Adding a QCTP to a 9×20 Lathe

It took me a while to find much information on this, so I’m paying it forward to hopefully make it more likely that the information finds its way to someone who needs it! If you’re fairly new to machining, I’m going to discuss toolposts in general a little bit. If you just want to see the 9×20 process, feel free to skip below!

What is a QCTP?

A QCTP, or Quick Change Tool Post, is a type of toolpost for a machine lathe which, as the name implies, allows for much faster tool changes than traditional lantern or turret type toolposts.

Four way turret toolpost
Four way turret toolpost boring out the stainless NN-14 barrel

It’s a pretty standard upgrade for most lathes which are used in any kind of production capacity, since the ability to move through a rapid sequence of tooling allows for efficient machining of multiple identical parts.

There are two main types of QCTP that tend to be used: two position “Aloris”, and 40 position “Swiss-type” or “Multifix” styles. The US tends to use the two position, and Europe the 40-position, though that’s not an absolute. They all use a lever that cam-locks a variety of toolholders in position against a post which dictates the angle they’re at.

The 40-position uses a spline pattern around the entire post which allows a tool to be set every 9°, and a loose pair of arms with pins on the end that engage with slots in the toolholders. The cam operates at the root of the arm joint, pushing the back of the joint apart, squeezing the pins together and drawing them back, pulling the toolholder (with a matching 9° spline) into the post spline. This is extremely accurate and repeatable (repeatability of a toolpost is defined by how close the tip of the tool is to its original position when the toolholder is removed, replaced, and re-locked). When I get a new, bigger lathe in future, this is the type I’ll put on it.

For now, I got a relatively low-cost two-position type. These have square bodies with dovetails on the inside and spindle faces (assuming it’s set square on the lathe, which they usually are). There are two variants of these; piston and wedge type. The piston has a cam inside the body which pushes a piston out of the center of the dovetail against the toolholder, forcing the dovetails to mesh in tension. These tend to be around 20-30% cheaper and are generally considered to have lower repeatability

Toolpost wedges
The silver bits are the wedges, and slide up and down by the big lever

The other type is the wedge type, where the cam action is a screw which draws down a tapered “gib” which replaces one side of the male dovetails. When the toolholder is inserted, the wedge is forced down, pulling the toolholder dovetail slot “apart” and drawing the holder into the toolpost body. These are a little more expensive and generally considered to have higher repeatability. This is the type I got, because being able to reliably replace a tool after removing it to measure the workpiece can be useful. Many people don’t much care and will re-touch off on the part on each tool change. Just personal preference.

There are size standards for QCTPs, so that there aren’t an infinite multitude of toolholders to manufacture. They’re generally categorised by lathe swing, with some overlap since the toolholders inherently have some vertical adjustment. Aloris styles have AXA (9-12″ swing), BXA (10-15″) and CXA (13-18″). There’s even a DXA for monster machines. Since the proliferation of mini benchtop lathes, there’s also an 0XA for 7-8″ swing. Most people reading this will need either an 0XA or AXA. I, of course, got an AXA. It looks quite large on a 9×20″ machine, but more is better when it comes to machine mass.

The toolpost looks big on the machine
Looks even more huge than this on the machine in person!

The second nice thing about quick change tool holding is that the holders have a screw stop with a jam nut to adjust the height that they sit on the post. This allows for quick and easy tool height adjustment, no shims required! If you use HSS heavily, this may be even more useful, since quick regrinds for rake angle, etc can be dialled in within seconds. You can also use this height adjustment as y axis indexing for sneaky little single-axis milling jobs- slotting, etc. on small parts that fit the tool holder. I believe some people have even made tiny mill vices with a lug base that can be fixed in a QCTP holder.

Adapting to 9×20″ Import Lathes

Where an old South Bend 9″ may have a traditional T-slotted compound slide, the import 9×20″ lathes (like my Grizzly G4000) have a fixed M8 stud protruding from the top. The QCTPs come with a large block that’s intended to be milled to size for a T-slot. This, of course, isn’t very helpful to someone who doesn’t have a T-slot and may not have a mill.

Machinable T-nut
The idea is to machine this block into a T-nut of the right size. Some just turn it down in the 4-jaw.

While it may be possible to disassemble the compound, remove the stud, and re-drill and tap for the M14-1.5 stud that comes with the AXA toolpost, this was a lot more work than I really wanted to go through- and entirely irreversible!

