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Cast Iron Plate

AHS

Aluminum
Joined
Jan 3, 2007
Location
N. Virginia
I have a 18 x 30 cast iron plate that is probably 50 y/o. It has seen very little use and hibernated on a bench covered by a sturdy wood cover for at least the last 30 years. For the last 20 years it wore a grease layer covered by plastic, which held up well so no rust. Pics below

No measurements taken of the surface, and no grade on the label. After cleaning, the few tiny dings on the surface quickly disappeared with light stoning. There is a surface blemish from some chemical on one end. No light showed below a ruler-type 24" straight edge moved around the surface, including over the blemish. Surface was not scraped.

* Would this be considered a layout plate or a surface plate??

* What would be the proper way to support this plate?? The bottom has 2 machined areas at the intersection of webbing, which are the same height as the machined perimeter walls (3/4" thick). The interior webbing is lower than the outside walls. Weight is ~151 #'s on the wife's bathroom scale.

Thanks!
 

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I'd call it a layout plate. It might do fine as a general surface plate, but IMO it needs more webbing that is laid out to rest on three points at roughly 1/3 from each end, to quality as a precision inspection plate. Iron surface/inspection plates of that size are often 6" tall or so to accommodate the webbing. Scraping or ground is more a preference, but scraped typically perform better for inspection as your parts and furniture slide across the surface much better. Ground surface is cheaper to make though and often good enough if your just lining up the sides of a gearbox or something. Layout plates are designed to be lighter and easier to put on whatever surface you desire with the only qualifications being that you can keep it from rocking and make it bubble-level. Inspection plates have to avoid sagging from just sitting by themselves and have no possibility of flexing when you move parts across the surface.
 
According to the Challange company it is a "cored bench plate that can be used as either a surface plate or to fixture bases. Here's a blurb from one of their catalogs.

APPLICATIONS Used as fixture bases or small surface plates.

GENERAL SPECIFICATIONS The top of plate is 5/8” thick and cored for lightness with numerous supporting cross ribs. All edges are 5/8” thick.

TOLERANCES The top surface is precision ground flat within .003” overall. NOTE: These plates will not normally hold accuracy in service unless supported by a firm, flat surface. Where an accurate surface is required, solid plates or thicker cored plates are recommended.

OPTIONS Challenge welded 34” steel stand (see page 21 for stands)

I have one of their hand scraped 24" x 36" cast iron 3-point surface plates. It weighs in at 475 lbs. It is supposedly accurate to within .0005". The ground model is accurate to within .001".

Here's a copy of the catalog. The cored bench plates start on page 17. I believe they quit making precision surface equipment several years ago.
 

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Thanks for all the info guys! The catalog is great, and per it I have the 2" cored plate at 18 x 30. It says it would weigh 193 and my bathroom scale showed 151, but since I have to move it I'm not complaining. :)

I did run a DI over it yesterday and I didn't see more that .002 deviation. But I was looking at an angle, so it could easily be double that. I'm not able to check for warping, etc., beyond the straight edge I mentioned above. The fact that it was stored on a flat bench probably saved it from serious issues. Now I have to find a place for it as the dolly is rather low. My buddy is gonna hate me when I decide on a place. Given that the bottom edges and the 2 web centers look machined, it makes sense that it should be full contact.

Thanks again! Alex
 
The fact that it was stored on a flat bench probably saved it from serious issues. Now I have to find a place for it as the dolly is rather low. My buddy is gonna hate me when I decide on a place. Given that the bottom edges and the 2 web centers look machined, it makes sense that it should be full contact.

Thanks again! Alex
Actually is a common and persistent myth that cast iron somehow sags over time. Your plate is still flat because that is the nature of cast iron---a stable crystalline structure. Don't take my word for it though. ASM has devoted an entire volume of the Handbook to cast iron. See Volume 1A. It can be viewed online for free.

