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BigBoy1
09-14-2007, 11:11 AM
Last night I parted a piece off of a 2.25" dia. steel round stock. When I looked at the back side of the piece, I discovered that the back side was slightly concave. The parting tool had "wandered". The parting tool was 0.1" wide.

My questions. Was the parting tool too small and apparenetly not rigid enough for the job? Did I try to use the wrong method, parting vice sawing to remove the part?

I chose parting as the piece was only 0.375" thick and I wanted a finished back on the piece. Appreciate any help. Thanks.

Bill

A.K. Boomer
09-14-2007, 11:19 AM
The face of the tool is important as it can "load" the tool to one side as you go to part, it however is not a consistant load as all kinds of factors come into play when the diameter of the part starts getting smaller when parting, this in turn can deviate even further as the parting progresses, a ton of other reasons as you will soon hear including just plain having the parting tool cocked slightly, many parting tools start wide at the top and narrow at the bottom so be sure your not just putting it in and tightening it down in the holder cuz that could be a fuqe...

JimH
09-14-2007, 11:49 AM
Bill,
Thanks, I was planning on asking the same question. I was parting off some 12L14 (1 inch) with a 0.1 inch HSS cutoff bit and the back of the part was convex when done. I didn't have time to check, but am guessing the tool was duller on the right corner causing it to deflect the blade to the left. Also need to check that the cutting edge is perpendicular to the sides of the blade.

As a note, the blade was an T style blade that had not been ground on either side and was properly aligned (pependicular) to the material.

Jim

cybor462
09-14-2007, 12:06 PM
Not sure if this the answer but I had that same thing happen when using a HSS cutoff tool. I found that when it was not as sharp as it should be (sharp not the right word) did not have the correct cutting surface and also I had the blade too far out away from the holder I found that to happen. A while ago it was posted here about a real nice insert style parting tool from Enco. I bought it and have not used a HSS since. I never have any trouble anymore and it lasts much longer than HSS. Actually I have only used 2 inserts and have cutoff/grooved so many parts I lost track how many.

I was always anxious using the HSS part off tool waiting for it to suck in but now I enjoy parting off. With the Enco tool you can run much higher speeds and feed rate.

Cost effective and is a hard worker.

Search the archives and you will see the discussion on that tool if your interested.

Sorry I kinda walked away from your question.:o

garyphansen
09-14-2007, 07:14 PM
Another cause for this is if the parting tool is not straight up and down. Gary P. Hansen

BadDog
09-14-2007, 07:55 PM
Blade not square to axis?

Blade tips can be slightly tapered one way or the other to control which side has the pip. But this can cause it to wonder, particularly on thin blades that have "too much" stick-out. Also seems worse on "T" cross section blades.

"T" cross section blades held in a flat side holder will have a slight angle on the top rake. This can cause them to "wander" toward the high side (usually against the holder).

So, check this:

1) Top rake is "flat" left to right. In other words, if the lathe is level, the cutting edge of the parting tool is level longitudinally.

2) Check that the tool is square to the axis. Either dial it, or I often just use a machinist square blade between the parting tool flat side and the face of the chuck (assuming using a chuck of course).

3) Last, if nothing else works, check the cutting edge and make sure it touches off flat, not on one side first. That will cause there to be 2 sides with a "pip", but better than concave.

In the final analysis, you generally have to face if you want it "nice" anyway, so I wouldn't worry too terribly much, as long as it does not break the tool. Deep parting (relative) is often somewhat difficult any way you slice it.

BigBoy1
09-14-2007, 10:03 PM
Thanks for the tips supplied so far. I did buy one of the Enco parting tools with the inserts and put it in the cupboard and promptly forgot about it. If I but it in the drawer with the tool bits, I'm sure it will be used the next time I need to part off a piece.

I think the uneven tool tip was my problem. I looked at the tool I used and made a test cut with it. The right side of the tool was a few thousands deep while the left side was just breaking the surface of the part.

Bill

BadDog
09-14-2007, 10:11 PM
That's not unusual. Generally you want it to cut clean on the part dropping, so you can face the remaining piece. If it's flat, the final little pip will typically break leaving a stub on both sides. If the right side cuts just a fuzz deeper (as yours seems to), then the part off piece generally has almost no pip.

I've only seen it be a problem when dealing with very thin parting tools. Specifically, a small "T" blade I have, maybe 0.062(?) wide? Anyway, if it's not perfectly flat, you can see it try to track off when you first touch off. That little blade is almost impossible (for me anyway) to use on anything above 1/2" (so just over 1/4" sticking out). And more, and it wants to wiggle. That's why I said, check the flat tip last...

Also, being a "T" blade, I have to put a piece of shim stock behind it (below the cross bar) to make it set vertical in the holder. I generally pinch it with a small parallel clamp while seating the wedge. Sometimes it works ok, sometimes I have to redo it even though it seems to have locked down vertical. <shrug> No idea if it helps you, just my limited experience.

I recently bought a Sandvik(sp?) insert parting tool, but have yet to make a holder for it. I've used one at a friend's, and it is very sweet to be able to part off that agressively (fast rpm, fast feed) with no problems. Kinda scary though. If you fool around with those (so I'm told), you'll ruin them. You have to squash that fear and "go for it". Still ties my stomach up though...

JRouche
09-14-2007, 10:34 PM
Not that I know any better but Ill chime in.. If you are looking to make a flat disk it sounds like you have enough meat left to be able to re-chuck the part and face it properly.

Three inches is a bit much to ask for a flat cutoff. Besides the concaved surface the finish prolly leaves much to be desired.

Donít forget to lock your carriage when cutting off. Carriage could be walkin.

JRouche

BigBoy1
09-15-2007, 06:03 AM
Not that I know any better but Ill chime in.. If you are looking to make a flat disk it sounds like you have enough meat left to be able to re-chuck the part and face it properly.

Three inches is a bit much to ask for a flat cutoff. Besides the concaved surface the finish prolly leaves much to be desired.

Donít forget to lock your carriage when cutting off. Carriage could be walkin.

JRouche


I had the bar stock held between centers as I did have a problem with it "walking" out of the chuck when I was using the steady rest.

Also, after the fact, I thought of a way to finish the back if I used a saw to "part the piece". Since the front face was finished, I could have put it in the mill and faced off the back side. Hind site if just so perfect!!!!

Bill

Al Messer
09-15-2007, 10:53 AM
Most lathes are built to cut concave when doing a facing cut, which is essentially what parting off is doing. BTW, when I was in school, I was taught that "parting off between centers" was strictly a NO-NO.

BigBoy1
09-15-2007, 08:21 PM
The parting was done while the stock was held between centers AND I had the steady rest in place too so the stock was supported and wouldn't go flying when the cut was completed.

Bill