Parting off in a Lathe

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KellyJones
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Re: Parting off in a Lathe

Post by KellyJones » Sat Nov 28, 2020 5:40 pm

Some very good comments here. Thanks.

Harold's comment about keeping the cut as close to the chuck as possible rings particularly true. Of course, it makes sense that the farther the cut is from the chuck, the more tool induced deflection for the same tool load, and of course, the greater chance for the tool to bind.

The spindle speed comments make sense, but the comments about increasing the feed surprised me. I had not considered that is was possible to feed too slow. One day when I'm feeling particularly brave, I will have to try it.

thanks.
Kelly Jones, PE
A life spent making mistakes is not only more honorable, but more useful than a life spent doing nothing.
George Bernard Shaw
(1856-1950)

Harold_V
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Re: Parting off in a Lathe

Post by Harold_V » Sun Nov 29, 2020 5:40 am

KellyJones wrote:
Sat Nov 28, 2020 5:40 pm
The spindle speed comments make sense, but the comments about increasing the feed surprised me. I had not considered that is was possible to feed too slow. One day when I'm feeling particularly brave, I will have to try it.
A light feed is often the source of chatter. An increase in feed rate has the potential to eliminate the problem, but a great deal depends on the machine, and why you're experiencing chatter. Changing spindle speed can often eliminate chatter, too. Slowing or speeding the spindle can serve to eliminate a resonant frequency. Break that pattern and chatter will usually stop.

Today I was faced with making a .040" thick spacer for the electromagnetic clutch I'm rebuilding for my Graziano. I chose to make the spacer, which is about 2-¼" diameter, from 17-4 PH stainless. I made that decision because 17-4 has many of the properties of chrome moly and is often used in its place in the aero-space industry, where corrosion resistance is desired. This material can be difficult to machine, although it does machine quite nicely when one keeps the tools sharp. My reason for the choice of material was because of its stability (little to no warpage) and ease of heat treatment, which, in my case, was to simply soak the spacer for a couple hours @ 900°F. There is no quench, as the material is precipitation hardening (thus the PH in it's identification). The material does not scale, due in part to the low temperature range to which it is subjected. It forms a blue/maroon/purple surface oxide and experiences only about .0006"/inch of size shrinkage in heat treat.

In order to part the spacer, I ground a new parting tool from Stellite and parted the spacer with power feed and slow spindle speed. It went without a hitch. No chatter, and a straight parting cut. One holds thickness by measuring the width of the parting tool with a mic, then picking up the face. Add the thickness desired to the width of the parting tool and step over, using either a DRO, or as I did, a dial indicator, to set the width desired.

H
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John Hasler
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Re: Parting off in a Lathe

Post by John Hasler » Sun Nov 29, 2020 8:25 am

I assume that you know that 17-4, being martensitc, is magnetic.

shootnride
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Re: Parting off in a Lathe

Post by shootnride » Sun Nov 29, 2020 9:01 am

Harold_V wrote:
Sun Nov 29, 2020 5:40 am
In order to part the spacer, I ground a new parting tool from Stellite and parted the spacer with power feed and slow spindle speed. It went without a hitch. No chatter, and a straight parting cut.
H
Harold,
I've had my lathe for about 10 years now and have yet to part with power feed because I'm chicken. I generally don't have issues with parting, but I always run at very slow spindle speed and use flood coolant (actually Mobil 766 cutting oil). I use HSS parting tools, most often .125 thick. I would like to try the power feed but am unsure of how to select the appropriate spindle RPM and cross slide feed rate. Can you provide some words of wisdom about this ?
Thanks
Ted
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BigDumbDinosaur
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Re: Parting off in a Lathe

Post by BigDumbDinosaur » Sun Nov 29, 2020 4:32 pm

Harold_V wrote:
Sun Nov 29, 2020 5:40 am
...Add the thickness desired to the width of the parting tool and step over, using either a DRO, or as I did, a dial indicator, to set the width desired.
Just so you know, Harold, the modern term for a dial indicator is ... wait for it ...

ARO (analog readout). :D

Us old folks need to keep up with the terminology being used by the kids. I'm here to help in any way I can. :P
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BigDumbDinosaur
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Re: Parting off in a Lathe

Post by BigDumbDinosaur » Sun Nov 29, 2020 4:45 pm

shootnride wrote:
Sun Nov 29, 2020 9:01 am
I've had my lathe for about 10 years now and have yet to part with power feed because I'm chicken...I always run at very slow spindle speed and use flood coolant...am unsure of how to select the appropriate spindle RPM and cross slide feed rate.
Much of this has been mentioned in early posts.

In most cases, you can run the same spindle speeds and feeds you would use for facing. A low spindle speed is not doing anything for you and in fact, may increase the likelihood of problems. Under-feeding may produce chatter. Using plenty of cutting oil is wise and will help to improve the finish and tool life.

What is important is your setup. Reiterating what has already been said, get your cutoff tool as close to the headstock as possible to minimize deflection. Be sure the tool is exactly perpendicular to the spindle and the cutting edge is aligned to the spindle centerline. Only expose enough tool to complete the cut. Make sure the tool is properly sharpened. All of this is standard lathe setup stuff.

Lastly, know your machine's capabilities. Try making some test cuts on scrap using various feed rates and find a comfortable rate for your lathe. You may be surprised at what you discover. I have a small note book in which I have jotted down feed/speed combinations for some things I have done in my lathe. That helps to keep me from re-inventing the wheel each time I make something in the lathe.
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Harold_V
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Re: Parting off in a Lathe

Post by Harold_V » Sun Nov 29, 2020 7:14 pm

John Hasler wrote:
Sun Nov 29, 2020 8:25 am
I assume that you know that 17-4, being martensitc, is magnetic.
Yes, I am well aware. Having come from the aero-space industry, I am well acquainted with the alloy.

