bigandy
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 posted on 17/2/05 at 12:54 PM |
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A machining question..
I'm currently designing a new set of pedals for my Mojo, so that they look better than the welded frankenstein ones I have at the moment, and
I've just come to the bushes that allow the pedals to rotate on a spindle.
The original pedal bushes are a hard plastic type material, and It worked well, so I would like to try that too. What is the best stuff to make
bushes like this from? I'm guessing something like Delrin would be suitable?
Also, what is the best way to machine plastics on a lathe? i tried with some Nylon last night, and I got an awful finish. I ha d amega sharp tool,
fast cutting speed, slow cutting, taking large cuts, small cuts, everything, and still it looked like a mangled mess.
Cheers
Andy
Dammit! Too many decisions....
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shortie
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| posted on 17/2/05 at 12:59 PM |
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I guess Delrin would be fine, but PTFE would be even better as it has less surface resistance, kind of an oily slippery finish.
I did some Delrin on my lathe and used the finishing tool with a standard cutting speed for ali (quite fast) and the finish was excellent. I found
the Delrin very easy to work with.
HTH,
Rich.
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David Jenkins
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| posted on 17/2/05 at 01:08 PM |
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I have limited experience of machining nylon on a lathe, but I found that medium speed with a very steep relief angle worked best. Too fast and the
nylon melted and did a hot-glue effect on top of the tool, too slow and the tool dug itself in and all hell broke out!
Even when the speed and cutting tool angles are correct, I always got a heap of tangly 'swarf' that wrapped itself round the workpiece and
got in the way of the cutting edge.
A big tough lathe helps, to overcome any tendency to jam - a 6" Myford just will not be stopped by a piece of plastic!  
David
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flak monkey
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| posted on 17/2/05 at 02:30 PM |
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Nylon would probably be the best material to make them from. If you can get some of the black nylon it has graphite in it, so it keeps itself
lubricated quite nicely, and isa little easier to turn.
Tool geometry, feed and workpiece speed are all crucial to cutting nylon sucessfully. You need a medium workpiece speed, but i cant remeber all the
other factors.
The swarf you get will always be long and stringy, and it loves wrapping itself around the work, a tool with a high back rake angle will keep it away
from the work a bit. A high end clearance angle will also help with the finish. You will need to regularly stop the lathe and remove the tangled
swarf...I would recommend you keep a sharp knife and a stong bit of wire handy to clear it though, it is deceptively sharp!
Cheers,
David
[Edited on 17/2/05 by flak monkey]
Sera
http://www.motosera.com
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nick baker
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| posted on 17/2/05 at 02:32 PM |
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Plastic PE (polyethelene)
Low friction
Very high wear resistance
High chemical resistance
High fatigue strength
Very high impact strength
Field of application
Sliding elements, bushings, bearings
PolyAmide Plastics (PA) are pretty much the same as above, but tend to absorb water and therefoe the sizes and characteristics change.
now.. an Acetal co-polymer plastic (POM) is probably what you want..
Field of application:
Gear wheels, cam discs, bearing rolls, transmission parts, snap fits.
Advantages:
Very high stiffness
High creep resistance
High fatigue strength
High impact strength
High wear resistance
Low friction
Ethylene terephthalate plastics (PET) basically suck... so, I'd be aiming for POM if I were you.
Or Teflon...
Are all the bushings the same? I could knock out a set for you in a couple of minutes this evening for you if you fancy... Sketch down what you want,
and all I'd ask in return is the postage costs.
It's about time I started giving something in return for all the advice and info I've got from this page!!
Cheers,
Nick
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bigandy
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| posted on 17/2/05 at 03:10 PM |
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I did happen to jam the lathe once or twice when the tool bit into the nylon. i was running a fairly slow speed, and taking off what was obviously
too much material per cut!
So how does this Delrin stuff hold up as a bearing material? It will be a 1.5mm-2mm thick sleeve with a flange at the end. And it is subjected to
the sort of forces you get with a 110kg prop forward stomping on the brakes!
Cheers
Andy
Dammit! Too many decisions....
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The Shootist
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| posted on 17/2/05 at 03:11 PM |
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Why make them?
Just use the purpose made bushes that are in your donor, or some other car.
Any good industrial supply, and here in the states any decent hardware store, should have a selection of sizes on hand.
For the price of Pennys each it's not worth the effort to make your own.
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bigandy
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| posted on 17/2/05 at 04:00 PM |
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The first question I asked myself (and indeed I ask myself in any design project, as I am a design engineer by trade) is can I use an off the shelf
part?
Basically, the answer is no in this situation because as far as i know, the spindle is a custom designed item. The original spindle is a little
small, so i have upped the size a little (from 10mm to 14-15mm off the top of my head). I also want to incorporate the spacers (required to locate
the pedals in the lateral direction along the spindle) into these bushes.
oh, and I also thought a bit of experience when it comes to machining a plastic bearing type material might be worth having 
Cheers
Andy
Dammit! Too many decisions....
