Chasing a warp on two parallels

[dsatchell]

I have been following a lot of the tips on the site to little avail. It's really perplexing.
My project is refinishing 2 1x2x18" parallels. these came to me used, and were generally in good condition with the exception of a <0.5 thou warp in them (top and bottom face (1x18" surfaces)), I can clearly measure both the hump and the concave sides, and a flatness friction test on a surface plate is consistent (hump side spins the part near the middle, concave side spins the part on the ends.

I set up on the grinder with a minimal magnet setting, so low that I can still measure the hump and push the part around by hand pressure. both sides are marked so I know which is high, and grind. it then measures flat. I'm grinding about 0.1 thou per pass with flood coolant.
Once done, I test the side for flatness on a surface plate by friction, all good (now spins at about 6" from the end), and then switch the sides, use full magnet, can again measure the ends now high, and grind. while still on the magnet with residual setting, I measure and it measures flat.
Remove from the magnet, clean and BOOM, I have the warp again.

I have noticed that the material does load in the wheel and have had to start resurfacing the wheel for each side. Generally finish is quite good.
I don't remove more than 1 thou total per side, and i've done this 4 times with similar results. I didn't keep good enough track to determine if the bow side is switching, but I think it is, as if it is springing from one side to the other.
the grinder has an 18" magnet, and I conducting the grinding parallel to the wheel. I'm going to try setting the pieces off angle and determine if that helps.

Other insights would great.

 

[Peter Neill]

It sounds like it's still pulling down or being pushed down, so put the concave face down on the mag chuck with some cigarette paper/papers directly under the bow to pack it out/hold it up, then try a light grind again and see what you get.
Are you marking the grind face with a pencil or sharpie to see where it's actually cutting? That may give you a good idea as to whether you're actually hitting the high spots first.

 

[eKretz]

Are you certain that the grinder is capable of grinding that flat? .0005" in 18" is not all that bad. If the grinder has any way wear you won't get a flat grind. Often on long "bendy" parts you can use shim or small ground in step (or even multiple steps) to compensate for some of the wear, as long as you're capable of accurately measuring flatness to a fine enough point.

Other than that, I can only give general tips, many of which you already seem to have a handle on. Make sure you're not using too fine a wheel, make sure it's dressed in a manner that keeps it open, cool and free cutting, and make sure the ground side going down and the mag chuck are both perfectly clean and burr-free. A quick slide back and forth will usually determine that if the chuck is clean; you can feel if there is any debris or if the part is not sitting perfectly flat. If there's any sort of "catch" use a flattened hone to dust over both the part and the surface of the mag chuck. Don't remove too much material from either side of the part when grinding, just enough to clean up. For a finish grind you shouldn't be getting much if any wheel loading.

 

[dsatchell]

Hi, thank you for the notes. I'll try the process with paper and low magnetic power. The machine is a Mitsui 6x18, with factory new ways. It's the largest grinder I've worked on so this is my first round of experience with parts this long. When working with the slide and chuck to resurface them, all seemed to go well. No measurable difference with a 0.5 thou dial indicator over the surfaces. I did notice some buildup on the wheel during grinding, given it is a 48 grit open wheel, I was surprised, but I've started to dress the wheel between sides to avoid issues.. I also run a ground stone over the surfaces as part of the cleaning process following measuring.

 

[eKretz]

Is this a hydraulic machine then? What kind of table speed are you running? How about stepover?

 

[???]

Use magnetic transfer blocks on the ends and pack them in tightly with parallels or scrap. Set the parallels at a 45 degree angle to the travel so that you have less time in cut for heat build up. Also check speed step over wheel grade etc as others have said. Another method is to grab them securely in a grinding vice with the ends only resting part way on the blocks.

 

[???]

Sort of like this

 

[michiganbuck]

You have to know the machine can grind to .005 flat/or what flatness you need..
If the chuck has been ground, then place an 18"+ long straight edge on the chuck with the mag off and with a light and eyeball, you can easily see a .002 error, blue it up and you can see .0003. Yes, stone the chuck with a larger size new condition hone.

If the chuck has never been ground in-house then you place a straight edge/gauge set off the chuck ..and run an indicator along the top of the straight edge. (X not indicating on the chuck.)

Having a flat grinding Surface grinder, you place the convex down on the chuck and shim the hollow. perhaps .005 at center and .003 at where that size fits.

The test: if you can push down on a part with 3 fingers then the grinding wheel will push it down during the grinding and you will never get a flat part.

The wheel has to be a cool grinding wheel, perhaps a 46h thorough L and an open wheel. A coolant and small cross-feed increments (.030 might be ok) less than .002 down feed used.

If me ..I would grind only .002 at the first grind to see how flat that grind came out in order to find out if internal part stresses would induce spring back. I might flip-flop the parts to reduce stress reliving changes if they appeared as the stock was removed

Heat will make the part swell higher in the mid section so with heat, you can't grind flat...good to have shims 1/2" long or longer for grinder shims.

Often a white or pink open AO wheel is better for low wheel pressure on the part...and lower heat.

Running an indicator off the wheel head along the chuck top does not tell that the chuck is flat, or that the matching can grind flat.

A pocket/folding shim set taken apart and the shims cut into smaller pieces is a good source of grinder shims. buy two shim sets and cut each size in half, for 1 1/2" long shims..

