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?

 

[Ken I]

I used to run a machine shop that produced thousands of turned shafts in EN8 - it tends to work harden so you have to take deep cuts at high feed rates using carbide tools and plenty of coolant. Requires plenty of horsepower and you can produce nice looking turned finishes.
As kd5qoq pointed out you have to allow for the give in the material - when producing one after the other on CNC's you can hold tight tolerances.
Doing a once off on a lathe requires that (at least) your last two cuts are the same depth so you can adjust your final cut for intended versus actual based on your experience with the prior cut.
I say nice looking - however the finish is quite scalloped by the feed rate and tool radius - so probably not good enough for plain metal journals.

So final cut with a very sharp HSS (honed) tool as described above - low speed, feed and depth of cut.

I'm tempted to say emery tape for final size and finish - but if that's a journal for a crank and you can't achieve both size and finish to your satisfaction you might need to get it ground.

Experiment on some material first to refine your approach and gain some insight as to how the material is going to behave.

I've found EN8 to vary considerably in terms of machinability - I suspect because of temper. You can buy it pre-toughened and that makes machining it even more difficult.

You might consider annealing the material but for a crank you might prefer machining it in the "tough as old boots" condition rather than post machining heat treatment.

Even this carry's distortion risks, the part can deform due to machining away asymmetrical portions of differentially stressed material as would be the case for a crank so you might rather start with it annealed and deal with the heat treatment later.

Examples I once cut a 50mmx50mm cold rolled steel bar down to 50mmx49mm - thus removing the compressively stressed layer from one side - it bent like a bananna. Similarly, I machined a keyway down the length of an EN8 bar and it too bent like a banana.

Good luck with your crank.

Regards, Ken I

 

[djc]

I'm tempted to say emery tape for final size and finish - but if that's a journal for a crank and you can't achieve both size and finish to your satisfaction you might need to get it ground.

A slightly more refined version of that, which is achievable in a home shop (takes time and patience, but not money), would be to make a circular lap. That will produce a fine finish and a good diameter. There is an old YT video by Suburban Tool that might spark some ideas.

Edit: 21 century version:

https://ethan.id/projects/2021/laps/

Stahli article refered to above here:

https://stahliusa.com/wp-content/uploads/2016/06/The_Technique_of_Lapping.pdf

 

[kd5qoq]

As kd5qoq pointed out you have to allow for the give in the material

Whew!
Thank you Ken!
I was afraid I was gonna be run outta town on a rail for my two cents.
As I said "I'm a scratch machinist" and know less than 1% of what the fellers on here know.
Most of what I can add here is due to mistakes I've made.
Not so much proper technique.

 

[indian]

May I suggest EN8M

 

[neil_1821]

I've got a new profile to grind on the HSS to see if that works, I've had a chap who has a mass of knowledge come over to have a look and the one thing that he mentioned, which I have thought about is, is it definitely EN8 that I've been sold. I saw him turn a piece of EN8 in his workshop and it machined a lot better than what I have.

A member of our model engineering society has some EN8 he thinks is the right size he'll let me have to try so hopefully I can get some better results. Thank you to all the comments and suggestions, I'm sure one way or another I will produce a good finish for the crankshaft.

 

[Peter Twissell]

Why are you sticking with EN8?
I make crankshafts from EN16T or EN24T. EN16T gives a nicer finish, but EN24T is higher strength.