from - my 10" radial saw turns about 3600 RPM which gives a surface speed of about 9500 ft/min. Is there some inherant reason why larger blades should be a lot lower?
I realize such a saw would have to be built solidly, with proper shielding etc. and operated with caution but I would be interested in specific reasons why it would be necessarily unsafe (18" is certainly not by a long ways the largest saw blade in use).
I appreciate your response but would be interested in more detail if you could provide it...
Laurie Forbes
Reply to
Laurie Forbes
I think you are right - if a 10" blade normally operates at a given cutting speed at say, 3600 RPM, an 18" for a similar cutting speed would be about 2000 RPM. This seems quite reasonable as I have noted that *max* speed advertised for the particular 18" blade I was looking at is 3600 RPM. The HP being proportional to the rate of chip production also seems reasonable so, particularly if the saw is not forced too hard, 3 HP seems like it would do it alright. As well, the logs I will be cutting are all aspen & balsam poplar which, as woods go, are quite soft (and, it's all cross cutting, no ripping).
As far as throwing wood, I am thinking of devising a long actuator "handle" which would be operated from the side of the saw rather than in front of it. I think that would be safer but would appreciate any comments otherwise. It also seems to me that operating the saw as a "chop" saw (pivoting feed as opposed to sliding) would also tend to make it safer as the blade would not tend to pull itself into the wood.
Laurie Forbes
Reply to
Laurie Forbes
3000 sfm is what is used on bandsaws when cutting wood I believe.
The horsepower I was gaging based on abrasive metal cutting. After all, this *is* a metalworking NG. The electric/gas hp equivalency I was told by an industrial air compressor company (Quincy) when I called to inquire about repowering an 18hp trailer-mounted compressor with electric.
The issue with safety is probably just my own fear. I have an incredible fear of large saw blades, or maybe I should rephrase that as respect.
GWE
Laurie Forbes wrote:
Reply to
Grant Erwin
I ran a 30" cordwood saw years back. Nasty beast. Used a sliding table rather than moving the arbor. I'm guessing it ran in the 900 rpm range (hard to know exactly since the drive mechanism was NON STANDARD!!) The 30" saw was used on huge piles of green birch running up to about 8", would slice it like butter. Saw teeth were on the order of an inch high, 1-1/2" tooth to tooth. (As in .6 teeth per inch?)
Found these pics:
formatting link
For your 18", 3hp would be a good start, 5hp would be better. You would want double 'V' belts, notched belts would be better. I'd try for around 1800 rpm which means finding an 1800 rpm motor so you could get a 1:1 pulley setup.
I don't think you will like the 18" blade, too small once you deal with the arbor, bearings, and pulley. Hard to get more that 6" to 7" of actual cut diameter. Keep in mind that logs have curves and knots. You really need some guards that both work to keep hands out and work with logs that are not anywhere near as straight as cheap 2x4's
I'll second Grant's phobia about saws like this. BTDT, still have all my fingers and hands. Not sure why I was so lucky.
Grant Erw>
Reply to
RoyJ
I agree on the respect issue.
Reply to
wayne mak
OH MY GOD!!!! I HAVE big saws so, don't listen to the guesses. My 14" is 12 hp. I wouldn't power an 18" with anything less than 15 hp preferably 20. Trust me, you DON'T want to stall it. Or: DO-YOU-HAVE-A-VIDEO-CAMERA???
Reply to
Tom Gardner
I'm pretty sure a chain saw is a safer, faster and cheaper way to cut fire wood. I also question the motor sizes others have indicated since a chain saw with an 18" bar does just fine with about a 3 hp gas engine. If you really want a "chop saw" style setup I think you could do just fine building a unit with a quality electric chain saw.
Pete C.
Reply to
Pete C.
Chip Load Formula Known: Feed Speed ------- 90 fpm. No. of Teeth ------ 60 RPM --------------- 3600 Unknown: Chip Load -------- ? Chip Load is -----0.005"
Feed Speed (fpm) x 12 / No. of Teeth x rpm = Chip Load
Tooth Quantity Formula Known: Feed Speed ----------- 90 fpm RPM ------------------- 3600 Chip Load ------------ 0.005" Unknown: No of Teeth ------------- ? No of Teeth is ---------- 60
Formula Feeds speed (fpm) x 12 /
Rpm x chip load = no. teeth
Feed Speed Formula Known: No. of Teeth ------- 60 RPM ----------------- 3600 Chip Load ---------- 0.005" Unknown: Feed speed ------------ ? Feed Speed is --------- 90 fpm Note: Feed Speed must be converted to inches. (90 x 12)
Rpm x no. teeth x chip load / (12.9inches per foot) = FeedSpeed
Reply to
president
I hear your concerns re large blades and am now not completely sanguine about it either (that's one reason I'm asking for advice). In regard to metalworking NG, the connection is that I plan to cut wood with this thing, not construct it using wood :)
Laurie Forbes
Reply to
Laurie Forbes
Interesting choice of words, sanguine. You want to avoid exsanguination with this saw.
Pete Keillor
Reply to
Pete Keillor
It is appropriate alright ("sanguine" - also defined as 'Colour. Of the colour of blood; red.').
I have accidentally deleted recent posts to this thread but thanks anyhow to the poster who seemed to agree that 3 HP and 1800 RPM would be appropriate. Someone else also posted that much higher HPs would be needed (can't remember exactly what numbers were given) - called the manufacturer of the blades (Dimar) I've been looking at and, while they did not have published HP data, he suggested looking at commercial saws to see what HP motors they come with for the same (18") blade diameter. I found a range of 5 to 7.5 HP for that blade size but no corresponding current draw figures were given for the motors so I wonder if they are quoting "Sears" HP units or similar. Anyhow, the guy at the manufacturer guessed that 3 HP might work but would be a minimum. Seeing I am only going to crosscut softwoods and don't need a whole lot of speed, hopefully 3 HP will be adequate (I suppose I could always add a bigger motor afterward if need be).
As to the suggestion that 18" might not be large enough, there is a 20"er also available but I'm thinking the 18 would be good enough as it would handle 90% of the logs I will be cutting and the large logs can always be cut from both sides.
As to using an electric chain saw, I asked here about that a few months ago and the consensus was that they would be unsuitable as they are generally used mainly for light pruning type of work. The local vendors I spoke to pretty much confirmed that. Chain saws electric or gas also need chain sharpening which I would like to avoid.
Thanks again for everyone's help!
Laurie Forbes
Reply to
Laurie Forbes
Chain saws electric or gas also need chain sharpening which I would like to avoid.
Not trying to be a jerk here. But, you're going to have to sharpen that 18" saw blade too, unless you just intend to run it dull and then burn it up. I'm with the rest of the folks here, get yourself a small chain saw, unless you are after doing it the way Great Granpappy did it, belted up to a small gas engine.
Reply to
whiskers69
You will not be able to safely cut larger logs from both sides. Guranteed to bind.
Laurie Forbes wrote:
Reply to
RoyJ
And that must be a Fairbanks-Morse single cylinder, hit and miss; or alternately, mount the blade in place of a rear wheel on a Model "T" drive line. Gerry :-)} London, Canada
Reply to
Gerald Miller
Yes, but the 18" (carbide) saw blade would last *much* longer than a chain saw between sharpenings (hundreds of times??). It would be great if you could buy carbide chain saw blades, AFAIK you can't.
I have one but am tired of the smoke, noise, stink and blade sharpening - just looking for an easier way and besides, I need a new metal fab project :)
Laurie Forbes
Reply to
Laurie Forbes

PolyTech Forum website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.