Thread: grinder build motor question

Hi All,well, for those of you who have followed my grinder build, the pool pump motor did not work out. The spindle is too thin and the fitter and turners i asked for help after i failed to extend and thicken it successfully, advised against it. They are sure the spindle would break under the lateral force of the drive bulit at some point soon while grinding - not a fun thing to have happen. least it was free.Sooo... i have looked about and not found anything i liked in the junkers, so am now looking to buy a motor.In Australia we have 240volt power, and i only have access to single phase, and variable speeds here are prohibitively expensive, so this helped narrow my choices. i have found some 2HP motors i am happy with. Two options to choose from. One runs at 1440rpm and the other at 2800rpm. i am using a 4 step pulley set up with a 5 inch diameter on the largest step. 2x72 belts. My gut says the slower speed is the better - more control, and excellent on the slowest step pulley for polishing. High speeds chew steel, but also make it really easy to stuff a blade right up in a spilt second.So - anyone who has built or uses ready made grinders regularly, please chime in with your thoughts. Is my logic flawed? Will the slower rpm be too slow to get the work done reasonably quickly?Appreciate your feedback.

My vote would be for the slower motor with two cone/step pulleys. I tend to grind slowly, I can't grind faster than I can think

The size of your drive wheel is also really important.

Sometimes it makes me wonder on the forums, everybody is obsessed with hp, but no one ask how slow can it go. With my style of grinding I tend to go slowly, I know others who tend to grind faster than me.

Charlie

Thanks Charlie,
you better expressed my thoughts. Slow for my brain to keep up.
Ta

On the 9x20 lathe boards I've seen a lot of guys take DC motors from old treadmills and use those as variable-speed drives. Whatever their HP or watt ratings, multiply by 80% and don't load them any higher than that. There are 746 Watts in a HP. If all they tell you is how many amps, you can figure out power (watts) with the formula P = I x E, or Power = Current (amps) times voltage. Sorry to insult you if you already know this stuff; just don't want to throw out a bunch of black-box info. Good luck.

I must have learned it from Charlie then. I got small hp high torque 4 step. I can almost watch the contact wheel go around. I did this mostly to learn on but I don't think I am going to need anything faster. A good 50 grit belt will do some metal munching fast enough for me. Also with speed comes heat. So how much more time are you going to spend dunking that thing than grinding it ? I am still learning mind you but those are my thoughts now :<0) I have a 3 inch drive wheel on a 1750 RPM motor. The step is 2" 3" 4" 5". I have had it up to 5 but see no use for it.

I also dare anyone to try and stop the darn thing, or even slow it down.

Slow is good, especially after the blade has been heat treated. Mine is about 1700 rpm's and I can't remember the exact drive wheel size, but I think it's about 3.5". With my 8" contact wheel, you can see it going round and round if running nice and slow.

Speed kills razors and knives...

Slow is better. I rarely run my grinder at full speed. Any grinding mistake or un-evenness is magnified with the speed. Slow -> even grinding. Fast -> error prone.
Besides, I've found slow grinding to be fairly fast as well, because you give the 'teeth' more time to sink in, and belts will wear slower due to the decreased heat and decreased mechnical impact.

Adam,

I like the slow grinding too. I have the Hare and Forbes motor (1HP) 1450rpm single speed with the 4 speed pulley. I don't like grinding on the fastest speed at all, not even for the rough grind. I would actually like to go just slightly slower than the slowest pulley combination with the finer grit belts.

Stu

How about a washing machine motor Adam any options there?

I pretty much agree with all of the above. My rig is variable speed. I have gone as high as 3000 rpm for rough grinding before heat treat, and some of the other things I use my belt grinder for, but most of my grinding is 1000 or less - much less post heat treat. I think you have all the info you need now to get that puppy spinning. Good luck, and keep us informed.