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taydin
05-09-2013, 09:49 AM
Was turning down a 170mm long, 120mm OD cylinder. The cylinder needs to be turned down to 80mm and 44mm, two steps. This would have taken some time, so I started to experiment with speeds/feeds.

First started with 280rpm, 0.20mm feed rate and 1mm DOC. Progressively reduced the speed to 105rpm because of sparks and smoke. 105rpm was smooth sailing. Finished the 80mm section and started on the 44mm section. But things were going slow, so increased the DOC progressively to 1.5mm, 2mm. Still too slow. Disengaged the autofeed and used manual feed at 5mm DOC. I was slow, but still advancing.

Then tried 3.5mm DOC and 0.38mm feed rate. After about 10 turns, the piece was ripped out of the chuck and my nice Kennametal insert broke. Was it the feed rate or the DOC? Or was it because the other end of the part was not secured by the tailstock?

http://www.taydin.org/web/chuck_escape/scaled_img_2889.jpg

winchman
05-09-2013, 09:59 AM
I would have put a live center in the tailstock, and tightened it pretty hard against the end of the piece. If you didn't want a center drilled hole in the end, a small piece of scrap would have worked. Rigidity is your best friend when machining. Things that can move always seem to move in the direction that makes things worse.

It looks like you got a good amount done by the time things went bad. The bit may have been getting a little dull by then, especially since you started off getting sparks. Coolant would have helped, too. A spray bottle works wonders if you don't have a coolant pump.

taydin
05-09-2013, 10:10 AM
Thanks for the spray bottle tip! I tried way oil as a coolant, but that created nasty fumes, so decided to cut dry...

I'd rather not fill the coolant tank, because after a while it becomes a source of stink.

Toolguy
05-09-2013, 10:18 AM
Yup - need to have the tailstock holding that firmly in place.

becksmachine
05-09-2013, 10:56 AM
Yes, if the workpiece allows, supporting with the tailstock is a simple solution.

Another possible solution would be to use a 4 jaw chuck.

When a 3 jaw gets opened up near it's maximum as this one is, the distance between the jaws gets excessive and causes some amount of mechanical disadvantage, not only because of the radial distance between the jaws but also because the jaws tip in their slots creating a bell mouth condition.

A 4 jaw will also usually achieve higher clamping forces thus affording a more positive grip, especially on solid workpieces like this. Also a 4 jaw will often have a greater protrusion of jaw from the chuck face and more agressive knurling or checkering on the gripping surfaces.

Dave

taydin
05-09-2013, 11:05 AM
Thanks for the replies everybody. I will try supporting the piece with the tailstock and will report back.

Steve Seebold
05-09-2013, 11:15 AM
With a part sticking that far out of the chuck, it's not a matter of if it's going to come out. It's when is it going to come out and how much damage is it going to do when it does.

MrSleepy
05-09-2013, 11:58 AM
Way oil is a fantastic lubricant , but terrible as a cutting fluid.

I have in the past grabbed and used my way oil container instead of the cutting one....and the difference is significant.


Rob

Jaakko Fagerlund
05-09-2013, 12:51 PM
I'd rather not fill the coolant tank, because after a while it becomes a source of stink.
Put a small air bubbler in the tank to keep it oxygenated, prevents the growth of those stupid smelly anaerobic bacteria :) Or get an oil separator, as it is the oil on the surface that prevents air from getting in to the tank.

At work some machines stand down easily 1-2 months in a row and the coolant gets life going in it. When it is started, the smell is awful, but after 3-4 hours of circulation the bacteria dies because of the oxygen it gets from the air.

Black_Moons
05-09-2013, 12:54 PM
Yea get a proper cutting fluid, Some of them even smell nice when they burn! At least I think so...
4 jaw should hold more securely yes.

Sparks and smoke suggest blunted insert or hitting hard inclusions. Either is very bad and stresses things excessively. Realise that you need a lupe or similar to see damage to your inserts cutting edge before it adversely affects cutting forces.

taydin
05-09-2013, 01:14 PM
Sparks and smoke suggest blunted insert or hitting hard inclusions. Either is very bad and stresses things excessively. Realise that you need a lupe or similar to see damage to your inserts cutting edge before it adversely affects cutting forces.

