Parting Problems - Backlash ?

Hi all,

Am struggling to part off generally, but at the moment specifically some 35mm EN3B. Have tried several different tools - normal hss, chipbreaker hss, glanze carbide tip, and also in both front and rear toolposts.

Lathe is Warco 290v (150mm centre height)

Have steady stream of soluble oil on it, have tried speed from 150-500 rpm, and both hand and power feed.

Some combinations cut ok, but all end up with regular dig ins, resulting in motor stall.

There is a bit of backlash on the cross slide - I can manually push it back and forwards about 0.3mm. Could this be causing the dig ins with the tool getting sucked into the work ?

Any thoughts gratefully received.

Regards,

Dan

I can probably move my 280 topslide a similar amount but don't have problems this is a bit of 40mm EN3B, about 400rpm, Glanze tool, brush applied soluable oil, power cross feed

Edited By JasonB on 19/01/2016 19:30:34

Jason,

Thanks - that's very helpful. I may get some new glanze inserts (all destroyed by dig ins) and try again as a direct comparison. Do you know what feed/rev the power feed was ?

Regards,

Dan

0.0015"/rev or about 0.04mm/rev

.

Is the tool tip dropping.?

If the tip drops below center height it will then dig in and as the work rotates be pulled into the job by any backlash there is making it even worse . Things will then go south quicker than you can blink.!

I learned this the hard way. My problem turned out to be the toolpost actually moving under load that was causing the dropping tip.

Nick

Nick,

It's possible, but same problem occurs with the rear toolpost, which is about as solid as it could be, so not sure. I'll try from the front using the original 4 way instead of the dickson clone - that is one combo I haven't tried

Regards,

Dan

The tool position is far left of the cross slide ways. I suspect the whole topslide is deflecting to the left causing the tool to drop. Can you reposition the topslide to keep the tool over the cross slide ways?

John if you are refering to the photos then thats my machine that parts with no problem, we have not seen Dan's setup.

John,

You mean in the pictures ? They are from JasonBs successful set up, not my failing version. Having said that, mine is similar, but not the rear toolpost version, in which the tool is inside the left t-slot of the cross slide and has the same problem.

Regards,

Dan

The tool position is far left of the cross slide ways. I suspect the whole topslide is deflecting to the left causing the tool to drop. Can you reposition the topslide to keep the tool over the cross slide ways? In my opinion the parting tool holders have far too much oberhang.

I had similar experiences with my South Bend 10K. I decided to use the HSS parting tools that fit the Aloris style tool posts. There is far less overhang and if something goes wrong I just re-grind my tool.

My parting tool is in the same place as that shown by JasonB, and I have no problem parting off. However I lock both the top slide and saddle. The backlash on my cross slide is rather worse than the figure quoted by the OP.

With an carbide insert parting tool I use a minimum feed of 4 thou per rev, otherwise the tool seems to chatter. Some years ago I bought some Glanze boring bars, which are fine, but the supplied inserts were very poor. I don't know if the Glanze inserts have improved since then.

Andrew

I would look for a loose or badly fitted gib strip.

Page 19 Using the Small Lathe by Len Mason " NEVER part off between centres unless you want to engineer a front- page smash up"

Brian

I think you will find Jason is "not quite" parting off.

Once you get to that point you can remove the centre and snap the bar off with no problems.

You would normally only do this to get a piece of stock parted from one that will not go down the lathe spindle.

Phil

Title of thread is Parting Problems not cutting a deep groove . I agree cutting a groove and then using a hacksaw is the way to go to cut through large diameter material . I solved my parting problems by tacking out the backlash .

Brian

Hi Guys
I find that parting from the back eliminates back lash as the lead screw is pushed on to the nut when parting off
Also any problems the tool digs out not in.

