Rear Tool Post

I am building a rear mounted tool post for an inverted parting off blade. On the designs I have looked at the blade is:

1. horizontal and the cutting edge fixed at centre height

or

2. at an angle so that ‘top’ rake is introduced and the centre height of the tool can be adjusted.

If I adopt 1, there is no top rake and if I ground one on the blade this would upset the centre height each time it was sharpened. An advantage though is that the tool overhang can be reduced to suit the job in hand.

With the blade at an angle, as in set up 2, this provides top rake and adjustment of centre height but has the disadvantage of maybe having too much over hang for a given diameter and conversely may also limit the diameter that can be parted.

My question is do I need top rake on a parting tool blade and which is the preferred option?

Regards John

Hi John,

I am thinking a rear toolpost with swivelling top cap with one angled blade and one level. The horizontal one will give zero `top` rake for brass and the angled one has `built-in` ` top` rake for steel, both blades being ground identically, just mounted differently.

Dont fully understand your remarks about overhang as if the front face of the cutter is at centre height when touching the circumference of say a 1" D. bar, then the tools leading cutting edge will still be on centre height when it has cut the bar down to 1/2"D. and will still be at same height when it reaches zero Dia. and the work is finally parted off. Having said that, I would probably stop and re-set tool projection when half-way through that sort of diameter.

John.

Hi John,

I made a rear toolpost for my minilathe. I angled the cutter in order to provide top rake. The top rake does make parting of steel easier. I can adjust the protrusion of the blade ,and using by using shims, also the centre height.

Details of my design can be found here:

**LINK**

Mike

I made an angled holder, but as it goes with my QCTP getting the height right is a synch.

Neil

DMB (John)

I think Michael's description and link illustrates what I was trying to say about overhang and keeping it to a minimum. With the horizontal blade it can be pulled in and out to suit the diameter of work without any further adjustment but using the angled method it has to be at a set protrusion to achieve centre height. Unless of course you add a further adjustment such as in Michael's design.

Interestingly the Eccentric Engineering parting blade (Advert on the right)) does not use top rake. For ease of use I would prefer option 1 in the original post but don't want to compromise on cutting performance which is difficult enough when parting, so option 2 is still possible. Any more views on the need or not for top rake would be appreciated.

Regards

John.

Is there a reason that you don't want to go the carbide insert route and part-off from the front?

Martin.

John (bog standard) & Martin

I do use a carbide insert tool and it is good for some but not all jobs, hence my need to use something else as well. I also believe that the rear tool post has some advantages, not least because it can be left set up and ready for use and this is useful even when a QCT is in use. I want do do things right and that's why I asked for specific advice. I certainly don't lack courage otherwise I wouldn't have ventured to ask for advice on this forum. I still hope someone will give me the benefit of their experience.

Here's an idea that might solve both of your problems.

Mount a carbide insert blade in your rear toolpost. Ta-Dah!

That way you get the advantage of it being 'always there', along with the correct geometry already built into the insert as well as having adjustable overhang without a change in centre height.

Martin.

hi john,

you pose a very valid question. it has been exhaustively and repeatedly catered for in ME over many many years.

it depends what lathe you have and how solid it is and what condition and type of bearings it has and how good everything else is, and also that degree of nerve/recklessness required to part off large stuff rather than partly do it and use a hacksaw, then face off instead.

it is just as easy to make 2 rear toolpost holders as one. you could make one for brass and non-ferrous stuff and another with an inclined tool - though the whole object of a rear toolpost holder is that you will do these these successfully and not have to worry about the tool wearing or getting damaged - and i would grind top rake on the tool for steel to fit in the same toolpost. drop the speed, tighten up the gibs and lock the saddle and see what happens! the hacksaw is always to hand for the fainthearted!

ive dont it in 1 1/2" steel in a normal (front) toolpost. ive done lots of things on big lathes such as cutting slides where the steel comes off like butter that i wouldnt dream of doing on say a Myford.

cheers,

julian

Edited By julian atkins on 29/11/2013 23:57:14

Have you seen the style of holder with a sloping recess and a matching sloped block that is moved along by a screw to effectively make a variable height base tothe tool slot. This could be made with an alternative block that is sloped.
For a lot of work, eg where the part is hollow, top rake can be achieved by moving off centreline. The amount varies with radius which is why I mention hollow items.

Hi John

I tried a number of different rake angles on various materials to see which cut better when I was designing the FoR holder.
The only difference I found in cutting ability was that a blade with zero rake angle required slightly more pressure to push than one with 5 or 7 degrees rake, there was no discernible difference in surface finish.
Having zero rake can be an advantage as it gives a bit of back pressure and there is less likelihood of the cutting edge hogging in to the work.

I think the advantages of being able to keep the blade on centre when moving it in or out, less chance of digging in, and the ability to part off a wide range of materials with the blade held horizontal outweigh the pros of having the blade held at an angle, namely that it is easier to push.
If you were using a wide blade on a big lathe it may be more important.

Cheers

Gary
(Eccentric Engineering)

Here is what I made, the base has a slight taper to set the centre height. The photos are in my album

Alan

Edited By Alan Jackson on 30/11/2013 11:43:05

The Empire Luers style blade that Eccentric use are IMO better than the tapered Eclipse style. They are only sharpened on the front face, since the top face is hollow ground which compresses the chip a little. MSC offer them with a carbide cutting edge. The holders are easier to make, since the sides are parallel. They work well with no back rake even though it's recommended. Make a toolpost at the right height and it won't need adjusting and you can easily vary overhang.

Ps you only need a couple of degrees clearance.

Edited By jonathan heppel on 30/11/2013 11:19:45

Gary

Thanks.

Funny enough I have just got in from the workshop trying a zero rake blade and one ground with top rake (both front mounted) and then read your post.

I only tried it on MS but agree cutting was a bit easier with rake but not by much. Up to then I was still not totally decided which way to go but you have now tipped me over the edge in favour of the zero rake option.

In any event it will not be my only tool for parting off so I will at least be able to choose the most appropriate one for the job in hand.

Julian

I think your post illustrates that there are so many variables connected with parting off that there is no universal answer other than to tailor for your own machine!

Regards

John

Hi
As Martin says the rear toolpost is always there. Mine is an home made one for my S7 Mk1 with about 2 degrees rake and a slight angle one front so the job parts off cleanly then just finish off the last angled pip

Nobby