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threading pre-hard on the lathe (having a hard time...)

thanvg

Hot Rolled
Joined
Mar 3, 2015
Location
Greece
Dear PM,

please advise on this. I had to make 4 long screws with 250 mm of 19 mm OD shoulder and 230 mm of M16 thread. For strength, we opted to use a prehardened steel, specifically Impax Supreme from Uddeholm, which is at 34 HRC.

I used my Colchester Triumph 200 for this (Clausing 15").

It turned out much more complicated than I had anticipated.

- turning down the shaft from 22 mm bar stock, was not trivial. I set up the follow rest (traveling steady) which, I was not able to use properly. Here is the setup:

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Some times, I was setting the roller for light touch and after a while I was getting chatter that when away if I removed the roller so that it wouldn't touch the shaft. Chatter like this:

attachment.php


Then I would try without the roller touching. Then I could get chatter again at some point, which went away if I moved the roller to lightly touch! Crazy stuff.

Was using at CNMG insert, running at 625 RPM and tried several feeds.

I never got the chips to break at all:

attachment.php


- as for the thread....what a nightmare. There were clearance issues so I wasn't able to use the follow rest. Tool was a ground carbide one. Feeding from the top slide at the correct angle.

The thread came out torn, dull, with burrs.

attachment.php


To my understanding, there must have been vibrations since I didn't have the follow rest on, but I am not sure if this was the reason for the lousy finish.
I run the (2 mm pitch) at 250 rpm, though I did try a bit faster as well.

So, I am wondering:
- my problems at thread cutting were mainly due to the dimensions of the part? If the part was short and fat one could get a nice, accurate and shiny thread on pre-hard? This thing, in regular conditions machines very nicely, but you have to take a bite. I am not sure if this is possible when single pointing a thread.

- would it make sense to switch to HSS (that would have the benefit of not chipping so easily) and go nice and slow?

Any advice welcome.

Thanks in advance.

BR,
Thanos
 
Yes, conventional follower rests are a PITA.

A "C" shape insert is prone to chatter. I would recommend using a T, D or V shape insert on skinny jobs. Or an Iscar style cutgrip with a very aggressive feed and not too much depth of cut is also worth a shot. But ultimately, you have to end up with a skim cut to get an accurate diameter and then you go to one of the other inserts, with a very high positive rake. I usually grind my own from an old insert. I gash a groove straight down behind the very cutting tip, producing a high positive rake of 30 to 45 degrees. This shape will skim with minimum chatter induction.

I would use a steady rest and turn half from each end. Easier to control what is going on than with a travelling follower.

As for the threading, this might be a case where I would finish with a die. Rough the thread out as best you can. Use a bushing in the follower, as the thread will wear a pattern in the end of the follower fingers (if they are soft). You might have to file the burr off the top of the thread between each pass, else the bushing may want to jam up.
 
I would have just left the stock slightly long and used a center in the end rather than mucking about with the steady.

Threading away from the chuck with the tool upside down might have helped too if you were worried about clearance issues with the shoulder and would have let you get up to a decent SFM. Either way I would expect to need a few spring passes to get all of the thread in spec.

An aluminum grade DCGT wouldn't be a bad choice either for a free cutting insert compared to a negative rake CNMG.
 
Yes, conventional follower rests are a PITA.

A "C" shape insert is prone to chatter. I would recommend using a T, D or V shape insert on skinny jobs. Or an Iscar style cutgrip with a very aggressive feed and not too much depth of cut is also worth a shot. But ultimately, you have to end up with a skim cut to get an accurate diameter and then you go to one of the other inserts, with a very high positive rake. I usually grind my own from an old insert. I gash a groove straight down behind the very cutting tip, producing a high positive rake of 30 to 45 degrees. This shape will skim with minimum chatter induction.

I would use a steady rest and turn half from each end. Easier to control what is going on than with a travelling follower.

As for the threading, this might be a case where I would finish with a die. Rough the thread out as best you can. Use a bushing in the follower, as the thread will wear a pattern in the end of the follower fingers (if they are soft). You might have to file the burr off the top of the thread between each pass, else the bushing may want to jam up.

Add the usual tricks: Keep heavy copper/lead hammer over the shaft, change to different feed rate for each pass (to break up the previous pattern and change the harmonics to different frequency) and play with cutting edge height to suppress chatter.

--
Thread doesn't look terribly bad in photos, maybe few last passes with very slow speed and tiny DOC would have fixed it. But threading a shaft with 13mm core diameter and 250mm lenght (L/D ratio=20) is almost warranted to chatter..
 
I would have just left the stock slightly long and used a center in the end rather than mucking about with the steady.
Unless my eyes are lying there is also live center on the end?
L/D ratio of 20 in threading is going to be lotsa fun even with tailstock support
 
Hi to all, thanks for the replies!

Yes, conventional follower rests are a PITA.

A "C" shape insert is prone to chatter. I would recommend using a T, D or V shape insert on skinny jobs. Or an Iscar style cutgrip with a very aggressive feed and not too much depth of cut is also worth a shot. But ultimately, you have to end up with a skim cut to get an accurate diameter and then you go to one of the other inserts, with a very high positive rake. I usually grind my own from an old insert. I gash a groove straight down behind the very cutting tip, producing a high positive rake of 30 to 45 degrees. This shape will skim with minimum chatter induction.

I would use a steady rest and turn half from each end. Easier to control what is going on than with a travelling follower.

As for the threading, this might be a case where I would finish with a die. Rough the thread out as best you can. Use a bushing in the follower, as the thread will wear a pattern in the end of the follower fingers (if they are soft). You might have to file the burr off the top of the thread between each pass, else the bushing may want to jam up.

