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Tipped Tools

Rough Finish?

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Dave Harris14/06/2009 12:55:06
28 forum posts
i am trying to use tipped tools for the first time. The finish I get is very rough . I picked up a book at Harrogate, produced by Sandvick which appears to suggest that for 8mm tipped tools I need a 1 to 2 hp motor and the cutting speeds should be in the hundreds of feet per minute!? Can someone explain why i am getting a very rough finish and are these tools suitable for use on a  Warco 918 lathe with a 3/4hp motor? how does one translate the cutting speeds into rpm on the lathe and should I be taking the fastest and finest cut possible by hand turning to get a good finish?
David Clark 114/06/2009 13:20:48
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3357 forum posts
112 photos
10 articles
Hi There
The Sandvik book will only be suitablefor commerical machines.
Small lathes are not rigid enough to withstand recomended speeds.
 
I would use a fine feed and a reasonably high speed. Certainly no more
than 1000 revs.
 
Finish might be down to the material.
Are you using a freecutting mild steel or any old crap lying around?
regards David
 
Dave Harris14/06/2009 19:21:34
28 forum posts
I am using free cutting mild steel. I dont know the manufacturer of the tool and tip, it was bought from one of the suppliers who advertise in ME. the tip seems to be 'blunt' in that it has no sharp edges - a 'duff' tip?
John Ladlow14/06/2009 20:09:33
17 forum posts
I use tipped toolsallthe time and have done for several years.
The one thing they need to do is to take a reasonable cut.They don't like small cuts because the tool tends to skim over the surface.
If you take a decent cut you should find the finish will improve.
The other thing is that some unbranded tips from some of the smaller suppliers are not very good anyway.
Recommend greenwood tools.
John
mgj14/06/2009 23:34:14
1017 forum posts
14 photos
John - I recommend Greenwood too. I think they supply tools with Sandvik tips. Seco tips are very good too. I think the tips supplied by Chronos are mostly Seco, though I don't know about their parting tips. Howver the CCMT -06 tips are. (Greenwoods Kit Q cut parting thing is super - that uses the Sandvik T-MAX Q cut tip N151-250 -5E. There is a 5F tip which I prefer - better finish for a lower feed rate. There are also right and left handed ones to drop the cut off bit, pip free (the designation is N151r/l xxxx etc) thats a 2.5mm seat.
 
Dave, pretty well all tipped tools will seem blunt - - The full designation for the positive rake lozenge shaped tips is a CCMT -06 (cutting length) - 04/08. Nose radius in mm. Most of ours are.4mm radius though the .8 mm are very good for roughing, or producing a good finish. However you do need to bevel  whatever is to fit over it..
 
Your final cut, in crude terms, should not be much less than the nose radius, or as John has said, it will tend to skid - that is the recomendation. However, that normally leads to faulty dimension, and not usually poor finsh. With good tips I regulary take cuts of a thou or so against the recomendation and it works fine - but I am pretty confident that the headstock bearings in both lates are 100%. However, I've never yet been able to skim say 1/4thou with a tip, which I can with ground HSS.  (theSuper 7 has a 3" scale graduated in 1/2 thou)
 
Finish - you can overdrive them. The Sandvik recommendation (for a CCMT 06 for normal finish), is a feed rate of  .1-.4mm per rev. The SECO is rather better. But you'll know because it goes rather graunchy. The chip stops being "nice" and the finish can go. If you are hand feeding you'll feel it instantly. As the tip gets older abrasion affects it, and the finish goes, so does that apply?.
 
i have a suspicion you may have found a slackness in your lathe. Assuming the tool is at centre height, and the top slide gibs are properly adjusted (or the top slide is locked during the cut) and feeds and speeds are correct, and the coolant is good, I'd look at the back of the saddle - the gib there.
 
I had exactly the same problem with a new 6" x37" lathe - actually with both ground and tipped tools. Finish and to a lesser degree dimension went to pot after about 3 months. I think the packing grease, which went hard, and probably wasn't really 100% cleaned off from new under the back of the bed, and within the gib blocks themselves, started to wear out or be squeezed out - anyway, outed one way or another. Another symptom was mild chatter on parting off, until you took a decent deep cut, when the chatter would go. 
 
What was happening was this. Load the tool, and the saddle tilts slightly as the tool bites. As it lifts, the cut reduces fractionally, so the saddle drops, increasing the cut, so the saddle lifts... and so on. All on a small scale of course. Anyway, checked all adjustments, cleaned off the rear gibs with white spirit, and under the rear bed where the gibs bear. Readjusted till I could just detect a drag with a set of spring scales (old fshing ones)and problem gone, and has stayed gone. 
 
