Thread Engineering Question

This sounds like a work-hardening issue, lots of up-and-down stress on bumpy roads. Additional fixings stopping this movement would help if you could find a place to achieve that. Early on I had one head bolt plus a U bolt around the steering head frame member holding my Bevel fairing on my 3c, the fairing holds the headlight and the instruments so is pretty heavy hanging way out there. The bolt would regularly snap. My fix was adding 3 other fixings tying into as many points as I could get access to. That stopped 95% of the fairing movement. Nothing has broken since.
 
Working with vibratory parts feeding equipment I had a situation where the leaf springs holding (m12mm skt hd cap screw) bolt regularly broke.
Querying this with the screw distributors their reply was " screws break because they are not tight enough". Following up on this advice I torqued the said screws up to twice their recommended torque value (my setting, not the manufacturers), hey presto, no more broken screws.
Not that I recommend torquing up to twice the recommended figure but the facts remain that incorrectly under torqued screws are more likely to break.
It stands to reason that the fairing mounting screw will loosen as a result of vibration and the softer mild steel bracketry wearing away.
Selecting the correct grade of steel is important, Stainless Steel bolts may look good but their tensile strength is similar to mild steel and shouldn't be used for load bearing installations.
 
Last edited:
Vince said:
The Die I used is split and held together with a grub screw, what's the deal with that. Adjustable to compensate for wear? What is its purpose and how do you use that facility in a die? More to learn for me.
Click to expand...

Firstly, I'm only a hobby machinist, not a professional. So I'm only speaking from personal experience rather than a proper trade education in how to use these tools.

As far as I'm aware, the reason for the split in the die is to allow for some fine adjustment in the finished size of the thread. Some split dies have a built-in grub screw that opens and closes the split, something like the pinch bolt on your triple clamps. Those type you adjust the gap with an Allen key before inserting the die into the holder (or die stock). Smaller dies that don't have this built in adjusting screw are adjusted using a screw in the die stock that has a conical tip which goes into the split to force it apart, opening the gap. There should also be a grub screw in the die holder either side of the adjusting screw. Those go into dimples in the outer edge of the die either side of the split. Screwing those in will close the gap in the split die.

If you're cutting a new thread in a hard bit of bar, it can sometimes be helpful to do it in two or more passes. Open up the split as far as it will go for the first pass. It won't cut the thread to the full depth, but it'll be easier going. Close the gap for subsequent passes until the thread will fit the nut or whatever you're screwing it into. Always use a suitable cutting lubricant.

Occasionally you might want to leave threads a little oversize so you get a tighter fit than you normally find with a nut and bolt. That can be achieved by adjusting the gap in the split die. A tight thread will be a more secure fitting for something that you don't want to unscrew, like a fixed stud.

As an addendum, dies are different to die nuts. Dies are usually round and have to be fitted into a holder to enable them to be used. They may or may not have a split in them. Die nuts are hexagonal and are normally used with a spanner. They don't have splits in them. Obviously anything without a split is non-adjustable. The finished thread they produce is a fixed size.

I mostly use dies for external thread cutting (if not doing it on the lathe). I find die nuts more useful for tidying up an existing thread that may have been damaged, although you can cut a new thread with them at a pinch.


EDIT: Since posting the above, I saw someone on a YouTube video using a die stock that takes hex die nuts. So I take back what I said above about needing to use a spanner on them. What I should have said is you CAN use a spanner, but you can also buy a die stock (die handle) that will fit the hex.
 
Last edited:
Good to see you back on line Cam, I did set out to see you with the guys, the bike had other ideas, must have sussed what happened last time and decided not to go any further.
 
G'day Chris. Yeah, I think your bike must be a bit gun-shy about returning to Tasmania. I caught up with the other guys though. My Laverda decided to play up too. Ended up chugging home on 2 cylinders. I haven't checked out what the problem is yet. Got another bike on the lift table.

About a week ago, some mates and I went for a ride to Derwent Bridge for lunch (I was on the Multistrada). While going along that bit of road near Tarraleah where the tree tried to kill you, I thought of you and kept a wary eye on the trees.
 
You must log in or register to reply here.
Back
Top