Not if youv'e got a screwed on chuck

Roy

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

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.

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.

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

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.

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

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.

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.

Today i had a play with some new parting tools – one from eccentric engineering and another from a local tool supplier The F.O.R tool by eccentric uses T shaped blades and the other the conventional wedge shape blade .

Both of these are mounted upside down in the toolpost ( QCTP) and i ran the lathe in reverse , if you have a screw on chuck you will have to use them in a rear toolpost . I started out with some 30mm 6061 T6 round bar running at 250rpm and neat soluable oil for cutting fluid . I followed the advice on sharpening the blades as supplid by eccentric enineering – use the side of the wheel to give a 4 deg relief , many use the front of the wheel which gives a concave grind and too much relief .

FIrst cut with the F.O.R was going ok but the chips looked like they were tearing out and not cutting well so i switched over to WD40 with an immediate improvement – the soluable oil didn’t seem to like aluminium and i also tried some trefolex ctd spray which worked ok and i repeated the same process with the other blade with similar results .

Next up was some bms at 20mm dia and as in the first test i tried the neat soluable oil and this led to my first jam up but fortunately I didn’t have the qctp clamped too tight so the holder popped up and stopped any damage ( mental note : don’t tighten the qctp lever too tight as this acted like a safety device and stopped the blade breaking ) . I reset everything and tried the ctd spray which worked better , I didn’t bother with wd40 as i never considered it as a cutting fluid for steel ( i was seriously mistaken on this ) .

Now that i seemed to have it going in the right direction I thought i better bite the bullet and try some larger dia steel , this time 40mm roundbar of unknown composition – i had machined it before so knew what it cut like . I didn’t try the soluable oil on this for the first cut as i knew from the previous tests it wasn’t the best so so ctd spray was used first .

I did the first cut in two stages with no dramas with either blade and considering the t shaped blade is only 2mm wide i was quite surprised it didnt wander or break ( i would learn a little later that this blade is one tough little blade ) and the other blade is 3/32 wide . Anyhow i parted of two disks with each blade with no issues and i did switch between ctd spray and soluable oil on both the last disks but i could hear a difference in how the blade was cutting so soluable oil went back on the shelf .

I did one last test with both blades using power cross feed and found the constant feed seemed to help a little , on the last cut using the wedge blade for sh#ts and giggles ( or possibly tears ! ) i thought i would try WD40 just to see what happens – You wouldn’t believe it but it worked better than any of the other stuff i had tried by a country mile ! I then set up the t shaped blade and tried it using WD40 and got exactly the same result so half way through i reached for some RP7 spray which caused an immediate seizure ripping the blade upwards along with the toolholder . Fortunately it didn’t damage the blade or holder but it did open my eyes ! You would think all those types,of sprays are all the same but not so ! I reset everything and started the cut again using WD40 and it cut all the way perfectly . On the last two cuts i cuts i did i cut through in one pass and was pleasantly surprised that the thin t blade came through with flying coulours ! The t blade has a slight concave grind along its top edge to help curl the chip in to clear the sides of the groove – the wedge blade has a convex v on top with the apex offset and I don’t know if this is intentional or not so i left it on there and it did seem to fold the chip and feed it out to one side of the groove .

Cutting speeds :

alloy 200- 300rpm for 30mm bar worked ok

steel 120 rpm for 20mm and 60rpm for 40mm dia ( i did switch up to 120rpm about half way through on one cut but didn’t see any gain other than alittle time saved but at the risk of a jam up so I don’t feel it to be worth it

So what did i learn from all this ?

Oils aint oils so don’t be afraid to try something else . I don’t know why WD40 worked so well on steel but RP7 caused a seizure ?

Speed isn’t everything so slow it down – with turning , speed and feed will give different surface finishes with parting it doesn’t matter I don’t think you can’t go too slow with parting .

The qctp can be used like a safety device by not cranking the locking lever closed too hard .

The relief angle on the front of the tool shouldn’t be too much , around 4 deg is more than enough will help stop dig ins , use the side of the grinding wheel to sharpen the blade not the periphery which gives a concave grind , more relief angle and a less support under the cutting edge .

If you have powered cross feed consider using it as it gives a nice even feed .

In my opinion the inverted parting tool running either at the from or the rear definitely lessens the damage caused from a dig in and it is even better if you use the qctp as a safety device .

Note: the F.O.R blade holder has a sliding wedge design so to can adjust the blade height ( about 1mm in height ) without having to use shims – this is a really handy thing for those who don’t have a Quick change tool post.

Disclaimer : i have no affiliation with any of the companies or products i have mentioed and ALL of the products mentioned were paid for by myself at full retail price .

I just found out something interesting as I was struggling to get a stable cut groove or even part off in a 50mm 6061 or 6082 aluminum bar, just using a Chinese 3mm carbide insert – rated for aluminum, which is just not the case as you will see later – tool in forward direction. Toolholder similar to the one in the post from Massimo above. I am using a 200x1000mm lathe with a 1.1kW motor and coolant flow straight spraying in the cut. The thing chattered all the way from 100rpm up to 1000rpm. N way to get it clean.

I converted the lathe to CNC a while ago and probably lost a bit ridgidity in the X-direction due to the limited dimensions of the ball screw, dictated by the small space underneath the cross slide. If i remember well, chatter on parting off or grooving was also there before the CNC conversion. So it had to be something else as many pro-CNC lathes are parting off at 1000-2500rpm without problems.

But then I read an article that makes a lot of sense from a dynamics point of view and it clearly shows how the toolpost dynamically flexes the wrong way ( most toolposts are fixed at the base side ) causing the tool to dig downwards in microscopically and release back accordingly. This elastic kind of spring-movement is the major cause of the chatter we see and hear. When using the normal setup that is.

So I reversed the tool and rotation, mounted the tool even a tad above center and tried again, at 1000rpm! Very much less chatter but still not satisfactory to my standards. It could have worked but it felt llike not cutting optimally.

Then I went on and sharpened the carbide so that the slightly convex front side below the cutting edge became flat to even a little concave. Leaving a razorblade sharp edge with negative geometry, theoretically the best one for aluminum. This time, no chatter, clean cuts, huge chips of 0.15-0.2mm thick, very often breaking in small pieces of a centimeter or so, avoiding birdnesting most of the time. Birdnesting was still there when cutting lightly though. So in order to get the chipbreaker in the carbide insert to do its work, one needs to push for heavy cuts, which makes sense.

So, this thing ran at 157m/min surface speed with coolant and it works just great. You need coolant because the one time i did not use it, the cutting edge has tendency to weld up quite fast with aluminum and the part became burning hot. The limit would be something like 250m/min cutting speed but for safety reasons I don't want to exceed 1000rpm on this CNC setup ( barely enclosed lathe and fairly large and heavy workpieces ).

Hope this helps for some of you facing similar issues and avoid time consuming research and trials.