Hi Marcus

 

Hope it doesn’t feel like I’m following you around…

 

If roller bearings don’t solve your parting troubles – parting is such sweet sorrow

 

Try this – a t-slotted table from continuous cast meehanite (from College Engineering)  and simple toolpost from the same (fixed by two 6mm screws into a double-hole t-nut). A competitor for Tubal Cain’s ‘Gibraltar’ toolpost.

 

Neil

 

Because its completely unecessary and puts the forces into the x-slide the wrong way.

 

ie it lifts the x slide, and is therefore less rigid than using it in compression down onto the the bed.

 

And it doesn’t work so much better if one has ones act together.

 

I should have mentioned above, along with having the right tool geometry, you need the tool set just a tiny bit below centre height at the front – that means that if you are a bit enthusiastic, it creates a component which thins the cut and unloads the tool. ie its excatly the same geometry as being a touch high at the rear. What you mustn’t do is be above centre at front (or below centre at rear) because then the tendency is for the depth of cut to increase with load, and the result is a jam up, front or rear.

should the tool be above or below the centre line

 

  In ALL cases, ON the c/l

 

  Regards  Ian

Not so Circlip (we are talking of a few thou).

 

As I said, if you are below centre at the front with any parting tool then the tendency is to push the tool (downwards)towards a thinner cut, so you have a “safer” set up. In other words it won’t drag the tool in or increase the cut so it wont jam. (Unless you have excessive rake which was my old trick – thankyou CS for the steer)

 

I set up on the C/l and then tweak the QCTH a tiny bit. Thats all it needs.

 

Michael – please think about it and the answer is obvious. If you are running forwards and you want to be below, then in reverse….Thats not being hard – its trying to get people to see where forces and thrusts are taking them.

 

But,  what are you doing with any parting tool running in reverse at the front. You are lifting the cross slide off its base and into free space, and de-rigidifying everything. 

 

I’m delighted you have a Kit Q Cut- so do I. Magic. Ploughs though everything at high speed, and never jams, but in the bowels of Christ, never once have I seen any cause to cut in reverse at the front with ANY tool (except when matching internal and external tapers which is hardly normal). For Heavens sake, put the forces into the bed the way the designer intended unless you have a very specific reason for not doing so.

 

Which is a very good reason for NOT cutting at the back. And if I can do it on a 30 year old Myford still on its original front bearing, so can anyone else. All it takes is a bit of thought about the geometry, (and rejecting the conventional thought about rear tool posts, which clearly is wrong, an oversimplification in engineering terms, and inconsistent in result) getting the rake right, and using the machine in the direction where it is most rigid.

 

Speaking as someone who could generate several jams in a single parting attempt in 1″ bar. 

 

 

 

Fine Michael – so why lift the x slide off the bed rather than drive it onto the bed as the good lord and the designer intended. If you look at your geometry and forces/vectors you will see that if you have the tool set incorrectly you will provoke a jam just the same.

 

What is the benefit? Most of the rest of the world has lathes that also have no facility for a rear toolpost, and they can part off OK at the front, so what provokes this?

 

My approach when I had all these horrendous parting difficulties was to say that if “they” can and I cannot, then I am doing something wrong, or I am setting up conditions which cause a jam. That being so, one can analyse and rectify. Which mercifully I have done.

 

To start running in reverse seems a little quaint?

Hi There

Psrting off upside down from the front is OK if you don’rt have a threaded mandrel like a Myford.

If you do, the chuck would probably unscrew.

regards David

 

Hi Guys,

I just noticed that the topic is about parting on a C3 lathe, silly me, I should pay more attention shouldn’t I? 

Parting off on a C3 is not as easy as you would think, for those who are used to rigid tools (make your own jokes, if you must). The “as bought” sloppily made gibs on the C3  cause all sorts of dig-ins when parting. I am not knocking the Mini-lathe, you understand, but as bought, it only works, but with some serious attention can be made to work well. There is a WPS book on the Mini-lathe which I believe covers all the usual improvements, such as changing the bearings and  fixing the gibs. Using a rear tool post or an upside-down-in-the-front-in-reverse is not the answer to any parting problems. Understanding the problem is always the first step to fixing anything. The main problem with the Mini-lathe is that it is built to a cost not to a quality. Some may say that “it should be perfect from day one” and it could be but then the price would nearer a Wabeco. Regrettably these machines are, almost, sold as a kit of parts to be assembled by the new owner.

 

I do not own a C3, or variant, but was I “teaching” kids at Ally-Pally on one and had great difficulty when parting. On close examination the top slide was moving quite markedly, and adjusting the gib screws was no help. In a quiet moment I took off the top slide and found that the dovetails were razor sharp and as everyone should know this is a bit of a no-no, when trying to match to a rounded valley. There was no way that the gibs could be adjusted correctly till things were worn enough for the main dovetails surfaces to meet. I might not have explained that well, but I know what I mean. Hopefully before our (SMEE) next teaching venture at shows, I shall have the time to bring the bits home  to my Engineerium to fix the problem. The fix should not be beyond the capabilities of anyone with Model Engineering skills, a bit of filing where needed and a bit of lapping with metal polish, together with a bit of gib strip pegging if found to be useful.

 

So,  to the OP, I say this the Mini-lathe is, with some work, a quite capable lathe for the Model Engineer. It does need some attention though and parting off is one of those functions that needs attention to work successfully.. A lot of what other posters have said may be of help, like the Kit Q Cut, but nothing will help if your slides need adjusting or re-engineering. Don’t assume that your machine is perfect straight out of the box, ‘cos it ain’t necessarily so.

christephens 

PS to any C3 owner might I recommend you read  the following;

 http://madmodder.net/index.php?topic=627.0  not only is the article a way to fix some of the Mini-lathe’s problems, it is also an education in the way to go about fixing  things engineering, written by a truly great teacher. IMHO

 

Thanks for all that mgj, more or less what I was thinking, normal tool mounting puts the load into the machine bed, which is where it should be. So have stopped making upside down holder and back to normal. Interestingly I never had much of a problem parting off, until I started reading the ME literature, but only did small bushes and the like. A tecnique I use for deeper cuts is to withdraw the tool, move over slightly to widen the cut, then go in again, this of course gives more side clearance. I feel lack of side clearance is often the problem.

I’ve been looking at the tipped tools ,which I think you are describing, not cheap but probabley worth saving for. How do you part 3″ bar ? Must be min. of 1 1/2″ overhang.