Parting off problems.

[Gas_mantle]

Hi all, I'm hoping someone can help with trouble I'm having parting off EN1a steel.

I'm trying to make 4 discs 7mm thick and 50mm dia with a 10mm dia central hole, at the moment I have the outer diameter and the hole done so I wanted to part off the 4 pieces at one setting in my lathe. I tried one and got only about 5mm in and had to resort to a hacksaw.

The photos show the set up I have, the part is protruding about 30mm outside the chuck - I have tried using a revolving centre to add a bit of extra support but it makes little difference.

Is the part sticking out too far possibly? maybe the blade isn't upto the job ?

I've checked the usual things - the blade appears sharp, it's on centre height and the carriage locked but I still get very bad chatter.

The tool is 2mm wide with 5% cobalt.





Any help greatly appreciated, thanks.
Peter.

[Tennessee Whiskey]

I'm not really one of the experts here,  but,  I would make sure that I kept it wet with cutting fluid or oil (I use a high sulphur content cutting oil) and decrease RPMs or increase feed.  I think a lot of us are leary of the parting process and tend to be a bit too gentle with it which can lead to chatter and work hardening in a few cases. 

Cletus

[Roger B]

What speed are you using?
Are you using any lubricant?
Is the blade firmly gripped in the holder?
Have you tried sharpening the tool? Just go straight into the grinding wheel, the radius of the wheel will sort out the clearance.
Is there any play in the headstock bearings?

[Bluechip]

All the preceding are very valid points. I don't know what lathe it is but you appear to be making quite an ambitious cut with what looks like a tool that is cantilevered out on a QC tool-post on possibly a non-locked top-slide.

I have a Myford S7 and I cannot use an Iscar inserted tip parting tool that is hanging out of the toolholder in such a manner on anything much tougher than PVC. ( Which it does rather well ).

Just not rigid enough ... 

Dave

[AOG]

Parting steel used to give me no end of problems. My lathe just isn't up to it except under specific conditions. This is what I have found that works for me. Use the largest (stiffest) parting too you have. Adjust the part in the chuck so that is sticks out the length to be parted off plus 1.5 X the width of the parting tool. For my first cut I extend the tool no more than twice the height of the tool and then make multiple cuts extending the tool by the same amount. The last thing is to run a the cutting tip of the tool against a sharpening stone before you start.

Tony

[Gas_mantle]

Thanks guys, I'm inclined to think as Bluechip suggested that I'm being over ambitious with a small 8x16 bench lathe and the cutting tool probably isn't beefy enough either.

I don't have a coolant supply to my machine but have been using cutting oil or light machine oil which helps a bit but I still struggle. I've tried various speeds from about 350 rpm down and find as low as about 120 rpm seems to work best but that sounds a bit slow to me ?

I'll give the tool a touch up on the grinder tomorrow and try again but beginning to think it will end up being a hacksaw job :-(

It's not the end of the world if I have to hacksaw then face off the reverse side but I was trying to avoid doing that as there isn't going to be much to hold onto in the chuck.

Peter.

[10KPete]

350 is definitely too fast. 150 is more like it. Make sure the tool is sharp, everything is as ridged as possible, then stuff the tool in without being timid! Make it cut curls. Lots of oil, as in wet at the tool contact point, constantly.

You have the right tool, with the curve in the top.

If there isn't enough power to make it do that then it's saw time.

Pete

[Allen Smithee]

I'm very much a novice around here, but what I would say is that when I first got my Myford S7 I had troubles parting off in steel, but then due to something I read on here I changed my approach.

I had been assuming that it was a matter of gentle, subtle cutting, but that made the tool chatter. So I tried pushing the tool hard rather than being subtle about it. I found that if I used a more rapid feed the material (probably EN1a or similar) just cut really nicely. One long curl of swarf from start to finish. The tool got hot, so I now use a carbide one, and I give it a quick wipe with a diamond stone before use, and I drop a little lube on it sometimes (but not always). I probably ran the (30mm) bar at something like 1,000 or 1,100 rpm because I often forget to think about speeds and just leave it running at whatever I was running before.

I'm waiting to be (justifiably) shot down in flames here, but try simply pushing the tool harder. Perhaps sometimes it pays to be unsubtle!

AS

[sco]

I've tried several different parting tools including inserts but the best so far is an old school Eclipse parting tool http://www.ebay.co.uk/itm/ECLIPSE-653S-PARTING-TOOL-FOR-METAL-LATHE-/201608531910?hash=item2ef0ce0fc6:g:fhUAAOSwFGNWRjdS with a 1.5mm wide blade.  The holder is a bit big for my lathe so needed adapting to fit in my tool post but it's easy to sharpen the blade and just seems to 'work'.

