Groove/cutoff tools wants to dig in too much

Another thing to be aware of is that you can try to part/groove at too low a speed. For the tool to cut there needs to be enough speed that in the small region where the tip touches the work the metal gets hot enough to soften. Manual feeding and going slow is particularly problematic since to start with the tool won't cut,you aren't feeding steadily because nervous about the whole process, so suddenly there's enough pressure for the tool to start cutting but then it pulls the slide forward so the tool digs in. I think it's George Thomas who recommends parting off at higher speeds using power feed - discovering that revolutionised parting off for me.

Maurits,

Don’t turn it around, in a 3 jaw chuck, if there remains any concentricity issues. Even removed and replaced the same way round is ill-advised, if concentricity is important.

The original Myford rear tool post was a solid lump of iron with a curved bottom to the groove for the tool holder and a curved part to fit, giving the ability to alter the tip height of the cutter. Depending on the tool angle effected the angles the tool had been ground to. A horizontal groove and shims might be just as easy ? The important thing is the rigidity of the set up ! Noel.

Make sure that the backlash of the cross slide is set to the minimum. Any movement in the cross slide causes issues. The leadscrew and nut are normal replacement service items. Also make sure the gibs are not too loose on the topslide or on the cross slide. Once you have those done, your lathe will be fine.

My S7 can is totally fine just grooving with the Dickson tool holders. I am after 36 years, thinking adding a rear tool post to save changing tools as much from just working on the front.

At work they were having trouble with parting and grooving on a Cohlchester 1800 lathe, the leadscrew and nut for the cross slide, were worn out. With it replaced and the gibs all good, everything is fine with it.

With parting and grooving tools, they are normally right on centre height, or just very slightly above, maybe 2 thou or so, 0.05mm for metric.

Worn 3 jaw chuck or worn can also cause issues when grooving. You can change to a 4 jaw, or sometimes hold the part with card thin card can help with the vibrations from a chuck that is stressed and is holding tighter on the back than evenly along the jaw length. Worn 4 jaw chucks can have the same problem as worn 3 jaw chucks too.

100mm stick-out with no end support is asking for trouble. Start with a bull-nose adapter and tighten the gibs. Raising the tool slightly to compensate for deflection can also help.

Maximum rigidity helps a lot, plus I always lock the saddle. Rear inverted parting blades fixed directly to the cross slide are best and gravity helps with swarf removal. If using blades with carbide tips, you may be able to get different grades of insert, and they like to cut and not rub. I have had better results with a bold approach, the main difficulty is keeping some lubrication in the cut while still advancing the tip.

Ok, I am making some progress:

- Made a bull nose adapter (basically a big 45 degree cone that fits round the 60 degree life center. This already makes a huge difference.

- Used the back gear. I had completely forgotten i had such, someone here mentioned it. It is the first time I use it. Now working in the 60~120 RPM range with plenty of torque.

- Put the cutter a bit more above centreline, as suggested.

Result: Much better. Some new problems, but much better,
Did not build a rear post yet, as I felt I wanted to see how this worked, and much better.

Now, what I am seeing, especially in the 60RPM range that my eyes can follow, is HOW MUCH everything bends. It's like the whole thing is made of plastic. My god, all the iron needs to be twice as thick! Not sure I can do anything about the cross-slide itself bending over backward, but that tiny 0XA toolpost that I selected because no other quick change post would fit between topslide and centreline, well, it is too tiny.

What stops me now however, is the fact the toolpost is only secured by a centre-bolt. The cutting tool is of course left of the bolt, so a rotation force is created, and I cannot tighten it enough to keep it from rotating, thus rotating the groove tool to the side.

I had seen it before, where a PIN prevent such rotation, but my topslide, nor my QCTP have those.

Edited By Maurits van Dueren on 23/06/2023 17:14:32

On my lathe (Bigger than a ML7 ) the front toolpost locates against, for want of a better word, a taper topped dowel.

My home made indexing four way rear toolpost locates against the rear of the cross slide with two 1/4" dowels. The toolpost is prevented from rotating on it's base, in each position by nothing bigger than a 1/8" dowel.

The parting tool, mounted inverted in the rear toolpost is 3/32" wide, and deep, on centre height, with no top rake.

Using power cross feed for parting off (Have become braver in old age ) is at a feed of 0.00225/rev

For parting off larger sized material, 12mm or larger, drip feed (Syphon fed from a bottle above the lathe ) of soluble oil is uded. Rotation speed is fairly low, minimum provided by bnthe VFD in middle belt position, so circa 90 rpm.

So, my advice?

Sharp tool, no top rake, mounted at centre height in a rear toolpost, wyth minimal overhang to maximise rigidity, , (Yes you can see things flexing, but my very old tool holder is intended to flex! ) Mounting above centreline menans that things flex to bring bthe toll to cednytre heihjt, or even slightly below.

Low speed, slow feed, (DON'T force it! ) and soluble oil feed. Some times, ordinary lubricating oil, applied with a brush.

Dig ins DO happen, but very rarely compared to the number of times that work is parted off, without breaking the tool.. And a slow resumption of feed gradually removes the "lump" and things then proceed as normal.

For my mini lathe, I made a rear toolpost located against the rear of the Cross Slidem by a rebate (Base is milled to leave a 3mm deepx 3mm thick rebate across the rear ). It carries a 1.5mm parting tool. Having no T slots, the post is secured to the Cross Slide (Unfortunately ) by an off centre M6 stud and nut, but seems to work on the rare occasions that the machine is used. Being small cutting is usually dry.

Being a mini lathe, infeed has to be manual.

Because the stud is off centre, the nut needs to be really tight.

HTH

Howard