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Questions on an MLA inspired toolpost

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Calum Galleitch02/04/2022 18:24:18
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194 forum posts
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As I mentioned in another thread, I have been doing some basic CAD modelling of my own version of a QCTP inspired by the MLA-23.

screenshot from 2022-04-02 17-40-51.jpg

The dovetailed toolpost as drawn is 80mm in diameter. The hole has a 1 degree taper, matched by the internal piece shown behind in grey. The diameter of the external piece at the bottom is 55mm and at the bottom of the internal piece 56mm. So if the internal piece were to bottom out the dovetail pieces would open by just over 2mm. Of course the plan is that the toolholders will constrain the amount of opening on the order of 0.1mm.

Two questions.

One. The narrowest part of the external piece opposite the dovetail is 7.5mm. Is this thin enough that the external piece will actually bend sufficiently under reasonable force from the toolpost handle? A cantilever beam of the same length would deflect several mm under 100N of force but of course the forces here are not normal to a 7.5mm cross-section. I could (and would) alter the distance or add a groove as Alan Jackson has done in his very attractive version, but I'd like to start off in the right ballpark.

Two: Rather than have the height adjuster in the centre, moving it back allows the toolholder to be a little narrower. Is there any reason this might cause an issue? I can't see that it would, but better to know now than later!

David-Clark 102/04/2022 18:47:54
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222 forum posts

That looks an interesting design. The height adjuster position should be fine although I would possibly add another height adjuster position at the other end so that you could have the height adjuster close to the tip of the turning tool.

How do you locate the radial movement of the tool?

Calum Galleitch02/04/2022 20:39:42
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Yes, I wondered if it would be better at the other end - I put it at the back so it wouldn't be getting in my sightline - but in theory the toolpost should be pulling the toolholder horizontal anyway. My only puzzle is to what extent theory actually applies!

There's no radial registration (nor is there in the original design, I believe). Maybe it's my inexperience, but I don't really feel the need for it. For something that required good alignment, like a parting tool, I'd indicate in anyway, and most tool alignments aren't crucial. I do have a ratchet-type pin on my top slide although offhand I'm not sure if it's in a useful position.

pgk pgk02/04/2022 22:16:59
2605 forum posts
293 photos

Naive question perhaps but once dogged down how do you get the tapers to release?

pgk

duncan webster03/04/2022 01:35:06
4122 forum posts
66 photos

I would have thought that not having angular location would be a royal pita. Without it you can't swap tools and keep diameter settings

John Haine03/04/2022 08:51:21
4718 forum posts
273 photos

I think a dowel to fix the angular position is essential. A right pain to have to dial in a parting tool, and you will be surprised at how much the post can shift over time with cutting forces.

Alan Jackson03/04/2022 10:54:14
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260 forum posts
146 photos

I chose the expander plug to be made from cast iron so it did not stick in position. It has an included angle of 40 degrees. The block has through holes and grooves to make it flexible and the adjusting screw has a half section that fits into the groove in the block. All this to minmise overhang. More pics in my album

Alan

Toolholder components.jpg

Through holes for flexiblity.jpg

Edited By Alan Jackson on 03/04/2022 11:03:24

John Haine03/04/2022 11:41:03
4718 forum posts
273 photos

So do I understand correctly - the central post is tapered, and the split sleeve around it, so when you force the sleeve down with the clamp screw it expands the dovetail into the tool holder? Doesn't that make tool height setting a bit variable?

Alan Jackson03/04/2022 13:31:45
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260 forum posts
146 photos

Hi John,

The central post is a parallel 1/2" threaded column fitted into the topslide. The expandable dovetail block fits over the post and firmly down against the topslide top face, then the solid tapered plug is fitted into its matching tapered bore in the dovetail block and over the post. The plug is then pushed down by the locking lever screwing down on the central post. This has no effect on the preset height setting, it just expands the dovetail block to lock the whole assembly in position. The locking lever only needs a slight force to lock it all firmly in position.

