Rear tool post vs front tool post

Im a complete beginner. What is the difference between the rear tool post and a normal one?

Many thanks

One cuts from the front of the workpiece and the other cuts from the rear. Nothing difficult in that. Sometimes the lathe might be driven in the opposite direction, sometimes the cutter may be inverted (definitely the way for screwed-on chucks).

Most use the rear tool post for parting off parts on the lathe.

Welcome to the forum Simon.

Wot NDIY said, but a picture is worth a thousand words:

The front-tool post is normally mounted on a compound-slide and can be moved sideways or swivelled; they conveniently allow cutters to be applied to the job in many different ways. Often as not, the front tool-post can hold more than one tool. In the photo, mine is a 4-way type. Another favourite is a Quick Change Tool Post in which cutters are held in cartridges pre-adjusted for height.

Front tool-posts are flexible in both senses of the word. Being able to swivel and slide means they sit on a somewhat spindly construction that's prone to bend under severe stress. Certain jobs can lift the whole saddle. As rigidity is important to machine tools - more the better - there are times when the convenient front tool-post is best avoided.

Rear tool-posts are much more rigid. As can be seen in the photo, they are usually a hefty block of metal bolted direct to the saddle. Being squat and heavy, with no moving parts, they are much stiffer than a front tool-post. Furthermore, if the chuck can safely run in reverse, the cutting forces go straight down through the saddle and into the lathe bed - much less chance of anything bending. Disadvantage is they're much slower to set up and not general-purpose.

Rear tool-posts are most used for parting-off. Parting is notoriously difficult because any adverse movement of the tool is likely to cause a dig-in. Any fool can part-off on a big heavy lathe, but real skill is needed on lightly built lathes with slim front tool-posts. On such machines, it's much easier to part off with a rear tool-post.

Dave

An inverted parting tool on the rear toolpost allows the chips to fall out of the groove, giving less chance of jamming.

Speaking from a position of very little experience, I've never been able to get my head around using a rear-mounted toolpost and normal forward rotation (with inverted tool) as the cutting forces would try to lift the back of the saddle. Surely not what it was designed to resist?

However, so many people swear by it, it must work

Also confused by SOD's assertion that in normal use a front-mounted toolpost can lift the saddle??

Rob

Ah well, looks like I'm a fool. Never felt the need to use a rear toolpost for parting off (*). But a rear toolpost is convenient when using a hydraulic copy unit:

A thought: in the same way that 'practicalmachinist' bans discussions of hobby machine tools should this forum should ban discussion of industrial machine tools?

Andrew

(*): Not quite true; on my repetition lathe I part off from the rear, but then again all tools are on the rear.

I am a beginner as far as the slightly larger lathes go as I have just got a Myford 10 having previously used a small Pultra 17/70 with a hand graver and apart from parting brass I found it difficult to part steel usually resorting to the hacksaw, now I have a rear tool post I don’t have any difficulty ( so far ) parting , I have parted brass, mild steel and silver steel, the reason that the rear inverted parting tool is better I think has to do with where the pressure is bought to bear on the various parts of the lathe and the tool not being forced into the stock but away, I am sure someone with more experience than me could explain it better, but I have found it is better to keep consistent feed and lubricant although I haven’t used lubricant on brass ( not sure about that ), as NDIY said above if your chuck is screwed on don’t run it backwards as it could unscrew the chuck hence having parting tool or other tool bit upside down in rear holder.

everything I have said above is just my own personal findings.

Dell

I think in one of the classic workshop practice books it explained that if you had any play in the h/s bearings then using a front parting tool the mandrel could lift slightly and cause the tool to pull in to the work, while an inverted rear tool would push the mandrel further down. But the two key things that fixed parting for me were first, using a sensible speed rather than back gear; and second using power feed. The first works as the tooth load is much smaller and at a higher surface speed the tool cuts much better. The second keeps an even feed going. I also bought a "kit q cut" parting tool from the (lamented) Greenwood tools which I use in the front toolpost, and I also have but seldom use a rear toolpost.

The problem often is that the operator is very nervous about parting so uses a low speed to "make it safer". Then they feed slowly and probably shakily, the tool doesn't cut at first as the pressure builds, then suddenly bites, digs in and stalls the lathe. This makes the operator even more nervous so next time is even worse!

Nowadays parting using CNC on the lathe I often just run the lathe at the last turning speed and feed at up to 5mm/minute and don't get any problems.

A front mounted parting tool "digs in" when an increased downward load causes a flexural rotation of its supporting structure towards the work

A rear mounted tool sees an upward load and "digs out" in similar circumstances thus avoiding a jam.

