You want ATP-5 or MIC6 as they have a really good flatness tolerance (tool plate). I’ve heard ATP-5 is better for this application, but I can’t find that source now.
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Did you do a surfacing pass on this? Not sure if that is the norm on aluminum beds.
We loaded the monster plate on a huge angle plate in a horizontal mill then indicated across the plate and it was within .01 across it. Thats the only reason I didn’t have to face it. That being said, my plan from this point on is to mount a vise, chuck, etc to a plate with the corresponding bolt pattern in the aluminum bed, so if you take that approach and mount a plate to the bed, then face that plate, as long as you mount that plate in the same place then it should be parallel to your spindle even in the bed is not flat.
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Midwest steel is the source for ATP5. I understand that above 1/2in, ATP5 has tighter tolerances than MIC6. These plates are cast while regular 6061 is rolled/extruded, as a result does not offer the same tolerances (maybe it does not even have flatness tolerances) and maybe the reason that suppliers are not as careful with the sheets as ATP/MIC plates.
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Thanks for the info guys. The price is right @ Midwest steel, however, the shipping to Canada is the same as the material 
The place I tried in Canada was almost 3 times the price, wowsers!
Don’t know where you live but I get some of my stuff shipped to an address in the US just across the border and bring it across, just pay the HST. If you want to know more, message me.
When I spoke to the mfg of ATP5, they check their flatness with the ATP5 laying flat on a large surface plate using a feeler guage around the edges.
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Do the flatness and thickness specs apply over the entire span (material length and width) or over some unit length (foot, etc.)? I haven’t been able to determine that.
A screen shot of the accuracy specifications (before and after resawing).
Having been in the Metrology career field, I asked why the specs are NOT in the units of measurement known as Total Indicated Readings (TIR). TIR is the overall flatness of the given length where the entire plate is scanned for the max / main deviation.
As I mentioned previously, that’s not how these plates are checked for flatness accuracy.
Whether the entire plate is whole or cut to a desired length or width from the whole plate sheet, the flatness specifications remain constant and based on the sheet plate thickness.
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Ohio Diesel’s plate which come from the same manufacture are spec’d even tighter, +/- 0.005 flatness.
Not sure how they are able to make such a claim. But in the world of Metrology, a flatness unit of measurement is not plus / minus but Total Indicated Reading. One would just have to apply a unit conversion, +/- to TIR.
In the field of traceability, if I were to spec"d a tighter spec than the original mfg specs, I would have to provide a certified charted value with each unit. This are the rules we have to follow and get prior approval up the many levels of management. We rarely go that route as it will open a new can of questionable worms.
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Maybe an error in the specs with an extra 0 added by the person who typed the description. You should contact Ohio Diesel point to the discrepancy and confirm the specs instead of relying on an Amazon description.
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Ohio Diesel says “Cast Aluminum Plates have a precision machined surface for superior flatness, flat within .015’‘, and +/- .005’’ thickness tolerance. If you need plates with a tighter tolerance, please contact us prior to purchase.”
That’s consistent with @agentwusabi’s 0.01" measurements.
I understand that the experts here claim this should be over the entire span. Can the drilling, tapping, countersinking, and shipping process impact that?
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I would think the only way that machining into the plate would have an effect would be if the plate was under stress and removing material would relieve it and cause it to warp.
Sorry, I agree that .005in is the thickness tolerance indicated by Ohio Diesel which is in line with ATP5 not better, I relied on the comment from Girard.
From the specifications posted early.
ATP5 tolerance greater than >0.5" = 0.005" or better.
ATP5 tolerance less than </0.5" = 0.015" or better.
Also to note specifications do not use a plus / minus or TIR tolerance notation.
Not sure what the symbol “/” mathematically equates to.
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The posted specifications taken from Ohio Diesel site not Amazon.
I did ask Ohio Diesel about the specs, its correct.
In Metrology, I come across a lot of mfg specifications that are interpreted differently thus requires calling to the mfg to confirm.
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CNC machined from 1/2” thick ATP-5 precision USA Aluminum plate with +/- .005" tolerance.
Perhaps I am misinterpreting Ohio Diesels specs. This was just taken from their website.
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This is what Midwest Steel says regarding ATP5
Cast Aluminum Plate
Aluminum Cast Tool & Jig (ATP 5) plate is vertical cast, stress-relieved, machined plate providing sound dimensional stability with minimal or no distortion even after extensive machining operations.
Cast Aluminum Plates have a precision machined surface for superior flatness, flat within .015’’, and +/- .005’’ thickness tolerance, and are protected with PVC plastic on both sides. Cast Aluminum Plate typically has a surface finish of 25 RMS or better. ATP 5 has a tensile strength of 41KSI a Hardness of 70HB and a Yield Strength of 18 KSI.
Cast Tool & Jig Plate ATP 5 is typically stocked in 48.5’’ x 144.5’’, 60.5’’ x 144.5’’ and 72.5’’ x 144.5’’, domestically partnered with Vista Metals out of Fontana CA, our Vista Metals ATP-5™ cast aluminum plate can be produced in custom widths and lengths with a very short lead-time. ATP 5 has outstanding machinability and responds very well in today’s high speed cutting environments. ATP 5 is the preferred choice for engineers that require a dimensional stable product that has great strength to weight ratio. Vista Metals cast aluminum plate (ATP5) is also available in metric thicknesses, in as little as 2-3 weeks lead-time delivered with a minimum order of only 2000 pounds. ATP 5 cast tooling plate applications include, but are not limited to, Computer & Electronics, Pharmaceuticals, Machining fixtures, Drill jigs, Index tables, Semi-Conductor, Automotive Molds.
Standard Cut Tolerance: -0’’ to +1/8’’
Precision cutting (+/- .005’’) available.
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