Need Help!- high failure rate with parting operation

[yang_cnc]

Hello:

I am, almost, keeping failing on parting operation.

I use a paring tool I grind myself from a 5/16 tool blank. It thickness is about 1mm.

I have used it to part a 3/8" drill rod, and two M5 screw. If I feed a little bit "fast"(actually, I believe I feed it very slow), it ends up that the workpiece ride above the parting tool and the lathe halts! So I am forced to do it with VERY VERY slow feed rate. I almost start thinking maybe using a hacksaw will be just fine. So far, I have run into the same problem 4 times.

Am I using a too thick parting tool? Also, keeping the parting tool square to lathe axis proves to not be trivial for me, any good idea on how to do that?

thank you very much!

[chalky]

Indexable parting off tools with carbide inserts work much better than hss tools, particularly on small lathes. I had the same problem as you when trying to part off with a Sieg 7x14 lathe using hss tools. Got an indexable parting off tool and the problem went away. Parting off is now easy peasy. I sometimes amuse myself by seeing if I can part off while producing one continuous roll of swarf - can do it pretty much every time with aluminium and most times with steel. Pretty sad pastime I know but it does illustrate how easy parting off is with a good tool.

[DXL]

You may wish to check the adjustments on the ways of your carriage and cross slide to be sure there is no excess vertical movement of your cutting tool relative to the work piece.

What kind of lathe do you use?

[yang_cnc]

I use 7x12 HF mini lathe, I think the cross slide is pretty tight, but I have not paid too much attention to carriage though. thank you for the advice, will double check them

[wildwestpat]

Best advice is to go with a replaceable tip type of parting tool. In these small lathes it is possible to use HSS parting tool BUT because of the possible play in the gibs on the cross and compound slides as well as the sadle it is best to mount the tool upside down. The lathe is then run in reverse with the tool cutting edge at cnetre height and the flank of the tool square i.e. lined up with clearance on both sides of the intended cut. The tendency of the tool to dig in due to any slop or vibration will now take the tool away from the work thus reducing the cut. The conventional mounting of the tool will cause the depth of cut to increase as the tool starts to dig in and this is responsible for the work riding up and possibly over the cutting edge.

It is good practice to keep a reasonably fast feed rate when parting off as this helps to avoid vibrations. The noise of the tool cutting and the feel of the vibrations on the tool holder are a good guide to the feed rate - get used to the feel and sound once you have mastered the operation of parting off.

Good luck and do try mounting the tool upside down and running in reverse!

Regards

Pat

[kawazuki]

Have a look at this - its a Sherline but gives the picture!
http://www.sherline.com/testcuts.htm - video's are well worth a look at bottom of the page!

[yang_cnc]

Originally Posted by wildwestpat Best advice is to go with a replaceable tip type of parting tool. In these small lathes it is possible to use HSS parting tool BUT because of the possible play in the gibs on the cross and compound slides as well as the sadle it is best to mount the tool upside down. The lathe is then run in reverse with the tool cutting edge at cnetre height and the flank of the tool square i.e. lined up with clearance on both sides of the intended cut. The tendency of the tool to dig in due to any slop or vibration will now take the tool away from the work thus reducing the cut. The conventional mounting of the tool will cause the depth of cut to increase as the tool starts to dig in and this is responsible for the work riding up and possibly over the cutting edge. It is good practice to keep a reasonably fast feed rate when parting off as this helps to avoid vibrations. The noise of the tool cutting and the feel of the vibrations on the tool holder are a good guide to the feed rate - get used to the feel and sound once you have mastered the operation of parting off. Good luck and do try mounting the tool upside down and running in reverse! Regards Pat

Hi, Pat:

I read your post, and sketch what you mentione(attched jpeg file). But I do not see the difference between the lower method(you recommend, install parting tool upside down and turn work piece reversely) and the upper method(the old method I use). As I see, both of them are inclined to have parting tool dig into the work piece because the work piece is running exactly toward the parting tool's tip with a sharp angle......

Am I understanding you right? or I miss some of your points......

[CountZero]

have you checked the height of your parting tool?

Also try to part off as close to the chuck as possible.

[escott76]

Originally Posted by yang_cnc Hi, Pat: I read your post, and sketch what you mentione(attched jpeg file). But I do not see the difference between the lower method(you recommend, install parting tool upside down and turn work piece reversely) and the upper method(the old method I use). As I see, both of them are inclined to have parting tool dig into the work piece because the work piece is running exactly toward the parting tool's tip with a sharp angle...... Am I understanding you right? or I miss some of your points......

It has more to do with the way that the machine itself handles the loads involved in cutting the work. It's not a difference in how the tool contacts the work, it's how the loads are transfered into the bed of the machine.
The way I've seen it done is to mount the parting tool behind the work, upside down, and run the lathe forward. Never done it this way personally, but I've heard it can help.
Understand you are dealing with a small machine, which is not too rigid. You have to do the best you can to increase rigidity. Lock the carriage, use the shortest stick out of the parting tool possible, do your parting op as close to the chuck as is reasonable, use coolant, and keep a steady feed. If the part starts to vibrate, it's usually because something is deflecting, either the work or the tool. Once it starts to deflect you get chatter, which worsens and the cutter digs into these marks.
I generally don't rely on a parting op to finish to dimension. I use it to get close, then re-chuck and finish. I try to plan my work so that last cut is just a facing op, so no need to work about getting things totally centered.

