Grizzly stirling engine plan symbols

[averagelemur]

Hello,

I am new to this forum and just purchased a lathe. I am looking to make one of the Grizzly stirling engine kits from their plans and don't understand a symbol that seems to appear on a few pages of drawings.

Can anyone explain to me what the upside-down check mark symbol around the octagon is supposed to indicate?

Thanks very much!

 

[TonyM]

The basic sign is for surface finish. That particular one means do not remove material on this surface.

 

[averagelemur]

Thank you. Any idea why it would be indicated at 0.8 at that point?

 

[TonyM]

The drawing says brass rod so I don't know why it's there. 0.8 μm may be the standard finish for brass round. Maybe it's just in case you are not using 6mm bar. Putting that particular tolerance looks overkill as it is just a connecting rod but then I don't know what purpose the part actually serves.

 

[Mechanicboy]

Here is the explain of the surface roughness.. Surface roughness - Wikipedia

 

[Mechanicboy]

And you need Rubert Surface Roughness Comparator Set if you want, but cheap?.. $261

 

[Richard Hed]

And you need Rubert Surface Roughness Comparator Set if you want, but cheap?.. $261 View attachment 116713

I have one of these I bought many years ago a fellow machinist sold me in a bundle of tools. I have never used it, other than to look at it an wonder. It's nice to know about and occassionally I look at it just to know approximations.

 

[Mechanicboy]

I have one of these I bought many years ago a fellow machinist sold me in a bundle of tools. I have never used it, other than to look at it an wonder. It's nice to know about and occassionally I look at it just to know approximations.

The roughness surface tester are to check the product under mass production due cost of production time, not for home model engineering.

 

[Chiptosser]

The surface finish is a way of conveying the required surface finish needed for the application.
You do not want a 125 um finish on a crank pin!. The surface finish samples are for reference to those call outs on prints. These can be found freqently used, and cheaply, most people don't understand the purpose of the tool. A home hobby machinest can benefit greatly by having a set on hand.
So, in your case, if you where turning the shaft diameter from a larger diameter material, you want the finish to be as indicated on the print. It may be a bearing surface and you do not want a rough surface to start with. If you have purchased bar stock, the finish is ether : ground, rolled or drawn to achieve its dimention. The surface finish is different, dependent on the process the material went through and the intended use of the material.