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What type of "plastic" rod machines best?

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Martin W09/01/2010 23:00:31
940 forum posts
30 photos
Hi All
 
While not strictly pertinent to the original post but more in line with Sid's post it is surprising what metals absorb from the atmosphere. While working with high vacuum equipment we had to wait an awful long time before we could establish a decent vacuum. This was due to the steel the chamber was made from and the plumbing etc 'out-gassing'. The metals, seals etc had absorbed gases from the atmosphere when at NTP and these slowly bled out as the vacuum was established. So it would appear that most materials, in one way or another, can absorb gases (includes water in molecular form) from the environment.
 
Another useless bit of info but possibly interesting.
 
Cheers
 
Martin W
chris stephens09/01/2010 23:55:21
1049 forum posts
1 photos
Hi Martin W,
"Another useless bit of info but possibly interesting"
Oh yes, indeed. Most people will only see things from their own perspective, but there are certain facts that only show up after very particular study. High vacuum or, say, Quantum physics might not have much relevance to the average Joe, but their effects are real!
Everybody should go through life with the philosophy that there is no limit to how much you can learn. You can never know when an odd titbit of knowledge will come in handy. Keep an open mind, always.
chris

Gone Away10/01/2010 01:43:20
829 forum posts
1 photos
Melrick - let me put it this way:
 
If a given part made from Delrin has a problem caused by water-absorption, I do not believe that replacing it with Nylon will solve the problem (far from it).
 
Conversely, if replacing Delrin with Nylon does solve the problem, then I don't believe that water-absorption was the problem in the first place. More likely the problem was caused by the Delrin stress-relieving under the conditions of service (often temperature related but vibration and working stress can affect it too). This should be addressed by proper stress relieving treatment during manufacture.
 
Now, so that I don't completely hijack the thread, I have had a lot of happy results in my shop with UHMW Polyethylene (often just called "UHMW"). I echo the sentiments about sharp tools (as with other plastics) although on the mill I've found that any regular end-mill gives good results, a nice finish and attractive parts. It has quite low friction which may be seen as an advantage or disadvantage depending on the application.
 
Locally, a number of tools stores sell inexpensive bags of UHMW as offcuts in various sections and colours, which avoids having to find a specialist plastics supplier.
 
Of course, the "wonder" material (in almost all properties) is Vespel (DuPont) but that's pretty expensive stuff (or used to be - I haven't checked in recent years). Probably about as stable as plastics come and easy machining to boot.

Edited By Sid Herbage on 10/01/2010 01:44:26

mgj10/01/2010 09:34:21
1017 forum posts
14 photos
Sid - you could well be right. I'm not a plastics man, and we didn't cover them in any huge detail when I did my engineering degrees. I'm merely conversant with a number of applications where things have gone wrong with some of them.
 
The driving band business is of course not my opinion. Its the result of a comprehensive investigation by those who deal with these things. You don't find, in the middle of the Cold War that your entire stock of anti tank ammunition (and that of many of your allies) is useless without getting a bit of twitch on about it. Especially when you thought you could store it for years.
 
As for the suspension/steering bit - I can only speak as I find. Damp day tight, dry day loose. Change to PTFE problem gone. Now thats just straight slabs of 5/8 plastic with a central hole and two clamp bolts. Each bit is very symmetrical.
 
I believe also without being certain that some tape recrders or record players had problems - broadcast quality ones like Ferrograph as were. Didn't some of them suffer form wow because of problems with Delrin bearings absorbing water and changing dimension? This was in the tropics with high levels of humidity. Distant memory. Not important.
 
I note that Mr Rose with his very fine spherical bearings doesn't use a Delrin coating - because dimensional stability and consistency of breakaway torque under a broad range of conditions is a very important factor in the high grade ones?
 
I note that in tanks on the very accurate and vastly expensive zero backlash  linkages connecting gun and sight mirrors they didn't use Delrin sleeves. Because of the high moisture content of the atmosphere with 4 sweaty soldiers all breathing away.
 
Pople must take as they find. Its not a topic which interests me further.
 
Martin W10/01/2010 10:57:10
940 forum posts
30 photos
Chris
 
Just a quickie, have you heard how Mr Schro's cat is doing ?? Also when our scientists get their Qbits stable there will be a real impact and, with apologies to Star Trek, one might hear the immortal words 'Its computing Jim but not as we know it!!'
 
If you are into this then a browse of http://physicsworld.com/cws/home is worth a look.
 
Best regards
 
 
Martin W
chris stephens10/01/2010 16:09:49
1049 forum posts
1 photos
Hi Martin,
Last I heard one of them disappeared in a puff of sub atomic particles down the nearest black hole, or did it? Closely followed by Mr Spock shouting "illogical, Jim" as he went. You flatter me if you think I would understand Physicsworld, do they do a version for "dummies" or do I have to wait to watch it on "Horizon"


Now to show an equal bias, It could be that PTFE works better than Delrin in some situations due to two of its properties, namely very low friction and its soft consistency. If it deforms due water absorption or due to the release of internal stress after machining, it will simply squeeze itself to conform to confines and due to low friction it will slip whatever happens. You will, I hope, forgive the simplistic approach, I find more people can understand it. If I am guilty of turning "PTFE" into "Hoover" please forgive me, I am thinking of the relatively soft white engineering grade stuff, not the hard grey stuff found in a frying pan. I am sure some smart Alec will point out there are "Heinz" different varieties, but I only use three, tape, string and big lumps, you could make that four if you count cooking.

