Cross slide lock...

[John Hill]

My Chinese 12x36 lathe  (type CQ6230A-1) has a very unsatisfactory cross slide lock that consists of a grub screw pressing on the cross slide gib.  It is effective enough but it is in a very awkward place with the grub screw on the side of the cross slide right where it can not be got to if the tailstock is moved up.

So I thought of this modification which would put the adjusting screw on the top of the cross slide..



As you can see screwing down the top grub screw would force a ball between two others and hopefully put pressure on the gib.


Any comments before I start drilling holes in my lathe?

John

[lordedmond]

John
Are you sure the gib bears on that face the normal way is on to the dovetail face not downwards

That face is normally the load bearing face

Stuart

[allanchrister]

It's not clear from your sketch but it looks like there are 2 gibs in place, the main one and a smaller one on the inside against which the BB's push.  Did you mean to put a slot in the top of the gib in which the 2 lower BB's rest?  If not, how is the outside BB retained?  It does look like a thread in the upper part of the gib, but it's not clear.

I think the concept is good and I trust the BB's are hard enough to resist any damage from the pressures involved.  Just meant to be some constructive observations........

[John Hill]

Hi,  sorry for the poor diagram.  The shape between the gib and the balls is supposed to represent a bit of brass.


The idea is that pushing a ball down will tend to force the two balls apart.

[awemawson]

John,

You may find a pointed grub screw more effective parting the two lower balls if you think of the angles involved.

[lordedmond]

John

I understand now the gib is the thin line on the left
The bit that I thought was the gib was I fact the cross slide body

Sorry but it was unclear as Andrew says pointed grub screw would work but normal hard balls would do after all ball races are OK

Stuart

[philf]

John,

Just an observation.

I assume the ball on the right hand side is retained by a grub screw. When you tighten the grub screw on the top, to press the ball between the two others, the right hand ball can't go anywhere and thus the top ball will try to embed itself in the left hand side of the vertical hole.

You need to make the vertical hole a good clearance on the ball so it's got somewhere to move.

Alternatively, instead of a ball on top you could make a hardened plug with a (e.g.) 45 degree angle machined on just one side.

 

Phil.

[John Hill]

Thanks, your comments have shown up a few challenges which I will have to think on some more!

[vtsteam]

I like the look of it. But it will only work well if the degree of movement is small for reasons PhilF mentioned. How small is probably important. Clearance in the hole is probably important, too.

Breakout (if any) would be at the bottom corner of the hole. All depends on the magnitude of the vector force when locked and the slide material properties (CI, I assume). Might be fine, might not.

Also, depending on the hole clearance to the ball,  it isn't just movement to the left wall of the hole by the upper ball that might be a problem as the lower ball moves, it is also movement fore and aft (in this profile view) if clearance allows it. These are balls so they slide in two degrees of freedom with pressure from above. Any movement that will allow the top ball to seat lower will occur, so that may not be just movement to the left. Depends on clearance, movement, etc.

But mental experiments take second place to real ones. I'm a great fan of reality! It wouldn't be too hard to mock this up in scrap to intended dimensions and see what happens with this kind of lock. 

[vtsteam]

ps. just remembered reading something way back about bathroom silicone rubber caulk used for a "hydraulic" lock.

Basically the brass pin bearing against the gib would be retained, but the balls would be replaced by silicone rubber caulking compound. Pressure from the screw transmits via "hydraulic" pressure to the pin.

I've never tried it -- just one of those memory things tucked away. Seems removable if it doesn't work. Once again a mockup would prove it out before risking the real thing.

[lordedmond]

Another point it's OK to tighten it up but will it release without the BFH

If the angles are not correct it could lock on

£0.02

Stuart

[vtsteam]

Found the reference -- "rubberdraulic" from Guy Lautard's The Machinist's Bedside Reader (TMBR #1)

[John Hill]

Thanks very much, now I must go and think about rubberdraulics!

[John Hill]

A block to test the idea of rubberdraulics.

The cap screw goes in the top hole and the piston goes in the side.  The passage between them is filled with bathroom caulk which I will leave for a few days to set.

[vtsteam]

Cool!

Can't wait to see if it works. 

ps. Tips: a little cornstarch added will make silicone rubber set up quickly and in heavy sections without air. It can also be thinned with mineral spirits or kerosene (paraffin), though this increases shrinkage after drying. Wetting fingers with water with a little dish soap added will prevent it sticking to you when handling

[John Hill]

I decided the caulk had had enough time so I tested the little test device.    It was no surprise that it did work.

However, I doubt I would be using this for my cross slide lock but it is something to be kept locked away for future reference!