Myford ml7 "parting off"and "max working size"

Hi people's,I've had my myford ml7 for a little while now, bought it cheap serviced it,performed maintenance and with the good help of people here on the forum got it to a state of good useful service.However parting off really grinds my ****,I find I have to have everything correct,sharp tool,correct geometry no overhang tight gibs,run slow with oil ( I still get belt slip/chuck stop)spindle has no play etc,and I still find I have to hope that eventually it will part off the material,this was 2" aircraft alloy with several plunges to make slot wider than the tool,3mm wide part off tool in rear toolpost.I have had success on smaller diameter work (hard steel as well )once I start trying to turn larger work 3" to 4" it gets a bit grumpy (could be the t link belt)stops the chuck etc light cuts blah blah takes forever.Its a lovely lathe and I have produced some good quality parts on it for my bsa project,but I wonder sometimes if I am pushing it's working limit.Be good to hear people's opinions/experience,forgot to add it does have a 3/4 hp motor as well.thanks Von.

Edited By JasonB on 22/10/2021 13:11:48

My ML7 had a long Cross Slide, so i made a Rear Toolpost, and mounted the parting tool on it, INVERTED, being careful to keep the cutting edge on centre height. It also has / had NO Top Rake.

Gave no trouble, as long as I kept a slow steady feed.

The same tool, mounted in a shop made four way rear toolpost , on a larger lathe, works so well that I now feel brave enough to part off under power!

HTH

Howard

Forgot to say that the largest item machined was a brake disc, which JUST fitted into the gap

My parting tool is 3/32" ( 2.38 mm ) HSS.

Tried a 2 mm wide carbide tip parting tool in the front toolpost and had TERRIBLE dig ins. So bad that the holder was damaged and the tips fell out!

Reverted to the reliable HSS in the rear post ever since.

Howard

Yes I agree the rear toolpost works much better, although I can use one in the front but only really works well on small brass or ally,still I find it's a slow slow process!,I wondered if things would improve on a more rigid lathe like a boxford or viceroy?(can of worms there!🙄or are they too similar.

Parting 2” stock on a Ml7 is a bit of an ask. I like the T profile parting tools. I use a 1 mm one a lot on small brass fittings. They come in 1, 1.6, 2, 3 mm and possibly more.

Yes Ive ground my hss tool with relief like that I did attempt parting a piece 3" Aircraft alloy from a 5" block that I acquired!,I know that was probably beyond the limit of machine now but it was free and needed for a pulley.so really then parting or working on stock above 2" is probably the limit for this lathe? Is what I was beginning to think?

Parting off is difficult because there is so much to go wrong. Higher than normal cutting pressures are applied and the cutter is liable to dip and dig in if there is any flex in the tool or tool-post, or if the saddle can shift, even slightly. Motor and belts are pushed hard, perhaps causing stalls and slippage because the cut requires more power than normal.

Also necessary for the tool to be at a right angle, for lubrication and cooling to be applied, and for swarf to be cleared out of the slot. Cuts have to be applied steadily at just the right rate by the operator because dig-ins and tool-blunting rubbing are likely unless you get the feed spot on.

All these problems multiply with increasing diameter.

Much easier to part-off on a heavy powerful lathe in good condition than a small bendy worn or maladjusted machine. Myfords aren't particularly rigid or powerful and need all the help they can get!

• Avoid parting off difficult materials especially in large diameters. Many Aluminium alloys are 'difficult' because swarf tends to weld to the cutter and blunt it. Any metals that work-harden, such as stainless-steels, are 'difficult'. Necessary to get the feed-rate 'just-so' before parting off. It comes with experience.
• Do everything possible to tighten up the lathe - adjust gibs, lock the saddle, minimise tool over-hang etc.
• Don't mount the parting tool in the top-slide, or even worse, a QCTP. Instead, buy or make a solid rear-mounted tool post, heavier the better. Top-slides and tool-posts are relatively bendy and they put a lever action on the saddle. A 'Gibraltar' tool-post being a short stubby lump mounted directly on the saddle transfers cutting forces straight down and is less likely to twist the saddle on the ways.
• If power cross-feed is available, use it. Power cross-feed is considerably steadier than a human operator.