H
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Harold_V
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Re: Parting off in a Lathe

Post by Harold_V » Sun Nov 29, 2020 7:53 pm

BigDumbDinosaur wrote:
Sun Nov 29, 2020 4:32 pm
Just so you know, Harold, the modern term for a dial indicator is ... wait for it ...

ARO (analog readout). :D

Us old folks need to keep up with the terminology being used by the kids. I'm here to help in any way I can. :P
Chuckle!
Well, I'm afraid I'll have to stick with the words *dial indicator*.
I don't tolerate well the clever use of words these days. One of them is the word **swarf**, which is commonly used in lieu of the proper word, chips. When was the last time you read literature about CNC machinery in which they offered a **swarf conveyer**?

Swarf is a valid word, and it closely relates to chips, but they are not one and the same. It is generally accepted that swarf is the fine byproduct of grinding, as an example. One does not generate swarf on a lathe or mill (although there may be some along with the chips).

Another buzz word that I find offensive is tram. One does not tram a mill head. It is "dialed in" using a dial indicator. The word tram has been stolen from the operation used in setting up timing on a steam engine, whereby a trammel is used. I do not recall EVER using a trammel to set the attitude of the head of my mill, but I regularly use a dial indicator.

Another term that I find misleading is the use of the word foundry. All too many say that they want to build a foundry. What they're saying is they want to build a FURNACE, not a foundry. A furnace is used to melt metals, while a foundry is a place where metals are cast. In a sense, if one builds a FURNACE, they will, in effect, have built a foundry as well, but the term is confusing. Say what you mean, not what you think you mean. A furnace is NOT a foundry.

I'm afraid I'm going to have to continue my practice of calling an indicator an indicator. As an old dog, I don't accept change well.

Thanks for the comment, though! :wink:

H
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Harold_V
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Re: Parting off in a Lathe

Post by Harold_V » Sun Nov 29, 2020 8:12 pm

shootnride wrote:
Sun Nov 29, 2020 9:01 am
Can you provide some words of wisdom about this ?
Ted,
For starters, what BDD has posted is pretty much spot on, although I do not agree with his comment about parting speeds. Here's why.
If you have been machining with carbide, you most likely are running way too fast to part, even with a carbide parting tool. There may be exceptions, and I've seen some of them, but, by and large, in almost all cases you will part slower than you machine. One of the exceptions is if you are machining too slowly, at which time the speed may be good for parting.

Assuming you're using HSS (for turning), you still most likely would benefit by slowing the spindle for a parting cut. There's good reason for that. As more and more surface area of a cutting tool makes contact with the work, it's easier for chatter to develop. Couple that with the fact that a parting tool is generally far less rigid than a turning tool, it's obvious that chatter most likely will be an issue. Slowing a spindle is one of the best ways to control chatter, and that applies to any operation, but it's especially true when any type of form tool is employed (due to the increase in tool contact area).

The material one parts plays a huge role in how much speed and feed can be tolerated. You most likely will discover that a slow spindle speed accompanied by a relatively fine feed works best for most of the stainless alloys--the exceptions being free machining grades. 302, 303 and 416 all machine very nicely and allow for greater speeds and feeds.

In aluminum, you'll find that 2024 machines extremely well, although it does not cut with a shine. It would be hard to over speed or feed a cut in that alloy, especially in the T3-T4 condition.

Want to start power feed parting?
Consider a starting point of about .003" feed, with a somewhat reduced spindle speed. If you get chatter, slow the spindle, or increase the feed rate. Experiment until you have a feel for what your machine and setup will tolerate. There's no hard rules, as each machine presents its own problems, each of which will generally require a slightly different change to achieve an acceptable cut. Just know that you can power part better than you can manually part. You just have to gain the confidence to know that that's true.

Flood lubrication for parting is a definite positive thing. I generally just brush apply my lubricant. Power feed makes that possible, as it frees both my hands.

Sorry I'm not more help.

H
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BigDumbDinosaur
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Re: Parting off in a Lathe

Post by BigDumbDinosaur » Sun Nov 29, 2020 10:58 pm

Harold_V wrote:
Sun Nov 29, 2020 8:12 pm
shootnride wrote:
Sun Nov 29, 2020 9:01 am
Can you provide some words of wisdom about this ?
Ted,
For starters, what BDD has posted is pretty much spot on, although I do not agree with his comment about parting speeds.
I should have clarified that I was assuming both the turning and cutoff tools are HSS. I generally run at the same speed for either operation without issue.
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Re: Parting off in a Lathe

Post by Harold_V » Sun Nov 29, 2020 11:53 pm

In that case I can't help but wonder if, maybe, you tend to machine somewhat slower than can be successful. If none of your chips come off discolored, and you understand what it means to rough machine, that's likely true. That, or your lathe is particularly robust. The broad nose of a parting tool is generally large enough in area to cause chatter, but, again, that depends on a lot of variables. If your machine is robust enough, that may not be the case.

Machine size is ultra critical when it comes to ability. I have memories of the time I spent on a 48" sliding gap bed lathe, where a facing cut of 7/16" in depth was taken on a chrome moly forging that was about ten inches long. Not once, but in a production run of hundreds (they were the axle housings for the launcher). You likely understand that that would tax the most rigid of 12" machines! :wink:

H
Wise people talk because they have something to say. Fools talk because they have to say something.

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NP317
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Re: Parting off in a Lathe

Post by NP317 » Mon Nov 30, 2020 1:27 am

My all time favorite cut off tool is a Hardinge one, mounted directly above the collet, and rigid as heck.
I once worked in a shop with 4 of those lathes Delicious.
'Wish I could afford a Hardinge lathe for my shop...
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