Too much rigidity?

[Bryan]

Since I got two good carriage locks working (yes two), I've enjoyed less anxiety about parting off. Today I found the limit, in a fairly dramatic way. I had a jam, but this time the holder snapped. Previously the jam would have happened sooner, but without major damage.

Also when milling now my spindle adapter seems to consistently vibrate loose, which it didn't before. I didn't think there was such a thing as too much rigidity, but I'm starting to wonder. Comments?

As a side issue, does the break in that holder look right? I mean it looks chrystaline to me, but I know nothing about metallurgy. It sure made a sharp snapping sound, almost explosive. My first thought was 'am I injured?' (no).

[bollie7]

I've never seen a parting tool holder break like that. I would have expected the blade to break before the holder.
Was the tool on centre? for parting it needs to be as close to on centre as you can or slightly below.The idea being if its above centre and you have a dig in it will increase the depth of cut.If its on centre or slighty below and you have a dig in it will decrease the depth of cut.
I have a couple of tangential holders which are quite good. Have attached a rough sketch. With this design if there is a dig in, the holder flexs at the top and the point of the tool moves away from the work.

regards
bollie7

[nadroj]

I've never seen a parting tool holder break like that. I would have expected the blade to break before the holder.
Was the tool on centre? for parting it needs to be as close to on centre as you can or slightly below.The idea being if its above centre and you have a dig in it will increase the depth of cut.If its on centre or slighty below and you have a dig in it will decrease the depth of cut.
I have a couple of tangential holders which are quite good. Have attached a rough sketch. With this design if there is a dig in, the holder flexs at the top and the point of the tool moves away from the work.

regards
bollie7

That's a "swan neck" holder - something I'm definitely going to investigate following my Indian made toolholder breakage. They might be hard, but they haven't got the heat treatment quite right - it's far too brittle. Note this blade also survived unscathed.

Jordan

[SurfinNev]

Looks like the business end of the parting blade moved sideways which put all the pressure on the back of the tool holder.

Nev

[Dave J]

Looks like the business end of the parting blade moved sideways which put all the pressure on the back of the tool holder.

Nev


Looks like that to me as well. If it is what happened, I don't think any parting holder would hold up to that.

Dave

[Stustoys]

I think you'll find the bottom of the tool holder failed first. I assume there is a large part missing off the bottom, the black part below the 6 should continue almost all the way across to the left.

But I could be wrong :)

Stuart

[Stustoys]

You can see the piece I mean in the overhead shot, its down on the carriage.

Stuart

[Dave J]

Stuart,
I see what you mean.
Bryan,
Time to use that milling attachment and make a new one.
I would say it is either Chinese or Indian tool holder that was mad out of crap metal.
If you bought it new take it back or if off ebay send them a picture.
As for your spindle adapter do you have a picture of it?

Dave

[Bryan]

Bollie7, I'm pretty confident I had the tool on centre.

Jordan thanks, I was trying to remember who posted that breakage.

Nev & Dave, Stuart is correct. The blade pivoted vertically, breaking off the front bottom and rear top of the holder.

The holder was pretty cheap, and I don't know if I'd try to claim warranty, but I might still let them know of the failure.

Dave, re making a new holder, I'm thinking about how to achieve the right angles for a tapered blade with only square cutters. Maybe I'll just buy one. :)
I'll take a pic of the spindle sleeve if you like. Are you thinking about why it's coming loose?

[Stustoys]

Bryan You're worried about the bottom corner being 90deg? That might not be a bad thing as it would tend to push the blade against the holder wouldn't it? Although it would mean the blade was only supported on one corner. If you dont like this, as the holder needs an angle over 90deg you just need to make 2 cuts. I've been looking to buy one for the big blades I have, having seen this I might just make one. I made one that uses hacksaw blades and works pretty well.
Stuart

[Dave J]

Dave, re making a new holder, I'm thinking about how to achieve the right angles for a tapered blade with only square cutters. Maybe I'll just buy one. :)
I'll take a pic of the spindle sleeve if you like. Are you thinking about why it's coming loose?

Yes I was thinking why it came loose, but with out seeing it it's a bit hard to work out.

With the cut off holder you could mill it out then set it up in the toolpost parallel with the main ways, make up a tool holder for a piece of HSS to put in the chuck (or between the chuck and the tail stock) with the HSS shaped to suit the angle needed and use the carriage as a shaper to take out the little bit that needs the angle. You could rotate the tool holder in the chuck to adjust the depth and with a sharp tool it wouldn't take long.

Dave

[Stustoys]

Ok I'm confused (not for the first time) so I drew a picture.
If Bryan can mill the one on the left, why cant he reset and make two more passes in the mill to square up the top and bottom?
Or are you guys talking about something else?
Stuart

[old_fella]

The parting off tool holders i have used have had like a dovetail slot which the parting tool can slide in and out in his may be like that

[electrosteam]

The HSS blade type parting-off tool I have is held in my 4-way toolpost for rigidity.
When mounted with the base of the blade holder flat, the tailstock side of the blade tool is vertical, no cutting clearance at all.

