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center / gun drilling bolts
FuryRebuild - 9/11/19 at 12:41 PM

I've been trying to find any formula that states what the ratio of metal to remove is safe when centre-drilling bolts?

Can anyone point me in the right direction?

Thanks
M

steve m - 9/11/19 at 07:06 PM

Why do you want to drill holes in bolts ?

coyoteboy - 9/11/19 at 11:04 PM

If youre going to drill out the centre, the only way of knowing if it's safe is to accurately assess the load on the bolt. There's no magic formula, no material removal will be "safe" unless the bolt is a unloaded cosmetic item; whatever you do you will be eating into design margin.

harmchar - 10/11/19 at 04:52 PM

This is going to be an interesting topic. Not sure if there is an actual formula for working out the lost strength in a drilled out bolt, but a simple experiment in the garage would be to torque up a bolt through a plate and see what torque the head shears. Now do the same for a drllied out bolt and I'll bet you 5 beer tokens that the torque is less. Bolt will be more likely to fail at head.

The bike forums are full of subject matter of saving weight by drilling material to the point that it is just stronger than it has to be. Problem with doing it on car parts is it's more difficult to judge the forces that each bolt will see.
Can we ask for what purpose you would require to centre drill a bolt for? If it's for saving weight, there is better places to do that. If it's for a college or uni project look up Drillium.

jacko - 10/11/19 at 08:23 PM

Can a bet it's something to do with suspension bolts and grease nipples gentlemens bet that is
Jacko

Bluemoon - 11/11/19 at 10:00 AM

quote:
Originally posted by jacko
Can a bet it's something to do with suspension bolts and grease nipples gentlemens bet that is
Jacko


Yep probably, may do that on mine.

Strength reduction will be less than you might first think, as the center of a bolt takes less stress most of it will be around the edge, i.e. think about tube and solid rod... BUT as Cyitebiy states, it will have some effect!

You can get drilled bolts for vacuum system use (experimental physics ect), not cheap but a common use for drilled bolts..

[Edited on 11/11/19 by Bluemoon]

[Edited on 11/11/19 by Bluemoon]

coyoteboy - 11/11/19 at 10:25 AM

Assuming thin drilling (relatively - 1mm in a 10mm bolt eg)

If your bolt is loaded in shear you're reducing it by the area reduction (approximation).
If your bolt is loaded in tension you're also reducing strength by the area reduction.
If your bolt is loaded in bending, eeeew design it properly.

If your drilling is larger, you start interacting with the stress field nearer the threads, at which point you can get odd deformations (think thread angle compressing the bolt and allowing it to slip through like a ratchet).

[Edited on 11/11/19 by coyoteboy]

nick205 - 12/11/19 at 05:11 PM

To the OP, can we ask what you need want to do with them?

When building my Indy I centre drilled 2x M8 socket bolts to feed the front indicator wires through for fitting to the GRP nose cone. No real load on the bolts and they were changed shortlyafter SVA as well.

907 - 13/11/19 at 07:53 AM

I've had hollow bolts and shafts on several of the bikes that I've owned, including wheel spindles and swing arm pivots. Proper stress points.


However; I'm sure that the o.d. will have been up-sized to allow for the missing centre.

i.e. A 25mm tube could be lighter and stronger that a 20mm solid bar. Not just a question of drilling the 20mm bar.



Paul G