Parting off question

[darkzero]

Below center

As mentioned already, dead center. Just wanted to pint out that it should never be below, that's asking for a disaster to happen as it's more prone to rolling under the workpiece & something bad will happen. Safer to be above than below.

 

[GK1918]

As for dressing the blade a couple of swipes on the grinder using its natural wheel curve, I use back
gears but I kick up the main belt a notch and feed like you want to break it (with oil)- course I found out
many years ago I was trying to baby it. I havent broken a blade YET. Parting blades do well in a
shaper too, for some operations. Another thing worth notice is, when setting up the blade it looks
crooked in its holder so I put it against the chuck then tighten, and the steel ruler to center it and I
have good luck that way. Since day one, I've never power parted not a good idea no time to run.

 

[Dunc1]

Should have included this in my earlier post but it slipped my mind.
As an alternate to the rear parting off tool you can achieve the functional equivalent by mounting the tool upside down in a conventional tool holder ( cutting point at centre height) and rotating the spindle in reverse. Not recommended tho for work held in a threaded chuck.

 

[Tony Wells]

A lot of us run a partoff blade, and other tools slightly below center. I know many of you don't, and that's fine. But to say never do it isn't quite right either. When I say below center, of course I'm not talking about 1/8" of anything. But when you get right down to it, center height gives you fine control of the rake angle. Granted, most tools are ground to specific rakes and are presented to the work at centerline. But if you need or want a little more negative rake, just lower it a bit. In parting, this isn't going to explode or anything. In fact, I have noticed a much reduced cutting pressure is required when you're hand feeding. The error of off center is far more bearable low than high. If the tool is high, you will rub on the heel of the tool and it will take much more pressure to get it to cut, and this tends to cause the blade to deflect easily. The only risk to running low, and remember, I'm only talking about a few thousandths, is when you get to the middle if solids, the work may, and I say may, try to climb up on top, but not much. And there may be a small nub in the center you can't cut off with the parting tool. Of course, in the case of parting a tubular piece, that's not a problem.

I'm not trying to start any arguments, but only stating my own preference and experience.

 

[revrnd]

I asked my brother (who's been around machine tools since '77, full time since '84) about parting from the rear. He had no answer. He said he's never had any issues parting off from the "front". Can someone explain why you would want to part from the rear rather than the front?

 

[Bill C.]

I asked my brother (who's been around machine tools since '77, full time since '84) about parting from the rear. He had no answer. He said he's never had any issues parting off from the "front". Can someone explain why you would want to part from the rear rather than the front?

A production setup, where one may want to chamber from the front then cut off the part in two moves. It is a setup person's option.

 

[mikey]

Parting from the rear greatly improves rigidity during a parting operation, regardless of the lathe involved. Instead of pushing the tool up and imparting a lifting motion to the carriage as experienced with a front-mounted tool, a rear-mounted tool pushes the carriage down, thereby removing any play in the carriage assembly and providing a solid platform for the tool.

A blade type tool like the typical HSS P-type blades has zero rake when used normally. However, when used upside down in a rear-mounted toolholder the tool now has back rake. This is the equivalent of side rake on a turning tool and will reduce cutting forces and cutting temperatures. In addition, due to the fact that the cutting tip is now on the bottom of the tool any cutting fluid contacts the tip first and is not carried off as it is with a normally mounted tool. This type of tool also has side clearance ground into the edge so it reduces cutting forces even more.

The result of all of this is a more rigid set up that allows a thin tool to cut much larger work pieces than it normally should, at much higher speeds, with better finishes and a greatly minimized risk of chatter. It has also been my experience that it is more accurate and allows very thin cuts to be made. I normally part 12L14 at 1200 RPM and 6061-T6 at about 1500-2000 RPM with no chatter, no dig ins, no problems. And I do this on a Sherline lathe.

 

[ScrapMetal]

Parting from the rear greatly improves rigidity during a parting operation, regardless of the lathe involved. Instead of pushing the tool up and imparting a lifting motion to the carriage as experienced with a front-mounted tool, a rear-mounted tool pushes the carriage down, thereby removing any play in the carriage assembly and providing a solid platform for the tool.

I'm a bit confused by this description. As I figure it, when I'm parting from the front, that is where the workpiece is pushing down on the cutter.

Am I missing something here?:thinking:

-Ron

 

[mikey]

As a front mounted tool encounters the work the forces on the tip push the tool downward, which lifts up on the tool holder that is attached to the carriage:



The front mounted tool will raise the carriage so that any play in the gibs may allow the carriage to float and move. This can cause significant chatter in some cases.

With a rear mounted tool, which is mounted upside down, the cutting forces push the tool tip up, which forces the tool holder and carriage down:



Any play in the gibs is taken up with a rear mounted tool and whole tool holder/carriage apparatus bottoms out on the gibs and becomes as rigid as the lathe is capable of being.

Does that help?

 

[Cheeseking]

Does not make sense to me. Cutting from front spindle rotating fwd the carraige is certainly not being lifted. Gibs? Do you mean ways??