And the machining of the base and the catalog recommendations both argue for supporting the plate on a (very) flat surface to maximize flatness of you plate. Alternatively, if the plate is lightly loaded, it is probably stiff enough in its own right to not deviate appreciably even if "improperly" supported. It might be interesting and revealing to examine it with sensitive instruments supported in various ways and see just how much deflection occurs due to its own weight. I doubt it would deflect more than a tenth or two over its length if supported only at the extreme ends. But again, don't take my word for it. Test it and, if you do test it, it would be very interesting to hear what you find. Your body heat or sunshine from a window are much more likely trouble causers.

Denis
 
If you are curious about how much your plate distorts under its own weight, you can check easily. I'm cutting-and-pasting from something I wrote back in November, 2023 to specifically describe an exactly-analogous check of a surface plate:

1. Let's call the corners of the plate A, B, C, and D "going around".

2. Arrange four screw jacks in a rectangle that's roughly 3/4 the size of the surface plate, extending the screws of jacks A, B, and C to their mid-travel points while leaving the screw of jack D fully retracted.

3. Set the surface table on the three extended jacks so that the surface plate center is roughly coincident with the four-jack pattern's center. The surface plate Center of Mass wants to be very close to the line connecting jacks A and C, so that only a small weight directly over jack B will be needed to prevent the surface plate from rocking over the A-C diagonal.

4. Raise jack D to contact the underside of the surface plate, without loading jack D.

5. Move the stabilizing weight to jack A or C, and lower the diagonally-opposite jack.

6. The surface plate surface directly over the lowered jack will sag very nearly twice the gravity-induced distortion.

There are a number of different methods to measure the double-sag at the lowered jack. I've done it -- on space-flight hardware, not surface tables -- with feeler gages, dial gages, precision sight levels, and laser trackers.
 
I would very much like to do further testing on this plate. I think a couple of local guys would have equipment to do the job. If, or when, I my scraping gets above the amateur level I would consider attacking it. The first thing is to find a location for it.

I don't have a place for it that is flat. I'm going to visit a local metal supplier that might still have cut up plates. A 3/8" plate (full width & length) would be my target, a 1/2" would be better but a bit much to man-handle. The bench I want it on is 28" wide, and the plate would be positioned across it, leaving the need to support 2", or 1" on each side of table. This may take a while to get together.

Alex
 
I would very much like to do further testing on this plate. I think a couple of local guys would have equipment to do the job. If, or when, I my scraping gets above the amateur level I would consider attacking it. The first thing is to find a location for it.

I don't have a place for it that is flat. I'm going to visit a local metal supplier that might still have cut up plates. A 3/8" plate (full width & length) would be my target, a 1/2" would be better but a bit much to man-handle. The bench I want it on is 28" wide, and the plate would be positioned across it, leaving the need to support 2", or 1" on each side of table. This may take a while to get together.

Alex
It will be interesting to decide how to attach or float the cast iron plate over the hot-rolled (not so flat) 1/2" thick plate. Bedding it on something like blobs of thickened epoxy will compensate for unflatness of the hot-rolled base. Of course, the epoxy even after setting up will compress a little (my guess is insignificantly) if loaded. I have 3-pointed straight edges on Wood's Metal for machining and that has worked well in that application. It is essentially incompressible. Still the metal will contract some as it cools. Straight-away bolting to the hot rolled would be the worst option I can think of. Just simply allowing the plate to float on a compliant pad like moderately dense foam rubber would allow the plate to evenly distribute the load bearing for the plate but also would allow weight-induced distortion. That distortion would likely be less than the random bearing expected on a hot-rolled plate.

For all but the most meticulous use, just floating the plate probably would work fine. If you start worrying about a tenth or two here and there (not usually a concern in my shop) then greater care and careful distortion measurement might be worthwhile.

Denis
 
Denis, I haven't thought ahead to the issue of how to set the plate. First will be to find something at a reasonable price. The company used to go to did a lot of business in this area and always had a good selection of cut sheets, but they have downsized. Cutting charges would probably be a deal killer.

I looked up Woods Metal, interesting stuff. Our community college recently got new automotive 2 post lifts and 1 alignment rack which were level positioned then a cement-type grout loaded under the leg pads. No idea on the composition or compressibility of the grout, but the racks are heavy. I think using any compound under contact pads is a science in itself.
 
