Machining plastic and brass

[Birley]

Hi
I hope you can help me I rebuild old magnetos and have to make a lot of spares myself as they are out of production, I have a problem with the HT slip rings I produce which consist of a plastic moulding with a brass segment set into it.
My metal supplier has sold me a very much softer brass than I usually use, this has only proved a problem on the last machining process. The plastic I use contains fibreglass to prevent warpage this wears a high speed tool very quickly so I have been using a carbide tool unfortunately because the brass is so soft the tool is tearing the surface of the plastic when it comes off the brass, do you know of a type of tool or another machining method that would help me cure this problem? The plastic I am using is crastin made by dupont.
Thank you in advance for any help you can give me.
Best regards
Mick Hall (Birley).

 

[MikeHalloran]

Do you later relieve the plastic between the segments?

How about doing that first?



Mike Halloran
Pembroke Pines, FL, USA

 

[Birley]

No the item is finished when turned.
The slip ring with the brass segment rotates with the HT coil on the armature, a carbon brush runs on the turned surface and distributes the spark, the plastic acts as insulation.
Mick.

 

[patprimmer]

Plastics are best machined with a VERY SHARP tool with minimal rake and good back clearance. Heat is your enemy, so go slow and let it cool if it even gets warm.

I don't know for brass.

Plastic turns easier if you can suport it in a sleeve or over a mandrel to hold it in place against the tool.

Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[btrueblood]

You say the tool is tearing the plastic as it comes off the brass;

...do you mean as the tool traverses the part, and reaches the join between plastic and brass, tearing occurs near this junction? Then, the problem might be chips from the brass fouling the plastic; try slowing the traverse speed as you approach the interface and allow the chips to clear (use a compressed air gun to clear chips if needed).


...or, do you mean you are machining the plastic, and it is tearing away from the brass? This issue is different, it sounds like the plastic and brass are not well bonded, and/or the glass reinforcement is so stiff that it pulls away chunks of the plastic as the material gets thin enough during cutting. Try making an interface layer with an un-reinforced resin (like a thick glue line)?

 

[SincoTC]

Many years ago when I worked in an auto-shop, I successfuly cleaned up some badly worn commutators and slip rings on some crane motor armatures by using a Crankshaft grinding machine. After a little experimenting with dressing speeds and coolant ratios it gave a very good finish on both the copper and insulator. If you have a lot to do then you would be best advised to get a special wheel to suit the material combination. It certainly avoided the problems we had (like you) when we tried to turn them.

If you haven't got cylindrical grinding facilities and need to stick to turning, then why dont you plunge cut the crastin insulators with a tool that can stand the abrasion to a diameter that is just below the finish size of the brass then you can finish the slip ring diameter with a tool that is optimum for the grade of brass used and even lightly chamfer the edges to remove any rags. It is quite common to undercut the insulation as suggested by MikeHalloran and is a whole lot easier with a few slip rings than with many commutator bars running parallel to the shaft axis!

Trevor Clarke. (R & D) Scientific Instruments.Somerset. UK

SW2007x64 SP3.0 Pentium P4 3.6Ghz, 4Gb Ram ATI FireGL V7100 Driver: 8.323.0.0
SW2007x32 SP4.0 Pentium P4 3.6Ghz, 2Gb Ram NVIDIA Quadro FX 500 Driver: 6.14.10.7756

 

[Birley]

Hello
Thank you for your replies. The slip ring on a multi cylinder magneto has the brass segment set into the plastic when it is moulded the segment is stepped to hold it fast. This type of slip ring differs from an alternator slip ring because 3/4 of the surface diameter the brush runs on is plastic I am sure it is a problem with brass damaging the plastic what I need to know is what type of tool will take the brass off cleanly but not become blunted by the glass.
Mick.

 

[MikeHalloran]

Look for carbide tools made from superfine powder.



Mike Halloran
Pembroke Pines, FL, USA

 

[patprimmer]

I think that removing the swarf and cooling are major issues.

Crastin PBT and is quite hard and the fibres are very short.

If you use a carbon fibre filled grade it will machine a lot easier but will have reduced insulation and tracking properties and will be a lot more expensive.

You might get away with non fibre reinforced material if you mould them in Stanyl nylon 4.6 or Ultem PEI or polysulphone or PEK

They are all expensive, but then again, it is expensive to have a slow difficult high reject rate secondary process.

Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.