 

[michiganbuck]

Folding shim set / buy 3 sets and take them apart.to have 3 of each size, 3" long..

Amazon.com

RE: The last chuck grinding often tells how flat a grinder might grind. Test the least used area of the chuck (near the operator is a good place to check).

The chuck center on the rail is often a well-worn-out place and may have a ,001 (or what) place/dip/hole..and the dip will show up on thin parts ground there.

A good grinding practice is to place parts all about the chuck so as to not wear out any one place.

 

[michiganbuck]

Still, there is a limit to grinding. One will have some heat, loading, breakdown, and machine error. There is no point in wasting time for Dead Perfect when switching to lap or hone is more practical.
.0005 flatness on a 18" part is darn close for a 6-18 Mitsui.
I would likely hone it to .0002 or so and call that good.

 

[michiganbuck]

My reasonable guess would be that it is a ballway Mitsui, and the ballway rack is .0002 (or something) washed out, and so at near the end of long travel, it grinds a little more.
Once the taper/belly is in the part it is difficult to grind the error out on such a machine because the part being only 1" thick, the mag would suck down the part on a not precisely flat chuck...

Possibly one could take a couple of balls out of the machine to get a little flatter at the end of travel...but that's a hack method because you would have to grind the chuck and then the part.
Sometimes a heat swell puts a belly/hollow in a part's first side, and the flipping makes the other side out also, which can be a bugger grind out of a thin part.

It is good to put a strip of tape along/overlapping the collum side parting line of a ball way machine when brand new to keep grits out of the balls..and still break it down at intervals for balls cleaning.

If I designed a grinder, I would have a wick or an overlap shield along the collum side of long travel.

 

[michiganbuck]

A cast iron lapping block can be used to take a half-thow50 millionths and less (or what)
A flat hone or a piece of automotive wet abrasive paper can also take a little stock.
Going back under the wheel to grind 50 millionths and less is taking the chance to scrap a part.

Oh, in the above post (#11), I should have added that an over-tightened mag chuck can bend a grinder table and make a grinder .0001 or .0002 off nearing the limits of long travel.

 

[stephen thomas]

Lots of good stuff always in your posts!

Going back under the wheel to grind 50 millionths and less is taking the chance to scrap a part.

Oh, in the above post (#11), I should have added that an over-tightened mag chuck can bend a grinder table and make a grinder .0001 or .0002 off nearing the limits of long travel.

Often people fail to appreciate that there are so many subtle influences to consider when you get down to a few tenths and under especially on parts that approach the size of the machine travels. Not even considering all the heat factors & stress in the part issues (both of which you did mention above)

smt

 

[gbent]

I know you can't put it at a 45 like ??? posted, but run it on as large of angle as possible. You dont chase the heat the length of the part that way. Put an indicator on various parts as you turn the magnet on and watch for deflection. You have to be on your toes to shim .0005.

 

[dsatchell]

Hi, Thank you all for the notes. The mag, and table are flat within the 0.5th, table, mag base and top were reground and honed with a precision g-stone. Meaurements of them at each state were consistent with the measurements. Also, I've been working on some blocks 5x5x2.5 and they all have nicely ground to less than 0.2th. I agree that the machine could be out, but I'm not seeing it in the measurement of the mag. no bump or hump over the surface. I'm going to try a shim using dykem. I've layered a thin coat in the middle of each parallel and it measures to about 0.5th (about three narrow, blobby coats) and see how that works. I'll also grind at an angle (as much as possible (~20 degrees). I'm beginning to think that the variable mag controller is not uniformly variable over the setting range. which may be causing the spring. at 20% I can move the parts, at 25%, really hard to move them. I'll need to take it apart and check the rheostat Or what ever it is that is controlling the mag power.

 

[dsatchell]

I'll try the setup with the mag transfer blocks next if the shim approach does not work. Tx.

 

[dian]

i cant believe the cigarette paper and feeler gage thing. everybody using a grinder should have an assortment of stainless spring temper shims. mine go down to 5µ. cut them to the size needed.

also i didt see any mention of bluing on the surface plate (hopefully aa). for really good results use a copper sulfate solution instead, especially on smooth parts.

 

[dsatchell]

The Dykem worked well. If I were to do it again I would apply a finer layer of it, and not apply it as blobs. When dried it formed a ridge along the edge of the applied film that I needed to scuff off for consistency. Generally, three layers of the solution provided about 0.25th thickness. I've added few images of the in process and final. Final measurement was well below 0.5th, est 0.2th. I suspect I could have done better with more time, but I liked the lapping suggestion above and will finish with that.

 

[dsatchell]

Is this a hydraulic machine then? What kind of table speed are you running? How about stepover?

Hi, missed this note, no the machine is manual, so no speed setting that I can remember..I try to limit my step over to 15 to 20th, but sometime I'll grab 25th depending if I feel that it's finishing or roughing pass. Each stepover is per pass (each side of the pass I push another step over). I think it was springing, and also I noted that if I didn't leave the coolant flushing the part BEFORE grinding, it would change size enough. My floor is cooler than the ambient temp, and it is definitely enough of a difference to affect the part size (something I tested on a separate part). I've since taken to elevating the tank off the floor.

 

[eKretz]

Yeah, coolant causing a temperature differential can definitely cause issues.