Depth of Cut - Southbend 9"

[Phil3]

I have a Southbend 9", with a square piece of 3" x 3" x about 4.5" long piece of aluminum in a 4-jaw. I need to turn this down to a 3" diameter round bar. How much can I take off per pass (would be 2X this for total diameter reduction). I am new, so not sure how far I can push the lathe per pass. It is equipped with a QCTP.

Also, how can I go about parting this? The aluminum will have about a .375" diameter hole down the middle, but that still means the parting tool has to reach in at least 1.313", pretty far for the QCTP parting tool I have. Or at least I think that is pretty far.

- Phil

[Harold_V]

Depth of cut will be controlled by several factors, so a reliable answer may not be forthcoming. You will be limited by the power of the machine, and its rigidity. Speed, too, makes a difference. One can often take a deeper cut at a reduced speed, as well as a deeper cut with reduced feed. Many factors must be addressed, although you can accomplish the task without addressing any of them by simply taking light passes at a comfortable speed and feed rate.

Aluminum machines quite nicely, especially with positive rake and a chip breaker. Pick a speed at which you are comfortable, then try taking .100" per side, per pass. Juggle feed rate if you find the machine runs without effort, or if it struggles. If it struggles, even when reducing feed rate, you obviously should take a more shallow cut.

Don't know if you are up to speed where roughing and finishing is concerned, but I'd recommend that you rough the turned portion completely before entertaining any ideas of drilling the hole. That way it will be where you want it when you're finished. Best to not drill after finishing the OD, too, which helps minimize scratches on the part.

Ensure that you have firmly chucked the material, using a four jaw. On a rigid machine, you most likely could take as much as .400" per side, per pass. That most likely isn't possible on your machine, but it should help you determine what is.

Parting? It's hard for me to see the problems you face, but if all else fails, part as deeply as you can, then finish the cut with a saw (band saw or hack saw). Use kerosene or WD-40 as you part, keeping the groove wet. You can adequately apply the fluid with an acid brush.

Hope some of this helps.

Harold

[Phil3]

Aluminum machines quite nicely, especially with positive rake and a chip breaker. Pick a speed at which you are comfortable, then try taking .100" per side, per pass. Juggle feed rate if you find the machine runs without effort, or if it struggles. If it struggles, even when reducing feed rate, you obviously should take a more shallow cut.

Wow..."100 per side. I had been doing ".025 and it seems fine with this. I just did not know what else it could do or what it would do in protest if too much. I'll creep up on it and see what it can do, but would be nice to know what a struggling machine does so I can recognize it.

Don't know if you are up to speed where roughing and finishing is concerned, but I'd recommend that you rough the turned portion completely before entertaining any ideas of drilling the hole. That way it will be where you want it when you're finished. Best to not drill after finishing the OD, too, which helps minimize scratches on the part.

Not sure I understand this. It may help to explain that I am making four flanges to go on my 8" Jet bench grinder. The existing stamped steel ones are about as flat as a potatoe chip. My plan is to face the aluminum block, turn until round and to a finish diameter, drill a hole deep enough so that I can part off four flanges. Machine a recess into the face of the aluminum, and part off. Repeat for flanges 2 - 4. The aluminum block will never leave the chuck until all parts are made. If there is a better way, I would love to hear it. I am a newbie, so appreciate any wisdom.

Ensure that you have firmly chucked the material, using a four jaw. On a rigid machine, you most likely could take as much as .400" per side, per pass. That most likely isn't possible on your machine, but it should help you determine what is.

Thanks. What warning signs should I look for that I am exceeding machine limits?

Parting? It's hard for me to see the problems you face, but if all else fails, part as deeply as you can, then finish the cut with a saw (band saw or hack saw). Use kerosene or WD-40 as you part, keeping the groove wet. You can adequately apply the fluid with an acid brush.

Since the flanges are just .375" thick, cutting off with a hacksaw means a very rough side that I have no good way to apply a finish. I guess I will need something like a pot chuck to hold it and finish it.

- Phil

[Harold_V]

Wow..."100 per side. I had been doing ".025 and it seems fine with this. I just did not know what else it could do or what it would do in protest if too much. I'll creep up on it and see what it can do, but would be nice to know what a struggling machine does so I can recognize it.

