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Stainless Steel Spikes (electrodes) (lemmy.ca)

submitted 4 months ago by [email protected] to c/machinist

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Hey folks, pardon the rookie question. We need to build a few hundred of these per year, so I thought I'd tool up, and wanted to figure out how to do this "cheaply".

Parameters. Stainless steel rods, with one end sharpened to a point, and the other end deburred. Typically 30cm long with no real tolerance issues, and no real parameters on the point other than "if you hit it with a hammer, you should be able to drive them into the earth." Typically made of 3/8" or 1/2" stainless.

My main problem is: stainless is fucking hard and destroys my bench grinder when grinding tips onto it. Is there a better grinder I could be using? Or perhaps I should be cutting these on a small lathe?

Also, when I buy stainless stock, I usually have the metal wholesaler cut them to length for us, but they charge quite a bit. The stainless destroys my bandsaw blades, so perhaps there is a better option? Is there a mitre saw blade that is rated for stainless? Or should I also be using a parting tool on a lathe here?

Thoughts are appreciated. Such a simple thing, but stainless so...

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[–] [email protected] 8 points 4 months ago* (last edited 4 months ago)

What grade of stainless are you trying to process?

Since it's clearly not a free machining grade and is probably an martensitic type, some care has to be taken when working with it. You need to understand the metallurgy behind the part before you start.

Non-free machining Stainless, with its high carbon and alloy content, does two things while cutting: Work harden, and heat treat itself. This makes it difficult to grind more than just a surface finish off of. (Also, grinding a lot of stainless releases hexavalent chromium, which is a very big OSHA no no without ppe and dust management!)

You have to hog big, deep passes at a relatively low SFM to avoid work hardening at the surface you are trying to cut. Never cut shallow unless it is THE last pass, and never allow it to dwell for even a whole revolution- power feed all the way. You need coated carbide. aggressive tip relief and lead angles. It also must be worked with heavy flood coolant, as ANY heat buildup at the surface (eg from a grinding wheel or an even slightly dull cutter) will simply harden the surface right into the Rockwell C range and you'll have a bad time. You could put the point ok using a lathe like this.

You could part off the stock in a lathe fine, a typical bandsaw blade probably isn't hard enough. Ideal would be a cold cut saw probably, but if you don't have one handy that's $$.

However, what this all leads back to is the design requirements: is a hard stainless really needed? Do they need something so tough, or are they mostly for corrosion resistance? You could switch to a free machining Stainless with similar corrosion/heat resistance and eliminate a lot of these special problems.

[–] [email protected] 4 points 4 months ago* (last edited 4 months ago) (2 children)

Our last metal order was 304 stainless, if that makes a difference.

Purpose is to make electrodes for use in scientific imaging. They are pounded into the ground with a 2lb sledge, and wired up to a scientific apparatus. Electricity is injected using the electrodes and then we measure where the current flows in the subsurface to make a map of the electrical conductivity/resistivity of the subsurface materials. We then remove the electrodes and reinstall them at a new location. They take a beating.

It's stainless due to being in a corrosive environment, often wet, sometimes salty. We also have to be a little careful with the metal choices so that there is not a strong galvanic self-potential created. 304 stainless (or similar) seems to strike a nice balance between corrosion resistance, reusability (standing up to being pounded into most materials), and lack of strong galvanic response.

[–] [email protected] 6 points 4 months ago (1 children)

Ahh, okay. 304 is austentitic and doesn't heat treat really but it does work harden like a mf. With those requirements it is a fine choice. So I'm guessing that work hardening has been your processing problems.

[–] [email protected] 5 points 4 months ago (2 children)

That makes sense. I just looked up cold cut saws and stainless blades on Amazon. Looks like I can experiment for about $500, which is great! Something like this blade: https://www.amazon.ca/Evolution-Power-Tools-14BLADEST-66-Tooth/dp/B004Q0994I/ref=pd_aw_fbt_img_m_sccl_1/134-0371646-0898112 in the matching cold cut saw.

Thinking aloud to myself: I wonder if I can put a pyramidal or tetragonal point on them, instead of a conical point, just using the 45° miter option. That would save me from having to use the grinder, and avoid the work hardening...

Thanks for your feedback. Simple things sometimes are not simple when you aren't already skilled in the trade :)

[–] [email protected] 4 points 4 months ago* (last edited 4 months ago) (2 children)

you might even try a single bevel 'point.' Just cut the end at an angle on one side - the point won't be centered, but it doesn't seem like you'd care if there's a little bit of drift as you pound these spikes in. Might want to go steeper than 45, but you go through and do a bunch of 30 degree cuts every 60 cm, then come back with 90s to length, and you might save a lot of work.

eta: you might also look at rebar cutters for just cutting to length. Basically hydraulic shears, maybe a little more expensive than the cold saw, but quieter.

[–] [email protected] 3 points 4 months ago

Shear cutting can work, but you run into the work hardening problem again where the round stock will squish on the sheared end and be very hard to machine a point onto.

[–] [email protected] 3 points 4 months ago

This is definitely worth testing. I worry that, in hard ground, it might bend the electrodes into a J shape if pounded in with a non-centred point. One they're bent too badly, we put them in the scrap bin as they become hard to pound and pull. But perhaps that worry is unfounded in practice and I can do some A/B testing on the points.

[–] [email protected] 2 points 4 months ago

You're very welcome, glad to help! I've not had to work with stainless too much in my job, but I did take some decent material science courses on my way to becoming a mechanical engineer- and quickly applied them to actual machining.

The pyramidal point may work, but rotary saw blades may want to deflect and start rubbing when going diagonal across a round cross section, especially one so small. It ends up being a pretty small bit to cut. It's worth trying though, as it'd really reduce the amount of labor and tooling you'd have to put into this.

[–] [email protected] 1 points 4 months ago

Get this harbor freight bandsaw.

Put these Starrett blades on it.

Use two belt sanders with a 36 grit belt on one and a 120 or so on the other to add your points and deburr the back. The 36 is for roughing to size, the 120 for finishing and deburr. You might use something like this combo belt and disc sander with a 36 belt and a 120 disc.