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nick baker
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| posted on 17/2/05 at 04:37 PM |
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Have you though of using grub-screwed rings to lock the pedals in position, therefore makig the lateral spacing adjustable?
l
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krlthms
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| posted on 17/2/05 at 05:09 PM |
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I have used most common plastics to make small, complicated shapes for micro plumbing applications; these required both turning and machining on a
mill. My advice is delrin always. It loves speed, and you should not get the swarf if you make your cuts small. I find it very hard to get precise
cuts on teflon, polycarb gets chewy, acrylic shatters, and polyE, poly P are too soft and don't have good chemical resistance. Further
observations:
1. When using plastics I use light mineral (paraffin) oil as lubricant.
2. Delrin is not resistant to alcohol in the long run; I don't know about substances (i.e., lubricating oils and greases).
3. When removing swarf to look at the workpiece make sure that the machine has stopped rotating, not just turned off. I once ended up in emergency
room with the end of my finger hanging off because I was in too much of a hurry to see the finish on a piece of polycarb I was machining! Luckily,
my finger healed ok. In fact, now, when I am in a hurry to get something machined I get someone who knows what they are doing to do the work for
me!
Cheers
KT
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nick baker
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| posted on 17/2/05 at 06:02 PM |
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I agree with teflon... it's terrible for machining: It ususally ends up looking "furry", but despite the poor surface finish, the
tolerances you can get are rather good.
With regard to "lubrication" I've never used any: partly as it's messy, but mainly as I've never been convinced that
they need the cooling. I DO however ALWAYS use a gentle jet of air (positioned just behind the tool, and pointing down) to take the string of swarf
down and away from the workpiece and chuck.
If acrylic's shattering, it might be becuase you're using a tool that's tool blunt. I find that I can only get a good cut if I use a
brand spanking new aluminium-cutting tool.
Bummer about your finger!! I once saw a friend get his hand cut to the bone by a large amount of stainless swarf that suddenly caught on the chuck
and came round like a flail, and another lad get his finger tip cut between a chuck-key and the rails for the carriage.... not cool, and both
mistakes that could have been avoided.
My most common mistake is to cut the BACK of my hand on the tool when cleaning swarf away  Daft, but I'll never learn!!
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bigandy
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| posted on 17/2/05 at 07:57 PM |
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Scary stuff this machining lark isn't it? I think the worst enemy is complacency! I always tend to be a bit wary of anything with a motor in,
regardless of whether it cuts or not. It only takes a momentary lapse in concentration to end up with no fingers!
Anyway, As this plastic stuff seems pretty tough to machine well, any suggestions for possible alternative bearing materials? I'm thinking
something like brass, or that oilite type stuff. Its heavier, but hey!
The bush is approx 25mm long (2.5mm thick flange) ID 14mm 0D 18mm and flange Dia is 25mm.
Cheers
Andy
Dammit! Too many decisions....
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flak monkey
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| posted on 17/2/05 at 09:57 PM |
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Brass is lovely to turn, bronze is not so easy to get a good finish on. Oilite bearings are made by sintering (to get the porosity) and as far as i
know they arent really suitable for machining.
I use brass a lot when making little steam engines, possibly one of the nicest materials to turn, easy to get a good finish etc.
David 
Sera
http://www.motosera.com
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bigandy
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| posted on 17/2/05 at 10:02 PM |
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IS brass suitable for a bushing/bearing material in your experience? I like the sound of something that is easy to get a good finish on!
Cheers
Andy
Dammit! Too many decisions....
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nick baker
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| posted on 17/2/05 at 10:12 PM |
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Like most metals, Brass will work as a bearing if well lubricated.
HOWEVER: the moment dust or sand gets into the area, it'll wear the brass away at alarming speed. (and your clutch/brake pedal will develop a
sweet crunchy feel)
The dust and sand are held captive by the oil/grease that brass needs: grinding paste.
EDIT: if the drawings that Andy has sent are good enough to work from, I'll be knocking out a set in POM tomorrow afternoon. Photos will follow
if I have time.
N. 
[Edited on 17/2/05 by nick baker]
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flak monkey
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| posted on 17/2/05 at 10:22 PM |
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If you use bronze there is not such an issue with the lubrication, it only needs minimal lubrication due to the lead in it acting as a slight
lubricant.
Brass has reasonable bearing properties, but it is soft and will wear away in a gritty environment. However with careful lubrication, i.e. sparingly
lubricated, then there shouldnt be any real problem
David
[Edited on 17/2/05 by flak monkey]
Sera
http://www.motosera.com
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bigandy
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| posted on 17/2/05 at 10:22 PM |
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That doesn't sound too good!
