Double Disk Grinding stock removal

[BobM3]

We have a cast aluminum plate about .55" thick that we have Double Disk ground to get nice parallel sides. Occasionally one side won't clean up. The grinder says the parts are bent too much. The foundry says they're fine. My question is: Can you control how much stock is taken from each side when Double Disk grinding? Or is the same amount taken from each side?

 

[kenvlach]

The warping is most likely* due to residual stresses through the cross-section of the part, which become unbalanced when material is removed.

*Precipitates within certain Al alloys can cause dimensional changes, and these precipitation & growth processes are sometimes induced by heat from grinding.

The usual remedy is a stress relief heat treatment. The foundry can provide in a stress-relieved or a stabilized condition. Sometimes, the best procedure is to do a stress relief after rough machining, before finishing to final dimension.

What is the alloy & heat treatment? Maybe the following threads will be helpful:

Properties of Al 319-T51
thread330-139413

Aluminum Alloy 319 or 356
thread330-138291

 

[BobM3]

The material is 356 and is heat treated to T6. I'll ask the foundry guys if they can do more to stress relieve the part. Thanks - I'll try the links.

 

[speedstan]

BobM3,

Since I'm new here, let me confirm that when you speak of "double disk" you are referring to double-sided grinding, correct? Yes, you can control stock removal from each side, provided your particular tool has independent control of the gears and plates. Usally this requires 3 or 4 drive motors, so that the carrier global (CG) speed can be varied w/r/t the individual plate speeds.

 

[BobM3]

Thanks Speedstan -

Yes, I mean double-sided grinding. Is there a good picture of the process somewhere? I have to admit I don't understand some of your terms (carrier global, plate, w/r/t). I'm done some machining - mills and lathes. This process is a bit of a mystery to me.

 

[Doublediscr]

Bob,
The way to confirm what the grinding shop is telling you, is to inspect the parts prior grinding. A double disc grinder requires enough stock removal to achieve your finish size tolerance. Take a random sampling of parts and lay them on a granite surface plate to check flatness with a surface indicator. Also check for surface deviations like casting dents or tapers especially on corners. A good rule of thumb is to have at 125% of the out of flat condition and lowest surface flaw to remove, over your finish thickness size. The next important thing to look at is the thickness variation from part to part. I worked at a double disc grinding job shop for 15 years, and ground a lot of problem parts.
If you find that there is plenty of material to remove another thing to look at is the area to be ground. Because the wheels are typically the same spec on each side the stock removal will only be even if the surface area is the same. Stock removal will go to the path of least resistance. This can also be an issue if the part is out of flat or has deep surface variations. There are things that an operator can do to equalize stock removal. One is to use to different grain sizes or wheel hardness, varying the wheel speed will change the cut too. It can also be as simple as a bad diamond used to dress a particular wheel causing one wheel to be more aggresive than another.
Hope this helps.
Brian

 

[BobM3]

Thanks Brian - That's exactly the info I was looking for. We just got a new shipment of castings in. I'll check the parts prior to sending them to the grinder. If the castings are good and he still has problems I'll be able to ask him some good questions.

 

[bklauba]

It also might be good to ask the builder of the grinder (Weldon?) or whoever they are. Oft times these folks have this kind of problem come across their desk.

BK

 

[unclesyd]

If warpage is a problem you might want look at doing a little a straightening prior to grinding until you can get the casting problem straightened out.

Your part is a little thin, but you might also look at duplex milling. Duplex milling allows a lot more operator input.