Why Doesn't the M6 Knob Tighten Properly?

[Nik_2213]

Another 'WTF ??' is that some Metric threads are available in several pitches. and, more disconcerting, tolerances. By analogy with 'USF' and 'USC', speciality Metric pitches exist. And, for a price, higher-precision, usually 'High Tensile' fittings for when you need utmost function from critical fixings...

I innocently acquired a 'clearance' baggy of such super-bolts, found they were correct pitch by my gauges and die-set, but seriously fussy about nuts. To prevent binding, I had to clean up their 'ordinary' nuts using tap-set. ,

 

[sophiecentaur]

I innocently acquired a 'clearance' baggy of such super-bolts,

I hope they didn't cost much. Items like that can find their way into the 'regular' drawer and cause no end of frustration. I don't actually buy stuff like that but I do have problems with some screws salvaged from old equipment.

But I don't think this thread will get us far until the OP tells us more about the actual situation. Grub screws are not intended to be used frequently. A component that's intended to be adjusted regularly will use an appropriate fixing. The picture in post #1 looks like it's from an astronomical device (holding a small optical tube? - are those rough dovetails at the bottom?)

 

[Baluncore]

The picture in post #1 looks like it's from an astronomical device (holding a small optical tube? - are those rough dovetails at the bottom?)

No, that is an open, linear-motion, bearing block, with circulating bearing balls.

The central side screw, is a clamp, used to lock the block in place. There should be an internal puck that is pressed against the rail when the clamp screw is tightened. The puck prevents the end of the clamp screw from damaging the surface of the precision-ground rail that the block runs along.

The clamp screw should have a polished flat end that presses on the puck. A sharp point would penetrate and split the puck, then damage the rail.

 

[sophiecentaur]

There should be an internal puck that is pressed against the rail when the clamp screw is tightened.

I made a similar comment; used the wrong word, though.

So you are saying that minimal force is needed to locate the bearing block in the tube (it's full of bearing balls so very little lateral force needed).
I have to ask WHY the OP needs to do any periodic adjusting, then. It's a bit of a muddle / mystery.

 

[Baluncore]

So you are saying that minimal force is needed to locate the bearing block in the tube (it's full of bearing balls so very little lateral force needed).

The bearing block contains the recirculating balls, and runs on a cylindrical rail. The polymer insert that guides the balls is held in the block, by an internal circlip at each end. End-play of the polymer insert in the block is not critical in a linear bearing. When the clamp is tightened, the end-play is also clamped. The clamping force is applied to the cylindrical rail through the polymer, between two lines of rolling balls.
For an M6 clamp screw, the holding force along the rail is about 30 N, with a torque on the screw of about 0.8 Nm.

I have to ask WHY the OP needs to do any periodic adjusting, then. It's a bit of a muddle / mystery.

A low profile grub screw would be used where the block only needed to be clamped while aligning the machine, setting up the axes, or synchronising the ball screws used for positioning.
A clamp with a manual knob would be employed where the machine operator needed to position the block and clamp it in place, before some process was undertaken.