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minerva

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Having come back to metal machining after a break of some 50 years the past 6 months have been to say the least somewhat of roller-coaster of a learning curve, leaving more questions unanswered than answered so I would be grateful for comments from you younger guys.
I took delivery of a Chinese Mini-lathe and duly took stock of what I thought would be required in the way of tooling for simple turning exercises (to bring me back up to speed) and purchased from my local supplier a set of carbide tipped cutting tools which contained a selection of tooling ranging from basic L/H , R/H ,Parting off tool ,thread cutting profile tool ,Boring bars etc. , You get the picture I'm sure,
Now from the outset finish was an issue for no matter how I tried varying feeds and speeds (the ranges of which far exceeded the lathes of old) the material was removed but fine finish eluded me. At this point I also felt that the tool post supplied left a lot to be desired needing an infinite amount of shim stock to set the various tooling to the correct height. A visit to ArcEuro solved this problem and I purchased an old fashioned "rocker" tool-post which proved to be a panacea to many of the problems I was experiencing. At the same visit I purchased a HSS parting tool and holder which I felt might help. My first attempt at parting off in the normal way was disastrous the whole set up was so unstable that it was abandoned in short order. I then read of rear mounted tool posts the theory of which made a lot of sense with a result I attempted the parting operation with the parting tool mounted upside down in its normal position and the machine running in reverse - the result was remarkable a parting cut thro' a 30mm steel bar as if through butter. Spurred on by my success I idly without thinking made a skimming cut of about 10 thou. on an off cut of 25mm stock the result was amazing at last a finish running in reverse with a tool being apparently abused!!
Now I have read somewhere that HSS tooling will give a superior finish as opposed to carbide tipped tools, but is the geometry of the set up the reason for this apparent move from the traditional set I learned so long ago? is the parting tool acting as a form tool? The only abberation appears to be the fact that the tool being only of the order of 1mm wide does tend to flex and on reaching say a collar will if permitted take a secondary cut but this can be exploited by using the lead screw to machine back out.
and a fine finish to high tolerance achieved.
Unsurprisingly I have continued to use this method for practice and will attach a couple of photos if I can fathom out the process!!
in anticipation,
kindest regards to all
terry


finish.jpg


new tp.jpg
 
thanks guys for your unbounded response !
An old engineer friend finally provided the answer to my problems, taking one look at the above mentioned set of TCT tooling pronounced them fit only for the bin, the clearance and rake angles being totally to c*ck. Not having a grindstone to deal with the carbide inserts, he duly sharpened a knife tool from a HSS blank and voila!! A finish in the conventional manner. I am still parting off in the fashion described above as I have yet to fault it but suffice to say for everyday turning I am sticking to HSS until I can afford some decent carbide tooling so fellow newbies to this fascinating hobby beware the cheap tooling offered by the most reputable retail outlets as they are sometimes not the quality they purport to be.
 
Terry,
Am i understanding that when using the cutoff tool from the back side and upside down that you are still running the machine in reverse? If so, it the cutting edge of the tool on the centerline of the stock? Maybe I misunderstood, and I can't argue with your results but have no idea how it worked like that.

As to the other point of HSS vs. carbide, I have much better results with HSS with just a slight radius on the cutting point to help the finish. I have also had good luck with a "diamond" (tangential) toolholder but again using a HSS tool blank.

In any case, welcome back to the hobby.

Bill
 
Terry
Your stated results seem to agree with those of most 7x ** users. These little lathes usually require a bit of tweaking before they can be called true working machines, but it sounds as if you've gotten the saddle strapped on yours. Once the kinks were all addressed, the small lathe 7x14 lathe that I have performs far beyond expectations.

They are capable of doing even more. I added more travel to the cross slide and fitted a 5 inch chuck to mine. I routinely turn and face metal as large as 6" diameter with only an occasional but minor chatter issue. I have yet to give the rear cut off a try, but everyone who speaks of it does so with great admiration.

Other mods I've made include DRO's, an extended cross slide screw, a spindle indexer and an independent power feed on the lead screw. Like I said... the capabilities can be expanded...LOL. I've become rather attached to mine and have resisted the temptation to upgrade as I found more an more things it could do.

I've tried carbide here too. It's great with some metals, but I've found, overall, I get the best results using HSS. The diamond cutter Bill mentioned is a sweet tool and one I too use quite often.

Steve
 
My experience with the Myford is pretty much the same, the rear tool post is pretty much an essential on small machines. HHS tooling is much better, and in some instances good ol' carbon steel even better still. If you must use carbide, stick to the indexable inserts - making holders for 'em is simple enough. There is some excellent info on the diamond tool in these pages - well worth the effort to build one.
 
Tel
While I've got you cornered on an active thread....I'll ask a favor. On another board you once posted a tool you made for reducing small rod diameters. Could I prevail upon you to post a thread showing the tool to the gang gathered here?

