Parting Problems - Backlash ?

As Phil says it was completed by hacksaw, also I would not want to be making that cut without tailstock support - the weight of the long length of bar would tend to tilt down and close up the cut which would definately cause a smash up. And as for the alternative of cutting at the far end

It was just a photo that was already uploaded, it was the same EN3B, its almost an identical lathe and dia was about the same so I was not far off on the other criteria. Maybe you could post a pic of yoursetup parting the same material?

For the benifit of the thread title here are a few bits of slightly smaller 32mm bar that were parted off without centre support.

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Jason,

This is a purely 'Armchair' question [because I have little interest in working lumps of steel that size] but: "as for the alternative of cutting at the far end" ... Might this be an opportunity to use the fixed steady ?

MichaelG.

From the picture, that is not parting off between centres. The work is held in the chuck and supported at the other end by a center. Whilst I would withdraw the centre for the last bit it's unlikely to cause much grief if it does part through.

Martin

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Quite so, Martin

[ this forum is not widely noted for linguistic accuracy ]

MichaelG.

Edited By Michael Gilligan on 20/01/2016 09:32:24

an we dunt always use the roite sort uv ritin an all.

:0)

Martin

If I am cutting a long bit in the lathe, I find it a good idea to support the free end with a lump of wood when using the hacksaw, it saves the bed from falling bits of metal.

Ian S C

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Martin [and Brian],

[now that we've done the jokes] ... I think that the importance of your remark needs noting. ... Brian's quote from Len Mason is very valid, but; it refers explicitly to working between centres, and is not entirely relevant to the process currently under discussion.

MichaelG.

Precisely Michael.

That's really why I picked up on it. I would not like someone to think that this is parting between centres, get away with it and then try and use two centres which the warning is really about. Two centres really is a no no.

Martin

As there seem to be plenty of parters-off about, how about this task?

It's a bar end of 5.5" (140mm) which was too small to fit onto the steel stockholders bandsaw, so it fell into my lap. "

Slightly too big for the vice on my little bandsaw, so how to get a 20mm slice off?

Pete

Michael Gilligan:

Yes, a job for the fixed steady as in this extract from DHC's Quorn book (the infamous ball handles):

Note the improvised extended cross slide handle.

• Use an indexable parting tool (blade is less brittle and fragile than an HSS blade and the holder will help to reduce overhang of the topslide)
• Minimise overhang of the tool from the topslide to an absolute minimum.
• Tighten up the gibs so they hardly move - and lock the top slide and carriage. The topslide is particularly prone to twisting due to the need for the tool to overhang so the cross slide clears the jaws. It's not ideal!
• Coolant if you have it
• Power feed - for consistency
• Rubber pants
• If you are superstitious, use the tool upside down and/or at the back of the cross slide, turn round 3 times, pat your head and say something lucky.

Or cut it from a piece of 20mm plate instead?

Michael I find that doing the parting (deep groove) at the chuck end works better as things are more rigid than working at the other end with either the tailstock or a steady where I get a bit more chatter. This is based on my findings in the workshop rather than thinking about it in a comfy chair

Looks like the pic Ega posted found the same issues and parted at the chuck end, either way you would not want to part all the way through.

J

Edited By JasonB on 20/01/2016 13:27:02

As far as chatter goes I suspect the main reason why the headstock end is better is that's where all the mass is.

Martin

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Jason,

1. Was it really necessary to 'have a dig' when I had already stated that it was an 'Armchair' question [and explained why].
2. Prof. Chaddock was using long bar-stock, so; interesting as ega's picture is, the arrangement is not what I had in mind when I posed the question ... I was thinking of a Chuck/FixedSteady/PartingTool/ and optional Tailstock, arrangement.

MichaelG.

Edited By Michael Gilligan on 20/01/2016 14:22:19

While I wouldn't part off at the tailstock end, chatter doesn't seem to be an issue when cutting deep grooves at the tailstock end:

Final parting was done with a hacksaw. Although not visible there is a 3/8" hole through the part, so not that much to hacksaw through. It's much safer to hacksaw the last bit on this type of part, as if you fail to catch it a damaged part results; typically usually the fine thread.

Chatter results from oscilatory motion of the tool and/or workpiece. If the tool is oscillating the extra mass and rigidity at the headstock end makes little difference. Parting tools will quite happily chatter away at the headstock end if cutting conditions are not right.

