Ideas for spindle guidescrew fix

[Naiveambition]

i have a zx 45 or rong fu clone mill, and the grubscrew or guidescrew for the spindle had excessive play that I wanted to address. The main issue is steel to steel contact. After initial adjustment it was much better, but if just a hair too tight it will not let the spindle retract. I tried greasing and polishing the contact points but was never really happy with it.
. I understand there should be no preload on this but would like to get the twist of the spindle and screw fitment minimalised as much as possible,

My thought is to somehow make the screw brass tipped at the contact point. I have read a couple people made the fix, but there were no photos to see. They just stated they made it fit better. As you can see in photo, my main issue is the hex head. I can't produce the pattern, so my thought would be to make a all brass one and put a flat for a screwdriver. If I have that thread on my lathe(metric I'm sure)

Second was to solder a brass tip on the end somehow. If it were to break off though, I'm thinking it's gonna be hard to remove, and may cause other issues making it a teardown job.

So asking if any of you all have messed with this or have some suggestions for a possible fix

 

[bazmak]

try building the end up with silver solder or braze

 

[Blogwitch]

Hi N,

Your problem is not with screwing up the dog nosed grub screw too much, but the concentricity of the dog nose in the slot when it is tightened.

If I remember rightly, the dog nose fits into a groove on the side of the quill to stop the quill rotating too much.

As the screw is tightened, it deflects the nose to one side or the other, depending on how accurately the threads have been cut in the quill or on the dog nosed screw. This deflection causes the nose to bind up in the quill slot because it isn't exactly 'straight on', so causing the binding you are experiencing. This is usually the same problem encountered on tailstocks of lathes, where no amount of adjusting and pressure will reduce the tiny rotation of the tailstock quill.

I have spent many hours making custom bronze tipped dog nosed grub screws in an attempt to overcome the problem, but when working with factory cut parts (slot in quill and screw thread for screw) it really is a toss up if you can get things to operate to closer tolerances than how it came out of the factory.

I would start with cutting a new screw that fitted the factory cut screw threads more tightly, then ensuring everything is running as concentric as possible, a new dog nose is cut to be a better fit into the quill slot. Fitting a bronze end into the screw poses no problems at all. Drill a hole into the end of the dog nose and then turn up a tip that can be loctited into the end of the screw, it doesn't need to be the same diameter as the dog nose, it can be a lot smaller. The insert needs to be only about 0.015" protruding at the tip so that it sits at the bottom of the slot and is used for putting pressure on while the steel dog nose part is used to control the side to side movement of the quill slot.

I don't think you will be able to completely eradicate the rotational movement of the quill because of the way the thing works, but you might be able to eliminate most of it by getting closer tolerances on that screw and tip.

Hope this helps and doesn't confuse.

John

 

[BaronJ]

Hi Guys,

I had a similar problem on a cheap Chinese drill press a while back.

In my case the threaded hole for the dog point was M8. As it happened I had a short length of M8 nylon studding laying on the bench. Using that with the end turned flat, I screwed it as tight as I could into the hole and then worked the quill up and down a few times. This nicely marked the end of the nylon. A few seconds in the lathe using the marking on the end of the studding gave me a 3/16" deep dog point. A couple of test fittings and a skim allowed it to seat nicely into the groove on the quill.

I didn't put the locking nut back on because the nylon studding was quite a tight fit. I just put a saw cut in the end after cutting it down to length.

It didn't completely cure the problem, but it doesn't rattle like it did and the quill is smooth but a little more tight than it was but the return spring does pull it back though, just

HTH.

 

[petertha]

Not sure if I have a clear picture or if this even a viable idea based on dimensions but - could you drill/tap a second hole a short distance away from the first hole and cone the set screw tips. That way each set screw would find its own independent 'rubbing corner' on the slot edge as a function of its own depth when it contacts. And its a bit tolerant of one/both being slightly off center. This could prevent rotation in either direction and give it desired slippage? Hope my sketch make sense.

 

[BaronJ]

Hi Peter,

Whilst your drawing is quite accurate, I found that the guide slot was much narrower than the hole diameter for M8. Forcing the nylon screw into the guide slot so that the guide edges marked the end, was how I got the end of the dog to fit.

However the screw hole and the guide slot don't quite follow an accurate centre line. The net result is that the dog point ends up a fraction undersized.


Using nylon is, for me, a good solution, the quill doesn't rattle anymore. Now how long it lasts...

 

[Nick Hulme]

This deflection causes the nose to bind up in the quill slot because it isn't exactly 'straight on', so causing the binding you are experiencing.

The solution for this is to use a radius cutter to make the pin on the end slightly "barrel shaped", this will allow for slight miss-alignment with the slot,

- Nick

 

[BaronJ]

Hi Guys,

Part of the problem is that the rack machined onto the quill and the guide slot are not always exactly at 90 degrees to each other over the whole of the quill travel. Any debris in the rack or the guide slot will cause binding somewhere. I also found the the depth of the guide slot varied along its length. Not by much but enough to cause a bind at the lower end.

 

[Chiptosser]

Here is a idea for you.

Mark the center line of the hole. Drill and tap to a larger size.
Shift the hole center line to correspond to the keyway. Retap to the size of your screw.