Steve
 
Hi guys,

Just a small point to think about on the parting issue: -

Whilst the method of using the parting blade upside down in a rear tool post is well documented, and works extremely well, especially on the smaller lathes, I would like to point out that using it in this manner maintains the normal forward rotation of the machine spindle.

Using the tool upside down in the normal (front) toolpost, requires the machine spindle to be run in reverse, as Minerva has done.

PLEASE NOTE

There is an inherent danger with this method if the chuck is a screw on type as is the case with Myford and many of the Chinese imported small lathes.
The strain of cutting can, and often will, unscrew the chuck.... you don't want to go there. :( :( :'( :'(

If I might suggest.... if you need to use the inverted tool method, then DO invest in, or make, the correct REAR tool post.

Play Safe guys, I would hate for one of us to have an avoidable accident.

Best regards.

Sandy. ;) ;)
 
The big problem with small carbide tools is the nose radius.
Most carbide tools 3/8" or smaller come with a .015" radius.

You will not find this in any book and it may be totally wrong,
but it has worked for me over 30 years.
I'd be disappointed if it is not debated here.

For rough turning I feed 1/2 the nose radius of the tool.
For a .015 radius the feed will be .008"
For finishing I feed 1/4 the nose radius of the tool.
Using that same .015 radius tool the feed will be .004"

HSS tools generally have larger nose radius on them.
If it's a hand ground tool you could call that an accidental advantage.
It would be very difficult to hand grind a radius as small as .015".

A larger nose radius will almost always give a better finish.
For carbide or HSS, the cutting point removes the material.
Just behind that cutting point the tip will have a burnishing effect.

Of course there can always be too much of a good thing.
Too large a radius will create a new problem. It will quickly teach the
meaning of tool chatter...

Rick


 
Sandy Thanks for the clarification and the carevat.
The 7 x usually have a bolt on setup to hold the chuck A bit of a pain to change chucks and face plate but no worries on the chuck unscrewing.
IMHO One of the most useful accessories for any lathe is a decent QC tool holder set up.It will save much time and frustration.
Tin
 
I believe the major problem with the cheap indexable sets is the inserts themselves.The toolholders are no better or worse than others,but invest in some brand name inserts.I think you will see a large improvement in finish.
 
Thanks guys for your invaluable input,

to clarify the parting off method: tool post in the conventional position ( front ) parting tool mounted upside down on centre height , with the machine running in reverse

As I see it the theory of tool loading/flexing may be compared with the rear mounted tool , and it certainly works.

My 7x12 has the bolt on variety of chucks so unscrewing the chuck is not a problem. As to the reason why a parting tool mounted in this fashion will give a superior finish using conventional techniques,God only knows and at this juncture he, isn't telling :bow:

As to the many other comments , I take them on board and Steve, I heartily concur every day I find myself wondering how I can "push the envelope and I am looking at the various steps needed to modify the machine ( I need to turn approx "5" ) and it sound as though it is well within the capacity of this very worthy machine.

Kindest regards and many thanks

Terry T
 
Cedge said:
Tel
While I've got you cornered on an active thread....I'll ask a favor. On another board you once posted a tool you made for reducing small rod diameters. Could I prevail upon you to post a thread showing the tool to the gang gathered here?

Steve

The Box Tool? OK Steve, I'll put something together in the morning
 
minerva said:
As to the reason why a parting tool mounted in this fashion will give a superior finish using conventional techniques,God only knows and at this juncture he, isn't telling :bow:
Terry T

One reason is that the chips fall away from the cut, not into it. That is one of the advantages of rear mount tool posts.

Regards,
Bernd
 
I have found that cheap cutters are a waste of time and money . I purchased a set of import indexable tooling for the mini lathe IMHO more trouble than what it is worth. I bought some "lathe bits" from HF got them home was disappointed. IIRC last January I finally spent a bunch of hard saved pennys on a indexable HSS set from A.R. Warner Co.
these stay in my QC tool holders. They are not cheap but they are made in the USA and top shelf.
Tin
 
Were you holding the carbide holders at an angle like that cutoff tool? If so thats why they were not working. The top of the tool should be flat at center height.

Parting on one of those machines is a lesson in futility. I usually used a hacksaw when I had one of those lathes.

One thing that will help on that lathe is ditch the pseudo lantern tool post and get a quick change tool post.
 
Well I don't usually have any trouble with parting off with my 7x as long as the machine is properly adjusted although that usually means doing it every 2 sessions and I haven't been working with particularly hard materials......

But I have been struggling turning 4.5" ring of Phosphor Bronze today (for about 2 hrs) and was just about to throw in the towel when I remembered this post...... Flip the tool upside down, adjust the height, I found that about .1mm below center worked best, run the machine in reverse and hey presto, decent depth cuts and no chatter or biting...go figure???

I haven't come to parting it off and I'm sure that will be a different story.
 

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