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Thread locking

How to keep an M8 screw tight into an M8 nut for adjustment.

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andrew lyner23/01/2019 21:56:26
254 forum posts
4 photos

I have made a steady rest for my mini lathe which has a max diameter of around 60mm and it uses three long nuts and three M8 screws (phosphor bronze pads on the end). It is quite rigid enough but I want to dissuade the screws from turning whilst it's running. I guess they could tighten or un-tighten as a result of vibration.

I am looking for a really thick grease or sticky stuff. I remember many years ago using a very soft Nylon compound for keeping magnetic tuning inductor cores in place. It would do the job fine, I think but I can't think where to obtains it.

I wondered about a nylon grub screw. Any other well known methods for this? Perhaps M8 butterfly nuts to lock the threads?


Bill Phinn23/01/2019 22:05:54
755 forum posts
113 photos

Would a compression spring (such as found on carburettor jet screws) between nut and screw help?

David George 123/01/2019 22:16:52
1839 forum posts
503 photos

You can use a grub screw at 90 degrees, with a bronze slug in between the grub screw and support screw. I have used a similar thing for ball screw nuts. You assemble the parts with a tap in the support screw hole and when you rotate the tap it cutts the matching thread in to the bronze slug end. When you tighten the grub screw it dosn't damage the support screw.


Pete Rimmer23/01/2019 22:22:59
1233 forum posts
65 photos

I was going to suggest similar - a grub screw and lead pellet.

andrew lyner23/01/2019 22:28:29
254 forum posts
4 photos

Cheers for the ideas.
I fancy the spring solution for a start. I will root around in my springs drawer and see what I've got.


Chris Trice24/01/2019 00:40:40
1375 forum posts
10 photos

What about a second nut on the screws you can tighten against the longer nuts? You could machine knurled brass ones as they only need to be finger tight.

Nimble24/01/2019 04:07:41
59 forum posts
4 photos

In the area where the nut will rest, cross drill your 8mm bolt with approx 2mm drill insert some weedline plastic and screw the nut up, this cuts off the excess length and is similar to a nylock nut and readily adjustable.


Nimble Neil.

Edited By Nimble on 24/01/2019 04:10:02

Robert Atkinson 224/01/2019 07:38:02
1209 forum posts
20 photos

The compound you used before on tuning slugs was probably Rocol Kilopoise 868S (or 868GW). It seems to have been discontinued.**LINK**

Robert G8RPI.

not done it yet24/01/2019 08:19:47
6809 forum posts
20 photos

Do you find your gib adjusting screws coming loose? Or any other fixings on your machine? A simple locknut seems to be adequate? So in agreement with Chris, apart from them needing a spanner, not fingers, for tightening.

Pete White24/01/2019 21:06:54
166 forum posts
16 photos

Third vote for keeping it simple, lock nut. smiley

ChrisH24/01/2019 23:08:13
1018 forum posts
30 photos

You can buy 'nylon' grub screws on the well know auction site quite cheaply, that said, you don't get that many for the price, but they seem to work.

Bazyle24/01/2019 23:50:40
6324 forum posts
222 photos

Try putting a bit of fishing line down the hole. Also possibly plumbers ptfe tape.

Hopper25/01/2019 02:43:57
6397 forum posts
334 photos

For a lathe steady you really want positive location of all three fingers under load so I would go for some kind of positive locking mechanism such as lock nuts or pinch bolts. Otherwise, vibration and load forces plus the movement of the job across the finger tips will manage to move something. And it only takes a tiny amount of slack on those fingers to induce chatter and inaccuracies.

A pic of your steady would help, but without seeing it, it would seem that locknuts on the bolts where they stick out of the long nuts would be the obvious solution. Something like my own solution below:


Edited By Hopper on 25/01/2019 02:46:08

andrew lyner25/01/2019 09:49:49
254 forum posts
4 photos

@Hopper That looks a nice and meaty piece of kit. I notice it seems to be split in two halves - i guess experience told you that flexibility of use would justify any compromise on strength. My version is much the same shape but uses 8mm plate and M8 nuts. I was thinking that just making the screws 'tighter' would be more convenient than actually locking them but comments from you and other members have convicted me otherwise. I haven't room to put locking nuts 'inside' the circle so I will just use longer screws and lock nuts.


not done it yet25/01/2019 11:03:53
6809 forum posts
20 photos


I don’t think anyone would try to fit locking nuts inside the circle. Takes up valuable working space.

Two halves fairly well maximises the size of bar that can be accommodated while allowing easy vertical placement or removal of the worpiece ( think here of heavy commercial operations, not us hobbyists). The steady is there to support the workpiece, not force it into another position. While rigidity is a good design feature, the bar is still basically supported on only two points... If the steady points were not adjusted to the correct relationship with the workpiece and chuck, something would have to give - strain at the steady or the workpiece moving (loosening) in the chuck jaws.

Hopper25/01/2019 11:32:12
6397 forum posts
334 photos

Yes, a little bit too "meaty" if anything. Cut from a piece of 1" steel plate with a plasma cutter that was not really up to the job. Never again. Took a lot of finishing work. The "flip top" adds convenience with no loss of strength apparent. Seems to be the way most commercial steadies are made in this size range.

But it is much more sturdy than the standard Myford steady, which has a tendency for the slotted brass fingers to move under heavy load, such as pictured parting off 2" steel bar. Myfords puny 1/4" BSF pinch bolts on the brass fingers are no match for the 7/16" grade 8 bolts used in this design. (Specially for those of us who tend to "push" our machine tools beyond what the chaps in the drawing office ever dreamed of. laugh )

Howard Lewis25/01/2019 19:53:04
6111 forum posts
14 photos

My vote would go for an adjuster capable of being held by a spanner or an Allen key, with a locknut on the outside.

A member of the "If in doubt, build a brick mausoleum" brigade..

(Having set the adjuster, hold to prevent rotation, and tighten lockinut. Even then, backlash in the thread is likely to produce a change of setting).

Failing that +1 for fine fishing line in the hole before inserting the screwed adjuster. Locks the adjuster, and easy to replace the locking medium when it wears/fails.


Paul Lousick25/01/2019 21:24:45
2043 forum posts
722 photos

Like all screws, there is slop in the thread and the clamping force on the bolts will push it in the outer direction. Having the nut on the outside, pulls the bolt in the same didection when it is locked-up.

If on the inside, the bolt will be pushed towards the inside, making it difficult to set the distance from the part being turned.


Neil Lickfold25/01/2019 21:44:21
862 forum posts
195 photos

A drilled hole with a piece of weed wacker nylon, works very well as a mechanism for keeping a screw or a nut from loosening off. An alternate to the nylock nut. A Ø2.6mm hole is plenty of clearance for the 2.5mm line X 3 to 4 mm deep. Make the nylon about 1/2 of the thread height. It will squash up and make the screw tight to turn.

Harry Crowther17/04/2019 16:45:49
18 forum posts

I would use longer screws and lock nuts. I read on the Accu website that left handed screws can be used to create deliberate incompatibility to counteract the screws working loose with rotation.

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