It is easy to check the side clearance by putting the holder on a flat surface with an engineer's square against the two sides.

I put a shim behind the bottom of the blade in the holder to equalise the vertical clearance on the two sides.
One day, I will mill the bottom of the holder appropriately.

An additional change is to mill off the top of the holder towards the rear so that the back toolpost screw bears directly on the blade.

The two changes above made all the difference.

At the limit, I broke (hell of a bang) the blade when I attempted, unwisely, to part off some steel tubing.
The blade was pulled forward as it cut into the bore.

John.

[Bryan]

John, your holder sounds similar to mine. The shim is shown as a red dot. I think I'd like to improve on it instead of reproduce it. Also I think that blade is a bit big for my machine so I'd like to go down a size. The little parting tool I ground out of a 3/8 stick seems to cut so much easier, presumably because it's half the thickness. It's just quite limited in depth.

Stuart, I'd like to know more about your hacksaw blade parter. Got any pics?

Dave, I like your 'shaper' idea. But I'm a bit limited at the moment as my apron and gearbox are both on the bench awaiting parts. I've rigged up a bracket so I can use the tailstock to move the saddle a bit, but it's a fiddle-fart, and it's certainly not up to shaping.

I've included a couple of pics of my spindle sleeve, one with the cheap chuck I was using. The chuck's never slipped at all, but it has some runout, which would contribute to vibration. The sleeve bore is MT2, finished with a reamer, and it holds very firmly. I made it that size so I could use my tailstock tooling in the headstock. The male taper on the sleeve is a pretty good fit in the spindle. It's hard to see in the photos, but the ridges of the machining marks have become polished quite uniformly over the surface. I guess the real answer is a collet chuck with a drawbar. But that's money I can't spare right now.

[Dave J]

I see your problem, you will need to have a draw bar setup through the spindle bore on that spindle sleeve to hold it in place and then it wont come out any more. It may mean making a new one or welding on the back of the one you have got.
Without a draw bar it will all ways work it's way loose with side load on it. It will only stay put if you use it for drilling like the tail stock.

Dave

[Bryan]

Ah so! I'm just doing it wrong. Again. :B
Thanks Dave.

[Dave J]

You could get a milling MT2 arbour to suit your chuck, it has a thread in the back instead of tang, or if you cut off the tang off, the centre might be soft enough to drill out and put a thread into it.
One thing I would be concerned about after the sleeve is secured properly, is if the chuck is going to stay on it's taper. If it's screw on it will be fine.

Dave

[Bryan]

Dave, both my small chucks have tapers. I can use the 4 jaw, but then what? It seems like (and this relates to my original question) I plug one hole so to speak and it just exposes another one. That's what you get for trying to hammer with a screwdriver I guess. :doh:

[Stustoys]

Couldnt you make something like this?(the undercuts are exaggerated) or do you mean you are worried about getting the angles correct?
I'm take some pictures of my holder tomorrow if I get time.
Stuart

[Dave J]

Dave, both my small chucks have tapers. I can use the 4 jaw, but then what? It seems like (and this relates to my original question) I plug one hole so to speak and it just exposes another one. That's what you get for trying to hammer with a screwdriver I guess. :doh:
I know what you mean.
You may be able to put a small thread into the end of the arbour for a socket head bolt to hold the chuck on. Cordless and reversible drills have that to stop them undoing in reverse.

Dave

[Anorak Bob]

Bryan,

If your cross slide has tee slots you could mount a rear tool post. Makes parting off a lot less stressful. The cutter in my post is held in position with a screw fixed top clamping plate and the tapered blade positioned vertically be means of a pair of small set screws. I don't know whether any of this is applicable to the problem that you have with your broken holder but it might suggest another way of rigidly supporting the cutter.

Bob.

[Anorak Bob]

Here is another way of dealing with the parting off tool holder issue.

parting off holder (http://www.rulezman.com/rulezman_workshop_2/parting_off_holder.html)

Bob.

[Bryan]

Bob, thanks for the suggestions & pics. The rear mount idea is interesting (and beautifully implemented as usual). I don't have T-slots, but I do have some threaded holes I could use. Do you find it gets in the way, or do you just pop it on as needed? I'm not real flush with big lumps of steel for things like that. Might be time for some serious scrounging.

As for quick change, that's a whole other place I'm not ready to go yet. I have my own QC system. It involves each commonly used tool having its own holder and shim stack, held together with a rubber band, ready to grab and pop in the 4-way. I only shim a tool after sharpening. It's quick enough for me, and cheap as chips. The 'holders' are just bits of 3/4 square stock with a 3/8 slot for the tool and a slit for closure.