My mini-lathe was so fiddly at parting off I preferred to saw and tidy up. My WM280 has no such limitations. With power cross-feed it parts-off OK from it's 4-way tool-post, and is almost fool-proof parting with the rear tool-post provided everything is snugged up. If there's going to be trouble it's parting off large diameters, like your 2"...

Dave

Edited By SillyOldDuffer on 22/10/2021 12:47:48

Wash the drive belt and pulley's with paint thinners dry off and get some belt dressing spray (Rocol) it will help with the belt slipping problem for a while until the belt gets contaminated with oil again ,have similar problems with my super 7.

John

Thank you both, informative replays there.such a nice machine I think I'm on its limit.There was a viceroy denford for sale near me with all the tooling power feeds screwcutting gearbox etc really good price I was tempted but I've done so much to my lathe and took ages acquiring all the tooling.Maybe Ill keep eye for something similar in future.

One thing that gets disregarded when parting off is the factor of confidence. Once you have had a few dig ins and failures there is a real tendency to get timid and slow the lathe down and use tentative infeed. Generally speaking this makes things worse and more confidence departs. If anything up the speed and be a little bolder regarding infeed. Clearly correct geometry, sharp tooling etc has to be in place but a little confidence is often the missing factor.

regards Martin

Remember, too that lathes tend, if anything, to cut a concave face (if convex, at all, the resultant part will ‘wobble’ when stood on a flat surface). If your lathe is cutting too dished, the parting tool could possibly be ‘following the groove’ as the cut gets deeper, necessitating making the parting kerf a bit wider.

Whatever lathe you have, by times you will go to its limits. Myford is for most of the work I do more than large enough. In the picture the exception, it cleared the bed but not by much. Niko.

I did manage to part off at "middle" speed(not backgeared) on the ml7 it suffered more belt slip than anything else,still I guess if I changed the sectional belt for the standard vee belt it could have a chance of "dig in" then.

ML7 and S7 Part off ok though better results are obtained with hand ground parting tools than commercial tools, hand ground tools can have side clearance so they dont jam in the slot, I was taught during my apprenticeship that a good guide for parting off was to use a spindle speed which is HALF usual turning speed for normal turning a lot of amateur users always seem to want everything done immediately and use far too high a speed, we never used power feed on for parting off on any size of lathe,even on the capstan lathes,you were expected to be able to operate the feedby hand at a steady speed, I agree that quick change tooling is not the best system for parting off ,most lathes in my early days had 4 way toolposts, a lot more rigid but requiring lots of packing to adjust tool height.Dont be cruel to a small lathe. Parting off is a skill which takes time to learn and lots off practice,when on a capstan lathe an operator might part off 400 or so parts in a day,though a centre lathe turner would not part off so much as sawn blanks were preferred. our model engineers lathes have to be multi purpose and there are problems with belt slip,tool breakage ,jamming of tool in the cut ,etc its not the lathes fault.

Fair points,I was considering buying a tip tool but to be fair the hss one I've ground does work ok,I guess i just need to take my time,funny how you take for granted those old Colchester and Harrison lathes during the apprenticeship.

One other factor to consider is that during parting off, becuse of the forces involved (relativly wide cuting surface) tend to cause the work to want to climb on to the top of the tool, any spindle wear will add to this. I suspect that rear tool posts do the opposite as the tool is inverted the forces now want to pull the spindle down.

Plus I agree I don't think I ever once used power feed to part off in my apprenticeship or was shown .

It does sound like it could be a belt condition / tension issue.. ..my ML parts 2" steel, maybe not devastatingly, but not unbearably weakly, either, and will take reasonable cuts, say, across a badly marked 7" faceplate without refusing or slipping.. ..it's always felt capable of chewing whatever will fit in it's mouth..

For parting I'd rather go for a 2mm tool rather than a wider section if I can, and for 'difficult' larger facing / turning work I might compromise surface speed in order to 'drop a gear' and take advantage of the extra torque a lower ratio provides instead - tho' it must be said that as far as partng speeds go, it's always chatter that gives the hint to change down rather than any slipping or stalling.

One other consideration is that the rear tool posts attach to the cross slide, thereby eliminating the compound slide from the equation, sorry I forgot to mention that in my previous post.

Parting-off by hand is a required apprentice skill because not all lathes have self-act.

Doesn't mean manual is the best or only way to do it. Use the facility if it's available because machines are steadier than humans, especially unskilled ones.

CNC machines always part off automatically ...

Dave