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Model Forum / General / Models / December 2008Parting with the rear tool post
I vaguely recall arguments that parting off from the rear tool post with the tool upside down is beneficial, but for the life of me I don't remember why. Naturally the swarf can escape the cut easier but I am sure there was another reason. Can anybody enlighten me please? Cliff Coggin. >I vaguely recall arguments that parting off from the rear tool post with the >tool upside down is beneficial, but for the life of me I don't remember why. >Naturally the swarf can escape the cut easier but I am sure there was >another reason. Can anybody enlighten me please? One reason that is often trotted out is that the natural flex of the tool/toolpost under cutting loads will tend to cause the tool to move away from the work in this configuration, rather than to dig in. Regards, Tony >>I vaguely recall arguments that parting off from the rear tool post with the >>tool upside down is beneficial, but for the life of me I don't remember why. [quoted text clipped - 4 lines] >tool/toolpost under cutting loads will tend to cause the tool to move >away from the work in this configuration, rather than to dig in. you can, presumably, get the same effect by putting the machine in reverse, for ones that have reverse, and inverting the tool in the normal toolpost. Only thing that strikes me, the (normal) toolpost is set up in general so that the top face of whatever size tool it's designed for is somewhere near the centreline of the lathe. The parting tool I use has about a 3/4" blade, so inverting it would require the holder to approx 3/4" higher than "normal".  SignatureAustin Shackles. www.ddol-las.net my opinions are just that Travel The Galaxy! Meet Fascinating Life Forms... ------------------------------------------------\ >> http://www.schlockmercenary.com/ << \ ...and Kill them. a webcartoon by Howard Tayler; I like it, maybe you will too! > I vaguely recall arguments that parting off from the rear tool post with the > tool upside down is beneficial, but for the life of me I don't remember why. > Naturally the swarf can escape the cut easier but I am sure there was > another reason. Can anybody enlighten me please? > > Cliff Coggin. The reason is because the tip of the parting tool is on the end of a cantilever that is supported by the tool-post. When the tool is held in the conventional way, the cutting force acts downwards and tries to rotate the complete tool, tool-holder and tool-post assembly forwards about the bottom of the tool-post. This results in the cutting tip moving slightly into the workpiece. If there is any play or flexing in the assembly the tip can dig-in with the usual result of a broken parting tool. If the tool is mounted upside-down on a rear tool-post, the same forces apply but the result is to move the tip slightly away from the workpiece and minimizes dig-ins. SignatureRegards, Gary Wooding (To reply by email, change feet to foot in my address) Pushes the spindle down onto the bearings instead of lifting. John S. > Pushes the spindle down onto the bearings instead of lifting. > > John S. I don't follow this John. Are you suggesting the spindle can move upwards more than downwards? Cliff. >> I vaguely recall arguments that parting off from the rear tool post with >> the tool upside down is beneficial, but for the life of me I don't [quoted text clipped - 13 lines] > apply but the result is to move the tip slightly away from the workpiece > and minimizes dig-ins. Hmm, I had to ponder that a while. It makes sense if you assume the tool and assembly are a rigid lump that pivots only at the bottom of the tool-post, but is that a reasonable assumption? Doesn't it make more sense for the tool to flex downwards and thus away from the centre line of the work? I ask because I want to decide if it is worth the effort and expense of fitting a rear tool-post. Cliff. > >> I vaguely recall arguments that parting off from the rear tool post with > >> the tool upside down is beneficial, but for the life of me I don't [quoted text clipped - 23 lines] > > Cliff. Oh it is Cliff , well worth the trouble. It has the added advantage it can usually be left in situ ready to use. AWEM >> Hmm, I had to ponder that a while. It makes sense if you assume the >tool and [quoted text clipped - 14 lines] > >AWEM I would have agreed until I got a Q-cut. This is so good that I don't have a problem any more, and with a Dickson toolpost it only take 3 seconds to fit anyway, so IMO the risk to skin of a permanent rear tool is no longer worth it. David SignatureDavid Littlewood >I would have agreed until I got a Q-cut. This is so good that I don't >have a problem any more, and with a Dickson toolpost it only take 3 >seconds to fit anyway, so IMO the risk to skin of a permanent rear tool >is no longer worth it. > >David I'll second that opinion on a Q-cut. Fit one of those and you can part entirely without trepidation. Only downside, and not really that big a deal, is that the parting insert tip is about 2.5-3mm wide. Peter > I'll second that opinion on a Q-cut. Fit one of those and you can part > entirely without