It is nice to know that our ammunition now fires straight , shame we cannot afford any. 

Just to go back to materials and their properties in earlier posts, and our perspectives of same. Think of a child's balloon, to a child it is a big round object that holds air and can be played with for hours, to an adult it disappointingly goes flat after a few days and has to be blown up again to keep young Jimmy happy, but to a physicist it is as porous as a sieve.
It's all down to Perspective. And with that in mind, I shall disappear into the vanishing point.
chriStephens


chris stephens10/01/2010 16:10:35
1049 forum posts
1 photos
Hi Martin,
Last I heard one of them disappeared in a puff of sub atomic particles down the nearest black hole, or did it? Closely followed by Mr Spock shouting "illogical, Jim" as he went. You flatter me if you think I would understand Physicsworld, do they do a version for "dummies" or do I have to wait to watch it on "Horizon"


Now to show an equal bias, It could be that PTFE works better than Delrin in some situations due to two of its properties, namely very low friction and its soft consistency. If it deforms due water absorption or due to the release of internal stress after machining, it will simply squeeze itself to conform to confines and due to low friction it will slip whatever happens. You will, I hope, forgive the simplistic approach, I find more people can understand it. If I am guilty of turning "PTFE" into "Hoover" please forgive me, I am thinking of the relatively soft white engineering grade stuff, not the hard grey stuff found in a frying pan. I am sure some smart Alec will point out there are "Heinz" different varieties, but I only use three, tape, string and big lumps, you could make that four if you count cooking.

It is nice to know that our ammunition now fires straight , shame we cannot afford any. 

Just to go back to materials and their properties in earlier posts, and our perspectives of same. Think of a child's balloon, to a child it is a big round object that holds air and can be played with for hours, to an adult it disappointingly goes flat after a few days and has to be blown up again to keep young Jimmy happy, but to a physicist it is as porous as a sieve.
It's all down to Perspective. And with that in mind, I shall disappear into the vanishing point.
chriStephens


chris stephens10/01/2010 16:27:57
1049 forum posts
1 photos
Hi Martin,
Last I heard one of them disappeared in a puff of sub atomic particles down the nearest black hole, or did it? Closely followed by Mr Spock shouting "illogical, Jim" as he went. You flatter me if you think I would understand Physicsworld, do they do a version for "dummies" or do I have to wait to watch it on "Horizon"


Now to show an equal bias, It could be that PTFE works better than Delrin in some situations due to two of its properties, namely very low friction and its soft consistency. If it deforms due water absorption or due to the release of internal stress after machining, it will simply squeeze itself to conform to confines and due to low friction it will slip whatever happens. You will, I hope, forgive the simplistic approach, I find more people can understand it. If I am guilty of turning "PTFE" into "Hoover" please forgive me, I am thinking of the relatively soft white engineering grade stuff, not the hard grey stuff found in a frying pan. I am sure some smart Alec will point out there are "Heinz" different varieties, but I only use three, tape, string and big lumps, you could make that four if you count cooking.

It is nice to know that our ammunition now fires straight , shame we cannot afford any. 

Just to go back to materials and their properties in earlier posts, and our perspectives of same. Think of a child's balloon, to a child it is a big round object that holds air and can be played with for hours, to an adult it disappointingly goes flat after a few days and has to be blown up again to keep young Jimmy happy, but to a physicist it is as porous as a sieve.
It's all down to your Perspective. And with that in mind, I shall disappear into the vanishing point.
chriStephens


V8Eng10/01/2010 16:48:28
1826 forum posts
1 photos
 
There is a chapter on plastics in the book 'Workshop Materials' by Alex Weiss.
 
 
Gordon W11/01/2010 10:17:43
2011 forum posts
If we want to use any matl. of unknown provenance it can be tested first. weigh, and measure the sample, put it in water,oil, brake fluid, or whatever, leave for some time, then remeasure. I know this is not to industry standards but we are not industry. This is especially useful for plastics, esp. "O" rings.
Ian S C11/01/2010 12:32:46
avatar
7468 forum posts
230 photos
Had one wee problem with carbon impregnated PTFE.Its a bearing through the piston of one of my hot air engines.I had the engine stripped down to replace a worn out crankshaft,and the PTFE bush sat for a month or so without anything in it,when I went to put things back together I found thatthe shaft would not fit through the bush.In its working life it had got oil on it and it swelled,after reaming it out everything was/and still is OK 15yrs later.Ian S C
Chris Trice05/05/2010 01:15:17
avatar
1376 forum posts
10 photos
For what it's worth, I've never had a problem with Acetal. I use the glass filled version where I can. I work in the film industry building animatronic and mechanical rigs and have used these materials a lot. Nylon on the other hand is another matter. I generally avoid it. BTW, the correct term for the less wise armchair experts is 'hygroscopic' with a 'g'. I know it's counter-intuitive given that we're discussing water being absorbed but it's NOT 'hydroscopic'. Occasionally we have rigs that need to be submersed in water for long periods and nylon is generally avoided.
KWIL05/05/2010 08:42:28
3681 forum posts
70 photos
To add another bit of useless information for Martin W, when we had finished pumping to a low vacuum, we switched on an ion pump and buried the gases again, to achieve an even lower vacuum.
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