JonL
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Post by JonL on Feb 1, 2019 22:30:00 GMT
I've been pondering the piston valve. I plan to make a bobbin with PTFE liners; downside being once its been run on steam it will leak on air, but I don't think this is a worry. I've been looking at the different techniques forum members have used for mounting the PTFE seal on the bobbin. Then it hit me, PTFE is cheap as chips. Why not make the whole bobbin from it? However after consideration I think the expansion will mean my valve timing will be compromised once hot. Anyone else have any input or opinions? As it stands I think I'll me making a brass bobbin with PTFE seals. I'm aware its been discussed to death on this forum, I've read as many posts as I can.
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Post by Roger on Feb 1, 2019 23:46:46 GMT
I think the idea of making the whole piston valve from PTFE is a non-starter, it's way too 'chewy' and will expand and deform all over the place. It's easy enough to make the ends of the bobbin removable so you can fit rings.
Unfortunately, PTFE expands enormously, about three times worse than Aluminium, so solid rings have to leak when cold else they will seize when hot. As you say, it's been discussed at length. Personally, I don't like the idea of solid rings, it's too hit and miss getting the size right. Some seem to have success using thin rings slit with a sharp knife. All these ideas can be made to work, but I don't like any of them!
I'll find you a piece of Fluorosint instead of PTFE if you want to make it from that. It has a coefficient of expansion that's a third of PTFE, so any solution that might use PTFE is going to benefit from using that.
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Post by Jim on Feb 2, 2019 3:38:20 GMT
Roger has a valid point regarding the bobbins. Just out of interest I made two trial valve bobbins from some gray plastic rod I had to check that I had the valve ports and valve dimensions correct. I tested them running the loco on air.
In the photo you can see the grey test valve bobbins alongside a yet to be completed valve body with a length of PTFE that will be machined and cut to form the valve 'rings' This is the completed valve for the driver's side. I think I posted this photo earlier here..can't remember now  The retaining collars have the front edge machined to a fine edge so as not interfere withe the valve's exact length.
Jim.
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Post by keith1500 on Feb 2, 2019 6:35:59 GMT
PTFE certainly expands when warmed up. As an experiment make a bore and into it place a bobbin with the PTFE idea/ design. If you stand it vertically you can add some kitchen weights to make it slide down the bore. Then warm the whole thing up and repeat the experiment, you be surprised how much more force is required and this force will be derived from the valve gear.
Of course you could now machine the PTFE until it works nicely at temperature and suffer the leaks when cold and I don’t think it would take much to heat the PTFE with steam once the cocks etc are open and you admit steam.
Keith
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Post by Roger on Feb 2, 2019 10:05:15 GMT
If you search my thread for the keyword PTFE there are 17 pages of discussion on the whole topic, exploring all of the issues and possible solutions.
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JonL
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Post by JonL on Feb 2, 2019 22:25:45 GMT
Roger, its an astonishingly kind offer for the Flurosint, but I've investigated the price of it, it's a non-starter! It's my first choice otherwise. More research required I think. I made two of the piston sleeve valve caps tonight. No big steps lately otherwise.  |
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JonL
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WWSME (Wiltshire)
Posts: 2,988
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Post by JonL on Feb 3, 2019 20:00:43 GMT
Made the cylinder end caps today (blind end only, I don't want to make the other side until I've got my piston rod). Also did something terrifying... drilled the steam passages between cylinder ends and the piston valve sleeve. Very nerve wracking, but I think I did ok. Checking it with an airline and there was good flow at the places I would hope to have it. Next up is marking off the bolt holes in the caps.  |
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JonL
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Post by JonL on Feb 6, 2019 9:00:07 GMT
I spent a while marking out the holes for the bolts on the cylinder end cap. To be sure I'd got it right I then did it again, from scratch. Finally drilled the holes (undersized just in case) and you guessed it, I'd still got it wrong. Luckily as I'd gone undersized I was able to rescue it with a rat tail file and its fine, but I've annoyed myself especially as I was really trying to take my time.
