Stock material size exact dimension?

[Novian]

I have bought very little in materials so far, so i do not have much experience in the raw dimensions of material.

I've bought 1" and 2" diameter Aluminum round, and when i brought it home my caliper said they were 0.06" oversize. This is good so as we have some leeway to get a good finishing surface if we wanted exact 2.000" diameter. Is this true for Hex bar?

Looking at crueby's 6 Cylinder Radial engine plans it calls for 2" diameter (measured from the flats) Hex shape, and since buying a piece of 2" hex doesnt have a precision surfaces, is there extra material to to mill off the surface clean. (use of hex bar would promote less waste than using round stock and turning

Also, for flat bar stock or plate, is that over sized as well?

With lumber it's the opposite; the physical dimension is smaller than advertised: a 2x4 is really just 1.5"x3.5" due to the wood shrinking.

Thanks,

Ian

 

[peatoluser]

Generaly speaking, and from my own experiance, cold rolled mild steel whether flat, square, round, or hex is usually about 0.001" - 0.003" down on it's stated size. IIRC there are industry tolerances for producing CRMS and they are unilateral . i.e. to size or minus not plus or minus a couple of thou'.
I don't know what they are for aluminium and I don't know the plans you are working from , but how important is the 2 inch diameter? would it effect other components if it was under size? if not , personaly I'd go for the cheaper option.
good luck with your builld.
peter

 

[b.lindsey]

While the specs on aluminum have always seemed a bit more generous in my experience, even that doesn't always translate into enough extra material to mill down to the stated size. In the case of hex stock, there may be enough to use a flat plate and abrasive paper to polish ite six sides down to the specified size but as Peter said, tolerances can be on the minus side too. Another factor to consider is the material (aluminum vs. brass vs CRS) as these of course vary in hardness and therefore are subject to differing degrees of "dinging" during handling or shipment. Deeper scratches or dings will require more material in order to polish them out. Ideally one could hand pick a piece for size and quality if you have a friendly local supplier but that isn't an advantage most of us have, especially when ordering metals online. Personally, I would go with larger stock and mill the hex from round, that way you are in control of the final size and finish both. With the time invested in making a radial, the little extra waste and added machining time are of less consequence than the accuracy and finish of the part itself.

Bill

 

[mu38&Bg#]

A steel suppliers catalog I have here shows for hot rolled: ±.007" for hex up to 1/2", with .011" max variation across flats. And, ±.010" up to 1" with .015" variance across flats. 1-1.5" allows +.021" -.013" 1.5-2" allows +1/32", -1/64".

Cold rolled is much tighter as usual. At 2" it's ±.005 to .009 depending on the alloy.

Greg

 

[Novian]

...I don't know what they are for aluminium and I don't know the plans you are working from , but how important is the 2 inch diameter? would it effect other components if it was under size?...

peter

http://www.homemodelenginemachinist.com/index.php?topic=12280.0

The 2" diameter hex would be for the crank case

Ian

 

[Tin Falcon]

Most of the suppliers give tolerances for materials they sell. these tolerances will differ based on the material finish . Mill finish material will have a bigger tolerance than ground stock. check the tolerances given by your favorite supplier. nothing is exact. although a ground finished rod may have a tolerance of plus or minus a few then thousands of an inch.
Tin

 

[arnoldb]

Hi Ian

There should be enough leeway on those specific engine plans to take of 5 thou per face on the hex for clean-up from what I can see.

Just for the record, hex stock dimensions are always given as measured across flats.

I'm not sure how far metrification has progressed in the area of Canada you live in - but just be aware that some mix-ups can occur if suppliers start selling both imperial and metric stock. I have quite a bit of brass stock that was bought as metric, but is actually imperial - especially 25mm (supplied as 1") and 6mm (supplied as 1/4"). This actually works well, as one has more leeway for cleaning up things. The other side of the coin is that sometimes stock can be undersize if there's a mix-up like this; I have about 10 feet of brass hex sold to me as 5mm, but is actually 3/16" (4.76mm).

Kind regards, Arnold

 

[tel]

Manufactuers, unless I am mistaken, work to + or - 2%