Parting Clearance

Hi All
I have made a fair amount of progress on parting off since I last asked about it. I 've parted off 2 inch steel bar (well actually I had to saw
the last quarter inch as the tool wasn't long enough) and I've parted off 3/4 inch steel under power crossfeed.
I just have one problem left to solve which is that everything I part off is concave.
I'm using a HSS blade in the rear toolpost angled down to provide the top (bottom at the back) rake and the tool is set in the holder so that the the taper of the blade is the same on each side. The tool is set parallel to a bit of bar faced off in the lathe. The top of the tool is as straight as I can grind it.
The setup is based on the rear toolpost described by George Thomas and he says "with the eclipse type of blade there is no back taper hence the necessity for making sure that the tool is mounted square to the lathe axis".
Following his advice I haven't ground the sides of the tool behind the cutting edge but I can't see any other possible reason for the tool to be deflected.
There are some pictures here http://www.hockerley.50webs.com/parting.html
I'd welcome any advice.
Russell
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Russell wrote:

That's pretty much the case with any parting off tool because no matter how rigid you try to keep things there will always be side deflection of the tool. Also you can't escape the fact that the SFM will be lower as you get closer to the centre of the bar unless you've got a fancy CNC machine which ramps up the rpm in inverse proportion to the cutting diameter. Similarly as you get closer to the centre the stock you're parting off is attached less rigidly so that changes things as well because the tool is not experiencing the same conditions on each side of the tip.
Always strive for rigidity. Part off as close to the chuck as possible. Tighten up the freeplay in any gibs which aren't actually moving during the op. Keep the protuding tool length as short as possible. However I'd accept the fact that parting is rarely going to provide a finish quality surface and that a final facing op is usually needed to remove any pips and achieve flatness. Just leave a little stock on everything to allow for that.
On his 5 ton CNC machine with top quality tooling and flood coolant my mate can part off near as dammit flat on just about any material but on my Student I resort to hacksaws half the time. I do have a proper Comorant blade and tips but never got round to making a holder for it and with brazed tip tools or bits of HSS I usually abandoned the attempt on anything tougher than aluminium or bigger than 1 inch in diameter. If you're parting off 2 inch steel bar successfully without breaking everything in sight I'd consider that a bloody good result regardless of how flat the final surface is.
--
Dave Baker



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wrote:

Dave I would definitely make or buy a holder. Since I made a holder and blade for the 2.75 tips that J&L sell I have not looked back. Best thing I ever did. Absolutly no part off problems since. Whipped through some 8mm mild steel bar the other day at 1100rpm sweet as a nut. Biggest I have parted so far is about 50mm ( little slower <G>).
The HSS part off blade and holder that came with the QCTP set is now used just for hollowing out the sides of flywheels.
Richard
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Eh ? Why do flywheel sides need to be hollow ?
--
Boo

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On Wed, 18 Feb 2009 18:44:24 +0000, Boo

They do not if you want very thick spokes <G>
Richard
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Richard Edwards wrote:

Thanks to all for the constructive suggestions - I'm afraid the pictures were deceptive the blade was set up vertically using two squares to try and get the clearance the same on both sides.
The saddle was locked and the gib tightened, the cross-slide gib is also rather tighter than is comfortable. I can reduce the overhang in spite of the built in back rake as there is a shim under the toolholder. The limiting factor is the cross slide travel as I have tried to get the back tool post as far out of the way as possible so I could only reduce the overhang by about 5mm.
I understand Peter's comment about getting the load between the Vs and I'll have a look at that - the issue will be where it is relative to the front toolpost.
Thank you for the positive comments too - I've made such a lot of progress on this that I feel sure the last bit is possible.
I'd be very interested to see a picture and drawing of Boo's home made tipped toolholder. I've thought about that but tipped parting tools seem very expensive.
Thanks again.
Russell
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This is my home made one ..you still have to buy the blade that holds the tips
http://img.photobucket.com/albums/v190/aboard_epsilon/smart%20and%20brown/fixedsteady.jpg
http://img.photobucket.com/albums/v190/aboard_epsilon/smart%20and%20brown/PART1.jpg
http://img.photobucket.com/albums/v190/aboard_epsilon/smart%20and%20brown/PART2.jpg
http://img.photobucket.com/albums/v190/aboard_epsilon/smart%20and%20brown/PART3.jpg
http://img.photobucket.com/albums/v190/aboard_epsilon/smart%20and%20brown/PART4.jpg
all the best.markj
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Russell, Correct me please but is this not the Sparey design taken from the Amateurs Lathe? I made one and wasn't very happy about its rigidity and fixings. I then went to Martin Cleeve- and I have the fabricated rear tool post. Finally, I went to George Thomas in the Model Engineers Workshop Manual or Vol 142 in ME. You will find that Thomas developed his from the Ian Bradley rear tool post.
The trick is rigidity, rigidity and then the ability to align 100% accurately each time- with a pegged unit and tool grinding to curl the ribbon that is being parted off narrower than the kerf being cut.
Mark, Aren't these fancy carbide things rather expensive- and uneccessary?
Regards
Norman

