Parting off - front or rear

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Not if youv'e got a screwed on chuck

Roy

Depends on your lathe. If your chuck screws on DO NOT try to run in reverse with an inverted tool! Chuck will unscrew.

I have an inverted parting tool for a rear toolpost - no modification needed to cross slide on my Super 7. I also have a Sandvik inserted bit tool in a conventional toolholder, and almost always use that in the front toolpost with normal rotation.. IMHO the secret to successful parting is NOT to run the lathe too slow, and to use slow auto feed. You need a decent surface speed to get the tool to cut, and a steady feed rather than a nervous jerky feed because you're apprehensive which in conjunction with slow speed causes a dig-in.

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So, yes you can run lathe in reverse and mount parting tool upside down. Still pays to keep plenty of coolant sprayed on the cut, or at least cutting oil on a brush or squirt bottle etc. Run the spindle a bit slower than you would for normal turning but keep the feed up to the cross slide good and steady.

Bill,

Although I am in the process of making a rear tool post for the mini-lathe, a couple of weeks ago I did try a 1.5mm parting tool upside down at the front and it was much better, give it a try you may find a big improvement.

Ron

This is mine,uses 1/2"hss blade, inspired by Hopper. Mick 🇬🇧

May not be as simple as turning tool/s upside down, you buy left or right hand.

I do have a rear tool post i bolt on for repetitive work ie gang tools up, these are LH opposite hand to the norm but do have to spin in opposite direction.
Put a RH tool in rear tool post upside down, just use normally (spindle anti clockwise) as if its in the front post - downside is you may not be able to see the tip and whats happening.

Will get a far better result Mick if it was whole like front post, assured that will bend away and or flex.

I'm a convert to a rear tool post for parting off. No dig ins at all since changing and I love the way the swarf drops clear. I use a tipped blade in a Dickson style inverted holder.

Running in reverse from the front doesn't give you much advantage over conventional way. Primary win is that any flex due to attempt jamming or shifting a sticky bit of swarf is upwards and relatively free. In conventional mode deflection is down with serious jamming effects as the work tries to climb over the tool.

Big wins with the rear tool post are :-

1) it bolts to the cross slide rather than the top slide so eliminating one set of joints and all the potential flex therein.

2) the feed screw operates in tension which is effectively much stiffer than compression. Especially with our skinny screws which will try to bend under load (I've measured the effect on a SouthBend 9 in decent nick) so the cutting force is a bit indeterminate as effectively being via a strong spring so the tool tip isn't under proper control should cutting force vary.

Another point in favour of the rear post is that being permanently mounted it's worth taking more trouble to set it up just so. Can key it in real solid too. Even with a QC system in the conventional position its worth making regular checks to ensure the post is dead square to the work. I pretty much never shift my topside but do find that occasionally the Dickson T2 has wiggled round a couple of degrees. Which makes a difference.

When swopping tools around odds are its gonna be close rather than dead nuts. Especially with the job in the way making access just that bit harder.

Clive.

This video puts a different slant on the parting off problem .

**LINK**

My father told me that he successfully used this method when working on a worn-out lathe in Portsmouth Dockyard in the '30s. He also fitted greasers in place of the oilers on the headstock bearings, and when parting off screwed them down so the bearings were chock full of grease.

George

I'm another user that likes the rear part off tool holder albeit only when I need to part off MS of a large diameter. I made up mine from a kit from Hemmingway's ( usual disclaimer ) & have successfully parted off 1-1/2" under power with neat cutting oil as a lubricant....



The original design mooted a slanted slot for the blade, which can be seen centre right of pic but found that there was too much flex & overhang so decided to make alternative use of the other side of the tool post design with alternative holder, works a treat. For the amount of small stuff I do it's not a problem to use the front mounted system & I can comfortably part off under power, brass, ( dry ) aluminium, at 400 - 500 rpm ( WD40 ) & dropping the rpm to approx 300 for MS., ( neat cutting oil ) with minimum feed rate. For extra rigidity I have the compound slide set back to provide more rigidity under the tool post, so far works fine for me.

Tool post set back...



George.

Definitely worth eliminating the top slide.

No, the feed screw is NOT in compression when parting off from the front, at least on the Myford and many similar lathes. The feed nut is fixed to the saddle, while the feed screw bearing is on the cross slide. To move the tool towards the lathe axis the screw PULLS the cross slide. A skinny screw might be an issue with a rear toolpost because then the screw is in compression when feeding towards the axis. I wondered about your observation when I first read it, then was idly watching my lathe part its way through a brass bar and realised the screw was pulling the slide.

The Dickson type holder has a dowel hole which should ideally match a similar hole in the top of the slide, with a close-fitting dowel to fix it in the right position. I found that removing the top slide and mounting the Dickson onto a rigid cast iron base with a locating dowel has made all the difference to being able to repeatably set tools for CNC turning.