[Anorak Bob]

Bryan,

You are better organized than me. I had my packers, offcuts of zincalume flashing, strips of aluminium and slithers of brass shim, scattered about the bottom of a drawer before I made my first quick change tool post. Unfortunately, I only made one tool holder and had to readjust the height of the thing with each tool change.

The rear tool post does get in the way but it is readily removable. I'm thinking about revising the design to enable it to rotate. G H Thomas featured a rotating and indexing version for a Myford in his "The Model Engineer's Workshop Manual".

The steel I used was cheap 1214. I bought it from Bohler Uddeholm for about 20 dollars. I will dismantle the tool post and take a couple of snapshots to show how the parting off blade is retained.

Bob.

[Dano48]

I agree with Bob , the rear tool post is the only way to part off, once you try it you'll never go back , it works well for me and it hardly gets in the way, and it is a real chunky one
Cheers, Richard.

[Stustoys]

Bryan, Here's my attempt at a parting tool. It was made on the lathe before I got a mill. It will go about 10mm deep in MS, althought I have parted 40mmDia Brass. It just uses carbon steel hacksaw blades.
Stuart

[Bryan]

Thanks Richard, good to know. Will seriously consider this option. Think I'll just some sort of parting tool organised first, then look at a rear toolpost later.

Stuart, sorry, thought I'd already replied. Yes, your sketch is what I had in mind too, except for proportion. The blade is 1/8 x 3/4, and my smallest cutter is 1/4. So the angle cuts would be a lot wider than your drawing. In fact there would only be 1/4" left in the middle for support. I'd try it anyway, but I need some larger material. The biggest square stock I have is 25mm. I think I really want a smaller blade.

[Bryan]

Stuart, that's a power hacksaw blade, right? How thick you reckon? Have you ground some side clearance? Rake would be 5 deg?

I had my rake at zero or even slightly negative and it still pulled in.

[nadroj]

I would say it is either Chinese or Indian tool holder that was mad out of crap metal.
Dave

Maybe not so much that the metal was crap, rather that heat-treatable steel needs to be heat-treated correctly. Hard is good, but too hard means brittle as well. So, a tempering process is also required, which reduces the hardness slightly but imparts toughness.
Mild steel could be used instead. It's not as hard wearing as HT tool steel, but won't smash like glass.
Alas, cheap Asian products still sell on image rather than substance.

Jordan

[Stustoys]

Stuart, that's a power hacksaw blade, right?
Nope, just a boring old normal hacksaw blade about 0.6mm thick. I ground a little side clearance on it but not much. The rake angle I can change, the less the blades sticks out the more rake.

How much front clearance did you have? Less front clearance should help stop self feeding, but then with neg top rake it shouldnt be self feedning anyway. Are you sure you had the blade at center height?
Stuart

[Bryan]

Ok, I think I see how that works now. That's quite clever.

[We need our own 'Homemade Tools / Neat Ideas' thread guys.]

[eskimo]

Ok, I think I see how that works now. That's quite clever.

[We need our own 'Homemade Tools / Neat Ideas' thread guys.]

As you have moved that motion, I'll second it

[Bryan]

Well here's my attempt at a quick & dirty parting tool, using a power hacksaw blade. It's inspired by Stuart's design, but I dumbed it down. Didn't even bother milling a slot, just made the screws a snug fit. I just wanted to test the hacksaw blade idea. The results are not encouraging so far. Worked fine in aluminium, but only got about a mill into the steel bar shown. I thought it was mild steel, it seemed to file ok. Will have to try turning it to see if it cuts normally.

Would the material be uniform in those blades, or harder at the cutting edge? Maybe I'll try the other side. There's something else I might try, an old cold saw blade, presumably HSS. It's about 3mm thick, where the hacksaw blade is only 1.6. I have a feeling heat is a big issue with such a small blade. I mean parting blades in general. I don't have coolant, just cutting oil. Parting is one time I would like coolant.

Sorry I think the pics have reordered themselves.

[Stustoys]

Hi Bryan "shatterproof" worries a little, I'd guess that means flexible i.e. not hard. I think most good hacksaw blades are only hardened on the teeth. So you may do better with the blade the other way up, dont grind to deep removing the teeth so you are using the hard side of the blade.

I think the cheaper blades are hardened all the way through, thats why the cheap ones snap like carrots.

As far as your holder goes, it works for me, as long as the blade is vertical(although if we can't get it cutting deeper than 1mm thats not going to matter ;))

Stuart

[Jekyll and Hyde]

As you have moved that motion, I'll second it

Thirded! I have one or two, not very interesting. Always great to see what people come up with in these sort of threads!