It's funny, some things I take to like a duck to water, other (often simple!) things I just seem to funk even after really taking my time and double checking!
I finished off the evening by snapping off another tap. So far thats one snapped by the previous owner, and two snapped by me. This one snapped whilst withdrawing, so I must have applied an angle by mistake, again annoying because I thought I was being careful. I think my mind was just elsewhere.
Tonight is a new start, deep breaths, clear head, try my best to apply best engineering practise and patience!
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Post by Roger on Feb 6, 2019 11:44:24 GMT
I spent a while marking out the holes for the bolts on the cylinder end cap. To be sure I'd got it right I then did it again, from scratch. Finally drilled the holes (undersized just in case) and you guessed it, I'd still got it wrong. Luckily as I'd gone undersized I was able to rescue it with a rat tail file and its fine, but I've annoyed myself especially as I was really trying to take my time. It's funny, some things I take to like a duck to water, other (often simple!) things I just seem to funk even after really taking my time and double checking! I finished off the evening by snapping off another tap. So far thats one snapped by the previous owner, and two snapped by me. This one snapped whilst withdrawing, so I must have applied an angle by mistake, again annoying because I thought I was being careful. I think my mind was just elsewhere. Tonight is a new start, deep breaths, clear head, try my best to apply best engineering practise and patience! Good progress then, despite those annoyances. Personally, I never tap anything without something to hold the tap vertically. Even if it's only a block with a hole the same size as the tap body that I can press against the face. That's a two minute job to turn up in the lathe, you might like to give that a try. The pillar drill is something else you can use to assist you. You could adapt a collet type tap wrench (or make one) so that a fixed part goes in the chuck and a sleeve holds the part with the collet. That way it's always held vertically. Sometimes the swarf gets stuck as you try to withdraw the tap, so you may have to go back and forth, particularly when you're deep in difficult material. Another tip is to keep swapping between taps and go say 1/8 of a turn if it's difficult material. Each time you swap taps, you're introducing different angles so there's less contact area, and as a result, less force required. |
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Post by Jim on Feb 6, 2019 11:59:29 GMT
Just to add to what has already been said, I like to use a tapping oil/paste to help the tap while cutting. The tap needs to be withdrawn regularly to clear the flutes of accumulated muck. I find lubricant helps ease the task and reduces the chances of jamming the tap and breaking it. Jim I just remembered this photo of tapping in the mill.
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Post by Roger on Feb 6, 2019 12:38:59 GMT
Just to add to what has already been said, I like to use a tapping oil/paste to help the tap while cutting. The tap needs to be withdrawn regularly to clear the flutes of accumulated muck. I find lubricant helps ease the task and reduces the chances of jamming the tap and breaking it. Jim I just remembered this photo of tapping in the mill.
Hi Jim, I agree with what you've said, and I use the sprung centre like that for all my machine tapping. However, it does assume that the hole is directly below the support, so that would have to be set and ideally clamped by lowering the sprung point into the hole before tapping. |
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JonL
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Post by JonL on Feb 6, 2019 14:42:24 GMT
Gents, thats all great advice. I'll make myself a tap guide tonight. I do enough 6BA tapping to make it very much worth my while, thank you. If at all possible I usually tap using the tailstock and the drive belt slackened so I can gently turn the chuck by hand. These more complex parts mean I've been having to do a lot more freehand.
I've got a bit of an annoyance with my pillar drill, its an old Walker Turner that in the past was cut down slightly (for reasons I'm not sure of). As a result for about a third of the drilling operations I want to do I can't quite fit the parts in under the drill bit!
