Well having used a mini lathe of late I'd guess it may be a good idea to clamp the saddle and if it happens to face a lot of dish people might still have problems.

They could try flexing the blade side to side a little with the saddle wheel but then clamping is not possible.

I recently bought a 10mm x 2mm HSS blade for the mini lathe but haven't used it yet. I'd guess from other work on it that clamping the saddle will help parting off but for straight turning it will produce a pretty high quality finish so the rigidity can't be that bad. I posted a shot a while ago.

The usual problem though is too slow a feed, fear and keeping an even and yet adequate feed. It's not the end of the world if a blade breaks. Personally I square the blade up against the face of the chuck. That's worked out ok on all lathes I have owned – so far.

I did have problems with an ML7 and at times had to ignore the noise and just carry on as a slight increase in feed didn't get rid of it. It had a rather loose bed and spindle. Most of the problem was the spindle.

John

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Ed, you might find what you are looking for in the pictures album I have on here. You are looking for the colourful displacement/stress analysis pictures.

Mark

PS. there was a thread on here a while back that looked at parting and how rigid everything was but cant remember when?

Cast iron PULLET!!! it was a pulley, cruelty to chickens I will not stand for!

I have the J&S version of that tool, but without the slit in it, surely the blade spans the slit and when clamped in place, will negate the spring in the tool?

I think yer reet an all! It wouldn't make sense if it wasn't, and using a short portion of blade negates the purpose of the design really. Not too difficult to make this type of mod to a standard tool, this has got me thinking, but I mustn't get too far ahead of myself because although there is enough meat on my J&S version to do this mod, the blade in is for a different make, which I have chamfered to fit the holder, again, pushing my luck, as if it moves down at all, the chances are it will grab, and we are back to square one! You certainly can't cut corners with parting off!

dont know about Phil, but yes i do, its a big bit kit though and it takes 31 mm blade, be ok on Phil`s big Cov-Mac, am using 3/32 x 1/2 inch.hss blades, dont have much trouble parting off , but it took awhile to get there all the usual problems one gets with old " used" lathe,- worn bed ( 4 thou dip usual place front shear l/h end ) worn saddle ( 7 1/2 thou difference between l/h front wing and r/h rear saddle wing.) also 3/32 inch difference between l/h and r/h bed risers , that came as a bit a surprise as the lathes originally first owner purchased it new from Myford in 1946-7, He was a regular contribute`r to ME and he did do a write up about the lathe , i think the title of the piece that appeared in the ME was " Myfords new 4 inch centre lathe " or something like that, sometime in the late 1940s i knew it was bit worn when i bought it , but it had some redeeming features anyway i thought i could sort it out , which i did eventually , a new chuck made a big difference when parting off , the mandrel nose adapter ( on the other thread ) was made using new the new chuck ( 41/2 inch Vertex) holding 2 inch steel bar stock parting off was no problem

So yes i think a good chuck goes along way to helping with parting off ops, i wouldnt have said any of the old s/h chucks i have were bell mouthed. But was surprised at difference it made when using the new chuck………………b

I reckon rear mounted parting off tools on a Myford 7 type work better for the simple reason that they reverse the loading one the slides, bed and saddle so if an even feed is used all the bits that can move about are held in place. Mostly because wear occurs in the opposite direction in normal use. Might be different when they ceased using a narrow guide on the front rail but what happens when a dovetail slide is lifted – it tightens automatically. Pressure on the front of the front rail is also increased – the opposite of what happens during normal turning. Most of the need for them was established when they all had front narrow guides. In a nutshell a rear parting off tool on these lathes makes use of guides in a way that avoids normal wear causing problems. The usual reason for them is convenience as they can often just be left in place. I can't see that the holders I have seen on them can flex. In fact they look more rigid than a qctp and could allow a deeper blade to be used.

When people mention grabbing it's probably noticing lots of noise and the tool taking bigger cuts from time to time. That's due to it not cutting cleanly and rubbing instead. Increasing the front clearance on the tool may help with that but grinding back rake on the blade is likely to be superior. If both are done too extremely it may literally grab and pull itself into the work. The principle is exactly the same as grinding tools specifically to take huge cuts using minimum power mentioned on here a while ago – taken too far and that can self feed. HSS will always beat carbide in that respect as carbide can't have such extreme angles.

During my training some people did manage to part off noisily. I suspect it was down to poor sharpening and setting but they were always told to carry on regardless and not to be scared by the process.

Insert blades do have back rake but my main interest in them is finish. My Boxford often doesn't leave the same standard of finish as facing does so I have to hold parts in paper or what ever and take a light facing cut. I don't add back rake to the blades and the lathe is ok like that. Some aren't. It seems that insert blades and tips can reliably leave a decent finish and also go to extreme depths.

John

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Edited By Ajohnw on 11/07/2016 11:10:23

Good example of back rake Brian. Hard to know how much but early tool bit holders often set the tool bit circa 15 degrees, maybe a bit less and due to that for normal work didn't need any back rake grinding on at all. Lots of back rake for a bar turning tool intended for very heavy cuts may not be a good idea though. Pre use of carbide that's when super weld tools came out – hardly any back rake at all.

The front clearance angle tends to push the tool away from the work / prevent it digging in so the 2 angles tend to work together.

It all an odd area really. There is plenty of info about using specific angles for certain materials etc but no explanations of what the angles actually do as far as I am aware. The old one – negative rake for brass has always interested me. I've tried it now and again and it doesn't work at all for me.

We used to get odd 10 min lectures on stuff like this. People were also sometimes given the job of making an angled tool bit holder and making a drift to square the hole up. That lecture started off pointing out that a square isn't far off the shape of a circle and that toolbits only needs holding at the corners. So drill the right size of hole and then hammer the drift into it.

John

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