The fixed compound stud between the M8 tap and the new 14mm stud.
The fixed compound stud between the M8 tap and the new 14mm stud in progress.

I did eventually find this handy PDF where a guy makes a replacement 14mm stud which screws into the M8 compound stud, but it seemed like a lot of work and the result would be slightly inferior unless I also used a heat-treatable steel to make it, which would result in even more work. Plus it still requires a mill to make the hex head, and I wanted to come up with a way that didn’t (plus I don’t have a hex collet block for the mill, so it’d be rotary table time!). I figured since they gave me an M14 stud in the package, why not just use that? It amounts to the same thing in the end (well, not quite, but I’ll get to that later).

Cutting off almost all of the bottom threads
Cutting off almost all of the bottom threads

So, I chopped off most of the bottom threads on the M14 stud and faced it off level with the beginning of the thread.

Faced off new stud
I love a freshly faced surface!

Next I needed to know what drills I’d need for the actual process, which turned out to be four: one, a 1/8″ for a through-hole to give the tap chips some clearance. The second would obviously be the 6.7mm tap drill for the M8-1.25 internal thread. The third would be something just a little larger than the shank of the tap. I went with 21/64″ or 0.3281″.

Measuring the tap shank with digital micrometers
Digital micrometers are the best.

That would support the tap and stop it wandering while it was so deep in the hole that I couldn’t support it. By definition, the threaded section needs to be concentric, or the toolpost will get tightened down cocked over.

Compound stud measuring
Not sure why they decided to go with a larger shank than thread, but hey.

The fourth would be the actual diameter of the M8 stud shank so that the M14 stud can sleeve all the way down to the compound for maximum support. I went with 25/64″, 0.3906″.

I drilled the 6.7mm hole slightly over-depth so that ideally, the M14 stud could be tightened directly against the compound surface rather than just hanging on to the M8 stud.

Measuring bore depth
Measure the stud, then see if it pushes in further inside the bore. Simple.

If you weren’t already aware, you can use the inner sliding bar of a caliper as a depth gauge. Apparently this is a thing that people are constantly just discovering, so I thought I’d mention it in case it was helpful!

Time to tap. My only tap wrench goes up to 1/4″, so it was crescent wrench time.

Tapping the bore
Tap tap tap.

Yes, I’m using it the “wrong” way here. No, I couldn’t care less. It’s irrelevant in any practically meaningful scenario.

This is why I drilled the base hole twice! Zero external support:

Deep tapping
Yup.

Then it was just time to open up the base bore, and it would be done!

Last drilling step.
Last step!
Fitting the new stud
It vuhrks!

I decided the first thing I tried would be parting off, since it’s the hardest operation to do on a small machine lacking rigidity and typically having a little more slop in the cross slide leadscrew than is ideal. Dialling in the height just took a few minutes, and the results were beautiful:

Fantastic parting tool chips
Those are the best parting chips I’ve ever seen!

Issues

For the most part, this job really is as simple as the pictures make it look. It’s an ideal process if you don’t also have a mill. There were however, a couple of things to note.

One, on the lowest end of the AXA capacity range, the spindle is naturally going to sit quite low. This means that to center up- especially with tooling larger than the 3/8″ typical of this size machine- the tool tip, the toolholder has to go below the level of the toolpost. This means relieving the compound slide itself to allow that room. I happened to have a large indexable mill that made short work of it, but a little time with a Dremel or file would do the job too.

Removing a little off the compound
Doesn’t need much, barely even 1/8″. I need to do it on the other corner when I get a chance

When it came to actually dialling in the toolpost- which I did with a locked-in toolholder and a test indicator- I found the problem I alluded to earlier. Turns out, the bore of the toolpost is not actually 14.05mm or whatever it should be, and tapping the post into squareness doesn’t purely rotate it like tramming a mill vice does, but wobbles it about the axis of the stud by quite a significant amount, which makes both squaring and tightening quite difficult. At some point, I may weld up a portion of the threads and turn it back down to a much more precise diameter, but since- fingers crossed- I’m very rarely going to need to loosen it back up, that may be a while.

The End

Well, that’s it. I hope this was useful to anyone wondering what a quick change tool post is, or debating the merits of getting one for their benchtop machine. Any questions or comments you can hit me up below or on the social feeds you see in the sidebar there.

Happy turning!

~Rob

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