Turning EN8

[neil_1821]

I'm trying to turn some EN8 for the crankshaft of the little Demon V8 and I'm having a hard time getting a good finish. I had initially tried with a carbide tip but after some research people recommended using HSS as it's supposed to give a better finish. I ground a fresh bar with roughly 15-20 rake on top and a small radius and although it was an improvement over the carbide tip I'm still not happy with the finish I'm getting. I don't know if the tip needs more refining or if the speed and feed isn't right.

The bar is roughly 1" diameter and running at 420rpm with a feed rate of 0.002", which is the slowest feed rate I can get on the Super 7 without buying another gear to reduce it even further. Wondering if you can offer any tips/advice on how to get a better finish, because I'm worried if the journals aren't great they'll most likely pick up on the bearings.

 

[Charles Lamont]

Those parameters seem OK, though the rake is perhaps a bit large. What coolant are you using, and how applied?

 

[neil_1821]

I'm using neat cutting fluid applied with a brush as it's really the only method I can use.

 

[kd5qoq]

How heavy is the cut you are trying to make?

 

[Charles Lamont]

I use a neat oil, Morris's Cora B, but I find it works best for me diluted 50/50 with paraffin. I realise it a long bar so each pass takes a while, but try going slower, if necessary, very very slow for the final cut.

 

[neil_1821]

The cut's I'm taking are 0.005", I have tried taking less and more but there seems to be no difference in the finish. I have tried different speeds and feed rates but it always seems to give the same finish.

I can try diluting the neat oil with paraffin, but I'd be surprised if that was the cause. I've also tried grinding a new HSS tip with less rake but it doesn't make any noticeable difference.

 

[kd5qoq]

I doubt the coolant will help much.
Like a "scratch golfer" I'm a scratch machinist.
None of what I say is gospel.
I was once trying to turn some 4140 and it would tear just like what you are seeing.
One of the machinist fellas at work told me that there are times when a finish cut has to be sacrificed.
That is, sometimes a heavy test cut say .020" to see how much 'push' there is.
So if an indicated 020" cut result is 019" a final compensated heavy cut of .021" to required dimension is the only way to get an acceptable finish.
I know what I said is probably poor practice but at the time it was the only thing that worked for me.
Side note, you don't have much cutter overhang, do you?
Also have you tried a much faster speed say 800 RPM?
That would be about 2400 IPM cutter speed.

 

[Charles Lamont]

Is the tool grinder finish or have you honed it? Could we see a pic of the tool?