Was at Sears yesterday, and they had an 8" for $99. Looked pretty sturdy to
me. When it comes to bench grinders, I found that those $20 miracle
grinders just don't cut it. No pun intended. With bench grinders, you
pretty much get what you pay for. IMHO, that is.
I've been watching the 8" harbor freight grinders for a sale. The seem
to have 2 non-lighted versions, 1/2 and 3/4 horsepower. I think the 3/4
was 54.95 at the local store. Dunno how many amps though, so I don't
know if it has any guts or not.
The "horsepower"/"HP" claims seem incredibly bogus in the US. Your
"3/4HP" looks that way. 6inch + 3/4HP would be unstoppable - half that
power but for-real would be capable machine.
Take this example.
I've seen "5~1/2HP" vacuum cleaners (the workshop ones which can also
draw up water) in the US, which plug into domestic 110V sockets. Now
that no more happens than "2+2=5".
1HPu0Watts (actually 748W?)
Amps = 4125/1107.5
37.5Amps from a domestic socket? I don't think so! The supply cable
would have to be about 5/8inch diameter (about 15mm), for another
thing! And that current draw is if the motor is 100% efficient (all
electric power -> mechanical power). Electric motors are quite
efficient, but still, you're talking over 40A current draw.
I think the current draw is a more accurate measure, as a contributor
suggested. You could deliberately throw away current with an
inefficient motor, but then the grinder would get hot doing nothing -
which would be obvious - so that doesn't happen (a bench grinder has no
vents or airflow, as have to keep out dust, so can't throw away waste
heat like in say a vacuum cleaner). For sure a good machine would be
icy cool doing nothing, for ever, while a cheapo one would get hot to
the touch if left on for 15mins doing nothing. But that's less that a
hundred watts wasted (less than an Amp at 110V).
current draw (Amps) * input voltage (110V or 220V - whichever it uses)
giving input power.
Then 750W = 1HP
HP Watts Current @ 110V
0.25 188 1.7
0.33 248 2.3
0.5 375 3.4
0.75 563 5.1
1 750 6.8
Hope this helpful
I saw a couple grinders that had low RPM's, I wonder if these would work
better that the 3400 RPM's. I saw another one at the about 3400 RPM that
said no load, would that mean it should not bog down?
I did a small job biulding a tie post for a friend and I am using the money
to buy a bigger grinder. I am looking at $80.00 - 100.00 if that helps
finding a decent grinder
The revs of an induction motor is "locked" to the mains frequency of
60Hz (N.Am.), 50Hz rest-of-world. Under load an induction motors
"skids" a bit with respect to this no-load speed. But not by much.
"electric drills", angle-grinders, etc. have DC motors with carbon
brushes (the AC frequency is too low to matter - they work just fine
on mains AC). Their response to load is more desirable for these
applications than induction motors - steady decline in speed with
load - but you have the brushes which have to be replaced. And
induction motors can be very very efficient (high-90's percent), so
very little waste heat, so can be fully enclosed. So no grit or
anything in, and no wearable parts apart from the bearings - which
are sealed, as we said.
There's good induction motors which are the right size for the job
and they are made as they should be, and there are not-so-good ones.
Hence this whole business about someone's grinder not doing so well.
I'm no electrician - anyone want to comment?
Steve Smith wrote:
Unless US horses are different from Swedish 1 HP is 735 Watts. But
newermind. I think the power specified for a grinder (or most other
machines is based on max current drawn under full load. For an electric
motor, that is when it is stopped. The higher the speed, the less curent
is drawn. Some sort of torque measurement would be more useful.
For high power machines here we use three phase wiring giving 400 volt.
Same with all larger welding equipment.
I have to make do with 220 V.
Don, the best bench grinder I have ever used is the one my dad and I built
out of an old 1/2 hp motor--a real, honest-to-goodness 1/2 hp motor, like
the kind you can scavenge for free out of a leaky washing machine. We bought
one of those threaded shaft adapters from Woodworker's Supply, put on a
wheel, and rigged up a guide/support. It does *not* bog down! It may be that
one of these days the grinding dust will wear it out, since the motor is not
sealed, but meanwhile it's been working great for quite a few years. One
downside is that it only has one wheel, rather than the two that are common
on store-bought machines; on the other hand, it runs at 1750 rpm, which is
better for most of what I do. I keep meaning to make another one just like
it for myself, since I've got one of those cheap tool-truck grinders that
bogs down if you breathe on it ... and I do have two or three old washing
machine motors lying around ... but I keep forgetting to pick up the shaft
adapter whenever I'm at a store that might have one. (HD and Lowe's
sometimes have shaft adapters, but only with the right-handed threads, and I
need a left-handed thread.) One of these days ...
I have a Sears 8" diam, 3/4" stone face bench grinder , don't see HP
rating printed on it but I recall 3/4 HP , it is a strong unit , I
grind 1/4 inch mild steel plates all day long and does not bog down.
paid $69 for the display unit since it was out of stock.
I'vealso bought in the past a 6" noname grinder that had a 3/4 HP
plate rivited on and it was a weak P.O.S.
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