[tinkera]

Hi Bryan,
A couple of years ago I discovered a parting off setup that works for me. It's a Mitsubishi holder with TC inserts They are only about 2.00 mm wide so that cuts down (pardon the pun) the kerf so therefore greatly reduces the power needed. they are double ended so 2 for the price of 1. A couple of days ago I replaced the first insert after about 3 years, but the lathe wasn't used for 14 months. I probably don't do a lot of parting but a box of 10 inserts is going to last for quite awhile. Last week I cut through a 100 mm diameter bar of mild steel with no problems. One of the secrets is that the profile of the tip causes the swarf to bend in a shallow v before it curls up (like a steel tape measure) this reduces it's width & therefore doesn't jam. There is a pic on page 2 under the "Parkerizing" Thread
Ian.

[Bryan]

Ian, that sounds pretty good. Did you buy that locally, and do you remember the cost at all? I couldn't find a holder like yours on the Mitsubishi website: MITSUBISHI MATERIALS CORPORATION##Turning (http://www.mitsubishicarbide.net/mmc/en/product/product_guide/information/turning/index.html). I'm guessing it wasn't cheap, but probably worthwhile in the long run. Being on a tight budget makes the short term more pressing though. And as soon as I start reading about carbide inserts I feel like I'm opening a Pandora's box of ever-increasing cost and complexity.

I had another look at my hacksaw blade today. Anyone with eyes and a brain would have seen that it was a 2 part blade, and I'd cut off the best bit. :doh: So I made another blade and it was better, but not good enough. It didn't wear the edge and completely stop cutting like before but it still wanted to overheat and dig in. So I'm losing interest in that idea. Maybe I'll keep it for al. [Yes it was just below centre. 7 deg front clearance, tried 0 and 3 deg rake.]

The seller of the broken holder has agreed to a discount on a replacement so I think I'll take that option as a stop-gap, and get a gravity coolant bottle set up just for parting. Later when I've got a few less projects nagging at me I'll look into a rear mount toolpost.

[Stustoys]

What Dia is the bar? What rpm are you using? It shouldnt be getting that hot unless it is rubbing somewhere.

Stuart

[Bryan]

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

[Stustoys]

How deep are you getting? you're sure the blade is vertical and at 90degs to the spindle? Is it MS? Are the gibs on your crossslide tight?
You might be right it might be time for it to go in the spare parts bin.
I've never tried it with a HSS blade or a blade that size.
Stuart

[eskimo]

. There is a pic on page 2 under the "Parkerizing" Thread
Ian.

it is?...is it the first one top left?

is this the type of parting tool your talking about

TAIWAN TOOL - Welcome to Taiwnan Tool, the professional for Machine Accessories, Tooling, Cuting, Hardare etc... (http://www.taiwantool.com.tw/product_view.asp?FkindNo=F000004&SkindNo=S000009&ItemNo=I00000132&tda02c=232099005)

[Bryan]

Stuart, the cross slide is probably the weakest link in my whole setup now. There's a ton of backlash in the leadscrew, and you can't adjust the gib properly cos it changes along it's travel. On one setting it will bind before it reaches full out, and be sloppy before it hits centre. I've got a t-screw fitted to the gib so I can ride it a bit but it's a fine line. I think I understand the issues - enough to know there's not going to be a quick fix. In time I'll start a thread about my cross slide. Have a couple of other fish to fry right now.

[Stustoys]

My cross slide could use some work, but that will have to wait until I have a big enough surface plate
Godd luck
Stuart

[tinkera]

Hi Bryan,
Sorry I didn't get back to you earlier, just a bit hectic, I bought the parting tool holder & inserts locally ( in Adelaide ) before I moved. The company was called Metal Cutting Technology, I think) They were the agents for Mitsubishi, the sales agent would call at my home with what ever tooling I phoned about. I don't think they exist in S.A. at the moment as there is no listing in electronic white pages. They do have a Melbourne No. & I can phone them for you if you want me to. I can phone them for no charge. I don't remember the cost but at a guess the box of 10 inserts would have been about $70.00 double ended so only $3.50 per tip.The first insert has lasted about 2 years of use so the box should last forever. I am guessing I paid about $130.00 for the holder & a special spanner that opens the holder to replace the tip. I had to make the holder to fit the blade to my tool post. hope this helps.
Ian.

[tinkera]

Hi Eskimo, Yes that is the parting tool, I have just posted another pic for
Bryan, probably just above this post. Sorry I didn't even brush some sawdust off of it before I took the pic.
Ian.

[Bryan]

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.

[lather]

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.

[Gavin Newman]

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/Products?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.

[Gavin Newman]

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

[Stustoys]

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

[Gavin Newman]

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.

[Stustoys]

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


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


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 :wink: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

[HavinaGo]

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)

[kwijibo99]

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.

[krisfarm]

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

[lather]

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.

[Dave J]

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