As for using tipped tools - well they are only a convenience, if you don't have a proper tool and cutter grinder. You wont take a cut of 250 thou, which is 1/2" on diameter on a Super 7 with a tipped tool in free cutting mild. (CCMT06), which you can with a properly ground up high speed tool. With the late set at 600 rpm it does it on a hand feed without even blinking - reasonably slack belts too! I tend to use them for roughing and cast iron, and grind everything else up on a Quorn.
 
Industry uses them of course because of repeatability, high cutting speeds and because relatively its cheaper to change a tip than to have to get all reground and reset. But for us, I think the convenience comes pretty expensive. 
 
Lastly cutting speeds  - about 600rpm for anything steel or so it seems!!!!
 
3.8 x cutting speed in FPM/job dia in inches. = revs. also applies to milling, but substitute cutter dia for job dia.
 
Cutting speed in mild - I always cut at 100FPM, because Tubal Cain (TD Walshaw) said so somewhere and Tubal Cain was never wrong. And it works well -  tipped or conventional tools.
 

 

 
 

Edited By meyrick griffith-jones on 14/06/2009 23:46:17

Dave Harris15/06/2009 08:25:27
28 forum posts
Gentlemen, thanks for the advice. I have tried what has been suggested and if I take  a deeper cut, the steel i am cutting just looks like a ploughed field! tried a very light cut and the tool just skims the surface without actually taking a cut. i have no problem using either hss tools, or the the tools with a brazed on carbide tip, so I think I will stick to what works for me. Advances in technology are not always a blessing! The one part of my question that remains unanswered is the point about working out the rpm for the lathe........how does one work out the cutting speed in fpm when cutting by hand?
Lawrie Alush-Jaggs15/06/2009 15:14:31
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118 forum posts
32 photos
Hi Dave
You will find the info you need on surface feet per minute here. http://en.wikipedia.org/wiki/Speeds_and_feeds
 
Another view on what is happening.  I recently purchased a decent sized lathe after many years of not using a Unimat SL.  I had the devil's own problems with surface finish using tipped tools.  In the end it turned out that the lathe was not set up correctly.  I dug out my 1960s copy of the South Bend "How to run a lathe" book and went to the section on setting up the lathe.   They suggest turning two collars on a peice of ~35mm steel about 125mm apart and measuring the difference in diameter.  It does work and it works well.  I did find though that the job of levelling the machine was made much much easier after I went out and bought a machine level and a prescision test bar.  All up it cost me about AUD$420.00 but the fact that I can use the bar on the milling machine and as a cosh makes the price reasonable.
The test bar is probably over kill but the level isthe thing that really does the work.  I got the bench (3.2m x 600mm) and so the lathe to within 0.01mm over 750mm and across the bed and it just cuts wonderfully.  I was recently cutting a peice of silver steel for a transfer punch using HSS.  The length of the shaft was 28mm x 2mm and I was able to take 0.04 cuts without additional support and get a good finish.
The final pint is that once the lathe bed is set up level, the tools really do have to be on dead centre. 
The South Bend book is available from Amazon, it seems to never be out of print.
 
Dave Harris16/06/2009 07:43:03
28 forum posts
Lawrie. Many thanks for the information and directions to the book and wikipedia information. This is certainly a help. Will let you all know how i get on when i get back in the workshop later this week.
many thanks for all the replies and it is nice to see that we can help each other even from the other side of our world with the help of 'IT' and this web site.
Many thanks to ME for the site and to you all for your advice
Peter G. Shaw16/06/2009 13:53:38
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1421 forum posts
44 photos
Hi Dave,
I've had similar problems as you and now don't particularly rate carbide tipped tools for my purposes. Apart from the rough finish, I don't like the inability to get into corners and the distinct likelyhood of the tip being chipped. Having said that I've got them, so I'll have to make the best of it.
 
What I've discovered works best for me is as follows. Initially rough turning with little regard for finish to get down towards approximate size. I then follow up using finer and finer cuts until at the end I may be doing as little as 0.02mm deep with as fine a manual feed as I can manage. I find the slowest fine feed of 0.1mm per rev is too coarse. Furthermore, I also repeat the cut a number of times at the same settings until virtually nothing comes off. By this means I can achieve a reasonably good surface.
 
Where I wish to cut into a corner, I use the carbide tool to roughly get to shape and then finish with a HSS knife tool with a slight flat on it. Again I use fine feeds and cuts with plenty of repeat cuts.
 
In respect of feed, I use the maximum speed I can (2000rpm) for carbide, and normal speeds for HSS. I do use a cutting fluid (Neatcut from Warco - usual disclaimer) for both carbide and HSS, but tend to cut dry for the finishing cuts on carbide.
 
Now, I'm well aware that what I've said goes against the usual recommendations, but it works for me.
 