Nobby

Edited By Nobby on 19/01/2016 23:03:27

For a start en 3b is not the best of materials to part off,I would rather part off tougher en 8 any day. The rigidity of the tool is a major factor.and a correctly sharpened HSS tool or a good insert tool from a proven supplier,both well lubricated makes further improvement. A genuine Dixon toolholder can all to easily deflect under load ,the tool dips and moves to the left then digs in, a rigid single tool holder or a big 4 way tool will improve matters. The design of the lathe is another factor,width and depth of the cross slide,type of gib fitted and stiffness of headstock. I have owned a round head Colchester master and now have a later master 2500, same centre height , the bed width is wider, the cross slide is wider and the v ways deeper, cross slide backlash about the same as the nut is adjustable, the bed is a lot deeper.with the same Dixon tool post ,when parting off with the Round Head Colchester dig ins were frequent and tools broken,worst machine for parting off that I have worked on, the square head master is a vast improvement ,parting off is easy , I did make a solid tool holder for the round head which improved matters. The basic rules often ignored is that the parting tool should only protrude as far as necessary to part off the job,the top slide should not overhang its bottom slide,tool should be lubricated,soluble oil is ok, the parting tool should be as close to the chuck as possible for maximum rigidity. Do not use too wide a tool.The work speed should be half of normal turning speed for that material (HSS tool) During my training I never saw anyone use power feed for parting off on a centre lathe, auto lathes and modern CNC is a different matter and they are vastly stiffer machines.Parting off with a tailstock centre in position was always regarded as a no no!! ,(cut off bar with a power hacksaw),for novices on here don't do it ,a lump of 40 mm bar rotating at speed can be dangerous if it breaks free.

Hi Dan

Sometimes the workpiece material is tough and maybe sticky creating little streaks as the parting tool rubs against the side of the work, particularly if the groove is deep. This can cause the tool to dig in.

One workaround is to cut in double steps. of say 5mm deep by cutting a second time slightly offset to widen the groove at each step.

My first lathe was a little less rigid than ideal and this method made parting off easier. there is less drag on the side of the parting blade.

In the photo above the work is supported between centres, needed with a long shaft, I prefer leave the last bit for the hacksaw, or just wiggle the piece to break it off (It may leave a small blemish) normally I reface the end anyway so no problem. Cutting right through can cause the work to wedge in when using the tail stock.

Lots of cutting oil will help.

Regards
John

.

It was a Dickson 'clone' that I had problems with moving under load.

I understand the genuine items are a first rate product. But the clone I had was a disgrace. So bad that I don't even have the audacity to shift it on ebay. It 'looked' the part with being nicely finished but in reality the internal cam action was a very poor in finish and bad manufacturing size causing the locking cam action to loosen under load.

Where out of curiosity did you get your clone from.?

Nick

My C3 used to and still does dig in occasionally. It depends on what's being parted, how long, how hard etc.

A dial indicator (after watching the oil film squeeze onto on the ways) soon revealed that the carriage itself was lifting slightly at the tailstock end (about 0.0015".

A new set of brass carriage gibs, a shortened Allen key (to permit adjustment in-situ) fixed most of that problem. Lapping the underside of the ways improved things a bit more.

The best results came when I mounted the parting tool upside down in a QCTP (for height adjustment) and ran the lathe in reverse (C3 has a bolted on chuck). A tailstock live centre was sometimes used for the initial cut but removed when the diameter reduced (It depends on the length of the work).

All other conventions were followed, ; Minimum tool overhang/protrusion, carriage locked, plenty of lube, cross and topside gibs a little tight, a freshly sharpened tool (with a tiny 'V' groove in the cutting edge) and the edge honed until it will slice paper.

Backlash can then work in your favour. If the tool is pushed away, back off the cross- slide handle about a turn and get stuck back in, don't be too timid about it either.

One redeeming feature of the C3 is the 'yellow light' (overload) on the control panel - it works!

Finally, if a lathe is used within it's limitations and one is aware of them, life is so much sweeter.

* Danny M *