I've just had a thought, how about ball-raced pedals for the ultimate in smooth operation??!!!
I foresee a spot of concept design work whilst whiling away a friday afternoon at work
Cheers
Andy
Dammit! Too many decisions....
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flak monkey
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| posted on 17/2/05 at 10:28 PM |
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The only issue is that to work properly the shaft they fit onto must be precisely to size. Known as a bearing fit funnily enough....
Also remember to use sealed bearings they arent expensive. (RS are usually the best place to get small bearings from).
David
Sera
http://www.motosera.com
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bigandy
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| posted on 17/2/05 at 11:07 PM |
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An even better bearing supplier is http://www.smb-bearings.co.uk/. They supply small quantities, and have a
brilliant help when I have been replacing bearings in my RC cars and helcopters!
As for bearing fits, then I know alllll about them. In a previous employment I was working as a dimensional managment consultant. Basically checking
complex assemblies (such as automotive engine front end accesory drives) will work properly by performing complex 3D tolerance analysis studies on
them. IT sounds quite dull, but it taught me a hell of a lot when it comes to applying tolerances, and assessing limits and fits.
I just wish I knew more about the practical side of engineering. Like the machining, materials technology etc etc!
Cheers for the help folks.
PS, the drawing I sent through should give you enogh info to make some bushes from POM. I would post it up here if I could figure out how to
(it's a pdf file)
Cheers
Andy
Dammit! Too many decisions....
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flak monkey
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| posted on 17/2/05 at 11:10 PM |
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I am into RC cars and boats....will bear in mind your recommendation!
You can post the pdf file as an attachment...
David
Sera
http://www.motosera.com
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NS Dev
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| posted on 18/2/05 at 01:17 PM |
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quote:
Originally posted by nick baker
I agree with teflon... it's terrible for machining: It ususally ends up looking "furry", but despite the poor surface finish, the
tolerances you can get are rather good.
With regard to "lubrication" I've never used any: partly as it's messy, but mainly as I've never been convinced that
they need the cooling. I DO however ALWAYS use a gentle jet of air (positioned just behind the tool, and pointing down) to take the string of swarf
down and away from the workpiece and chuck.
If acrylic's shattering, it might be becuase you're using a tool that's tool blunt. I find that I can only get a good cut if I use a
brand spanking new aluminium-cutting tool.
Bummer about your finger!! I once saw a friend get his hand cut to the bone by a large amount of stainless swarf that suddenly caught on the chuck
and came round like a flail, and another lad get his finger tip cut between a chuck-key and the rails for the carriage.... not cool, and both
mistakes that could have been avoided.
My most common mistake is to cut the BACK of my hand on the tool when cleaning swarf away Daft, but I'll never learn!!
I've got a couple of scars thanks to that one as well!!! Sounds daft but very easy to do!
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nick baker
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| posted on 18/2/05 at 03:44 PM |
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Hey Andy..... Here you go fella, I just knocked them out for you. U2U me your address and I'll pop'em in the post
http://www.locostbuilders.co.uk/upload/bushings_1.jpg
http://www.locostbuilders.co.uk/upload/bushings_2.jpg
Edit: The bores and outsides are H7 and h7 tolerances respectively.
A satisfying half half hour
N
[Edited on 18/2/05 by nick baker]
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Rorty
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| posted on 20/2/05 at 12:23 AM |
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I use PTFE impregnated acetal for bedal and gear linkage pivots. Best of both worlds with moisture absorbtion less than 0.2%. Nylon will swell far
more in a damp environment, which can result in a tight bush.
Is Delrin (acetal) available in the UK? I thought it was a US brand.
Cheers, Rorty.
"Faster than a speeding Pullet".
PLEASE DON'T U2U ME IF YOU WANT A QUICK RESPONSE. TRY EMAILING ME INSTEAD!
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Hellfire
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| posted on 22/2/05 at 01:37 AM |
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quote:
Originally posted by Rorty
I use PTFE impregnated acetal for bedal and gear linkage pivots. Best of both worlds with moisture absorbtion less than 0.2%. Nylon will swell far
more in a damp environment, which can result in a tight bush.
Is Delrin (acetal) available in the UK? I thought it was a US brand.
Have to get my knob made from Nylon then
On a more serious note - we're considering putting bearings into our gear selector... we've never had a problem with the pedals... but if
we do I feel some bearing finding there way into there too....
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flak monkey
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| posted on 22/2/05 at 08:27 AM |
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As I am sure you are aware PTFE (Teflon) is actually classed as a toxic material for machining due to the gases released if it gets hot.
If you arent machining very much you should be ok, but just make sure the area is well ventilated.
I dont know all the ins and outs, but a quick search on google will tell you if you want to know.
Cheers,
David
Sera
http://www.motosera.com
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