Sanity check - thread size for linear rails

[Doddy]

Quick, daft question: I've been eyeballing the installation of some 20mm chinese linear rail - similar spec (on paper, at least) to Hiwin, who identify that the 6mm mount holes should be bolted down with M5 bolts.

Having dry-fit these, an M5 feels sloppier than a night out in Essex - I can appreciate that it offers a little wiggle-room to align the rail, but 20% undersize? I've got various M6 cap bolts - some of the heads fit, some don't, and the M6 thread aligns very securely within the 6mm mount hole; whereas M5 gets lost somewhere in the bolt-head recess on the rail.

Before I order the bolts I'm just looking for a positive noise that M5 is correct for 20mm rail?

[Clive S]

Before I order the bolts I'm just looking for a positive noise that M5 is correct for 20mm rail?

I used M6 but found that I had to put each bolt in a drill then spun it against a grinder to bring the dia. of the head down a bit, it only take a second or two.

[Neale]

An alternative view - I used M5 very happily. These bolts clamp the rail to the frame, they are not dowel bolts for location. The +-0.5mm wiggle room for adjustment is no bad thing (allows for slight drilling/tapping errors) and does not reduce clamping force (or at least not enough to matter).

So, there's two conflicting views from two different builders! Lot of help we are...

[Clive S]

So, there's two conflicting views from two different builders! Lot of help we are...

Quite right, that's the beauty of forums In my case I just preferred the bigger dia. But wiggle room is important.

[mekanik]

Have to admit i never liked the idea of using 5mm screws just seems to small to me, if you are clamping the rail and using a drilling jig you should be able to get away with using 6mm as Clive has(much more better)

[magicniner]

Good quality M5 fixings will be up to the job.
You should never be relying upon tapped bolt holes to provide precision location unless you counter bore the two mating parts and use a cylindrical insert.

[Ger21]

When the manufacturer specs 5mm bolts, why even consider something else?

And you really think you can drill a series of 6mm holes in a perfectly straight line, with absolutely perfect spacing?

[Doddy]

When the manufacturer specs 5mm bolts, why even consider something else?

And you really think you can drill a series of 6mm holes in a perfectly straight line, with absolutely perfect spacing?

I quoted the hywin spec, which on paper looks the same as the chinese, but it is enough for me to see that this is a point considered by others here already.

It remains a sloppy fit, but as alluded to here that might be of benefit. M5’s ordered yesterday.

(On the actual Hywins that I fitted into the Z on the Starmill I did, out of ignorance, drill and tap for M6 using the rail as a guide)

[Ger21]

It remains a sloppy fit, but as alluded to here that might be of benefit.

It's to prevent the actual hole locations from preventing proper alignment.
Technically, at least one rail should be mounted against a machined shoulder, and you don't want the bolts pulling the rail away from that shoulder.

And the fact that many others do it that way, doesn't make it right. Just look at the thousands of poorly built machine on the internet, with thousands more people eagerly copying their designs.