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wrote:

Having contacts with professional machinists (Alwyn of oak engineering and many others) is a revelation ..that keeps you ahead of the times and up to date.
The fancy parting tools were demonstrated to me ...and prooved far Superior to the HSS blades ,...that make you have racing heart and hold your breath.
send it in under power ....dont worry about it ........gets the job done with no aggro........once you have it ...tips are quite cheap ... thats cheaper than doing the job again ..if you mess up with a HSS blade.
All the best.markj
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On Thu, 19 Feb 2009 08:50:46 -0800 (PST), mark

I totally agree with Mark. Once you have tried the replaceable tip PO blades you will never look back. Parting off becomes "just another simple job" as opposed to that scary operation that is likely to bu**er the job or your machine.
Richard
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On Thu, 19 Feb 2009 00:16:50 -0800 (PST), ravensworth2674

Norman
Whilst I agree that some designs of Tool post or tool holder for parting off are better than others, one is still left with the basic inadequacy of the parting blade itself. Granted it has side clearance designed in, and depth to give strength but that is where it ends IMHO.
The holder may fit the tool at an angle implicitly supplying top rake, again in my opinion a better design BUT once one grinds the tool and touches the top then the cutting edge becomes narrower than the shank of the tool. More grinding on the sides to give relief, and back by an inch or more to allow parting a two inch bar.
IMO the whole thing is a basically bad design.
Compare that to the insert tool. The shank IS narrower than the tip. The tip is formed to reduce the width of the swarf. The thing cuts without chatter and as Mark has said power feed is quite possible (with left hand on the VFD speed control knob to give constant SFPM). No dropping to low speed to part off. I can part off steel at 1000rpm on my old Viceroy, the hacksaw used to come out when I had the HSS blade.
Cost wise, depends. I made my own blade and Dickson tool holder so cost was 2.75 each for tips from J&L. Blade is usually about 30+. Though I noted an advertisement recently for a free (Sawman) blade with 10 tips at 3.67 each (2mm wide). Parting block (Korloy) at 42.91. All plus VAT.
<www.cutweltools.co.uk>
I did wonder about a purchase as I have never heat treated my blade (gauge plate).
Best Regards
Richard
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BUT once one grinds the tool and touches the top then the cutting edge becomes narrower than the shank of the tool. More grinding on the sides to give relief, and back by an inch or more to allow parting a two inch bar.
IMO the whole thing is a basically bad design.
Whilst I agree about grinding the top of a parting blade will cause problems and should be avoided, once the top surface is ground flat and NO more, it will never require grinding again All further grinding is carried out only on the front ,the top rake is built into the holder. Ned Ludd I.
--
ned ludd
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I think that Ned is nearer to my way of working. Richard, at no point are the flanks of the blades ever re-ground. In the GHT original, the blade is set exactly at 1" from the holder and therefore cuts 2" diameter. The Vee or concave rounded 'top' is never re-ground - it is only adjusted back to lathe centre height when the front is re-ground.
It is obvious that this is the cheapest tool and the simplest to maintain in first class order. The foregoing is my conclusion despite having a Clarkson etc etc- because I don't need them.

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On Thu, 19 Feb 2009 14:06:15 -0800 (PST), ravensworth2674

I have the casting and drawings to make a GHT parting tool holder, but I've never built it. A significant problem is the 2" diameter thing. With my insert parting tool in the front toolpost, I can do 5" EN24 on the ML7 and not worry about it. I picked up a couple of spare blades and boxes of inserts of Ebay last year. But if the setup is right (cutting edge needs to be two or three thou above centre) then the inserts last a very long time. Conversely, I have snapped an HSS parting blade when pushing too hard with a lot of stickout. It tends to raise the heart-rate a bit ;-)
regards Mark Rand RTFM
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'The average time for 4" stock to be cut to a 1/2" core is 12 to 14 mintes whhereas a power hacksaw takes 40 minutes' from
Page 364 Model Engineer No 2858 vol 114 1st march 1956- on a ML7 by 'Martin Cleeve'
I, too, am reading the manual- and I haven't tried to do it
Cheers
Norman
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ravensworth2674 wrote:

Thanks to all for the interesting comments on grinding HSS blades, this was one of my original concerns - I have lined up the blade as accurately as I can and I still get a concave cut - I'm wondering about grinding the side of the tool to get more clearance - though I also have an idea for aligning the tool accurately. Interstingly once the part is parted off the tool starts to cut the convex end left in the chuck - I find that very peculiar as it seems to me to suggest that the blade is being pulled to one side rather than pushed off line.
I'm still interested in pictures of home made tipped holders - unfortunately I wasn't able to view Mark's pictures.
I looked at the Sparey, Duplex and GHT designs before I made the toolpost. I rejected the Sparey design because I thought the mounting to the cross slide was too small in area and used only a single bolt. The lathe is a Boxford and other Boxford users had commented on rear toolposts being in the way so I wanted to hang the toolpost off the back of the crossslide - both Duplex and GHT have variant designs for that purpose - originally intended for the Drummond. I decided that as I was making a separate base for the post a separate turret would add another joint without much gain. I didn't like the GHT mounting for parting blades because I don't want to use a 1/16 blade with a 1" overhang - I've tried this and I can see the blade deflecting. I was however keen to try and present the blade to the work in the way GHT descibes so I decided to make a separate toolholder to hold the blade - it uses GHT's method of adjusting the vertical position of the blade with grubscrews in the tool holder. I also reasoned that a separate tool holder would give me more flexibility if I wanted to change to carbide parting tools. There is an indexing pin in the base at the back left - similar to GHTs but shorter and so probably less accurate.
The only design I looked at during construction was GHT's.
Russell
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Russell, I have been making 2 replacement 18T 20DP gears from the gearbox on a Myford Super 7 ie without a lead screw. The work is being carried back and forward between the lathe and the Warco MillDrill and the gears ganged on a bit of 1" bar. Consequently, I have had to both part off and recess with the GHT tool. I used a new Eclipse blade which had only been corrcectly ground at the front but had NOT been vee'd. The lubricant was lard oil which had collected foolish wasps over several summers. I had no problems although the lathe is past its best- and is repairing itself.
Having said all that, I would love to know how you ground and sharpened your blade. It sounds a bit like 'flaming' but what do you use to inspect your finished blade? I use a watchmaker's loup but have a traditional microscope and a cheap children's one. Again, I use a waterproof felt tip to check that I have ground away the worn part of all my lathe tools- and then hone so that I can see my dirty finger nails reflected in the work.
Apart from my poor efforts at cleanliness, GHT and Martin Cleeve both went along similar lines.
GHT said- there are no hairs so fine that cannot be split
Cheers
Norman
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ravensworth2674 wrote:
(lots snipped)

Hi Norman
My tool grinding technique is quite simple - I've not been doing this long enough to find a complicated way. For the parting blade I clamped a straight edge to the side so that I could see how I was presenting it to the wheel. I used the side of the wheel (which I generally try and avoid) to finish the top surface of the blade and ground the front on the front of the wheel finishing with a vertical pass to get the grinding perpendicular to the edge. I didn't inspect the edge too closely but relied on making sure that my grinding reached the cutting edge on both faces. I honed it lightly - no corner radii.
When I mounted it in the lathe I used a straight edge on the top of the tool to try and see how well the top surface was aligned to the lathe axis. It looked OK - within the limits of how well I could see the gap between straight edge and cutting edge.
My latest plan is to turn a bar parallel so that I can use a square to check the alignment of the tool but I've been distracted by playing with the Adept shaper I just bought.
Russell
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Shades of 'Baissez-moi et la'- yep, I do speak French. Russell, You now have a little Adept shaper- and very nice too! You can now make nice little jigs, perhaps out of ally so that you keep your biceps down to reasonable size. You now can make the little holders that George Thomas suggested in MEWM( yes?) You could make the 140degree front vee far more simply and accurately than before. If you do, you will find that it will not wander so much as a straight cut blade. With a carefully made holder to hold your parting blade, you could do the 140 degree top or a curved one using a drill press. I'm not sure that I could hold a Dremel steady enough! A baby jig would ensure that using a flat surface that your work would be parallel. After all, for a 2" cut you only need to 'vee' an inch.( sounds like I am trying to teach you to suck eggs, but I am definitely not)
Presently, I'd love to have half an hour with your baby Adept. You don't live in Geordieland, I suppose?
Let me know how you progress
Norman
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ravensworth2674 wrote:

Hi Norman
Looks like it's back to the library for me then. I'd wondered about the grooved tools but haven't experimented (yet).
I lived in Geordieland in the early 80s for a while but am now in rural Derbyshire.
The Adept looks like it should be useful - I have a Tom Senior hand shaper too - but I don't have space to leave it set up and it's so heavy that I'm always reluctant to get it out. That shouldn't be a problem with the Adept!
Russell
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