Probably hard inclusions, because they were intermittent sparks and there was occasional grinding noise. But once I reduced the rpm and after a certain depth, no sparks anymore.

taydin
05-09-2013, 01:16 PM
Way oil is a fantastic lubricant , but terrible as a cutting fluid.

I have in the past grabbed and used my way oil container instead of the cutting one....and the difference is significant.
Rob

Mr Fenner over at Youtube is using some fluid at the lathe. I wouldn't know what the equivalent is here, but I probably will be just fine using a sprayer with my usual Shell Dromus coolant :)

taydin
05-09-2013, 01:20 PM
Put a small air bubbler in the tank to keep it oxygenated, prevents the growth of those stupid smelly anaerobic bacteria :) Or get an oil separator, as it is the oil on the surface that prevents air from getting in to the tank.

At work some machines stand down easily 1-2 months in a row and the coolant gets life going in it. When it is started, the smell is awful, but after 3-4 hours of circulation the bacteria dies because of the oxygen it gets from the air.

Hmm yea, I noticed that, too. Thought I was getting used to the stink, but in fact those little bastards are dying, which makes the coolant more bearable...

Jaakko Fagerlund
05-09-2013, 11:54 PM
Oh and always use the tailstock for support where ever possible, especially when roughing out lots of material. I have small cutoffs that I've center drilled and use them if I can't drill the end of the workpiece. I just face as close as possible to the support piece and finish it later with fine cuts. Sometimes I even cheat and don't use a scrap, just adjust the center against the workpiece, it will dig in a little for some support at least.

And last one I do with large pieces that will have a hole: first take a truing cut from the end and OD, then drill and tap the end, flip the workpiece around in the chuck and pull it against the jaws with a long bolt through the headstock. Makes it easy to work the whole piece in one setting, roughing, finishing etc.

Black Forest
05-10-2013, 01:14 AM
Just curious. What material was the workpiece? What is the recommended surface speed for the insert at what DOC and what mm/rev? What is the nose radius of the insert?

taydin
05-10-2013, 04:28 AM
What material was the workpiece?

I "think" it is low carbon steel. When I go to the steel vendors here, I really don't have the chance to ask specific details. The guy there just knows how to operate the crane to bring the piece to the band saw, cuts it and takes the money :)


What is the recommended surface speed for the insert at what DOC and what mm/rev?

Hmm, well, I'm not really sure what this insert really is. The seller gave this to me as TNMG432RP. I looked this up in the Kennametal catalog. There is no such thing, but there is a TNM220432RP. This has a nose radius of 3.2mm, but I am sure mine does not have this radius. Mine is probably 1.2mm. So I am a little confused...

I don't know the grade of this insert, either. The common grade here is KCP25, which is an average grade. But for this grade, the recommended surface speed is 150m/min - 360m/min, which translates to 405 rpm - 970 rpm. I was getting sparks and dark blue chips even with 280 rpm, so I would definitely not try the "recommended" rpm :)

But if this insert has a grade of KCP30, which is closer to the roughing side of the scale, then things make sense. I was using 105rpm (40m/min) and the insert was happily chugging along, removing 5mm material with manual feed. The catalog also gives a surface speed range of 20m/min to 250m/min, so this makes sense also.

So, based on this analysis, I have a TNMG220412RP, KCP30 (nose radius 1.2mm). This insert can handle a DOC between 0.64mm - 9.6mm and a feed rate between 0.24mm - 0.82mm

It seems my settings (20m/min, 3.5mm DOC, 0.38mm feed) are well within the specs of this insert. The workholding was the problem.

When I buy an insert from now on, I will buy a set in an unopened box, because I am suspecting that the clueless sales clerks just move around the inserts among the open boxes. So what you think you get isn't always what you're really getting. And the designation on the insert doesn't help, either...

Jaakko Fagerlund
05-10-2013, 04:56 AM
TNM220432RP would mean TNMG inser that is 22 wide, 04 thick and has a 3.2 nose radius. Probably is something very else than the TNMG432, but I don't know that much of the shorter notation.

But for myself I just try to steer away from triangular inserts, it seems they just cut badly no matter what. DNMG, VNMG and WNMG inserts I've been using with success and the DNMG gets 99% of all turning jobs done.

