As a beginner, I have found it hard to select a suitable tool post for my small ML10 lathe. This Instructable is intended to hold my own notes to remind me about the main considerations when I pick another tool post in the future! I have settled on a four-way tool post, but the dimensions given here will obviously be relevant for other types as well.
Why get a new tool post?
The standard elephant's foot tool post is very tiresome to use when you have a selection of cutting tools to change. Each tool needs to adjusted so that it cuts on the lathe's centre line - this need shims underneath it, which get lost or muddled, etc.
I can't afford a quick-change tool post, so I am using four-way tool posts. Getting one to fit can be tricky. This Instructable is intended to help choose/adapt one which fits.
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Step 1: What Is the Problem?
The ML10 is a small lathe. Most of the affordable four-way tool posts I have seen advertised are too big to be of use without modification.
The critical dimension is 13.5mm! This is the distance between the top face of the cross-slide and the centre-line of the lathe. 13.5mm is fine if you mount the cutting tool on the top face of the cross slide (as in the elephant's-foot tool post). In this case, you could use a tool as big as half inch (about 12.5mm) and it would need about 1mm of shim underneath it to bring the cutting tip up to the centre height. Typically on a small lathe like this, you would use a smaller cutting tool (some of mine are 5/16" [7.9mm], others are 3/8" [9.5mm]) and pack it up higher by using more shims.
13.5mm becomes awkward when you move to a more convenient four-way tool-post. This already has the tools mounted at centre height by holding them in a steel-tool-steel sandwich. The difficulty is to get the thickness of the steel underneath the tool small enough to let you fit a tool on top and still come out under the 13.5mm dimension.
Both of the small four-way tool posts that I have bought had a bottom face thickness of about 10mm - only leaving me room for a 3mm tool (not sensible).
Advice to myself when buying the next four-way tool post:
Remember to check the thickness of the bottom flange. Anything over 3mm or 4mm is unsuitable - or will need adapting (see the next step in this Instructable). Stick with cutting tools of size 8mm or 5/16" or 3/8". Smaller tools are OK. 10mm or bigger cutting tools will not fit.
Why buy a four-way tool post instead of a quick-change tool post?
I'm sure that the quick-change tool post is the most desirable solution - a huge number of experienced lathe operators have invested heavily (and continue to invest) in their QCTP. For me, the cost is simply too much to justify, and I find my four-way tool posts to be excellent. Both of my four-way tool posts have cost under £20 (US $30) and I'm guessing that I would have to spend at least five-times that amount to hold 8 QCTP tools. The cost of a QCTP tool post and a carrier for each tool I use would probably exceed the cost of the lathe itself.
I keep my elephant's foot tool post handy for the occasional cutting tool which I don't have mounted in a four-way tool post.
I have seen a lot of criticism about four-way tool posts, but for me they work well. I have never cut myself on 'jutting-out tools' (having read this, I started out covering my unused tools with rubber tubing - I soon gave that up). I keep my 'main' tool post loaded with four tools - I can change from one to another in a few seconds. You cannot mix tools like boring bars (which stick out towards the chuck) with 'normal' tools (which stick out in the opposite direction). I think many people use them as two-way tool posts, to avoid some crowding hassles; even this is still sensible because you can swap one four-way tool post for another in 20 seconds - and then you have all of the tools already set on centre height and ready to go.
Please don't add comments below on your opinions of different styles of tool posts
I'm trying to forestall a mindless 'discussion' about the merits of different tool post designs. That is not what this Instructable is about.
Step 2: Modifying a Purchased Four-way Tool Post to Suit an ML10
To make my four-way tool posts usable, the bottom flange had to be reduced to 3mm - 4mm. To do this they had to be mounted in my four-jaw chuck (take out the tool post clamping screws, push the tool post right into the chuck, check that the bottom face is running true and square) and skimmed until the bottom face was around 3.5mm thick. I don't have a mill, so the tool posts had to be turned on the lathe. For the first tool post, I had to hold the cutting tool under the elephant's foot!
The cuts were pretty brutal being intermittent, but I took it slowly. I was worried that the tool post might be hardened in some way, but this was not a problem.
Both four-way tool posts had to have their central hole bored out from 10mm to 11.5mm in order to fit over the ML10's tool post bolt. Again, the four-jaw chuck was used for this.
If you want more information about the dimensions, study the comments on the photographs.