If you are cutting with the cross slide near it's outer limit and you lathe bed has a lot of slack at the back of the saddle then the tool will pivot the saddle on the front way and lift it's rear off the bed, especially if the bed is a prismatic one. I don't see it happening when the saddle is locked for parting off

Just another thing to be aware of: RTPs also restrict space for other purposes - for example they can limit the position of the crossslide when drilling with a chuck from the tailstock or get in the way when you want to use a manual tapwrench with tailstock centre guidance.

OTOH, the more capstan-like work you're likely to be doing, the more useful they're likely to be. A big spindle-bore tends to favour this sort of approach.

Edited By Mick B1 on 22/03/2023 11:27:13

This forum can't even agree on what constitutes an industrial machine, according to a recent thread on the topic.

You do need to lock the carriage down when using the rear parting tool, for best results. And there are lift plates under the bed ways to hold the carriage down.

One of the old books ISTR says the cutting forces on the headstock bearings with the inverted rear parting tool are downwards, so the force is taken by the lower half of the bearings, solidly supported by the headstock casting. The front parting tool puts the cutting forces upwards against the relatively flimsy, flexible and moveable top bearing caps. That's one theory to add to the mix anyways.

I just know from long personal experience first on the old Drummond M Type and now on the Myford ML7 that the inverted rear parting tool works much better than the conventional front parting tool. I even managed to snap the end off an almost new Eccentric Engineering T section HSS parting blade running it in hard and fast from the front without much for oil on it on the ML7. Could not manage to do the same rear mounted and inverted. The old Drummond is the same on the old conventional shaped parting blades. Chattery and dodgy from the front. Competently parts 2" diameter steel from the rear.

I put it down to the chips falling out of the groove on the inverted tool. No scientific basis for that, just that I can observe the chip behaviour on the two methods vs the number of jam ups and heat generated. Whereas I can't really see or measure forces and flexes in the machine parts so can't really know if that  is a factor, or just a theory.

Edited By Hopper on 22/03/2023 11:42:40

Rear toolposts purport to solve a number of problems that should be a long way down a beginner's list of things to worry about. The only claim that isn't mostly subjective is that they add another tool position for complex production work.

As a beginner you need to concentrate on making whatever tools you are using - which is another can of worms - work as they are designed to. Rear toolposts, quick-change toolposts, complex insert holders etc etc are expensive distractions from achieving that.

not got a rear tool post. all my parting off problems come down to one thing. blunt tool

I agree generally with Nick that you should focus on what you need to do the current job in hand - and acquire tools as the need arises. I speak as someone who has been guilty of diving into all sorts of very interesting (but not always very useful) side projects.

I have two rear mounted toolposts but rarely use either of them these days. The most useful is a commercially made holder for an inserted parting tool that can cut through 2"+ diameter mild steel with ease, something I wouldn't attempt from the front on my aging S7. Having said that, I mostly use my small bandsaw for this work these days, especially for for any stock larger than my headstock can accomodate.

The other RTP is a George Thomas design that is useful for the very rare occassion I need to make multiple small parts, when it is very useful as part of a (repeatable) process.

But in truth most parts I make are singular in nature and not repetitve. So my advice to anyone starting out would be to focus on making the parts they need as well as they can and not be tempted by too many 'bells and whistles' along the way. Of course, making tools can be a very enjoyable hobby in it's own right but everything takes time and time is something that gets more precious as you get older.

Regards,

IanT

Edited By IanT on 22/03/2023 12:26:20

Welcome to the forum I use a rear tool post for parting off next used it for anything else but I struggled at times parting off since using my rear tool post not had a problem

H

I don't personally think they're that much of a game changer for parting off on a Super 7 (or bigger), more of a nice to have.

The thing they can offer which is game changing on many smaller lathes (and even on big ones), is using an upside down threading tool to thread tight up to shoulders.

You can both reduce the size of thread relief needed to do so, and increase the speed of the lathe whilst being entirely confident you won't crash.

Thank you so much for all the informed replies.
I shall re read them when I have time. Work is really busy at present but I am very grateful for the replies.
The reason I was asking the difference between a rear tool post holder and a front one was because I was wondering if they are made differently.
I have a Taig lathe that has many parts missing and I can not find a front tool post holder for sale in the uk! But on eBay there are some rear tool posts that look similar. I was wondering if I could use the rear tool post at the front of the job.
min fact the Taig lathe I have acquired from a family member has so many parts missing it may be more cost effective for me to buy a new complete lathe.
my reason for wanting a lathe is to make parts for old reel to reel tape machines and also dabble in making jewellery rings and bands from wood and metals.

thank you.

Is this the thing?;

Rotagrip - Peatol Extra Toolpost