Recommended stainless grade for espresso machine

[Mick BaileyParticipant @mickbailey28509]

My Ascaso Uno espresso machine has a brew head that is part nickel-plated brass and part aluminium alloy. This is prone to heavy corrosion and was discontinued by the factory in favour of a solid brass unit along with a stainless steel removable insert that retains the seal and holds the shower screen. I’ve obtained the main casting, but the stainless insert is no longer manufactured so I need to turn one up from a short length of 60mm diameter stainless bar.

My question is, what would be a suitable grade to use? the part is in contact with nickel plated brass and held with stainless screws and on one side it’s mainly hot water and the other the coffee grounds. Easiest to machine is perhaps 303, but is 304 a better choice in terms of corrosion resistance?

[Clive Brown 1Participant @clivebrown1]

304 has slightly the better corrosion resistance, but I suspect, for your purpose, 303 would be adequate, especially since it’s easily inspected for possible deterioration.

[bernard towersParticipant @bernardtowers37738]

Why would it corrode?

[duncan webster 1Participant @duncanwebster1]

Some grades of stainless can be subject to stress corrosion cracking. Mainly under high temperature, tensile stress in the presence of chloride. I wouldn’t get worked up about it in this instance

[Mark RandParticipant @markrand96270]

303:- Good machining, lousy welding, passable corrosion resistance.
304:- Passable machining, good welding, good corrosion resistance.
316:- More care needed to machine, excellent welding, best corrosion resistance if chlorides are present.

[bernard towersParticipant @bernardtowers37738]

Its coffee not silver soldering

[Paul LousickParticipant @paullousick59116]

316 stainless steel is the preferred grade for the food industry and marine fittings. It has a higher resistance to salt water and chemicals but is harder to machine.

[Nicholas FarrParticipant @nicholasfarr14254]

Hi. both 304 and 316 are food grade, 304 should be good enough for appliances like your espresso machine, but preparation surfaces for food, should be 316, as regards to metal.

Regards Nick.

[noel shelleyParticipant @noelshelley55608]

I would go for 316, it may be more difficult to machine, but I often use it on a Myford so it’s not that bad. If drilling use a sharp drill AND cutting fluid. Good luck. Noel.

[CirclipParticipant @circlip]

304 – General purpose

316 – Hygiene

321 – Heat/exhaust

Regards  Ian.

[FulmenParticipant @fulmen]

It’s hard to argue against 316 if you have the stock and can machine it. Then again, if plated brass is good enough then 304 should hold up just fine. I wouldn’t use 303 unless I really had no other choice.

[noel shelley]

On noel shelley Said:

I would go for 316, it may be more difficult to machine, but I often use it on a Myford so it’s not that bad. If drilling use a sharp drill AND cutting fluid. Good luck. Noel.

Me too.   The issue isn’t how easy or not the steel is to machine, it’s avoiding corrosion.   Not because corrosion shortens the life of the component, but because some of it ends up in the coffee, perhaps enough to damage the health of the drinker.

Stainless steel contains a lot of Chromium, and the salts of this, i.e, ‘corrosion’ aren’t nice.   They cause ulcers and many are carcinogens.   The metal is safe when locked inside the alloy,  but not if a chemical reaction with hot coffee releases it.   Also, lots of Nickel in stainless.  Also a health risk, but not so bad.

Looking up 304, Wikipedia says ‘It is subject to pitting and crevice corrosion in warm chloride environments and to stress corrosion cracking above about 60 °C’  My bold.  304 isn’t suitable for hot coffee!

316 is a much better choice for this application: higher corrosion resistance, and not temperature sensitive.  Molybdenum locks the Chromium and Nickel more firmly in place, the disadvantage of 316 being increased cost and reduced machinability.

Dave

 

 

[Martin ConnellyParticipant @martinconnelly55370]

I would just say that 2″NB is Ø60.3mm so if the insert is not a solid lump then a piece of 316 pipe may be a good starting point. Where I worked cut-offs of 2″NB 316L were easily found in the scrap bins.

