Surface Grinding

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Glenn Brooks
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Re: Surface Grinding

Post by Glenn Brooks »

I have only minimal experience with surface grinders - except to know probably you will discover you need a magnetic chuck to hold work pieces. Lots of used grinders seem to not have any accessories or tooling. So maybe a good idea to watch for one with the chuck included - or figure on an extra cash outlay right away to buy one.

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John Evans
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Re: Surface Grinding

Post by John Evans »

John Hasler wrote:
John Evans wrote: Steve is right on the 3 Phase. I have run my grinder on both a static and rotary converters and the rotary finish is better.Forget single phase if finish is critical.
DC is just as smooth as three-phase. Since you don't need speed control you could run a treadmill motor directly off a simple full-wave bridge.
A treadmill motor would be a poor choice if finish quality is important. Grinder motors are carefully balanced not so cheap treadmill motors.
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Harold_V
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Re: Surface Grinding

Post by Harold_V »

John Evans wrote:A treadmill motor would be a poor choice if finish quality is important. Grinder motors are carefully balanced not so cheap treadmill motors.
Yep! Changing the motor on any precision grinder isn't a great idea unless a high precision balanced motor can be procured, as even going to single phase presents problems that may be impossible to overcome.

Three phase motors don't "hunt" the way single phase motors do, plus the even torque created by the nicely spaced phase angle is hard to beat. Finishes tend to suffer when single phase motors are chosen, even if they're well balanced.

Harold
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John Hasler
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Re: Surface Grinding

Post by John Hasler »

John Evans wrote:
John Hasler wrote:
John Evans wrote: Steve is right on the 3 Phase. I have run my grinder on both a static and rotary converters and the rotary finish is better.Forget single phase if finish is critical.
DC is just as smooth as three-phase. Since you don't need speed control you could run a treadmill motor directly off a simple full-wave bridge.
A treadmill motor would be a poor choice if finish quality is important. Grinder motors are carefully balanced not so cheap treadmill motors.
I'm suggesting that *if* you must move away from three-phase *then* DC is a better choice than single-phase.
A DC treadmill motor will be just as well balanced as a generic single phase and will lack pulsation. There are also pretty good DC motors on Ebay from time to time. If you've already got a good three-phase motor on the machine and just lack three-phase power it makes more sense to buy a VFD.
earlgo
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Re: Surface Grinding

Post by earlgo »

SteveM wrote:That brings up a question I have wondered about:
What do you use for dust collection?
If there are hot sparks flying off the machine, that could have the potential to set fire to things in a vacuum.
Steve
My Delta is placed against an outside garage wall and I poked a hole thru the wall and built a small enclosure to hold a shop vac. The vac sat outside and was plugged into a switched receptacle near the grinder. Some innovative cutting of the flat plastic vacuum nozzle and it would suck the swarf out into the vacuum with out too much escaping into the grinder enclosure. Never had a fire in the vacuum, but a couple of holes were melted into the vac hose.

ctwo: When you get your SG you will need to have a surface plate and some metrology equipment to see how you did. I'm sure you already have it so this remark is really for others who may not and are contemplating a SG.
Oh, and I have never had a magnetic chuck in the 30 years I've had the grinder, so it is really not an issue, but then again, I don't grind large flat plates. I do have a small 3 x 6 perm mag for small things. So it depends on what you are planning to use the SG for.

--earlgo
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Harold_V
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Re: Surface Grinding

Post by Harold_V »

John Hasler wrote: If you've already got a good three-phase motor on the machine and just lack three-phase power it makes more sense to buy a VFD.
Well, that depends.
With the opportunity to operate a grinding spindle at speeds that are open to variation, there's the risk of blowing wheels. That could spell death to the operator, depending on several factors.

By the same token, having the ability to over-speed a grinding wheel offers the opportunity for smaller diameter wheels to improve their performance (it's well known that grinding wheels lose efficiency as surface speed is reduced because of diameter reduction). Key to success is geared to one's ability to make a conscious effort to verify speeds before operating a different wheel, and to ensure that proper peripheral speeds are demanded. Rule of thumb for vitrified wheels is 6,000 sfpm. Resinoid bonded wheels tend to tolerate greater velocity.