trepidation. > Only downside, and not really that big a deal, is that the parting > insert tip is about 2.5-3mm wide. > > Peter 3 mm? That's bigger than some the whole parts I turn. Cliff. > 3 mm? That's bigger than some the whole parts I turn. Then you also don't need a rear tool post. :-) Nick SignatureThe lowcost-DRO: <http://www.yadro.de> >> 3 mm? That's bigger than some the whole parts I turn. > > Then you also don't need a rear tool post. :-) I use a parting tool ground from a 6x6 mm toolbit in the rear toolpost on my Cowells 90ME lathe. It works like a charm. SignatureVenlig hilsen/Best regards Erik Olsen http://www.modelbaneteknik.dk/ > >I would have agreed until I got a Q-cut. This is so good that I don't > >have a problem any more, and with a Dickson toolpost it only take 3 [quoted text clipped - 9 lines] > > Peter I considered buying a Q-cut - but as a beginner my experiences with carbide tooling have been expensive - indeed I recall threads here on how HSS was a benefit. I had no desire to spend money on an expensive tool if it was likely to go the way of my other parting tools. So I recently made a rear toolpost and at the first test parted off half a dozen discs from a 1" bar which i hadn't managed to do from the front at all. Perhaps now I'll think about a Qcut again and put it at the back. Russell > go the way of my other parting tools. Now you know why they are called *parting* tools. :-)) Nick SignatureThe lowcost-DRO: <http://www.yadro.de> > I ask because I want to decide if it is worth the effort and expense of > fitting a rear tool-post. I think its easier to buy a good parting knife. Nick SignatureThe lowcost-DRO: <http://www.yadro.de> John S is the only one that got it right. A word of warning if using the rear tool post, PLEASE use a safety screen, if the tool snaps it will be coming towards you. Have seen a few one eyed turners in my working life >I vaguely recall arguments that parting off from the rear tool post with >the tool upside down is beneficial, but for the life of me I don't remember >why. Naturally the swarf can escape the cut easier but I am sure there was >another reason. Can anybody enlighten me please? > > Cliff Coggin. > John S is the only one that got it right. > A word of warning if using the rear tool post, PLEASE use a safety screen, [quoted text clipped - 7 lines] >> >> Cliff Coggin. I'm still not sure I understand why pushing down on the spindle is better than pushing up? Sort of implies that if the lathe was mounted upsde down then the front toolpost would be best. What does seem certain is that one reason the rear tool post works better than the normal tool post is its rigidity. If I look at the parting tool in the normal tool post, it's quite a way to one side of the toolpost bolt (one lever) and then the tip is quite a way forward from the toolpost bolt (second lever), which seems exactly not what to do with a parting tool. The rear toolpost reduces both of these significantly. Merry Christmas Steve > I'm still not sure I understand why pushing down on the spindle is better > than pushing up? Makes a difference when you do have play in the spindle bearing. But who has that, or even wants it? When the parting tool is on the rear, the spindle is pressed down by gravity and cutting force. Normal position, gravity forces spindle down, cutting force up. Nick SignatureThe lowcost-DRO: <http://www.yadro.de> >> I'm still not sure I understand why pushing down on the spindle is better >> than pushing up? [quoted text clipped - 7 lines] > >Nick Not true for Myfords. Vee belt tension holds the spindle UP. Jim > Makes a difference when you do have play in the spindle bearing. But > who has that, or even wants it? There's always a little play in the spindle bearings as you still wants the spindle to be able to turn. Even the tiniest amount of play matters. SignatureVenlig hilsen/Best regards Erik Olsen http://www.modelbaneteknik.dk/ > There's always a little play in the spindle bearings as you still wants > the spindle to be able to turn. I'm certain, that my spindle bearings have 0.000 mm play. They are preloaded. Nick SignatureThe lowcost-DRO: <http://www.yadro.de> On Dec 24, 10:36 am, "Cliff Coggin" <clifford.cog...@virgin.net> wrote: > I vaguely recall arguments that parting off from the rear tool post with the > tool upside down is beneficial, but for the life of me I don't remember why. > Naturally the swarf can escape the cut easier but I am sure there was > another reason. Can anybody enlighten me please? > > Cliff Coggin. Cliff As someone who has fitted a rear toolpost, mostly used for parting off, I can confidently say they are worth the effort. I fitted a long T slotted cross slide mainly so I could fit a rear toolpost and it has been worth every penny. On my elderly lathe, before I fitted the rear toolpost I often broke parting off tools and I had to reduce speed to part off successfully. Since fitting a rear toolpost I can part off at the same speed as I am using to turn a part and 'touch wood' I haven't broken a parting off tool in years. The theory is right, dig ins reduced dramatically, parting off is easier.. John |
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