Pro Tips - February 2010
Raw Material Sizes
Last month's newsletter was a case study showing changes that were made to a real customer part to save them money. This month we are going to look at the available sizes and forms of raw material, and their related costs. I would like to say thank you to John Hunt from Lockheed Martin for the suggestion of this newsletter. John, I hope you like your Pro CNC baseball cap.
Back in December 2008 we discussed how costs can be saved by trying to minimize the size of raw material that is used in your design. Well the question came up, what are the available sizes that one can design around. This answer certainly depends on what type of material you are using. It also depends on what your local material distributors happen to stock, or else you will be looking at getting something shipped in specifically for you which may not save money. Out here in the Pacific Northwest, in Boeing's backyard we are fortunate to have a very wide variety of aerospace grades of material available to us. It may vary around the country but we will discuss what is available around our area which hopefully is fairly representative of the country. We'll start with the materials we most often work with.
In the most common grade - 6061-T6 or -T6511 there are a wide variety of sizes available. In rectangular bar shapes, the thickness generally starts at 1/8" and goes in 1/8" increments up to 1.5" thick and then goes in 1/4" increments up to 6". The widths generally start at 1/2" wide and go in 1/4" increments up to 12" wide depending on the thickness. 2024 and 7075 alloys come in fewer sizes, and tend to follow 1/4" or 1/2" increments. Square bars are generally available in 1/4" increments with some smaller increments available under 1.5". The amount of material that is required to clean up a machined face is a consideration when trying to optimize the size of material that will be used. As we recommended in our December 2008 newsletter, it is advisable to leave 0.1" on the width dimension for square or rectangular bar stock unless you are expecting to leave the stock surfaces and tolerances in your finished part. On thickness, .125" is the minimum amount of extra material needed, primarily for work holding reasons. But the thicker the part, the more extra material is needed. On a part made from 4" or thicker bar, as much as .25" might be needed to hold on to the part. Consult your manufacturer to see what they suggest.
In aluminum round bar, a very wide variety of diameters are available. The sizes start at 1/8" diameter and go in 1/16" increments up to about 2", after which the increments are 1/8" up to about 5", and then 1/4" or 1/2" increments after that up to 20". The amount of excess stock that is needed to clean up is much smaller compared with rectangular shapes. But it varies considerably depending on the diameter of the material and the type of material (ie - extruded vs cold finished). As little as .020" can be anticipated to be cleaned up on the outer diameter for smaller diameter materials. So if you were going to design a part to fit into 1.0" diameter bar stock, then try to make it no larger than .975" on the OD. This is a safe rule of thumb. There are cold finished grades of material which come with much tighter mill tolerances on the OD, allowing even closer dimensions to be achieved. Cold finished aluminum in a 1.0" diameter has a +/- .0025" tolerance, making it possible to design a .990" diameter part to be made from cold finished material. Extruded material on the other hand, has a much looser stock tolerance - +/-.012" on a 1.0" diameter, which is why it is prudent to leave at least .020" for clean up. The larger the material becomes, the looser the material tolerance, and you need to leave more room for clean up. 6" diameter extruded aluminum for example has a +/- .044" OD mill tolerance. So it would be advisable to leave at least .062" between the finished part diameter and the raw material. There are so many factors at play, the best option is to ask your manufacturer up front in the design phase so you can optimize the size needed.
6061-T651 plate starts at .25" thick and with the exception of .3125", it comes in 1/8" increments up to 1.5" thick and then comes in 1/4" increments up to 3.5". After that, it is available in 1/2" increments up to about 8". With plate, similar to round bar, the thicker the size, the looser the mill tolerance will be and the more you should anticipate will need to be cleaned up. The thickness tolerance range is approximately .023" on 1/2" plate and .075" on 3" plate to give a rough idea.
There are such a wide variety of stainless steels, all of which seem to come in different sizes. The more common grades such as 303, 304, 316 and 416 come in nearly every size of round bar you can hope for. In rectangular sizes, the choices are more limited as some alloys come primarily in round and square sizes (for parts typically made on CNC turning centers). Tolerances on cold drawn round, square and rectangular bar are generally very good. Hot rolled variants have much lower tolerances and this needs to be accommodated for in your design.
Being a popular grade for turning, brass is available in very small increments up to about 6" round bar. The selections for square are decent, but your options are very limited for plate and as far as we know, rectangular sizes are not available at all. Mill tolerances are fairly good so not a lot of excess material is needed to ensure your part gets cleaned up from the raw material.
There are so many varieties and alloys of steel that it would be an enormous undertaking to describe them all. Similar to stainless steel, tolerances on round, square and rectangular bar are generally very good, especially in the cold drawn varieties. So allowance for material tolerance is not a big factor. Our recommendations for rectangular sizes of .1" on width and length and .125" on thickness would generally still apply. Recommendations on round parts are similar to those above for aluminum.
We have now published 17 issues of Pro Tips, we have covered much of the lowest hanging fruit that we could think of, but we would love to hear some ideas for new topics. Anyone who submits a topic that we use will get a Pro CNC T-shirt or baseball cap. If you have an idea, the link to email us is on the right hand side.
Until next time!
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Part of the Month:
Every month we feature a really cool part that we have made. February's Part of the Month is a mold for spinal vertebrae. It is a very complicated mold made from several pieces with extremely complex geometry required in order to accurately model the shape of a vertebrae. In order for the mold to work properly, the tolerances are extremely so flash isn't created during the molding process. It was a very fun project to be involved with.
See more cool parts here. >>
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