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Post by Roger on Feb 6, 2019 16:33:18 GMT
Gents, thats all great advice. I'll make myself a tap guide tonight. I do enough 6BA tapping to make it very much worth my while, thank you. If at all possible I usually tap using the tailstock and the drive belt slackened so I can gently turn the chuck by hand. These more complex parts mean I've been having to do a lot more freehand. I've got a bit of an annoyance with my pillar drill, its an old Walker Turner that in the past was cut down slightly (for reasons I'm not sure of). As a result for about a third of the drilling operations I want to do I can't quite fit the parts in under the drill bit! Personally I wouldn't use as small a tap as that in the tallstock, it's too easy to put a lot of force in it, especially in a blind hole. I use a sprung centre in the drill chuck even up to M10 in the lathe, I much prefer the control of being able to feel what force is being applied |
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stevep
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Post by stevep on Feb 6, 2019 16:39:02 GMT
Nobby, One of the best workshop projects I ever undertook was to make George Thomas' tapping and staking machine, as well as the sensitive drill attachment.  The drill is excellent up to about 1/8", and with the calibrations around the table, is useful for dividing (e.g. putting holes in cylinder covers). The tapping guide (swung to the right in this picture) keeps all taps perfectly upright, and the small knob ensures that you don't exert too much torque on the tap. Well worth making. |
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JonL
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Post by JonL on Feb 6, 2019 16:54:30 GMT
These are some great suggestions, however as desirable as the tapping machine is I can't quite justify the outlay or space of making one! It does look a phenomenal piece of kit.
I worry I'm coming across as a clumsy halfwit; I'm honestly not as bad as these last few posts make out!
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uuu
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your message here...
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Post by uuu on Feb 6, 2019 18:01:55 GMT
It's worth referring to Tubal Cain's arguments in his Model Engineer's Handbook, with alternative tapping drill sizes. I find that the process of tapping a lot more forgiving using his recommendations.
Wilf
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Post by ilvaporista on Feb 6, 2019 19:52:47 GMT
Being a cheapskate I made my universal tapping tool out of one of those small chinese drilling machines. Take off the motor, take off the return spring and add a handle on the top of the spindle. Works a treat and broken taps have been much reduced.
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Post by Roger on Feb 6, 2019 20:00:42 GMT
These are some great suggestions, however as desirable as the tapping machine is I can't quite justify the outlay or space of making one! It does look a phenomenal piece of kit. I worry I'm coming across as a clumsy halfwit; I'm honestly not as bad as these last few posts make out! Don't worry, sharing occasional cockups is preferable to hiding them, we all do them. Perhaps you can make a sprung loaded centre though, that's useful on the lathe, mill and pillar drill. Once you've had one, you'll wonder how you managed without. It's just two parts with a spring and grub screw, no more than an evening's work and well worth the effort. I modified one that I bought to have a better fitting centre piece and a lighter spring so it would be useful on very small taps. |
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JonL
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Post by JonL on Feb 6, 2019 21:02:56 GMT
I will spend an evening making some parts to help me out based on these recommendations. I'm all for making my life easier!
I had a much better evening; I made the other cylinder cap and this time used a paper template and it worked a treat. No errors, no filing required to make things fit! An evening like that balances things out.
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Post by Deleted on Feb 6, 2019 21:23:02 GMT
Hi Nobby Sorry to hear of your trials and tribulations with taps...we all snap a tap at some time or other. To add/agree to the advice given,I would always use a tapping fluid no matter what size, I use Rocol myself but there are plenty of others out there. A tapping attachment is a worthwhile investment, yes you can make one but they are cheap enough. IIRC I bought mine for less than £15...it may have been from RDG Tools?.. it's basically a chuck with MT2 sleave (to fit my mill) and a 'T' bar. When using it on my lathe, I remove the MT2 sleave and hold it loosely in the tailstock chuck, works a treat. Another point is quality of taps, there are a lot of cheap carbide taps out there that easily snap. Worth spending a little more on good ones... Keep up the good work, we all have bad days...I've learnt not to even start unless I'm feeling good. You'll be surprised how one day a job can seen a real pain and yet, sleep on it and it all goes smoothly the next day...  Pete |
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