Andrew

If some one can push their cross slide moderately easily by the amount mentioned I'd be inclined to adjust it. It's nice to have sufficient resistance to ensure the nut is more or less fully home when the cut starts. It's best adjusted with the lead screw out to that some resistance to movement that's even across the whole travel can be detected by hand. Often impossible on used lathes that have done even normal rates of work for some years.

It's important that the nut is fully home when parting off - usually done by the feed rate. Jason posted a shot that shows a significant thickness of swarf coming off. Pussy foot about and parting off generally wont work out. To fast a feed and the blade may break. If there is too much play in various parts of the lathe parting off can make a lot of noise. Usually a short sharp increase in feed gets rid of it, sometimes it wont go away so just grin and bear it. Even a well set up lathe may do this if the feed rate is too low.

It's not a good idea to have a parting off tool above centre and not too far below either. They need setting up with more care and square to the axis of the lathe. I usually do that against the chuck but something a bit more extreme might be beneficial on lathes that face a lot of dish.

Personally I think that the top of HSS blades are best left flat as they come. The front clearance angle is about right too. This is inclined to increase cutting pressures and help keep the lead screw hard back on the nut - an even feed wont grab then. The HSS used on these is sometimes a load of crap so best not temper it when sharpening it. They do need to be sharp. I wanted some recently and had to buy from the USA to get the size I wanted and a reasonable amount of cobalt in them.

A modern lathe bed shouldn't need a rear parting off tool but they are convenient as they can be left in place on lathes that have sufficient cross slide travel. Myford 7's often benefit from them because the cutting forces tend to lift the back end of the saddle and press thing home reducing play.

A variable speed lathe is likely to have to be run at a speed where it still has a reasonable amount of torque available. Most have a lower speed belt setting which should help but people might find that they need to run at a pretty fast speed. That might make a disposable tip parting off blade attractive. These are usually 1" deep and come in various widths. RDG do a holder which should fit many lathes providing they have a Myford sized tool post. On some lathes the side of that may need to be very close to the end of the compound slide in order to get the tip low enough.

John

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Thanks all for replies. I have been trying a few things, without any notable progress:

Setup as in photo - now with 4 way toolpost, compound locked, cross slide gib tighter than I want it, carriage locked. Moved on from En3b to a bit of EN1A leaded. Should be a doddle right ?

using hss tool (http://www.axminster.co.uk/axminster-parting-off-tool-holder-800377) as no tips left.

Still no Joy. Constant stops. If I use power feed (3 easily available feeds something like 0.035, 0.017, 0.009 mm/rev) it jams almost instantly, so is not an issue with rubbing on the sides. If I feed manually I can get further, but not for long.

It does not appear (as JasonB said) to be the backlash - I can see the X value on the DRO does not jump when the jam happens. There is now no perceptible movement anywhere in the carriage/slides with my not inconsiderable bulk tugging on them.

Could it be a motor/vfd problem ? I noticed that it is not particularly difficult to pull the chuck around by hand after it stops, which I would not have expected from a proper dig in - also the tool is not damaged, deflected etc.

This is my second 290v , and I did notice that it took a lot longer to spin up than the first one, but didn't think much of it at the time.

Someone asked earlier in the thread about my dickson clone - although that doesn't directly seem to be the problem here, I am pretty sure they do sometimes move under load. It is the Warco one marketed specifically for the 290V.

Sorry for the long post ...

Dan

If I remember correctly Jason mentioned 400 rpm and I assume in the low speed range. Have you tried that?

Angle the tool and take a facing cut with it so that you can check what the centre height is. Say around 30 degrees. Also looking out for the work lifting - loose headstock bearings but that would be more noticeable if the tool was nearly square on but it will bend. The pip usually breaks off if this is happening when facing if it's extreme. Don't have much tool sticking out so that you can vary the feed rate with less risk of breaking it. It might be a good idea to sharpen it first.

John

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Dan

I think you have answered your own question, the VFD is not coping with the torque required for the cut, perhaps it needs some adjustment for low speed output.

Jam up is either too much infeed or a too heavy cut whether you are turning,facing or parting but all are related to available torque at the spindle.

Emgee

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So are you saying the machine is stalling.?

Lets get away from the parting for a little test. - Put some of your en3 into the chuck and the revolving center in the other end.

Take some aggressive cuts along it gradually getting deeper. (with a known good sharp general purpose turning tool) Then let us know what the chuck speed is, the depth of cut and the feed rate is when the machine stalls doing this.

By the sound of it (to me) you may have a torque issue with the motor / inverter combination. If so this may be a simple fix of altering the inverter parameters.

Nick