High spot in plates from surface grinder

BrokeStatus · Apr 28, 2017

So I'm fairly new to surface grinding, and I've only been doing it about a year. Its GRIND-X from Okamoto, and it has been leaving high spots in my plates. 420 stainless, about .003 high in the middle of the plates on a 15x15 plate. I switched out wheels a month or so ago, and have been getting good plates (+/- .0005, but this is the second time the plate has been out. I usually take a .0005 cut up until everything is cleared, since it is rough stock. Then jump down to a higher table speed, .0002 cut, and a small stepover until the plate is done. I know tolerances can be shit if the hydraulics get too hot, but .003 seems very excessive, especially since it is only in the middle of the plate. Any ideas or insight is welcome.

Johnny SolidWorks · Apr 28, 2017

How're you holding the workpiece? Mag chuck? Fixture? Using coolant?

Chances are, the lots and lots of light passes are putting a lot of heat into the part (and 'dulling' your wheel) so if you're taking your finish cut on a hot workpiece, then unclamping it, the distortion isn't showing up until after you're letting go of the part, and/or it has come back to room temperature.

Lots of techniques to avoid this, depending on the situation. You can try:

-Taking finish passes carefully with the chuck off (assuming you're using a magnetic chuck)
-Unclamping/reclamping the part to allow it to 'spring' if you're holding it mechanically
-Taking much heavier rough cuts, and allowing the part to mostly cool before finish passes
-Oodles of coolant will help, if you're currently cutting dry

All of this assumes the obvious stuff - make sure you're stoning the chuck every piece, etc.

If something is off in the grinder (and considering it's one piece every now and again, I don't think it is) you can usually still get a flat part by spinning the workpiece (with the chuck off) between every finish cut, instead of feeding across the piece in Y.

Don't try to take heavy cuts with the chuck off!

Hazzert · Apr 28, 2017

What condition is the chuck in? Have you resurfaced it since this issue has cropped up?

BrokeStatus · Apr 28, 2017

I am using a magnetic chuck, and I wet stone it before I set up the plate and after that side is ground. I use flood coolant, and there is plenty. I ground the chuck roughly 2 months ago.

DMF_TomB · Apr 28, 2017

BrokeStatus said:
So I'm fairly new to surface grinding, and I've only been doing it about a year. Its GRIND-X from Okamoto, and it has been leaving high spots in my plates. 420 stainless, about .003 high in the middle of the plates on a 15x15 plate. I switched out wheels a month or so ago, and have been getting good plates (+/- .0005, but this is the second time the plate has been out. I usually take a .0005 cut up until everything is cleared, since it is rough stock. Then jump down to a higher table speed, .0002 cut, and a small stepover until the plate is done. I know tolerances can be shit if the hydraulics get too hot, but .003 seems very excessive, especially since it is only in the middle of the plate. Any ideas or insight is welcome.

.
.
with magnetic chuck released use .001 feeler gage stock and see if part bowed up. if it did fill space with feeler gage otherwise magnetic vise will suck it down or distort part from its relaxed shape
.
by using
.001"
.0012" (.03mm)
.0015
.003"
feeler gage stock you can even put .001 and .0012 on opposite ends to get a .0002" taper and or eliminate taper. use .0001 indicator and when magnetic chuck turned on measure how much part goes down.

BrokeStatus · Apr 28, 2017

How would that be able to help? Wouldn't that just push the swarf under the plate and make it worse?

DMF_TomB · Apr 28, 2017

BrokeStatus said:
How would that be able to help? Wouldn't that just push the swarf under the plate and make it worse?

.
if you have a bowed or curved part when unchucked and if you turn on magnetic chuck it sucks it down it will just go back up when unchucked. you have to support part so it will not suck down (distort) when magnetic vise turned on. if you use .0001 indicator you should measure it not going down when magnetic vise turned on
.
similar to milling. if i got a bowed or twisted part i DO NOT beat it down to parallels if i want to remove the bow or twist. if i did beat it down as soon as unchucked it would distort to its relaxed position
.
Starrett feeler gage shim stock is also used if you got 2 right angle blocks that are not perfectly 90 degree and you want to eliminate the error. Never seen a machine shop that did not have Starrett feeler gage stock in rolls in the shop. like how else do you shim a part .0002 or .0005 or more as needed ? again if i use .0010 on one end and .0012 (.03mm) on other end thats .0002"

Johnny SolidWorks · Apr 28, 2017

Just remember, the bow you're seeing isn't from the chuck being out of flat, the machine not being true, or anything getting under the workpiece (because you've told us you've eliminated all of those issues.) What you're seeing is the part reverting to its 'relaxed' state when the chuck is turned off. Whether the internal stresses causing this are from heat or another source isn't really relevant.

Tom gave you one very good approach to resolving the problem. But what I've typically seen done (and done myself) is to take the finish pass (of a couple of tenths) while relaxing/turning off the magnet.

