Thread: Too much rigidity?

Ian, thanks a lot for the info, but $200 is not a sum I can spare anytime soon. Will keep it in mind for later though.

Its been a while since i posted.
having probs parting as well.
Do not have a proper parting tool yet, still waiting for tooling purchased from chronos to arrive from the U.K, "its been over 4 weeks".

had shaped hss to use for parting, lined it up e.t.c.
while parting a 20mm bar, close to the chuck the tip of the bit snapped, almost looked like the lathe jumped, scared the crap out of me, didn't expect a 3mm tool bit tip to have this sort of effect, hate to think what could happen if a 1/2' bit had jammed.

ended up making a parting tool holder, to hold a power hacksaw blade.
spent a whole day trying to figure out why it kept breaking, the blade was correctly sharpened and lined up.

the cut was not consistent, it felt jerky on the hand wheel, it would cut then stop, would then need to move the hand wheel in a touch to take up the slack, even though i was turning the wheel at a consistent rate, it seems like the slide moved forwards a touch, due to play in the slides screw, then suddenly it would break the tip of the blade.

checked the play of the cross slide, its about 0.3 mm, could this play cause the problem, so when feeding the tool in, it could grab and try to move the blade into the work ?

decided to turn the parting tool upside down, as the holder was large enough to do this, ran the lathe in reverse, this setup parted well, was smoother, and no jerking.
The hacksaw blade works better than i thought, when it does break theres no drama

don't understand why it works well upside down.

Originally Posted by lather

don't understand why it works well upside down.

Consider the geometry, when you are parting conventionally and the tool digs into the work it tries to move downwards but it cannot do so because the forces are down towards the bed of the lathe which will not move (hopefully). As a consequence something has to give and it's usually the weakest part, the tip of the tool, closely followed by the operator's pucker factor.

If the tool is running upside down and the work turning in reverse then when it digs in to the work it will try and move the tool upwards. The toolpost will flex slightly and the tool will cease digging in and then it will return to the cut and resume cutting. Often lathes with T-slots on the cross slide will run a second toolpost at the rear of the work with a parting tool there (that way the lathe can turn in the normal direction - important when the lathe has a threaded chuck mount).

The play in the cross-slide leadscrew shouldn't be an issue. A lot of people don't drive the tool in hard enough, that's often a problem. Parting off a 20 mm bar shouldn't cause issues though, I do it all the time with a normal HSS tool.

What speed are you running?
Are you locking the saddle when you part off?
Are you making sure the tool is exactly at 90 degrees to the work?
Are you sure the tip of the tool is exactly on centre?
Are you using oil in the cut, lots of it?

I'm using one of these holders https://www.machineryhouse.com.au/Pr...stockCode=L074 and a Sutton's brand blade. All I did to sharpen it was to grind a slight dip in the top face to encourage the chip to curl away from the cut.

Originally Posted by Bryan

The bar is 28mm. Speed is 280 rpm - my slowest.

Bryan, I suspect the speed may be part of your problem, that's fast for parting off a bar that's over an inch in diameter. The other issue may be that there will not be any side relief in your hacksaw blade parting tool. Remind me tomorrow when you come to look at the saw and I'll give you a parting blade to try in your holder.

If you are not sure about the bar bring that along as well and we can try it out on my lathe to see how it parts off.

Rgds - Gavin

Sounds like it is self feeding to me. Pictures? Are you sure the tool rake angle and center height are the same when you turn it upside down?

Gavin Newman The people with rear tool posts I have seen claim extra rigidity not reduced. I'm not sure I belive them. Sure it removes the compound slide but you are pulling up on the cross slide dovetail, that can't be as rigid as pushing down on it. I wonder if pulling up on the cross slide dovetail helps lock it from self feeding?

Stuart

Originally Posted by Stustoys

Gavin Newman The people with rear tool posts I have seen claim extra rigidity not reduced. I'm not sure I belive them. Sure it removes the compound slide but you are pulling up on the cross slide dovetail, that can't be as rigid as pushing down on it. I wonder if pulling up on the cross slide dovetail helps lock it from self feeding?

Stuart

Stuart

I've always been under the impression that it was the increased flexibility of the rear toolpost that did the trick. I've just gone back through my books - surprise, surprise, some say that the rear toolpost gives increased rigidity and some say the opposite - go figure.

My thinking would be that the front toolpost would be more rigid as the forces are acting down on the lathe ways whereas the rear toolpost is pulling up on the t-slot as you've said.

If the rear toolpost was more rigid why don't lathes use that setup for all turning...

It's academic for most of us anyway as not many lathes have t-slots on the cross-slides these days.

Originally Posted by Gavin Newman

Stuart

I've always been under the impression that it was the increased flexibility of the rear toolpost that did the trick. I've just gone back through my books - surprise, surprise, some say that the rear toolpost gives increased rigidity and some say the opposite - go figure.

Well they do have to make life interesting

Originally Posted by Gavin Newman

My thinking would be that the front toolpost would be more rigid as the forces are acting down on the lathe ways whereas the rear toolpost is pulling up on the t-slot as you've said.

It maybe something the depends on the lathe and how new or old the top slide etc are. I'm not sure the rigidity is the answer a couple of my books talk about tangential tooling where the parting tool effectively comes vertically off the top of the cross slide or making a support that goes under the parting blade straight down onto the cross slide, now that is going to be rigid. I think its more self feeding but he has it working and thats what matters

Originally Posted by Gavin Newman

If the rear toolpost was more rigid why don't lathes use that setup for all turning...