[wildwestpat]

Hi

As 'escott76' has responded the tool mout is not that ridgid in a small lathe. If you consider the movement of the cutting edge at the moment the work piece starts to mount the parting tool the resulting movement in the case of your two sketches is very differnet.

A conventional tool will dip down as it starts to dig in. This causes the tool post to pivot downwards. The tool dips downwards about a pivot point that is determined by the geometry of the cross slide and or compound. If you consider this the tool path is now down below centre height and intowards the work resulting in an ever increasing dig in until the lathe stalls or the tool break or the work bends and rises up over the tool - or some combination all three. Result is frustration and a situation that cannot be recovered easily.

In the case of the upsidedown reverse rotation the action is similar when the cutter is acting normally - BUT - when the cutting edge starts to dig in the tool pivots upwards about a point that will now take the tool away from the work piece and release the cutting pressure and helps avoid the catastrophy in the previous case. However it is still possible to dig the tool in if you do -NOT - listen to the cutting noise and alter the tool feed rate to match the sweetest noise. You need to listen as well as feel the lathe to appreciate whent he tools are cutting nicely and when disaster is about to strike! With CNC operation the use of insert tool tips and more ridgid tool posts is highly recommended as these all serve to increase that vital ridgidity.

Suggest you thing in terms of the tool post pivoting abot the rear dove tail slide to see the reason why upside down tooling is better for parting off and probably a host of other operations.

On lathes with chucks that are not bolt and flange mounted reverse operation is not practical as this would cause the chuck to fall off. On these lathes there is often a rear tool post provided and this has a similar use. But requires a much longer cross sadle dovetail. But note that the tool is mounted rake side up in the rear tool post and advanced by withdrawing the cross slide.

Hope this helps. Draw the arcs on your sketch and you will see the tool moves very differently at dig in.

Regards

Pat

[yang_cnc]

Originally Posted by wildwestpat Hi As 'escott76' has responded the tool mout is not that ridgid in a small lathe. If you consider the movement of the cutting edge at the moment the work piece starts to mount the parting tool the resulting movement in the case of your two sketches is very differnet. A conventional tool will dip down as it starts to dig in. This causes the tool post to pivot downwards. The tool dips downwards about a pivot point that is determined by the geometry of the cross slide and or compound. If you consider this the tool path is now down below centre height and intowards the work resulting in an ever increasing dig in until the lathe stalls or the tool break or the work bends and rises up over the tool - or some combination all three. Result is frustration and a situation that cannot be recovered easily. In the case of the upsidedown reverse rotation the action is similar when the cutter is acting normally - BUT - when the cutting edge starts to dig in the tool pivots upwards about a point that will now take the tool away from the work piece and release the cutting pressure and helps avoid the catastrophy in the previous case. However it is still possible to dig the tool in if you do -NOT - listen to the cutting noise and alter the tool feed rate to match the sweetest noise. You need to listen as well as feel the lathe to appreciate whent he tools are cutting nicely and when disaster is about to strike! With CNC operation the use of insert tool tips and more ridgid tool posts is highly recommended as these all serve to increase that vital ridgidity. Suggest you thing in terms of the tool post pivoting abot the rear dove tail slide to see the reason why upside down tooling is better for parting off and probably a host of other operations. On lathes with chucks that are not bolt and flange mounted reverse operation is not practical as this would cause the chuck to fall off. On these lathes there is often a rear tool post provided and this has a similar use. But requires a much longer cross sadle dovetail. But note that the tool is mounted rake side up in the rear tool post and advanced by withdrawing the cross slide. Hope this helps. Draw the arcs on your sketch and you will see the tool moves very differently at dig in. Regards Pat

Pat:

I greatly appreciate your patient help and clear explanation, I think now I understand it I think your post will also help other people with similar confusion as mine to understand this issue.

Also thank all other guys who helped me... this is a great community

Cheers
long

[wildwestpat]

Err - NO.

The point I am trying to make is that the sketches you have produced only relate to the static or cutting ok. As and only when the tool digs in do the two ways of mounting the tool differ. The upside down and normal way up tools have a common pivot point due to the slop in the dovetail slides. The normal mounted tool dives under the work in a trajectory that causes the tool to bite harder into the work and if the tool has a negative rake this makes matters even worse. The upside-down tool however tilts the tool rest upwards. In moving upwards the feed of the cut is reduced. Also the tendency to hook the work on the negative rake of the parting tool's cutting edge is also reduced as the angle is reducing and would rapidly approach zero or even become positive. This further stops the upside-down tool digging in. Use of a rear tool post or upside-down cutting is standard lathe practice and I strongly suggest that you try it and see as there is no cost involved and it is so easy to do. This can also help out with difficulty to turn materials so is worth while learning.

Regards

Pat