The greatest possibility is that the motor can't pull the load. It's not uncommon for the tool to break if the spindle stops, so if you see it struggling, pull the tool out before the spindle stalls.

It may help to explain that I am making four flanges to go on my 8" Jet bench grinder. The existing stamped steel ones are about as flat as a potatoe chip. My plan is to face the aluminum block, turn until round and to a finish diameter, drill a hole deep enough so that I can part off four flanges. Machine a recess into the face of the aluminum, and part off. Repeat for flanges 2 - 4. The aluminum block will never leave the chuck until all parts are made. If there is a better way, I would love to hear it. I am a newbie, so appreciate any wisdom.

Yep, sure does help to know what you're trying to do.
The point I was trying to make is that it's good shop practice to rough all dimensions before finishing any dimensions. In this case, I'd turn the stock until it was round, stopping shy of the target diameter about .03". I would then drill the hole to full depth, which would allow me to make the four flanges in question. The hole would not be drilled to size (yet another roughing operation), but would be drilled undersized by at least .03", so you'd have enough material present to bore each flange before parting. That guarantees the flange OD and bore are concentric. Use a telescoping gauge to measure the bore diameter, and make them an easy fit on the spindle, but not loose. I'd suggest something like .002" clearance.
Do the finish turning and boring on each flange as you progress----don't do them all at once. That way if the material shifts in the chuck, you're still getting good parts. Takes a little longer, but assures the proper outcome.

What warning signs should I look for that I am exceeding machine limits?

A lot depends on the machine and the setup. If you have the material firmly chucked, resting against the chuck face, and the jaws are tightened adequately, nothing to worry about as far as the material goes---it won't come out of the chuck. That's one of the real hazards of taking serious cuts, but you work accordingly. It's rare for pieces to come out, but it does happen. I've seen a piece that weighed a couple hundred pounds come out of a fixture and roll over a guy's head. He was lucky---got a bad bruise and nothing more. Crushed a Kennedy toolbox behind him, though.
Generally, especially with small machines, all that happens is the spindle stalls. Breaks the tool (the tip), although with HSS you may escape that problem, especially in aluminum. Beyond that, and the material coming out of the chuck, not much can happen. If you have a quick change, start a deep cut with moderate feed, then increase feed as you run. You'll know when you get in trouble by the way the machine responds. You won't (can't, really) learn this by reading---it's something you must learn by experience. Unless you have a miserable lathe (like a 109 Craftsman), there's not really much you can do to harm the machine----just make sure the chuck is tightened properly, so the part can't shift under cutting pressure. You're likely to be pleasantly surprised in how large the cut can be in aluminum---it's very forgiving.

Since the flanges are just .375" thick, cutting off with a hacksaw means a very rough side that I have no good way to apply a finish. I guess I will need something like a pot chuck to hold it and finish it.

This is a perfect example of why a guy needs soft jaws.

What may work for you is to turn a piece of material (it can be wood if you choose) that is slightly smaller in diameter than the flanges. Place it in your chuck, then seat the flange on the spacer. That will square the flange, even though it may have radial run-out. No big deal---you're just going to face the flange. You can debur with a three cornered scraper, so the bore need not run true.

If you choose wood, make sure it cuts cleanly, so you don't have whiskers preventing the flanges from seating perfectly well.

You can do it.

Harold

[zetec7]

...as for parting it off, I did the same thing on a 3" diameter aluminum solid stock. I won't do that again. It's just too deep a cut, and the blade flexes way too much, so you end up with a dish instead of a flat surface at the cut.

The next time, I cut the material off on a bandsaw, then re-chucked it in the lathe & faced it true. Worked perfectly (did it several times), and was waaaay faster than trying to part it in the lathe.

[Harold_V]

While I agree that a deep cut can lead to a dished parting line, you can often address the condition by paying strict attention to the cutting edge. It should be uniformly sharp and parallel to the spindle centerline. The amount of relief on the sides can play a role, too. Often, the side with minimal relief will push the cut away. Doesn't hurt to do a little experimenting, so you'll have a firm understanding for future use.

Harold