Sparks do not always tell of bad condition, some materials just tend to spark when the carbon burns. For example, MC 212 or 16MnCr6, a fine grained low carbon steel will give small sparkling all the time. Nothing fancy, it is just burning carbon when you take a heavy cut and the heat is thus high. Best would be to run with small flow of coolant on top of the insert to give it the best life span and to cool down the workpiece also somewhat.

taydin
05-10-2013, 05:54 AM
TNM220432RP would mean TNMG inser that is 22 wide, 04 thick and has a 3.2 nose radius. Probably is something very else than the TNMG432, but I don't know that much of the shorter notation.

But for myself I just try to steer away from triangular inserts, it seems they just cut badly no matter what. DNMG, VNMG and WNMG inserts I've been using with success and the DNMG gets 99% of all turning jobs done.

Sparks do not always tell of bad condition, some materials just tend to spark when the carbon burns. For example, MC 212 or 16MnCr6, a fine grained low carbon steel will give small sparkling all the time. Nothing fancy, it is just burning carbon when you take a heavy cut and the heat is thus high. Best would be to run with small flow of coolant on top of the insert to give it the best life span and to cool down the workpiece also somewhat.

Below is a picture of the insert. I have been told about using DNMG for general purpose, all around machining. I will definitely get me a set of those. Are there any subcategories of these for heavy roughing and fine finishing?

http://www.taydin.org/web/insert_gone_bad/scaled_img_2872.jpg

Shuswap Pat
05-10-2013, 09:58 AM
If in doubt - Four Jaw it !!! The opposing jaw forces exert much more holding power (and precision) than the 3-Jaw. Also - use the tail stock and a center. If you don't want to c-drill your part, use a button. If you have a hollow section (pipe), or hole larger that your center, use a bar accoss the opening. I have used that option many times turning pipe (8"-16" diameter) for rolls.

Patrick

jdunmyer
05-11-2013, 07:47 PM
Thanks for the spray bottle tip! I tried way oil as a coolant, but that created nasty fumes, so decided to cut dry...

I have a little squirt bottle of Tap Magic that is my default cutting fluid, but it smokes like the dickens when hot, and is absolutely choking to breathe... Sooo..

I grabbed an old electronics cooling fan, 4" size, 120 VAC. Put a machine screw and nut in one of the corner holes and connected a cord. When I need it, I just put the end of the screw in the T.S. chuck to position/hold the fan to blow towards the H.S. and keep the smoke moving away from me. Yeah, it's a redneck fix, but it works.

taydin
05-12-2013, 06:38 AM
Ok, prepared a spray bottle with coolant, center drilled the end of the piece and secured it with a live center. Then I did make 3 passes, 105rpm, 3mm DOC and 0.38mm feed. The total cut length was about 100mm. During all passes, I sprayed coolant into the cut. But still, the insert got so hot that the coolant was evaporating upon contact. I increased the rate of pumping as fast as I could, but the heat was just too much and it didn't help the insert. After the three passes, the insert tip lost its shape completely. One noteworthy detail, the chips were coming off small and almost completely black, which is consistent with the observed high heat of the insert.

I guess this type of heavy cut calls for a flood coolant. But I would rather sacrifice one edge of an insert instead of filling the coolant tank any time.

So, I already had cut the bulk of material, and I just rotated the insert to cut the final dimension. Job complete!

John Stevenson
05-12-2013, 06:45 AM
Just a general tip but if you want support but can't or don't want a centred drill just pop an old bearing on the end of the work and support this with a centre.

Decreases the pucker factor knowing you won't be wearing the part.

http://www.stevenson-engineers.co.uk/files/largerotor5.jpg

J Tiers
05-12-2013, 12:13 PM
use a 4 jaw chuck, and if possible turn a small ring on the work to fit in the jaw grooves. later you can cut it off, using small D.O.C. because it is small.

Search for posts by Forrest Addy that reference the "death grip" technique with a 4 jaw.

Some things you just have to be careful with.... if it looks risky, it probably is. Those times are not the place to try pushing the cut to maximum.

Jaakko Fagerlund
05-12-2013, 01:22 PM
One other tip that prolongs the insert life: Make sure it is a tad lower than at the very center. This way it is always cutting and not rubbing against the workpiece on feed start.