I don't know the answer to the question in the OP, as I've never had any ongoing problems with parting off conventionally from the front, once I'd got over some initial hiccups.

John Haine has hit the nail on the head - the key is a steady feed combined with a minimum rate, especially with insert tooling. I never part off at less than 4 thou per rev, using power cross feed. One can part off at slow spindle speeds, but I'm normally in the ballpark of 500rpm.

Andrew

Having once used an old HSS blade, inverted on a rear toolpost never want to go back. On the rare occasions parting off from the front, had dig ins so bad that they eventually wrecked the holder for the carbide bit.

HSS is cheaper than Carbide. The HSS bit was given to me when I bought my ML7 about 30 years ago. and still using it on my 15 year old larger lathe.. A smash up with a Carbide tip means about £5 each time.

I have a bottle of soluble oil arranged over the lathe to provide a drip feed to the tool. The Jet is held on a small magnetic base. on the Cross Slide. Lubrication definitely helps the process, and minimises problems cutting steel.

Last parting off job was a piece of brass, 2.75 inches dia, obviously. done dry.

Howard

Rear toolpost for me every time.

No mods to cross slide and easily removable if necessay but stays in place most of the time.

Made this to GHT design years ago but modified for a tipped insert (from Warco IIRC) and never looked back.

Invered chamfering tool on the other side - just takes a second to turn it round.

I think the vid above (xd 351) says it all. Rigidity is the main problem. Support under the cutter or fixing to the cross slide is better than a floppy top slide mounted cutter. Reverse direction, and an upside down cutter on the cross slide ,is likely an even better option. End of story for me, I think. Case proven, QED and all that.

Yesterday i made parting tool support along the lines of the one in the video i linked to in an earlier post .

I decided to do some experiments to see if it did help rigidity and it did but did it stop dig-ins no it just broke the parting blade so i loaded a new insert in the other end and tried again - same result .

I then tried HSS and got the same result with it starting out ok and with pleny of cutting oil i got half way though a 25mm bar of steel then i would feel some resistance on the feed handle and bang a broken blade , i left it for the day and went to have a good suck on my thumb while i lay in the foetal position in a dark room - the insert parting blade cost around $80 au so i was not a happy camper to say the least !

Next day : I thought i would go over the machine just to check everything was adjusted ok and all was well so back to the drawing board! By now i was getting desperate and the only thing that sprang to mind was to try a different piece of steel ( I wasn’t sure what it was but assumed it to be free machining steel or BMS ) I had a dig around my stockpile of steel bar and found a piece that i knew was BMS and chucked it up in the lathe and most of the problem disappeared! I then went through all the different partting tools again although this time i had an old carbide tipped grooving tool to try as well and didn’t have another dig in , i tried the grooving tool and it flew through the BMS so i thought i would go back to the original piece of bar and try it but first i wanted to check that all of my insert tooling was set to centre. This was fairly uneventful for the most part except for one toolholder - a R/H turning and facing tool which regardless of what i tried just wouldn machine , it grabbed and chattered so i tried a new insert with no change. I had a L/H tool with the same sort of insert that worked fine so i switched inserts and the issue dissapeared ! I have a few sets of this style of insert that i got of ebay and a tool seller i regularily use and looking at them there is almost no difference in appearance except one is slightly darker so i did a comparison between the two with the result being one set worked 100% the other set just wouldn’t cut . Why I don’t know - need a comparitor or microscope to check them over - i have neither and as they were cheap they found a new home in the bin !

Back to the other bit of steel that gave me so much grief - the the grooving tool did part it of and instead of breaking of it actually stalled the lathe so it wasn’t without incident ! I then tried turning this stuff with the now sorted insert tooling and it wasn’t pretty ! The chips wanted to weld to the insert and some cutting oil did improve this a little but i still don’t like it so this stuff is now separated from the rest of my stock until i work out what it is ( i thought it may have been 4140 that i picked up off ebay but i have machined a bit of 4140 and this stuff is nothing like it ) .

One thing i did notice with one toolholder was the way it clamped the HSS blade allows the back end to work loose as the slot for he blade has parallel sides and the clamp is only at one end 😳. See pics .

Still sort of scratching my head as to why the much wider grooving tool did the job ? Maybe the thinner tools are picking up something on the tip that is causing the tool to be forcd off centre and the tip is biting into the side wall of the groove ?

I put this up here to show that a rear toolpost may or may not solve your problem , sometimes it will come at you from a different direction and blindside you ! Cheap inserts or cheap HSS along with what you are trying to part off can all play a part .

Unfortunately my South Bend lathe has no provision for mounting a rear toolpost. The top of the saddle wings are not machined so I can't rig a support under the parting tool. Damn.