The "cement-type grout" AHS speaks of is, almost certainly, what is called "precision" or "non-shrink" grout. High-end versions are sold under brand names such as "V-1" (the name V-1 is used by at least three different companies around the world) and Crystex. Economy versions are sold by Big-Box stores such as Home Depot, Lowes, and Menards.

I've used both the high-priced and low-priced stuff that meets the Federal specification, and haven't noticed any real differences in general use. On the other hand, there's probably a reason that Waldrich-Coburg specified the German-made V-1 while Ingersoll Milling Machine specified Crystex for installation of their machines.
 
John, good info on the grout, didn't know much about it but now I know more.

dgfoster mentioned potentially using a rubber pad to even out the irregularities in the base plate. So I did some googling and rubber is readily available up to a Shore Hardness of 60, and goes up to 90. The material available thru H. D. seems to be 45-50. Harder material is available on other sites, some having sheets and some strips of 2" - 4".

Keep in mind that I am not outfitting a legacy shop, so I really don't have to lose sleep over the sag issue. Rubber under the perimeter and middle contact areas would be worthwhile I think, and will look into it. I will do what I can to minimize the potential sag issue.

The bench I'm looking to place this on is an ancient saw table from a lumber yard. It's ~5' long with of a width of 28" from 3 boards of 1.5" thick. The 2 full width legs are heavy cast iron. The plate would lay just to the inside of one leg. The top is well worn and not smooth, so would require a base under my layout plate.

If I don't get a steel plate I'll probably consider an MDF board. At a weight of 193 # and a perimeter length of 96" the weight per perimeter inch for my plate would be about 2 lbs. Actually slightly under because there are 2 machined contact points in the middle of the plate. So I dug out my certified 2 kg x 2" od scale calibration weight and set it on a piece of truck tire innertube last night, and no visible indentation this morning. This highly scientific test tells me that rubber would work to take up surface irregularities of the base plate, within reason.

Here are 2 urls for those interested rubber and Shore Hardness.
A Simplified Look at Shore Rubber Hardness and Its Application
https://swcpu.com/blog/shore-rubber-hardness/

The main point for me was, "In layman’s terms, shore hardness indicates the measure of a specific material’s overall resistance to indentation or penetration."

This give a little more info on usage.
Good basic descriptions of suggested usage.
https://www.rubbercal.com/industrial-rubber/rubber-durometer-chart/

So there it is! I know the destination, just have to finalize the route. :) I appreciate all the info provided, that's what make these forums valuable.

Alex
 
Rubber from inner tubes may be about the right stiffness but in my limited experience can vary a lot in thickness. Might be worse than nothing.
 
Gard, the inner tube rubber was a "wonder if" test to see if it would dimple. Based on the rubber info I provided above, they are probably a 50 - 60 hardness rubber, which is somewhat easy to get. If I did this, I would buy it in a roll with a width of 2" - 4" (maybe 3/16" thick) so I could run the full perimeter and center points. As I mentioned, the weight per inch is not very high.

I did look for a steel plate, but the supplier didn't have anything close. So now I think I'll use a piece of 3/4" (or 19mm in todays dimensions) MDF. I might add the rubber, but whatever I do I'm only lifting/hoisting/positioning that thing once, The 150# or 190# must be atleast 80# heavier than I remember from a few years ago (age? never!).

Alex
 
Since rubber came up, Amazon and eBay list rubber rolls in various thickness and width x 10'. The perimeter walls are 5/8" so I was looking at either 1" or 2" width, in either 3/16" or 1/4" thickness.

Amazon is out of 3/16" but has 1/4" at $16 for a 1" x 10' roll. eBay has a 3/16" x 3" x 10' for $22.75.

I'm thinking of the 1" width, in either thickness. I figure the 1/8" thickness would be too fiddly during install. Not sure I need it on the MDF so just thinking right now.

Thoughts???

Alex
 








 
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