Parting Clearance

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.

Hi Russell. In your first photo, the P/O tool seems to be cocked to the right slightly at the top, giving clearance on the left and none on the right, which will not help. The second photo seems to agree with this, and your last photo shows a "lot" of overhang, which I would keep as small as possible. Also, I would think about reducing the top rake, if you can, it looks very steep in the photo. Cheers, Norm5.

Hi Russell, Obviously things are nearly right if you can part 2" stock, all you need is slight tweaking to make "perfect". Looking at your pictures, two things occur to me. First the "top" of your blade still seems to have its original bevel. Best remove this and make the "top" level (headstock to tailstock wise). Don't grind anymore than needed otherwise you will end up with jamming problems in later life. The second point is, is the blade vertical, ie. are both side angles equal. Come to think of it this should be the first point as it has an effect on point one. If the parting tool's cutting edge is not square and level there is going to be a slight bias when cutting, this shows on a thin blade more than a thick one, also why you want as little of the blade protuding from the toolpost. This is a little difficult to adjust on your holder due to the built in back rake. you have to decide before hand what size you can part off when you are making the thing. Good effort though. Ned Ludd

Make sure your carrage is locked .

all the best.markj

Second and third that.

The overhang seems to be a bit long too.

Also, can you move the blade back on the cross-slide? By that I mean move the blade holder across the top slide, in the direction away from the chuck, so the blade tip is vertically above the area between the cross-slide V's.

That way vertical forces on the blade tip have much less of a twisting, and thus sideways-moving, effect. Hope that's clear, it's difficult to describe.

However, if something isn't tight ...

-- Peter Fairbrother

That, I have to say, is NOT George Thomas's design and it does not follow his guide lines. Sorry

Norman

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 ).

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

Eh ? Why do flywheel sides need to be hollow ?

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

This is my home made one ..you still have to buy the blade that holds the tips

all the best.markj

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 >

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

They do not if you want very thick spokes

Richard

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

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.

I did wonder about a purchase as I have never heat treated my blade (gauge plate).

Best Regards

Richard

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.

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.

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

'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