Martin C

[CirclipParticipant @circlip]

Forgot to add the ‘L’ suffix ie 316L  – – – Low Carbon content, applies to other grades too.

Regards Ian.

[Mick BaileyParticipant @mickbailey28509]

The insert is a solid disk so I need a short offcut of bar. I did machine up a replacement in aluminium alloy for the previously heavily corroded part and anodised it. I left it in the bath for two hours to get a really thick layer. With the test piece I did a file skidded off it and I though at least this part would have stood up to corrosion. Rather surprisingly after 4 years of daily use the faces are perfect, but the perimeter that’s in contact with the main brew head rubber seal had corroded around the edge.

It isn’t just hot water and brewing coffee that needs to be considered – there’s also the hot descaling solution of either citric or sulphamic acid that needs to be periodically used.

I’ll get hold of some 316L and give that a go. I have some nice new inserted tools that I’ve been using lately on difficult materials and they’re a revelation compared to my old brazed types.

[Martin ConnellyParticipant @martinconnelly55370]

I find the polished inserts designed for use on soft alloys work well on stainless.

Martin C

[old martParticipant @oldmart]

Whichever grade you use, before final fitting, passivate the stainless steel. This involves throughly cleaning and degreasing before immersing in an acid solution which dissolves the iron from the surface layer on a microscopic scale. A suitable acid which is biodegradable is citric acid, which should be safer to use and easier to get than the ones that spring to mind such as nitric or hydrochloric.

[Mick BaileyParticipant @mickbailey28509]

That’s something I haven’t come across before, so thanks for the advice. I have a fair bit of hydrochloric acid that I use to etch pcbs, as well as a few kg of citric acid crystals.

[Neil LickfoldParticipant @neillickfold44316]

316 is prone to work hardening. So plenty of coolant, and sharp tools. We used to say, half the speed and twice the feed. Their are very nice sharp coated carbide inserts for cutting stainless steel these days.They also work well with Titanium grades also.

Sharp hss works just as well, but will require regular touching up to keep it sharp and prevent the work hardening. Cobalt hss drills that have the web thinned or the centre pointed, are the easiest to drill 316 with. Where possible, drill with the finished size instead of trying to step drill. If a tap is required, go to the next size 0.1mm larger than you normally would and the tap will be alot easier to use.

316 does cut very well using the sulphurised  cutting oils and tapping compounds, like the old Rocol or the Garia B or C cutting oils. Nasty smelling though. If you are using a soluble oil water mix, having the mixture at 10% to 15% works alot better than the usual 6-8% mix.

Neil

[duncan webster 1Participant @duncanwebster1]

When I was still in paid employment we used a lot of stainless. One particular job of machining some stainless forgings went to Peter Brotherhoods. I can’t remember what grade it was, but it was big lumps. They found the best way of milling it was with HSS and literally painting the job with Trefolex or similar (long time ago, memory isn’t that good). Then reasonable depth of cut and feed, but keep the speed down, and at the slightest hint of the cutter getting blunt resharpen it. Brotherhoods were a great company to work with, sadly I think they have gone as well.

I often use HSS in a tangential holder for turning SS, it is easy to keep sharp. Do it frequently or you’re on a hiding to nowhere

[Howard LewisParticipant @howardlewis46836]

Over the years, Peter Brotherhood have undergone several changes of ownership, but like the other old established engineering companies in Peterborough, Baker Perkins and Perkins Engines, are still operating.

The original Brotherhoods site is now a huge shopping complex, as is what used to be the Perkins Walton factory, next door, (Used for heat treatment, gear manufacture, and aluminium machining).

Howard

[Nicholas FarrParticipant @nicholasfarr14254]

Hi Howard, when I was at college day release, we got to visit Peter Brotherhood’s, Baker Perkins and Perkins Engines, but not on the same day, all very established engineering companies back in the 60’s & 70’s, and Peter Brotherhood’s were doing some very large fabrications when we went.

Regards Nick.

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