Finish

Morning folks, I need some advice: I am presently making the valves for my Vega Twin. The plan recommends making in one piece from Stainless steel I have some 8mm rod which is perfect, my trouble is trying to get a good finish on the stem? ( issue one), however I am contemplating making the valves from 3/32 silver steel and brazing the actual valve head made from SS, could anyone tell me if this is a good idea or helping me get a really smooth finish on the one piece valve. I am using an 8mm collet and cutting a 1/4" at a time and extending the rod through the collet thereby minimising any taper.

What are you cutting with?

I prefer to ctr drill with a very small Ctr Drill and use tailstock support so the shaft can be turned in one go. I use a DCGT insert with 0.2mm tip radius and that allows fine cuts to be taken without deflecting the work. If that does not work then polish to finish with fine emery backed by a block.

You can leave extra length on the stem if needed for clearance and to take the hole then cut off and face to length.

I've only silver soldered larger valves but doing it all at one setting from soliid metal gets everything concentric

Personally, I would machine the valves from solid. The valves I made for the Nemett engine just needed a super sharp HSS tool. It does need to be the correct grade of free cutting stainless.

The Nemett was my first engine, Malcolm Strides highly detailed articles make it ideal for a beginner. (I won't talk about scrap...but it gets less with experience )

Mine is a bit smoky but it does run well otherwise!

Edited By Steve Withnell on 11/04/2021 10:29:35

There are other very sharp inserts with a 0.1 or 0.08mm radius that are very good for taking most of the cut in 1 pass. I will often leave 3mm on diameter for the last pass and use the 1st pass to take the outer skin off the stock and use it as a gauge for the diameter it is cutting at. Making a Vee block polishing tool , works a bit like an external hone. Allows the polishing while keeping the part round. Often patience is a good thing and not too high a rpm. Keeping the speed down, will keep the heat lower and less distortion of the part.

Exhaust valves do get very hot in use - so I suggest that brazing is not at all a good idea.

Modern valves are often made of two different 'recipes' of steel, butt welded by friction. If your lathe is fairly robust, this might work - spin the stem fairly quickly, and press on it the non-rotating head. All held firmly concentric, of course. The metal in the join will get red hot very quickly (or your flat belt will come off) and leave a flange of metal to be turned or ground off at the join.

Worth a try?

Tim

Thanks Lads, Good advice.