a couple of question regards stainless steel tapping.

[Steve HuckinsParticipant @stevehuckins53362]

I am attempting a PM research No1 and have chosen to substitute the screws etc for metric because I cannot stretch to a whole new set of taps and dies etc., as many others appear to do.

I have just about managed to tap the Stainless steel piston rod (that I swapped out for 5mm), to M5 but struggled with the die I possess. I use an ordinary carbon steel die with the rod held in an ER32 collet. I have not yet tried single pointing and don’t feel confident enough yet to change all the gears etc on my SC4. also, my 70 year old eyes don’t see quite so well these days !!  Stainless is obviously very hard so I guess this is why I am having difficulty.  Would a split die be a better option ? and/or would carbon steel or cobalt die do better ?

I also managed to get an M3 thread on the other end of the piston rod after turning it down to size but it was rather rough……although serviceable.

Is stainless steel used as piston rod because of its use in steam environment ? Or could I use a softer material as I probably will never run my models on steam, only air.  My joy is mostly in the making and producing a model that runs. Once finished I go on to another.  This now being my fifth.

Thanks for any feedback

Regards  Steve

 

[Steve HuckinsParticipant @stevehuckins53362]

Appologies, I should have mentioned High Speed Steel as alternative die not Carbon Steel at the end of paragraph 2

regards Steve

[Thor 🇳🇴Participant @thor]

Hi Steve,

I have used Stainless steel for Piston rods, the free-cutting 303, and have never had problems. The 316 Stainless steel is more difficult to machine. If your die struggles with your 5mm Piston rod, try turning the part of the rod you intend to thread slightly undersize and see if that helps. If you run your engines only on air, try EN1A.

Thor

[noel shelleyParticipant @noelshelley55608]

Stainless comes in various grades, some easier to work than others. You will need sharp tools to work it what ever. You could use a bronze, or even brass as the piston rod. Good luck. Noel.

[JasonBModerator @jasonb]

£)£ should be OK with Carbon or HSS. This is one done a week or so ago.

 

M4 x 0.7 on the piston end, Split HSS die by Volkle

For finr adjustment I tend to go with M5 x 0.5 at the other end but would not have thought twice about M5 coarse. This cut with a solid HSS die by ARC

Stainless is the usual choice as you don’t want any rust pitting which will make the rod rough and it wil lwear your gland packing.

[jimmy bParticipant @jimmyb]

I cut a lot of threads in stainless, mainly 304 grade.

 

I use spiral entry dies and Rocol RTD as cutting fluid. Always power feed, around 200 rpm on an M6 thread for example.

 

A split die will cut exactly the same as a solid die. When you but high end brands, they will spilt dies if you need a (very) small amount of adjustment.

 

In summary, best dies and cutting fluid you can afford.

 

If you need a “decent” M5 die, let me know as I have a few spares

 

Jimb

[Steve HuckinsParticipant @stevehuckins53362]

I’m not sure what grade of S/S is provided by PM research but I may just purchase some of the free cutting recommended above. I should invest in some HSS dies as well and see what difference it all makes. Thanks to all responses so far and any others on the way.
I am following a couple of YouTube builds for the PM1 and it’s interesting to see the differing approaches. However, with the limitations of hobby type machinery, some of the techniques are beyond me. Wish I’d known what a wonderful career being a machinist could be when I was looking into the future as a teenager.
regards. Steve

[old martParticipant @oldmart]

A high quality split die used as large as possible for the first cut, and 303 stainless as already mentioned, but you can use other metals if they are easier for you to work.

[SillyOldDufferModerator @sillyoldduffer]

Stainless Steel is a large family of alloys with different properties.   They’re individually tuned to meet a range of different manufacturing needs, and many don’t machine well.   The type of stainless used to make sinks doesn’t cut nicely because it’s designed to be pressed and stamped in a single operation.  And it’s an advantage for the sink to be extra hard and tough as a result.  Sink metal often has low resistance to salt, so don’t use it in boats or to make dishwashers!

Many of the stainless family work-harden.   If approached with a sharp tool at the right feed-rate, they cut reasonably well because the metal is removed before work-hardening occurs.  But the “right feed-rate” isn’t obvious, at least to me.  Going in too fast and hard, or too slow and gentle, or not lubed suitably, or with a blunt tool will work-harden the metal and it can easily become as hard and as tough as the cutter.  Watch out for the symptoms: a tap or die driven into a work-hardened metal is soon blunted, and then using that tap or die on new metal causes instant work-hardening.  Woe, woe and thrice woe!