So then, a VFD may not be a wise choice, although I agree fully with your suggestion that the pulse free operation of DC motors is an advantage.

Harold
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John Hasler
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Re: Surface Grinding

Post by John Hasler »

Harold writes:

> With the opportunity to operate a grinding spindle at speeds that are open to variation, there's the risk of blowing wheels. That could spell death to the operator, depending on several factors.

If you are afraid that you will not be able to resist the temptation to overspeed the motor put the VFD in a box with just start and stop buttons accessible.
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Harold_V
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Re: Surface Grinding

Post by Harold_V »

John Hasler wrote:If you are afraid that you will not be able to resist the temptation to overspeed the motor put the VFD in a box with just start and stop buttons accessible.
This has nothing to do with temptation. It has everything to do with reality--the reality that one may be operating a wheel that is smaller than the one to be used in a different operation. One may unwittingly change wheel size and ignore speed, resulting in a blown wheel.

That it may or may not happen isn't a question, as it will. It's just a matter of when.

Smart people don't use variable speeds on grinding machines. If it was a great idea, they'd come that way.

They don't.

Harold
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John Hasler
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Re: Surface Grinding

Post by John Hasler »

Harold writes:
> This has nothing to do with temptation. It has everything to do with reality--the reality that one may be operating a wheel that is smaller
> than the one to be used in a different operation. One may unwittingly change wheel size and ignore speed, resulting in a blown wheel.

How does operating the grinder from 60Hz three-phase from a VFD make it any more likely that one might install the wrong size wheel than if the grinder was running from utility-supplied three-phase?

> Smart people don't use variable speeds on grinding machines. If it was a great idea, they'd come that way.

Where did I suggest varying the speed? I suggested making it impossible to do so. A VFD allows one to run a three-phase motor from a single-phase supply and is small and inexpensive. I agree that the variable speed feature is superfluous on a grinder. So what? You configure the VFD to put out 60Hz and then never touch it except to turn the machine on and off. This is no different then paying the utility to bring in three-phase except for cost.
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BadDog
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Re: Surface Grinding

Post by BadDog »

It strikes me that you could say similar things about a lathe, or mill. Run it fast for that 1/8" EM on the mill or small diameter collet work in a lathe, and switch to an 8" (relatively) unbalanced fly cutter or large 4 jaw chuck without changing the speed appropriately would also be a disaster. I've considered the utility of adding variable speed to my surface grinder for the same purpose. My grinder will fit an 8" wheel (on it now), but I've also got smaller cup wheels and the like, including CBN and diamond. If the speed is correct for 8" and 7" wheels, what does that do for my 3 or 4" cup wheels? Smaller vitrified wheels may have suitable rpm requirements, but not sure about others. I haven't used them yet, so haven't studied up on those, maybe a non issue?
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ctwo
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Re: Surface Grinding

Post by ctwo »

Thanks to this forum I have learned of the dangers of grinders. The extend of the risk has been very enlightening. Caution to the extreme may be warranted.
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Harold_V
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Re: Surface Grinding

Post by Harold_V »

BadDog wrote: I've considered the utility of adding variable speed to my surface grinder for the same purpose.
That's the very issue I've raised in my previous comments. It's only a matter of when, but you'll eventually install a wheel that won't tolerate the selected speed.
My grinder will fit an 8" wheel (on it now), but I've also got smaller cup wheels and the like, including CBN and diamond. If the speed is correct for 8" and 7" wheels, what does that do for my 3 or 4" cup wheels? Smaller vitrified wheels may have suitable rpm requirements, but not sure about others. I haven't used them yet, so haven't studied up on those, maybe a non issue?
Such wheels don't suffer the same problem vitrified wheels do. They don't behave softer, so while they may not remove as much metal in a given portion of time, they don't lose their ability to perform, very unlike vitrified wheels, which behave softer and softer, resulting in extreme wear and poor performance.

Introducing variable speed (without considerable effort to change) to a grinder is asking for problems----problems that may have fatal consequences. I will never endorse an idea that offers unnecessary risk.

Harold
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