It's academic for most of us anyway as not many lathes have t-slots on the cross-slides these days.

Well you cant see whats going on for starters but also the area of the ways is much small when you pull up on them they would wear quickly. I had been planning on making a T-slot crossslide but I'm not sure I need one anymore.

Stuart

Originally Posted by Gavin Newman

Stuart

I've always been under the impression that it was the increased flexibility of the rear toolpost that did the trick. I've just gone back through my books - surprise, surprise, some say that the rear toolpost gives increased rigidity and some say the opposite - go figure.

My thinking would be that the front toolpost would be more rigid as the forces are acting down on the lathe ways whereas the rear toolpost is pulling up on the t-slot as you've said.

......

One aspect my Dad passed on to me was that the plain bearings on the spindle in the old Southbend had some clearance. The reaction forces for a rear tool-post is not inclined to cause the spindle to lift which is in contrast to a front tool. All I know is that parting using the rear tool-post worked like a treat on the old machine.

For my machine I have had some dig-ins when parting steel - still working on it but the comment about top rake - i.e. none or even a little negative - on centre or a smidge above, along with slow speed, good even feed and lots of cutting oil - seems to be the combination for me .....

It is a process I still do NOT enjoy! (but it beats a hack saw)

It's funny, parting off is probably the scariest thing for most people using a lathe, and as Gavin says, has one of the higher pucker factors. It's probably the big bang you sometimes get when the machine has enough power to shatter the blade or the fact that more often than not a good jam up will riun the work piece, or at least move it off center and make things more complicated.

I have found the following usually works for HSS parting tools:
Neutral or slight negative rake for brass, bronze, plastic etc.
Slight positive rake for free cutting or mild steel, aluminium.
Bigger positive rake for black steel (gummy).
Neutral or slight positive for high aloy steels
Neutral for stainles steels (these are probably the hardest thing to part off with HSS, better to use carbide)

I always set my tool a smidge below center height so if a jam up occurs the deflection will tend to drive the tool away from the work rather than into it.

Speed is also important, for anything up to 30mm I usually run at around 180 to 300rpm for FC and mild steel, faster is generally better than slower I have found.
For high alloy steel 150 to 200rpm and for stainless 50 to 100rpm.

Always make sure the tool is sharp and square to the work, I usually lock the carriage and lock the gib on the topslide just to keep things as rigid as possible.
Also keep the feed even and avoid letting the tool rub on the work (especially important with high alloy and stainless which can and will work harden causing all kinds of bother) if you need to stop feeding, back the tool off a little.

Try to prevent chips building up in the groove, this can allow bits to be forced down the side of the blade or back into the cut and cause a jam. If chips start building up, back the tool out and brush them away.

Last but not least, use lube, although dry parting off is possible, cutting oil always helps.

For a good confidence builder practice parting on brass, this is the easiest thing to part off and is actually fun if you don't get any splinters.

Cheers,
Greg.

This is an interesting subject, I made a rear parting tool for my lathe (Al 1000D) as I found that the front mounted parting tool for steel with HSS parting blades was a mine field, as soon as your blade lost a little bit of its edge, disaster was not far way. I found the rear mounted parting much more successful.My reasoning was that the combined clearances in the compound and cross slides were made available as the load was applied, the force on the tool is compressing both the compound and cross slides downwards, effectively disengaging their dovetails.They are then capable of quite a bit of movement, a no no for parting.On a rear mounted parting blade you eliminate the compound entirely - clearly the weakest link in the chain and as the direction of rotation is trying to lift the cross slide in doing so it is engaging the dovetails making a much more rigid setup. I now only use a carbide insert parting tool and can part of 25 mm dia steel at 1000 rpm using coolant with no problems.
Bob

Gavin,
the speeds ranged from 110-420 rpm, the faster the better the feel when parting.

saddle and top slide are locked. the tool is set at 90 degrees to the work.
had checked the chuck face to see if its sqaure. It's perfect, so i place the blade against the face of the chuck to check if its at 90 degrees.
didnt use oil, had the coolant running.
Had tried various rake angles, as the tip broke about 30 times when testing.
no matter what i did, when the blade was in the normal upright position, it eventually felt like it dug in and broke.

Have now made many parts using it upside down, it works well, there are times i can feel a little roughness during parting, so it is probably the extra bit of flexibility when using it upside down that saves the tip. It runs smoother when running it at higher RPM's, except the heart rate and "pucker" factor increases waiting for a breakage, i wear 2 underpants now.
A positive rake angle cuts better than no rake.
will try the proper HSS blade when ever it arrives, though the hacksaw blade is very safe when it breaks, as its only the very tip that breaks, doesnt make the lathe jump.
will post a pic of the holder soon.

Not game to run the lathe in its highest speeds after the dramatic effect of a only small tip breaking, as well as watching some of the youtube vids of lathe accidents.

may make some type of shield, had once worked in a place using a lathe without any training, made sprocket blanks,
suddenly one day the toolpost on the lathe behind me went flying off a solid old type english lathe. That definitely scared the crap out of me.

WARNING
If you have a weak stomach don't look at the above photo's.

Those photo's went around a while ago and feel for his family. Evidently he was filing on the lathe when his shirt got caught and dragged him in, a mess to say the least.


Dave