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Rear mounting parting-off tools

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mgj18/10/2009 13:58:47
1017 forum posts
14 photos
Chris - tip radius will have quite a lot to do with it. They always say that your final cut should never be less than the tip radius, to stop rubbing. Or so I was told. I don't think Sandvik actually do .2mm tips. .4 and .8 I have.
 
80-300m per minute. I'm not sure my machinery can get to that speed, and certainly not without spraying coolant everywhere.
 
However thanks for that - I have noted the tip numbers.  I'll see what i can do, because sometimes a sharp corner is quite helpful.
chris stephens18/10/2009 17:42:44
1049 forum posts
1 photos
Fear not, I have used them on my Bantam which has a top speed of a mighty 800RPM! Oh and I used them dry.
You are welcome, see you in the ether.
chris stephens
PS I was machining the cast iron table, for a GHT Pillar tool, and was not happy with finish. I was getting that mottled effect, anyway I dug out a used rhomboid tip and used the only sharp corner left, the 80 degree one, and it cut great.  Note to self, after five years it's about time to finish it.
Mark Smith 303/11/2009 09:43:51
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175 forum posts
36 photos
I have read with great interest and not a lot of understanding this whole thread; am I confused? of course I am, I am a shoe maker with pretentions of being an model engineer.
I find parting off to be a chore I approach with fear as I have an ancient Barns Lathe designed about 1880 and manufactured around 1930. It has a tiny spindle, poured bearings and an overhang of the chuck of 7" from the head stock to the jaws. Now, when I get a dig in the whole chuck and spindle lifts very high and the flat belt is thrown off its pulley. The bearings seem ok so long as I check the screws for tightness on the bearing cap, but it still does it. I have found that copious quantities of wd40 helps which also keeps the bed from rusting. I would like to try a rear mounted tool post but there is simply nowhere to mount it as it has no T slots on the cross slide. Woe is me I think I need a new lathe, what think ye?
Mark
Geoff Sheppard04/11/2009 11:45:19
80 forum posts
1 photos
I carried out quite a few experiments with a variety of parting tool systems for an article I prepared for MEW quite a few years ago now. I ended up acquiring/making some eight different forms of holder and several dozen blades. One of the more effective (and least expensive) was the holder described by the late Len Mason in his book "Using the Small Lathe". This consists of two plates, between which is sandwiched a short length of hacksaw blade ground to the correct form. Some of the nicest to use were the small Churchill blades which produce a thin groove.
Quite definitely, using the Myford Super 7, an improvement was made by going to a rear toolpost and the ultimate system was a Q Cut from Greenwood mounted directly into a rear QCTP. For use with steel, though, I have found it necessary to provide a continuous drip of coolant (I use neat cutting oil) right on to the tip of the insert. This seems to keep the swarf moving away and prevents overheating. Power cross-feed ensures a continuous cut and prevents rubbing which, on some materials is fatal because of the work-hardening tendencies.
Hope this helps
 
Geoff
Peter G. Shaw04/11/2009 19:34:19
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1531 forum posts
44 photos
Geoff Shepherd mentions Len Mason's parting tool system using two plates and a bit of hacksaw blade.

I tried this, and didn't have much success - my blades kept breaking.
 
What I did do though, was to use the same idea but instead of hacksaw blades use an old 4" file ground down to a thickness of 1.8mm, (1.8mm because that's where it ended up being successful!)  and a blade length of about 10 to 12mm. Using slowest feed and plenty lubrication, It worked perfectly despite the fact that the clamping device was rather poorly made.
 
Incidently, it's used as a front mounted cutter working the right way up.
 
Regards,
 
Peter G. Shaw
calder percival 120/02/2010 23:37:52
19 forum posts
1 photos
the rear toolpost originally came to counter the split cap bearings found in most model engineers lathes in the early days the effect was to thrust the weight of cut into the casting and not against the bolts that held the bearing caps down. You will find that a keen tool with the correct rake and set ever so slightly below centre will part clearly all the time coolant is essential! i have just parted off two rings 4" diameter with hand feed tonight in b.m.s for backplates on my traction engine hubs a little care goes a long way.
KWIL21/02/2010 09:50:55
3681 forum posts
70 photos
All very interesting, in the end its what works for you.  Personally I am a Sandvik/Iscar person, front mounted on all sizes of lathe, when used with fluid the wet stuff stays on the top surface where it is wanted!
Dunc22/02/2010 02:42:07
139 forum posts
Regarding the use the (front) toolpost but run the lathe in reverse...
Here is another commentary
http://email.villagepress.com/pub/HSM/20091211/20091211.html
Nobby10/01/2011 22:55:17
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587 forum posts
113 photos
Hi Tony
I part off from the back as Michaels says it digs out . The reason is because as you wind in from the back the backlash does not come in as the nut pushing on the front of the thread.and no movement can accour. Ps this may have been said already
Regards Nobby
KWIL11/01/2011 09:01:49
3681 forum posts
70 photos
Nobby, As far as I can see, if the tool digs in it still has the backlash freedom to move into the work.  Working from the front you are pulling the leadscrew on the back of the nut and therefore backlash allows it forward towards the work, if working from the back, the leadscrew is pulling from the front of the nut and can still  move towards the work??
 
As I understood the theory, from the front you are pressing down on the slides which "loosens" the dovetails, thereby making their movement easier, whereas at the back you are pulling the dovetails tighter,  (but are also trying to lift the carriage).

Edited By KWIL on 11/01/2011 09:05:38

blowlamp12/01/2011 10:46:42
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1885 forum posts
111 photos
Provided that the workpiece itself is mounted rigidly, there can only be two causes for a cram-up.
Either the tool digs in somehow, due to something like backlash or bending of the support system, or there is chip crowding of the groove being machined.
 
In any particular situation, it might be worth experimenting to find which is the cause of this bother, by cutting a groove just a few thou' in from the end of a bar, such that the full width of the tool is utilised, but (apart from the thickness of this flimsy shim) one side remains open for swarf clearance.
 
If a cram-up still happens, then it must be that the tool is moving into the workpiece for some reason. If the problem disappears, it must be chip crowding.
 
Martin.
Nobby12/01/2011 16:11:10
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587 forum posts
113 photos
Hi
Backlash ?
With m/c turned off .Try this?  Wind x slide in stop then push forward and see it move forward    Then wind in as if parting off from rear try pulling ?  be carefull of any sharp tool in toolpost .

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