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Plastics Expert

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Plastics Expert

Postby Cncpaul » 13 Jan 2021, 15:35

l’m looking for a some advice on a plastic that would be suitable replacement

My laser rise and fall table developed a worn threaded bearing, these are 50mm dia x 20 mm thick

They are available at £85 each plus shipping and vat, when it arrives in Ireland it attracts another 23% vat
making a grand total of 152 euro’s :shock:

I can make them on my cnc router, so l plan to machine 4 replacements and some spares, my problem is the best choice of material either Nylon, Nylon 66, and Acetal

458814F1-9CB4-4813-B5A0-3AE82728A8B2.jpeg
(219.56 KiB)



Any advice would be greatly appreciated
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Re: Plastics Expert

Postby sunnybob » 13 Jan 2021, 19:37

You should be able to buy tax free for export with the appropriate invoice from the supplier, only pay VAT when it arrives in your country. Will that make it worth buying rather than making?
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Re: Plastics Expert

Postby novocaine » 13 Jan 2021, 20:15

Nylon.
Carbon fibre is just corduroy for cars.
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Re: Plastics Expert

Postby droogs » 13 Jan 2021, 21:45

If you can get it then Delrin or Nylon 6
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Re: Plastics Expert

Postby Trevanion » 13 Jan 2021, 21:58

Sweet Jesus, an expensive piece of plastic!

I'd personally be making them in Phosphor Bronze if it was something that needed to last a long time without wearing other components, is it literally just a plain piece with a threaded bore and four other holes or is there something more to it than meets the eye?

Alternatively, Acetal/Delrin would work well.
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Re: Plastics Expert

Postby Cncpaul » 13 Jan 2021, 22:49

sunnybob wrote:You should be able to buy tax free for export with the appropriate invoice from the supplier, only pay VAT when it arrives in your country. Will that make it worth buying rather than making?



Apparently not, at the moment things are up in the air here, if you buy a car in the uk and bring it to Ireland
you will pay 21% tax even on the vat you paid in the uk, you have 30 days to register and pay the tax if not they will impound the car if you are stopped by the garda.
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Re: Plastics Expert

Postby Cncpaul » 13 Jan 2021, 22:53

Many thanks to to Dave and Alan for the suggestion on material, l will see what i can find locally.
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Re: Plastics Expert

Postby Cncpaul » 13 Jan 2021, 23:06

Trevanion wrote:Sweet Jesus, an expensive piece of plastic!

I'd personally be making them in Phosphor Bronze if it was something that needed to last a long time without wearing other components, is it literally just a plain piece with a threaded bore and four other holes or is there something more to it than meets the eye?

Alternatively, Acetal/Delrin would work well.


Yes, Phosphor Bronze would be the best choice but l would have to find an engineer to make those, the bearing do not sustain continual use, the rise and fall is only used when you change material thickness to focus the laser. l can machine Nylon/Acetal/Delrin, they only a 50mm dia x 20mm disc with 4 x 3mm holes and a M20 thread in the center, so nothing exotic
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Re: Plastics Expert

Postby novocaine » 13 Jan 2021, 23:09

Nylon is self lubricating where as delrin (which isnt the same acetal) isnt, what is is though is highly abrasion resistant. For cost id go with nylon. For ability to machine it id go with delrin.


Tough call,if its something you dont mind swapping out and have the machines to work it, then it doeant really matter.

Looks like an aluminium head so phosphur bronze isnt suitable really,coef of friction being to similar and wear on both parts being more likely. Id rather have a plastic bearing to replace than a whole shaft.
Carbon fibre is just corduroy for cars.
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Re: Plastics Expert

Postby Cncpaul » 13 Jan 2021, 23:17

[quote="novocaine"]Nylon is self lubricating where as delrin (which isnt the same acetal) isnt, what is is though is highly abrasion resistant. For cost id go with nylon. For ability to machine it id go with delrin.


Tough call,if its something you dont mind swapping out and have the machines to work it, then it doeant really matter.

Looks like an aluminium head so phosphur bronze isnt suitable really,coef of friction being to similar and wear on both parts being more likely. Id rather have a plastic bearing to replace than a whole shaft.

The aluminium housing each end of the shaft has ball bearing, the nylon nuts just support the table

EEF1F6DA-9382-4020-A356-601D6FC21285.jpeg
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many thanks
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Re: Plastics Expert

Postby novocaine » 13 Jan 2021, 23:25

Ohhhhhh its not a bearing as such, its a nut, I see now.

Bronze doesnt like threads.

Aluminium would be my choice metal but the reason they are plastic is to reduce backlash by deforming slightly on direction change and to reduce noise an vibration from thread movement. Delrin will be to hard, they actually look like hdpe from the machine marks but nylon will be perfect.
Carbon fibre is just corduroy for cars.
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Re: Plastics Expert

Postby Woodster » 13 Jan 2021, 23:44

novocaine wrote:
Bronze doesnt like threads.


Very common on machine tools.

https://www.abssac.co.uk/c/Bronze+Nuts+POWERSCREW/16/
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Re: Plastics Expert

Postby Trevanion » 14 Jan 2021, 00:25

Cncpaul wrote:Yes, Phosphor Bronze would be the best choice but l would have to find an engineer to make those, the bearing do not sustain continual use, the rise and fall is only used when you change material thickness to focus the laser. l can machine Nylon/Acetal/Delrin, they only a 50mm dia x 20mm disc with 4 x 3mm holes and a M20 thread in the center, so nothing exotic


There are a few people on this forum that could help you out with that if you want to explore that path :)

novocaine wrote:Bronze doesnt like threads.


Really? I've always thought Bronze would be the go-to material for this kind of thing where the screw might be in continual use as the female threads on my lathe are bronze as far as I know but an acme thread form rather than a regular coarse metric which might make a difference.
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Re: Plastics Expert

Postby Lons » 14 Jan 2021, 01:40

I'd use nylon in any of the grades Paul or even ptfe if you can find a bit. Depends on your method of machining whether block or rod, from memory we used to sell 50mm round bar, you may be able to persuade one of the semi finished plastics suppliers to send you a free sample thick enough. £85 seems extremely expensive but I guess it's down to labour and profit.

Just a thought another material could be SRBP (Tufnol), most grades are suitable for bearings though I don't know for this application, possibly a bit of lignum vitai is another option?

If you get stuck I could possibly post you a bit enough for 1 bearing from my stash from when I was in the plastics industry but I don't know the cost of postage.

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Re: Plastics Expert

Postby novocaine » 14 Jan 2021, 09:13

Sorry let me clarify

Phospher Bronze is fine with larger thread forms like acme, large pitch etc. And I'm confusing it with oilite because im an idiot. :D

The other reasons mentioned still stand.
Carbon fibre is just corduroy for cars.
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Re: Plastics Expert

Postby Lons » 14 Jan 2021, 11:53

I forgot to say though you likely already know, search for plastics stockholders, engineering or semi finished plastics suppliers, there will be some in Ireland.
If you want to try for samples I would phone rather than email as you'll get a salesperson looking for future business and if your marals allow just say you're experimenting making up bearings, looking to compare materials and could they send you samples of what they recommend, in the size you need to make up prototypes.it will work most of the time foc or at worst a minimal cost.
It's only a teeny lie and I've heard the Irish can tell a good story. :eusa-dance: ;)
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Re: Plastics Expert

Postby Woodster » 14 Jan 2021, 12:14

One of the plastics that I like to machine is Ertalyte TX. I bought some 25mm rod for a particular job about 20 years ago and still have a tiny bit left. It’s nice to machine, dimensionally stable and has a very low coefficient of friction due to included PTFE.
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