Top slide stud too short?

I've replaced the existing static tool post for a new Dixon quick change on my ML7. However the existing nut only half covers the protruding stud - see photo.

Given that this is the genuine Myford quick change tool post, am I right in saying either

a) the stud needs to be extended - is it threaded into the top slide casting

b) is this the correct nut

c) do I need to a (new correct) length top slide stud altogether?

As ever, any help is gratefully received.

Edited By choochoo_baloo on 11/07/2017 18:55:34

The washer under the nut looks very thick.Substitute a thinner one and see what that looks like.

Maurice

Looks like it's got plenty of thread engaged, you should be fine. Isn't the extra height just to allow you to fit a spanner without fouling everything else?

Neil

Is that the thick domed washer that comes with the standard tool holder you have used?

The stud fits into the topslide from below, it pushes through, not threaded. It has a wider base with a pin into a small hole in the topslide, which stops rotation, and it is all located by 3 tiny screws.

The top of the standard stud should be about 66mm high above the topslide and this lets the whole length of the original nut fit completely over the thread with a bit left over.

Check the height of your toolpost block - the "standard" size block for a Myford is 36 - 38mm.

The correct nut is a long one, 2-3 times the size of a usual nut.

Some posts have beaten me to it while my brain was ticking over (very low revs).  You don't need a thick washer under the nut, usually a "top hat" washer is used to ensure a good fit on the stud, but if its a good fit anyway then an ordinary washer will do.

Edited By Robbo on 11/07/2017 19:23:35

And whilst I have my pedant's hat on, if it is a 'genuine Myford' QCTP, it won't be Dickson, and it could never be Dixon - he was the geezer in Dock Green .

I can confirm the tool post and stud are of the lengths stated above.

So as to avoid any misunderstanding, are you all referring to the thick shiny washer at the bottom of the stack -I assumed that was part of the tool post piece -in which case removing it would make sense. Side on photo attached for clarity.

What purpose does it serve? Once again I am a newcomer - so which is the best way to do this?!

Edited By choochoo_baloo on 11/07/2017 20:25:24

Yep, the thick shiny bit that looks like a bearing housing.

What purpose does it serve? - none, it shouldn't be there, you only need a normal plain washer under the nut.

At the risk of making a fool of myself, was the thick shiny washer part of the QCT? I had a very similar "washer" with my QCT which came from the bloke who used to make them for Myford.

It isn't a washer, but a bush which is a damn good fit into the base of the QCT block. You are supposed to bore it out accurately so that it will just slide over the Myford toolpst. It should be bored accurately, so that there is no shake of the QCT block on the toolpost. Something to do with accurate machining or rather repeatable machining.

GHT had a thing about 4 way toolposts not being accurate enough because of this shake. He designed a 4 way toolpost with a huge plain chamfered bearing to seat the similarly bored 4 way toolpost block. I never did understand the reason for it!!!

Andrew.

Edited By Andrew Tinsley on 11/07/2017 21:57:14

Robbo:

Aren't those screws for the fourway ratchet?

The ratchet on my ML7 is actually an 8-way, but yes, the 3 countersunk screws hold that bit in place on top of the tool platform (if you have it, and it works with your qctp. Mine doesn't.)

A good hold with a thread usually relies on about three turns of thread. Any more does not add to the strength of the assembly - but doesn't really do any harm in cases like this.

Tim

Tim Stevens:

Thanks for the clarification. I was actually thinking of the (maximum) number of tools the post will hold. That said, I don't own the Myford device and am interested to learn that it indexes eight positions as opposed to the four that my GHT version provides.

Right I'm confused! As suggested, I removed the 'thick shiny washer'. It is actually a step down bush. So I'm wondering why is the post block bored larger in the first place, to take this interference fit bush to bring it to the required stud diameter. The bush is only about half as deep as the tool post too, which agains seems poor.

I'm sure this is done on purpose. I cannot see why though.

So to maximise rigidity I plan to replace the 'thick shiny washer/bush' with a new flush full length bush; stud is then held along whole post, and the toolpost nut grips along its whole thread.

