Face or turn?

[Houndogforever]

I have a part made from HT 4140. 4-1/4" dia. After working the front, the part is flipped and all extra material needs to come off.

That is about 0.350 in Z and from 4-1/4" down to 1.18 hole thru the middle.

So, should I take multiple facing cuts down to center line, or take short OD turning steps down to size?

This is a larger part than I normally make, yeah yeah, and I wasn't real happy with chip control on facing, but maybe that was just my insert.

So how would you guys do that?

 

[sinha]

Facing cuts, theoretically

 

[Orange Vise]

Turn.

Generally a more rigid operation and reduces power consumed by the spindle motor, as you're not constantly ramping the RPM up and down to maintain surface speed.

 

[guythatbrews]

Turn for that amount of stock. Especially if you are chucking on the shallow side.

How'd you wind up with that much stock? We had a customer that supplied sawn blanks with a length to diameter ratio less than 1. They always left at least .25 stock, and that .25 was crooked as shit. What a PITA! With a good saw .1 stocknis plenty. We'd leave that much on 12" diameter. One rough facing, one finish facing pass. Sweet.

 

[MaxPrairie]

If you had a good hold on it, I would prefer facing cuts. Wide enough cut to break nice chips. Probably at least 50 thou. Nice with hole in it, you aren't going a million rpms when ending the cut.

Also prefer facing over turning OD in case that unfinished face is uneven. Insert has a uniform cut after the initial face pass rather than an interrupted start every time.

For me, I typically think which ever dimension is larger is the most efficient direction to cut.

 

[MaxPrairie]

Turn for that amount of stock. Especially if you are chucking on the shallow side.

How'd you wind up with that much stock? We had a customer that supplied sawn blanks with a length to diameter ratio less than 1. They always left at least .25 stock, and that .25 was crooked as shit. What a PITA! With a good saw .1 stocknis plenty. We'd leave that much on 12" diameter. One rough facing, one finish facing pass. Sweet.

My saw doesn't cut that straight. If I left 100 on a 12" round I would be cutting a new one. Usually order my material as saw cut blanks for that reason. Too cheap to invest in a really nice saw.

 

[sinha]

The cycle time with turning moves would be slightly more, as there would be too many turning passes, and each pass involves acceleration and deceleration.

 

[SeymourDumore]

Well, specially with a hole already through it, my vote goes for facing with one of these babies: https://www.toolmex.com/itemdetail/6-705-012R

The angle the tool is presented to the material pushes the part into the jaws, so not much worry about it flying off.
To boot, once you buy the holder the inserts to use in them are free for life.

 

[Houndogforever]

Before I had a CNC lathe, I used to purchase these. Now that I am turning them myself, I'm learning things.
The part finishes at 1.06 wide. I ordered 1-1/4" long pucks to make them since I can't material handle a bar of that diameter into my saw. I received the pucks and they were consistently sawn at 1.375-1.400.
So that is how I ended up with so much material.

 

[Nmbmxer]

I have a stubby 6mm grooving tool that rips material off when making facing cuts if you have a good grip on the part. It cuts way freer than facing with a CNMG, I typically take a corner radius less than the full width every pass.

 

[mjk]

If there is any question about the part being held securely, I will turn to within 005 and then do a face cut for a better finish.
My thought process is that turning will "push" the part into the jaws as it is cutting, the final facing cut will have very low pull out potential

 

[jbs007]

I think I would rough face about .08-.100"doc, .012" feed, 350 fpm. I'd leave about .01" to finish with a different tool and about .006" feed. That's a good size ID so I wouldn't worry about the RPM spinning up too much (at least on my machine).
Take it with a grain of salt; I've never done high production work where I have to worry about tool life or cycle time.

 

[EmGo]

The cycle time with turning moves would be more, as there would be too many turning passes, and each pass involves acceleration and deceleration.

What ? no need for any of that, in fact no need to withdraw at the end of the pass, just first pass rpm=100, z move at feedrate to end point, rapid back in z with simultaneous rpm=120, rapid in x, feed in z, rapid back with speed upgrade, etc etc etc.

I would probably do that rather than facing moves just because I am not a fan of facing, but definitely not much more time-consuming. .350" to remove in a modern lathe, you're not going to take that off in one pass either, so still have multiple passes involved.

SIx of one, half dozen of the other in practice. In production you'd certainly try to cut that blank shorter, unless you were using that portion for holding or some other reason.

 

[sinha]

Let us consider a rather extreme case of producing a step on the diameter.
Assume that the rectangular region to be machined has 3 mm width and 30 mm height.
If the maximum DOC is assumed to be 1 mm, then machining would be complete in just three facing cuts.
On the other hand, in turning, a lot many small turning passes would be needed.
Each pass is associated with acceleration and deceleration. Therefore, even though the total length of the feed motion would be nearly same in the two cases, the turning method would take some extra time.

 

[EmGo]

Let us consider a rather extreme case of producing a step on the diameter.

Let's not and say we did. The op had a question. From a little over 4" to 1" with .350 stock removal. It's up to personal preferences. I don't care much for facing but what you said about acc and dec was not correct. There's no need for that.

 

[sinha]

No start/stop is without acceleration/deceleration. It is in-built and automatic.

We might be on different pages.

 

[guythatbrews]

Before I had a CNC lathe, I used to purchase these. Now that I am turning them myself, I'm learning things.
The part finishes at 1.06 wide. I ordered 1-1/4" long pucks to make them since I can't material handle a bar of that diameter into my saw. I received the pucks and they were consistently sawn at 1.375-1.400.
So that is how I ended up with so much material.

Too much stock has also been my experience with purchased blanks. We had a customer that supplied saw cut blanks for their parts. Great, right? Not so much. 1/4 stock and so crooked they almost wouldn't clean up.

 

[EmGo]

No start/stop is without acceleration/deceleration. It is in-built and automatic.

We might be on different pages.

I'm confused ?

Turning method - rapid to part at fixed rpm. Feed in z to desired dimension. Rapid back in z. Jack up speed. Rapid down in x. Feed to stop point. Rapid back, increase speed. Rapid down in X. Repeat repeat.

There's no (relevant) acc and dec involved with feedrates. They are at whatever ipm you ask for.

Facing method - move off part in X, engage css, face down in X while spindle speed increases. Rapid back in X, should put in a short dwell to let spindle slow down because 12" chuck doesn't stop instantly. May not need this with smaller lathe. Move over in Z, repeat repeat.

There's acc and dec of spindle involved in facing method, none in turning method. There's maybe more wasted rapid motions in z with turning but there's also wasted retract motions in x with the facing method unless can do it in one pass.

Basically, it's a wash. I don't care for the woooooooo waaaaaaaa of facing, so prefer turning. But others may not care, not enough to argue over. There is no "best way", just "whatever you prefer".

 

[rjwalker1973]

No start/stop is without acceleration/deceleration. It is in-built and automatic.

We might be on different pages.

Are you referring to the acceleration from stop to Max programmed feed rate and the deceleration from max programmed feed rate to stop to perform what ever retract move you have programmed?

 

[sinha]

Are you referring to the acceleration from stop to Max programmed feed rate and the deceleration from max programmed feed rate to stop to perform what ever retract move you have programmed?

Obviously!