Simon.

[Florian Eberhard]

Hi Peter

Three things that I think do matter quite a lot:

1. you need very low rpm! (I would say no faster than 200 to 300 rpm though It could be even lower; actually I do not know exactly what speeds I am using)

I went down to the shop for recording a short demonstration:
<a href="https://www.youtube.com/watch?v=gLJmKf7bUTY" target="_blank">http://www.youtube.com/watch?v=gLJmKf7bUTY</a>

You can hear the vibrations at the beginning. Then I reduce the speed and have less vibrations. Another reduction finally lets the vibrations almost dissapear. It is very important that your feed rate is not too low and even more important, it should be as continuous as possible.

2. Your Rake angle is actually 0°. I would give at least 5° for any kind of steel. I do use my parting blade (2.3mm) for everything and I have in about 5° which seems to work quite fine.

3. The blade overhang is quite long (especially for the beginning of the cut). For deep cuts, I would first have it stick out like 15 to 20mm and then cut until you can't go further. Then move the blade out of the holder just as much as it needs to cut the part off (in your case I would choose 26mm to 27mm)

Oh and I almost forgot - the steel I have been parting is some kind of structural steel - so not the easiest to machine.

Cheers Florian

[Gas_mantle]

Thanks everyone, the replies have been very helpful. It sounds like I need to keep the speed low and feed in a bit more aggressively.

At the moment I've been very cautious with the feed rate (my machine doesn't have powered cross feed) as when I attempt feeding faster I can feel the machine struggling and the rpm drop - on couple of occasions the tool has dug in and stopped the machine.

Florian, thanks for taking the time to make video, that is very instructive    When you say I have 0* rake do you mean I'd possibly get better results if I grind a 5* rake on the top of the blade ?

Peter.

[Florian Eberhard]

Hi Peter

Its only a thought, but I think it could help because the blade is sharper and therefore cuts easier through the material. I would consider it worth trying.
The price you pay for this is that you will have to cut away a piece of your blade as soon as you reach the bottom of the wider top part of your blade. (due to resharpening)
But if you keep the radius small, it is not going to be too much. And the (hopefully) increased performance is worth it definitely.

To shape that radius, I ususally first grind away excess material with the edge on the bench grinder and finally use a small stone for finish.
You can also grind away a small wedge instead of a radius (but round off the end to enable the chips to "flow" away)

Florian

[Gas_mantle]

Thanks, I'll give it a try tomorrow - I still have 3 of these discs to part off so it's worth a try 

[jadge]

It is possibly ambitious to part off completely with the tool being used, but it ought to be capable of getting a considerable way through and thus easing the effort needed for the final hacksawing.

Provided you have tailstock support the distance from the chuck to cut is fine. With tailstock support I've parted off (nearly) 5" out in 80mm diameter cast iron with no problem. Obviously the last bit was done with a hacksaw.

Feedrate is the key. I'm gung ho, or just plain lazy, and part off under power feed. For my carbide insert parting blade anything less than 4 thou/rev causes chatter. Above that it's fine.

Speed isn't that important, I normally run at 400-500 rpm, for anything over about 1".

On my Britan repetition lathe, which uses 5/16" deep HSS tooling, I part off on the top speed I currently have the belts set for, about 1400 rpm. Fedrate is by hand lever but measured swarf thickness is about 8 thou upwards.

Parting off is one of those techniques where pussy footing about is precisely the wrong thing to do. 

Andrew

[Bobsmodels]

Hi

About 8 or 9 years ago I was introduced to a parting off technique that I have used ever since.  I always had trouble with the feel, the feed, the rpm's.  The largest I have parted off with this method was a 2.75 Dia, my more normal range is .125 to 1.0.   I use a HSS blade or a Carbide just like in the first photo.  I set the RPM at about 75.  Set the power cross feed for about .001 or so.  I then let the machine do the work.  Add lub, brush out the chips.  I can change my RPMs easily (VFD) so I may move that around a bit as I see what is coming off and how it sounds.  I am in no hurry.  My friend tells me to up the infeed but I like it slow.

Anyway works for me, and I use it on my Myford S7 and a Clausing 12, both work fine.

I attached photos of the most complicated parting job that was all done under power. Its a pump I carved out of a solid blank.

Bob