Quite right - memory failing again

I asked the very same question of the man that makes them, so you are not alone Choo Choo! The answer was quite simple, he said that the Myford sized QCT also can fit other similar sized lathes, that have different toolpost diameters.

Hence the block was bored oversize and a damn good fitting bush was provided so you could bore it out to suit your own lathe.Simple when you ask!

Andrew.

Yes, I remember that. I should check, but from memory the difference with the 4 way toolpost is that it is rotationally indexed unlike most QCTPs so that once all your tools are set to the right angles for your particular job and the cross-slide and topslide micrometer readings are noted each tool can be returned to it's exact position. This way you can do repeat operations such as turning to diameter, chamfering and then parting off in sequence without losing any settings. Quick change toolposts don't need this refinement so long as the main block is not unclamped. If it is you loose all the settings anyway.

I seem to recall the other issue was it didn't suffer from accumulated swarf as it was slightly clear of the table apart from the tapered seating.

As I say I should check the book to be sure. George Thomas didn't often do stuff without a good reason.

regards Martin

There are good, albeit somewhat arcane, engineering reasons why its correct to have the tool post bore rather larger than the stud diameter with a short, top hat style, bush to both span the difference and apply clamping pressure. Its also correct to have a large diameter annular relief in the centre of the tool post base rather than making it flat. Typically the diameter of said relief is around half the length of the block sides.

Probably the most obvious example of why this arrangement is a good idea is to consider fitting the post to an elderly machine whose stud is no longer completely straight and slide top no longer perfectly flat. On smaller machines it doesn't take much more than occasional careless over-tightening to pull the top slide surface up a little where the stud threads in. Due to the relatively coarse threads involved such pulling will almost certainly shift the stud out of perpendicular too. Relatively small studs are easily bent a bit too.

Clearly if the tool post is bored a close fit on the stud and its base made dead flat it becomes very hard to get a firm, rotation free, seating of the tool post. Resistance to rotation being very important with a QC system because not only do all the loads feed to the slide via the post but also the tool tip is considerably offset from the post centre line so has considerable leverage. With the oversize bore and adapter bush at the top small deviations from perpendicular or small bends in the stud will tend to be pulled out when the nut is tightened down. Obviously the securing force will not be quite as great or evenly distributed as it will with a perfectly perpendicular and perfectly straight stud but small deviations can be accommodated in an adequately satisfactory manner.

Similarly a flat bottomed tool post on a distorted or damaged surface may only have proper contact over a relatively narrow annulus surrounding the stud. Not good for resisting rotation as the contact area will be small and the tool leverage large. With the relieved base not only is the contact area much larger, even if the actual width is no greater than before, but its a lot closer to the tool tip correspondingly reducing the leverage. Even if the actual mounting surface on the slide is a little uneven, so not all of the available surface on the base to the tool post is actually in proper contact, the actual holding area will almost certainly still be much greater than with the flat base contacting only around the stud.

Many folk, myself included, consider a thin washer of stiff card or aluminium alloy underneath the tool post gives a useful improvement in grip and allows a lower torque to be used on the mounting nut. Need a new washer every couple of years but thats no great issue. On some of the smaller machines high torque on a QC tool post mounting stud can distort the slide enough to impede its operation. Not enough to lock things up but can certainly destroy the nice even feel so important for precise work.

Clive.

Edited By Clive Foster on 12/07/2017 14:32:33

Hello Clive,

An interesting dissertation on the subject of bent toolposts. I can certainly see where you are coming from. I have always checked that my topslide is flat and the toolpost is vertical, otherwise you can certainly come a cropper as you stated.

My input was purely based on what the gentleman who made the original Myford QCT recounted to me when I asked what the "washer" was for.

Regards,

Andrew.

Hello Clive,

An interesting dissertation on the subject of bent toolposts. I can certainly see where you are coming from. I have always checked that my topslide is flat and the toolpost is vertical, otherwise you can certainly come a cropper as you stated.

My input was purely based on what the gentleman who made the original Myford QCT recounted to me when I asked what the "washer" was for.

Regards,

Andrew.

Thanks Andrew. That's reassuring to know I am asking sensible questions!! ATB Matthew