Carbon Steel for Machining 3

[kgwhipp]

I'm making this part and ones like it, and my customer is flexible with regards to material and even slight design changes.

Part is finished to 12" on one side square and 2" thick. The stock will need to be oversized for machining. Customer initially specified Cold Drawn 1018 which isn't available in this thickness or size.

Customer is most worried about uneven distortion as the temperature changes and have called out flatness in a few locations. We're going to inform them of the unreasonableness of the flatness callouts (they're currently .0003" on a 12"x12" surface) and recommend that we can likely hold .003" or better with a grinding pass after finish machining.

I'm trying to recommend a better alternative, I'm concerned about A36 having inconsistent hardness with machining, and thought that A572 might have better consistency and machine less gummy. I'm also open to stress relieving and will budget this into the cost/lead. I'm also considering 1045 but don't know whether the extra carbon will slow down the machining too much.

Take a look, let me know any thoughts you might have!

-Kevin

 

[EdStainless]

How many are you making?
Just 1, or more?

If it is more than just one then you should consider welding the sides to a plate, giving it a good stress relief, and then finish machining.

Or you could look into high machinability (free machining) grades.
In the old days these were leaded, but now there are Ca treated steels with very good machining characteristics.

Either way I would rough it, then stress relieve, and then finish grind.

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Plymouth Tube

 

[kgwhipp]

Good question, small lots. 1 or 2.

Would the free machining materials come in the large blocks that I'd need?

-Kevin

 

[CoryPad]

Yes, the free machining grades are available in plate. For example, KLEEN-KUT 20 FM from Peerless, Clean-Cut 20 from Alro, and E-ZCUT20 from Ryerson.

 

[btrueblood]

What about making it from a casting, ductile iron or steel?

 

[tbuelna]

I would agree with CoryPad's suggestion to use a free-machining mild steel plate for best results. However, given the convoluted shape of your finished part I don't think you will be able to hold a .0003" flatness over a 12"x12" area. If you are using rolled plate stock, the direction of the grain in the rolled plate relative to the finished part shape will have an influence on how much the part distorts in manufacture.

You would rough machine the part from plate and then stress relieve it. Then you would do a secondary machine operation on the part followed by another stress relief. Finally you would do a finish grind operation on the critical surfaces. If you check the ground surfaces right after final grinding they will probably meet the flatness requirements. But if you check the ground surfaces again after the part has relaxed and been exposed to a few daily ambient temperature cycles, you will likely find the ground surfaces are not as flat as they were right after grinding.

 

[mrfailure]

Personally not a fan of free machining steels - every time I've looked at part failures, root cause turns out to be directly related to the choice of free machining steels because of the high sulfide or lead content. If you go that route, make sure this does not cause a problem for your customer's application..

May be a silly idea, but would using cast carbon steel cast to the basic shape and then machined be suitable?

 

[arunmrao]

Cast steel can be a good option, casting them near to shape with about 5mm/tool point machining allowance. Hope, the quantities are adequate, to make it feasible.

I'm just one step away from being rich, all I need now is money.
( read somewhere on the internet)

 

[jwhit]

This part could be made as an investment casting. ASTM A732 Gr. 1A or 2A would probably be a good choice for material. 1A is a cast 1020 and 2A is cast 1030, both in the annealed condition.