Toolpost mount for Acorn lathe

[dmbgo]

A while ago, I was talking on this forum to Uwe, I don't remember the context of the discussion, but I think it was regarding lathe accuracy etc. Uwe said that I'd be better off getting rid of the cross slide on my lathe, which was redundant after the Acorn had been fitted, and instead using a solid steel block with the tool post on top of it, for greater rigidity etc.
I have mulled over this for a while and decided to proceed.
I have milled a block of steel square to a couple of mm narrower than the width of the cross slide. At the moment the steel block is 117.00mm square by 58mm high. I can now alter the dimensions of the block with the mill to the optimal size. My toolpost block is 75.75mm square by 66mm high, this is without the toolholders for 16mm tools fitted. The toolholders add 25.59mm to the width of the toolpost, taking it to 101.34mm square.
The toolpost (or tool block) is a QA150 as per the following photo.

This brings me to my questions:

1/ The mounting bolts for the (now redundant) compound slide that the steel block is to replace, are back a bit from the edge of the tool block. I think there is enough size in the new steel block to mount it onto these bolts, but I want to make sure before mounting it, that I am able to move the tool far enough forward to accommodate any future positioning requirements. Is there any foreseeable need to be able to move the shortest lathe tool past the centre line of the lathe?

2/ Do you think I need to reduce the width of the steel block to the same width as the toolpost? Or doesn't it matter? With the current width of the block, I won't be able to drop the toolholders more than about 10mm below centre height.
Here is where I'm at....

I'll await a reply to this post before I do anything else.

Regards
Dave

[suntravel]

Hi Dave,

i would keep the solid tool post as big an heavy as possible.

I mounted it with 4 screws, needs some drilling and tapping, but it is rock solid.
I kept the position in X like it was with the topslide.

Uwe

[dmbgo]

Thanks Uwe,
I suspected that it was beneficial to have a heavy mass for the toolpost. I like your idea of mounting. I was going to mill away an area of the block, to allow the original compound slide mounting bolts to fix the block to the cross slide, but 4 x countersunk M8 bolts holding it on would be even more rigid and would save me a bit of milling. Thanks.
Another question, if you don't mind: What mechanism are you using to allow your toolpost to turn and then lock? I have a circular plate, which is recessed into the bottom of the tool post, but it still allows a a bit of movement before the lever on the top (like your tool post) tightens it down. That means that there is no exact repeatability there. This is a bit of a pain, since I need to reset X: every time I change tools. It's normally less than 0.1mm, but still, I think it could be a lot better.

[suntravel]

I use a Multifix toolholder, the base is fixed in place, and you can mount the toolholder every 15°. They are also adjustable in height individually.

Precision on parts after a change is unter 0,01mm.

Here is a good source for high precision copy's, since the originals are no longer made:

https://www.stahlhalter24.de/axa-schnel ... oesse-11-a

Uwe

[tblough]

Along the same lines is Dorian's Quadra toolpost:

SEC-B-TOOL-POST-TOOLHOLDERS-2016.pdf

(10.59 MiB) Downloaded 19 times

[suntravel]

Along the same lines is Dorian's Quadra toolpost:
.....

They also are looking good, maybe more rigid than Multifix.

But in Germany everyone and his dog have Multifix on a lathe

Uwe

[Muzzer]

On my Colchester Bantam conversion, I made a toolpost that can be fitted in 3 alternative positions on the cross slide - this required 3 sets of tapped holes but I'm not put off by the idea of making holes in this old machine. Because the post is offset from the centre of the cross slide and it can be flipped 180 degrees about the vertical axis, it also allows me the option of positioning the tool at the tailstock side - handy for when you have a really long workpiece.

[tblough]

Because the carriage is so wide on my Hardinge, I made my block with three positions as well, but mine were to move it across the carriage. Closer to the front edge for collet work, closer to the back edge to reduce stickout on the tailstock when working between centers.

The Dorian tool post has provisions for two 6mm dowel pins for location. This keeps my diameter offsets from changing whenever I move the toolpost.

[dmbgo]

..... There is a lot to think about, and more than I expected - thanks.
There are some really nice toolpost mounts here.
I will start by replicating Uwe's toolpost mounting block, I think 4 x cap head 8mm screws fixing the steel block to the cross slide would be a good starting point - it shouldn't move.
I like the idea of a permanently fixed toolpost with the toolholder being the object that is indexed, but unfortunately, I have to stick with the toolholder I have - I've invested too much money in the toolpost and associated toolholders to change now.
My next challenge will be a rigid, indexable mounting system. At the moment I use a circle of ball bearings that are supposed to fit into corresponding indentations in the bottom of the tool block and lock the toolpost into the indexed position. I don't think this is satisfactory. I am tempted by the picture that Tom posted, using needle rollers to find the index. I feel that this may be more rigid than the ball bearings I currently use.

[tblough]

Dorian's smaller Quadra toolposts like the QITP25N (AXA) one I use has 24 ball bearings for location (every 15°). They tout indexing repeatability within 0.00005".