Alan

Edited By Alan Jackson on 03/04/2022 13:33:40

John Haine03/04/2022 14:40:53
4718 forum posts
273 photos

Aha! Thanks for that explanation.

duncan webster03/04/2022 14:57:03
4122 forum posts
66 photos

But Calum's is different, very slow taper on much longer expander plug

Calum Galleitch03/04/2022 16:28:31
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194 forum posts
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Posted by duncan webster on 03/04/2022 14:57:03:

But Calum's is different, very slow taper on much longer expander plug

Well, this iteration is :D

pkg's question about releasing made me realise (a) they are quite right, releasing a gentle taper would be a pest and (b) there would be practically no downforce to hold the toolpost against rotational forces. I also did the trigonometry and realised that the plug would sit about 30mm proud when the toolpost was in a relaxed state, so some redrafting will be in order, starting with a self releasing taper.

On radial location, I take the point about rotation under load, but my four-way toolpost has no locking either and in my experience so far the tool itself slips before the post does.

Howard Lewis03/04/2022 16:53:11
6311 forum posts
15 photos

To maintain radial position of the Main Toolpost body, why not drill and ream the post and the Cross Slide, and use a dowel to locate them to each other?

If you wanted to use the toolpost in a 90 degree orientation, just drill ream another hole in the bottom of the toolpost

(Basically what I did for my 4 Way Indexing Rear Toolpost )

Or am I misunderstanding something?

Howard

Calum Galleitch30/05/2022 22:36:42
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194 forum posts
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screenshot from 2022-05-30 21-41-54.jpg

Not much change, really just an update to the wedge shape and I've added a 5mm hole at the opposite edge to the dovetail to act as a sort of hinge.

screenshot from 2022-05-30 21-42-48.jpg

I'm not sure if I'll want to use the current toolpost handle; I suspect it might be a faff. I might make a long hex nut and washer and keep a dedicated spanner on hand for it.

I'm still not convinced of the necessity for radial registration, but it occurs to me the 5mm through hole would take a 5mm drop-in shaft nicely, so if I change my mind it's just a case of transfer punching a hole.

One question I will need to answer quite soon is: what do I need in the way of milling cutters? I already have a dovetail cutter, as it was on sale, and a cheap 12mm end mill from the auction site. With a lot of metal to remove in the tool holders, I suspect I want a decent sized roughing end mill, within the capacity of my lathe.

Nigel Graham 230/05/2022 23:30:08
2287 forum posts
33 photos

A slot-drill rather than end-mill, surely, to cut those big slots?

(Use the end-mill for the block's exterior.)

It's a neat design but I think I'd prefer a rectangular block to support the tool-holders' full length. I'd also worry about unwanted rotation when slacking the device to change holders; and whether the taper would release as intended.

Still, try it and see how it all works out.

Regarding putting the height adjuster at the back (towards your side of the tool-post) rather than the front, I think that is the right approach. Mechanically there is no difference, but the less clutter near the tool itself the better.

You might in the end find a separate spanner more of a faff than a handle, so if 'twere I designing this, I'd do so to keep my options open - allowing simply swapping nut with handle or vice-versa. It may prove possible to adjust the handle's operating arc by washers to keep it in the optimum position for your use.

Calum Galleitch31/05/2022 22:12:07
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194 forum posts
65 photos

Posted by Nigel Graham 2 on 30/05/2022 23:30:08:

> A slot-drill rather than end-mill, surely, to cut those big slots?

That's what I thought but the more I read the more confused I get, and I've read a couple of comments that rippa-style end mills are preferred or at least equal for hogging out stock like this. I'd definitely like to take advice on this as I will have a few of these to do and given that it'll be done in a vertical slide setup I want to make it as pain free as possible! I don't want to waste money but for something like this I don't mind buying the right tools for the job.

> You might in the end find a separate spanner more of a faff than a handle, so if 'twere I designing this, I'd do so to keep my options open - allowing simply swapping nut with handle or vice-versa. It may prove possible to adjust the handle's operating arc by washers to keep it in the optimum position for your use.

Yes, I'm retaining the original toolpost stud and I'll be using the original handle in the first instance. It's a bit of a pest as is as the handle can foul the tool-holding studs, so we'll see how it goes.

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