So I finally built my 2x72 Grinder...

L

Lethal289

Joined
Apr 10, 2013
Messages
349
I started this project over a year ago, and finally got it done this weekend. Ok, not done, but useable. I still need to finish the platen and build a work rest. But i have a few questions. First, I'm using an old (read Free) motor that has a little wobble in the movement. I made the belt tension adjustable by moving the spring location over 3 inch area of predrilled holes. Its currently set in the middle and as tight as i can get it, the belt has about 1/16" of wobble in each direction, 1/8" total. Would more tension fix that, or am i just too picky?

Im stepping up from a blazing fast craftsman 2x42 with no power. Because i could never master the craftsman, i purposely went with a two inch drive wheel. Now i think that may have been a bad decision. Is there such a thing as too slow for a 2x72? It's not crawling, but i'd bet it's less then half the speed of the craftsman. Should I step up to a 4 or 5" drive wheel? Im not sure about the belt speed calculations, and all the science behind it.

I still need a work rest. If anybody would like to share a picture of your favorite tool / work rest and why you like it, i'd appreciate it greatly. I'll be building mine soon, so all the tips i could get would be appreciated. Im not sure if the super adjustable ones are necessary.

Thanks in advance.

Heres the obligatory pictures

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Looks good so far. You have a stepped pulley and you are set on the slowest combination without swapping the pulleys - so just try the other (faster) grooves and see how you like it.
 
Tension won't fix a bent motor shaft.
You can increase the grinder speed by changing the belt position on the pulleys.
 
Look at the photos on the kmg grinder site. It uses pulleys to double the speed of a 1725 rpm motor and then uses a 4 inch drive wheel. This makes about 3690 surface feet per minute. Try to get to that speed at least. You may need to shop for a 1 1/2 to 2 hp motor. If you canfind the money get a 3 phase motor and a VFD drive. Larry Lehman
 
Thanks guys. I realize the picture shows it on the slowest speed, I have had the motor at all three locations.
 
Thanks Guys! Larry, i figure at top speed I'm around 1800 SFM. Looks like i may need to step up the drive wheel. VFD is not in my immediate future.
 
What is RPM of motor? "IF" it's a 1725 rpm motor, then your speeds will be as follows (assuming those are 2-3-4 pulley):

for 2" drive wheel: (in round numbers)
2" motor X 4" shaft = 450
3" motor X 3" shaft = 900
4" motor X 2" shaft = 1800 SFPM

for a 3" drive wheel you'd have (in round numbers)
2" motor X 4" shaft = 680
3" motor X 3" shaft = 1350
4" motor X 2" shaft = 2700 SFPM

for a 4" drive wheel you'd have (in round numbers)
2" motor X 4" shaft = 900
3" motor X 3" shaft = 1800
4" motor X 2" shaft = 3600SFPM

for a 5" drive wheel (in round numbers)
2" motor X 4" shaft = 1130
3" motor X 3" shaft = 2250
4" motor X 2" shaft = 4500 SFPM

I think I got all those numbers correct.... and that is "assuming" a 1725 rpm motor.

edit to add: The 2" drive wheel speeds are not really too slow - you'll really like the slow speed for sensitive work. 1800 SFPM will make for slow profiling. A 3" drive wheel is a pretty good compromise.

Ken H>
 
Good math, Ken. 1800 is plenty fast for most tasks, except it'll be slow on the profiling.

As far as wobble goes, more belt tension is a good place to start. The other thing to check is that all the wheels are in line with each other, and all the wheel bolts/shafts are square to the belt. Clean looking grinder.
 
Hey Bro,

If you are in Pittsburgh, come by my shop and you can look at the different rest I have, maybe give you an idea, I have a couple tilting ones. Shoot me a pm or email if interested mike12_nguyen@yahoo.com, I'm near Murrsyville. Great job on the grinder man.
 
your spring is out of extension in the pic's. you need to pull the platen out farther and put tension on the belt. try this, while the grinder is running put pressure on the tension arm manually and see if the wobble goes away. you will find the harder you push on the arm the straighter the belt will run. nice grinder.
 
Thanks guys. Ken that's perfect info. I appreciate the effort. My motor tag has 1690 rpm stamped on it. So it's close enough.

Mike, I will be in touch. I'm by West Mifflin.
 
If the shaft is bent you might be able to staighten it - or at least improve it, but at the risk of messing up the bearings. A larger pulley on a bent shaft will make it vibrate more. Just saying. Ultimately you are gonna probably get another motor.
 
I'd get a bigger drive wheel not only for speed, but for surface contact with the belt. The smaller the wheel, the easier your grinding belt will slip under pressure. It may not be as much of a problem with a new wheel, but I bet it will be after it wears in a little more.
The other concern I would have is the length of shaft you have hanging past the bearing on the drive/motor side. Eventually, the laws of physics will prevail, and you'll start getting a lot more vibration in your shaft, and eventually a bent or broken drive shaft. If you can space that motor side pillow block a little closer to the drive sheave, your shafts will last longer, and your grinder run a little smoother.

As for the wobble in your motor, is it a bent shaft, bearing slop? balance issue? If it's just confined within the shaft, you MIGHT be able to straighten it some with a dial indicator and some patience. Depends on if it's just bent on the end, or goes further into the motor.

At any rate, vibrations are a machine killer, and eventually they always get worse. The better you can control them at the start, the longer and better your equipment will run.
 
Wow... Cool idea with the stepped pulley! I've been contemplating building a similar grinder and was sweating having to install a three phase motor and a VFD but this might just work for me to keep the $$$ manageable
. Great job.
 
While the stepped pulley does allow a change of speeds, and with my first grinder I was convinced that was the best way to go due to cost of VFD/3 ph motor. BOY have to eat crow after I built the next grinder with variable speed option....

When you've got the motor 'n shafts on hand at no cost, it's does save a bit of money. BUT - it just don't run as smooth and I never imagined how much better the variable speed was. It cost me about $250 for a 3 ph motor ($127 shipped) and VFD ($116 shipped). Granted the VFD is a NEMA 1 open frame which I had to put filters over the air intakes. Some folks mount in box for dust protection. With 56C direct mount motor the grinder is so much quieter and smoother. I had a photo of a nickel balanced on tooling arm with grinder running at 4,000 SFPM - that is smooth!

Ken H>
 
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