Give this a try: finish a part with the magnet on, then turn the magnet off, keeping in mind there will still be residual magnetic force holding the part. Without changing anything else on the grinder, take another pass, and watch/listen for how much you're cutting.

WARNING: If you try to cut more than a half thou, or a thou with the magnet off, you may very well throw your workpiece. (Which is why Tom's method might be the safer and smarter way, but does necessitate taking the time to shim the piece properly.)

DMF_TomB · Apr 28, 2017

even on big parts weighing tons if inspection says something is off they put measurement numbers on part and i shim til when part on machine and clamped i measure the same error numbers with a .0001 or .00005 indicator then i machine the error off.
.
quite often i am only cutting .0005", check part with indicator on the machine if possible. obviously if magnetic vise turned on if part goes down it indicates something is moving.
.
even on a mill vise often a part will bow up when mill vise tightened. you often need a indicator to see it bow up

michiganbuck · Apr 28, 2017

How thick the plates?
Small step over..How small, does the machine have time to settle.
Does the holding stress the part.
Place in indicator at head to see if head is going up or moving to side..(not likely on that grinder)
.003 is .0015 per side if all else fails can a .0015 shim be place under... if plate is thine enough for that to matter.
Qt: [I switched out wheels a month or so ago,] from what to what?
Does a full wheel do better than one half diameter?

Got to go .. hope someone finds the answer (s) and I will look next week.

BrokeStatus · Apr 28, 2017

You can understand me being sketched out by grinding without the magnet on, but I will try. The plates are Blanchard ground to roughly 1.5".

DMF_TomB · Apr 28, 2017

BrokeStatus said:
You can understand me being sketched out by grinding without the magnet on, but I will try. The plates are Blanchard ground to roughly 1.5".

.
often there is a percentage of power on a magnetic chuck where 100% is turned down lower if needed. i would not turn magnet totally off myself

Johnny SolidWorks · Apr 28, 2017

BrokeStatus said:
You can understand me being sketched out by grinding without the magnet on, but I will try. The plates are Blanchard ground to roughly 1.5".

Yep, definitely some pucker factor. But chances are, whoever the experienced grinder-hand in your shop is probably does this without thinking about it. Have them walk you through it the first couple of times you do it.

BrokeStatus · Apr 28, 2017

Johnny SolidWorks said:
Yep, definitely some pucker factor. But chances are, whoever the experienced grinder-hand in your shop is probably does this without thinking about it. Have them walk you through it the first couple of times you do it.

We are a very small machine shop that does production runs mainly, but as of the last couple years have been making specialty injection molds. This is the first grinder that has ever been here, and I am the first one to operate it extensively.

DMF_TomB · Apr 28, 2017

BrokeStatus said:
We are a very small machine shop that does production runs mainly, but as of the last couple years have been making specialty injection molds. This is the first grinder that has ever been here, and I am the first one to operate it extensively.

.
you every see picture of after a wheel blows up ?? and picture aint the same as when standing next to machine

BrokeStatus · Apr 28, 2017

DMF_TomB said:
.
you every see picture of after a wheel blows up ?? and picture aint the same as when standing next to machine

My boss had ran a manual grinder for a couple years that was not of good quality, and he has talked about the one wheel that blew. I don't think I'd really like to have that experience.....

Johnny SolidWorks · Apr 28, 2017

BrokeStatus said:
We are a very small machine shop that does production runs mainly, but as of the last couple years have been making specialty injection molds. This is the first grinder that has ever been here, and I am the first one to operate it extensively.

The Suburban Tool Company has a YouTube channel you might be interested in - some pretty solid information on grinding.

SuburbanTool Inc
- YouTube

gbent · Apr 28, 2017

If you are getting your parts rough ground on a Blanchard, talk to your Blanchard grinder. Have them block the part in and spark it out with the magnet off. Its no fun to throw off a part on a Blanchard either, but thats why they have 1/4" or thicker water guards.

BrokeStatus · Apr 28, 2017

They are just blanking them to 15x15, but do not touch the faces.

JNieman · Apr 28, 2017

We do blanchard grinding and sled surface grinding in house and what Tom says about the mag chuck working against you is the most frequent problem we have when someone complacent runs parts on the big grinders. They would just throw a part on there, maybe block it in, and turn on the chuck. Grind away, then release. Indicators / CMM tells them it's out of spec on flatness and they throw their arms up like it's impossible. Then they go back and do the exact same thing, and take a light pass over it, and get exactly the same results, but a little thinner, now.

Then someone asks if they shimmed the bow on the raw/rough side and they facepalm and walk away. Happens way too often. It's easy to miss if it isn't your everyday thing.

ETA: It's extra tough if your shit comes in bowl shaped.

High spot in plates from surface grinder

Banned

Hot Rolled

Stainless

Banned

Diamond

Banned

Diamond

Hot Rolled

Diamond

Diamond

Banned

Diamond

Hot Rolled

Banned

Diamond

Banned

Hot Rolled

Diamond

Banned

Titanium

Similar threads