Thanks for the tips,
I did, indeed, finish with a CCGT insert, highly positive rake, aluminium grade. This was the way to achieve my target OD. No chatter with that one, shallow passes though.

Regarding steady rest, I, indeed, turned half from each end. It might make sense to turn smaller portions, like 50 mm stock each time down to close to the target OD and then extend stock and repeat. Then a finish pass on the whole half of the shaft with a sharp positive rake tool.

Regarding the threading, I failed to mention that, I swallowed my pride and did, indeed, finish with a die....not that it made too much difference. Only my arms felt the difference, I did 1000 turns in total...(1000 mm threaded length in total, 2 mm pitch, back and forth)

I would have just left the stock slightly long and used a center in the end rather than mucking about with the steady.

Threading away from the chuck with the tool upside down might have helped too if you were worried about clearance issues with the shoulder and would have let you get up to a decent SFM. Either way I would expect to need a few spring passes to get all of the thread in spec.

An aluminum grade DCGT wouldn't be a bad choice either for a free cutting insert compared to a negative rake CNMG.

Thanks, see above, there was tailstock support and sharp positive rake for finishing.
Regarding threading in reverse, you are right, I didn't try that...Though, since I was metric threading on an imperial machine, it's a bit more complicated to start from a shoulder with a short relief. But doable.

In the picture it looks like you are cutting this dry. Have you tried this with lube?

Thanks, did try lube as well. Especially for the thread, it was soaking in cutting oil :)

Add the usual tricks: Keep heavy copper/lead hammer over the shaft, change to different feed rate for each pass (to break up the previous pattern and change the harmonics to different frequency) and play with cutting edge height to suppress chatter.

--
Thread doesn't look terribly bad in photos, maybe few last passes with very slow speed and tiny DOC would have fixed it. But threading a shaft with 13mm core diameter and 250mm lenght (L/D ratio=20) is almost warranted to chatter..

Thanks for the tips,

I did try my dead blow hammer on the shaft. It might not have been that heavy....
Thanks for the tip on changing the cutting conditions for each pass, makes so much sense!

Photos don't do the thread justice, in real life it's even worse :) For sure it's not a nice shiny thread, like this prehard is when properly turned. I did finish with shallow spring passes...

Unless my eyes are lying there is also live center on the end?
L/D ratio of 20 in threading is going to be lotsa fun even with tailstock support

Yeap, lot's of fun....:D
 
A couple more things not mention is when you use the follower rest, back off the tailstock pressure where the live center is barely loaded against the end of the part. Compressing the long thin part from tail stock pressure could cause the part to flex like a rubber band, causing it to chatter when cutting. Next, how worn are the bearings on the follower rest? If they have any slop or flop around from wear, replace them. Personally, I find brass/bronze/cast iron tipped jaws do a better job keeping the chatter down than a roller bearing jaw IMO. Ken
 
I have been threading a lot of prehard 4140 recently and maybe something I do may help you..... or give somebody a good laugh. I hand sharpen my carbide ( real sharp!) use cooking lard ( works better than commercial lard based cutting oils)take light cuts after passing half of thread depth, as light as you can with plenty of spring passes when nearing the final cuts. I often have to sharpen mid or near end of threading to keep it sharp enough. turn it slow, sharp cutters do not require high speed. be sure you are not getting tip welding of material on your cutter, it happens pretty easily. did I mention SHARP cutting tools?? it can be cut using HSS but tip welding is more likely to occur and more frequent sharpening required. great patience is required this way but it comes out shiny! happy threading!
 
A couple more things not mention is when you use the follower rest, back off the tailstock pressure where the live center is barely loaded against the end of the part. Compressing the long thin part from tail stock pressure could cause the part to flex like a rubber band, causing it to chatter when cutting. Next, how worn are the bearings on the follower rest? If they have any slop or flop around from wear, replace them. Personally, I find brass/bronze/cast iron tipped jaws do a better job keeping the chatter down than a roller bearing jaw IMO. Ken

Hey Ken, thanks for the tips.

your comment on pressure from the tailstock is really interesting. This might be the root cause of chatter induced by the follower rest, tailstock pushing the stock out of alignment too much (and I did push too much with the tailstock, never thought about this aspect....)

Regarding rest bearing, I suspected them and went on and replaced them with brand new ones, no difference. I am with you with the fixed jaws, my feeling is also that they could work better in supporting the part. I may find the time to make up some at some point and give them a shot.

I have been threading a lot of prehard 4140 recently and maybe something I do may help you..... or give somebody a good laugh. I hand sharpen my carbide ( real sharp!) use cooking lard ( works better than commercial lard based cutting oils)take light cuts after passing half of thread depth, as light as you can with plenty of spring passes when nearing the final cuts. I often have to sharpen mid or near end of threading to keep it sharp enough. turn it slow, sharp cutters do not require high speed. be sure you are not getting tip welding of material on your cutter, it happens pretty easily. did I mention SHARP cutting tools?? it can be cut using HSS but tip welding is more likely to occur and more frequent sharpening required. great patience is required this way but it comes out shiny! happy threading!

Thanks for the tips,

I did retouch the cutter a couple of times throughout the whole procedure, afraid that I didn't have the geometry correct, but not much changed (this doesn't mean, though, that I had the geometry correct I guess :) ). Spring passes as well, and shallow cuts when the engagement starts getting serious is what I did. So, I guess, if the stock was not long and thin, I would have had better results. Next time...:)
 








 
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