Easiest answer is to buy a free-cutting stainless as identified by earlier answers.   Alternatively develop the skills needed to thread nasty metals by practising!  Although well-made taps and dies stand-up to work-hardening metals longer than cheap tools, they’re not a panacea, and it’s a shame to spoil them by forcing them to cut unfriendly alloys.  Counter-productive, I feel, when an alternative free-cutting stainless threads more easily and the alloy is “good enough” for the job in hand.

Dave

[bernard towersParticipant @bernardtowers37738]

I have a metre length of 303 ultra turn and that is nice stuff to thread but not always easy to find.

[noel shelleyParticipant @noelshelley55608]

As has been said, use a good cutting compound. I use RTD but it’s not cheap ! Noel.

[Martin ConnellyParticipant @martinconnelly55370]

You can always single point some material away and finish with a die for final thread form. With stainless do not try such a fine cut that you rub a bit of material away, you need to keep cutting all the time because some grades will work harden and cause further headaches. As others have said a good cutting compound is always a good bet. I have both the grease RTD compound and, for brushing on a thin coating, the fluid RTD.

Martin C

[Steve HuckinsParticipant @stevehuckins53362]

There’s a lot of good advice here thank you. The part I have completed seems to be ok but fought me all the way.  So better dies on the xmas list as well as note to self regards free cutting S/S and good cutting fluid.

Happy xmas all.

Regards

Steve

[PeteParticipant @pete41194]

At one time there were still very good quality high carbon taps and dies available Steve. Possibly there’s a very few manufacturers left that might continue to offer them, but none as far as I know with the better and more well known industrial brands. Those now seem to be only produced in high speed steel. Overall the high carbon taps and dies available today are pretty much at the low priced consumer grade, poorly heat treated and ground end of the market. And those reasons are exactly why I no longer even consider buying them. If any high carbon tap isn’t already sharp and well heat treated out of a known high quality tool steel, the risk of breaking one off in the hole rises exponentially. And having that happen in an expensive casting or a part that already has a lot of hours invested in it can make those low cost high carbon tools extremely expensive. Those lower quality dies also won’t last very long, and in general produce much poorer quality threads because there tearing the metal more than cleanly cutting it. And unlike taps, a die has no real positive guidance. Yes they can produce usable threads good enough most times. But a slightly wandering or as some call it, drunken thread is usually the result. For something as critical as a piston to rod thread, it’s thread quality, pitch accuracy and condition, fit to the female piston thread, and properly aligned to the rods long axis. I think I’d always choose to single point thread that piston rod. And just like anything else, you only get half way good at that by doing it enough. Some hrs practicing on scrap isn’t wasted at all, and that should then allow you to be confident enough you can properly do so any time it’s required.

There’s also a perception that the common varieties of stainless meant to be machined are hard. They aren’t really, yes there tough or tougher to cut than the more common varieties of mild steel, and tough would probably be more accurate or a better descriptive term. And that work hardening can be a serious issue. Excess frictional heat from the cutting tool is what will cause that, so you can’t allow any tool to rub or dwell on the work at all. And as others have already said, a very good cutting oil meant for the job wouldn’t be optional to me. The best method for drilling or cutting stainless or any work hardening material is to stay below the maximum recommended cutting speeds. Those are for industrial use where time is money, and those recommended speeds assume that at least flood coolant would always be available. At most, I’d use about 80% of what my reference books recommend as the suggested maximum cutting speeds for stainless. 60%-70% might be a better choice to start with. Sharp, good quality drills are the minimum. When having to peck drill to clear the chips, never allow the drill to dwell at all in the bottom of that hole. It’s either producing a proper chip, or it’s instantly on it’s way back out. Being timid or too light with the feed pressure isn’t the method to be using. You want that drill or any cutting tool to always be producing a proper chip. The cutting oil and the frictional heat being produced should be getting removed both by the cutting fluid and in the chip itself. I suspect most issues with stainless are caused by using duller tools than they should be, cutting rpm too high for the equipment and tooling most of us have available, and not being aggressive enough on the feed rate. Once I finally understood that, I can’t recall having any more work hardening issues.

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