One other thing is that the rough finish prevents accurate measurements being taken, hence another reason for multiple cuts to obtain a good surface for measurement.
 
I also have doubts about the accuracy of my lathe, it's graduations and adjustments - yet another reason for multiple cuts in order to, as Len Mason says in his book Using the Small Lathe, "to work the spring out".
 
One final thing. I now use diamond hones to keep my cutting edges sharp. (And to repair the inevitable chipped carbide!) It takes time, but does seem to pay off.
 
Good luck.
 
 

Edited By Peter G. Shaw on 16/06/2009 13:55:51

James Jarvis16/06/2009 14:14:29
1 forum posts
Dave
 
Many excellent and useful response all ready and probably don't have much more to add. I too have some cheaper tipped tools that leave terrible finish I have BH600 so plenty of power and I mainly use them for roughing out. I also swear by Greenwood tools but due to expense only use for finishing cuts, they once came to give a lecture many years ago at Plymouth miniature steam it was really good and they explained the grade of carbide/cermet they use has been designed for ME on the basis that we take shallow cuts at lower speeds, they saw a niche in the market and worked with the producers of the tip to come up with something. I brought the parting tool as well and at the time had an old hobby mat lathe which I couldn't get setup to work with any parting tools. Anyway this parting tool worked a treat and I still use it now nearly 8 years on.
 
If you have something that works for you then stick with it, but tips are very useful and I do think greenwood IMHO was a good investment for me, even got my father in law to spend money on a set and even he was impressed and that's some going
 
Also on the cheaper tips, a higher speed and cutting oil gives better finish but no where as good as brazed or HSS or the Greenwood tools.
 
BTW I do not work for greenwood.
 
have fun
mgj16/06/2009 18:32:07
1017 forum posts
14 photos
Lawrie - that is a very good tip - if you are trying to set your lathe up straight. ie ensure that it does not turn taper. Its in several books - Ian Bradleys Myford Manual, Tubal Cain covers it somewhere, and I think Myfords own instructions cover it too. A Myford has proper screw jacks with which to bring the bed straight.  After doing that check, one can then align the tailstock.
 
Its a standard check that everyone should do when a lathe first arrives, or is moved. And about monthly or so thereafter if accuracy is to be maintained. 
 
BUT with the greatest of respect, it isn't going to sort out finish, unless the lathe is so far twisted that, and it is allowing, or making a clearance for the saddle, and the saddle can then permit chatter. 
 
Of course, all this may just be a matter of the tool being set a touch low, and at the point of cut there is a cross-tip vector which causes tearing.
 
As for measuring and accuracy- I was always taught to take as many roughing cuts as needed. The final margin was always to be divided by 2. so if one had left 20 thou by measurement, you'd take a .010 cut, remeasure and take a final cut. Not too thin so it doesn't rub, but importantly of a size that closely resembled the last measured cut. Thus any spring or whatever in the work would be the same for both measuring and finish cuts and both should cut the same amount. (so what you set as a cut is what you get as a cut!)
 
The only time one should need to take shaving cuts is when deep boring and then the tool springs, so it should come out under power on the reverse cut before measuring, or the hole will come out oversize of course.(as we have all done I guess) Anyway, one won't really get away with shaving cuts in workhardening materials like stainless, should one use that for bits of steam engine, and even some gunmetals. (Same result, different reason)
 
Achievable accuracy. Well George Thomas reckoned that anyone with a lathe in sound condition and decent tools ought to be able to hit a diameter +/- .0002. Within 2 tenths of a thou. 
 

Edited By meyrick griffith-jones on 16/06/2009 18:34:41

Dugson17/06/2009 07:13:20
11 forum posts
Dave and anyone else interested,
Your trouble with finish may be due to the tip having too little front clearance.
I purchased a set of tipped tools in a wooden box (made in china I guess) and could not get them to cut properly. On careful investigation I discovered that the tool holders did not hold the tips at the correct angle, the result was that the tool rubbed a little on the cutting edge, hard to see but definitely a problem.
My solution was to re-grind the front of the tool, (holder and all), to provide extra clearance.
It is important with small lathes to present a sharp corner to the work this does not mean a sharp point, the point can be radiused, but the corner between the top and the front must be sharp.
It may be woth having a close look at this.
Dave Harris19/06/2009 19:29:19
28 forum posts
Dugson, thanks for the suggestion!  Having established correct tool cutting height etc I have now checked the tool as you suggested.... and hey presto.....the same trouble as you you have described. I have bought a new tool from a local engineering supplier who happened to 'just have one left' and it cuts a lot better than the tool i originally bought from a model engineering supplier! Will be looking a lot closer at the offerings in future!  Again many thanks to all for the comments/suggestions. Hope this sort of helpful contribution continues .        

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