Gibs, what material?

[VicParticipant @vic]

I’ll likely replace a gib on a small compound table. Is the material very important? I’m thinking ground flat stock at the moment.

[Brian WoodParticipant @brianwood45127]

If it’s only small, a very suitable gib can be made from thick brass strip. Ground flat stock could be too hard and might wear the component it beds to. Usually these things are made from decent quality steel strip which can be readily scraped to fit and thus provide an oil retaining surface.

Brian

[bernard towersParticipant @bernardtowers37738]

I would think that steel for a cast table and brass for a steel table .

[noel shelleyParticipant @noelshelley55608]

I go along with Bernard and Brian ! Noel.

[Mark RandParticipant @markrand96270]

Ground flat stock isn’t hard until you harden it.

Grey cast iron is a far superior material to both carbon steel and brass, but it depends on whether the gib is sufficiently supported to avoid cracking it.

Every steel gib I’ve come across has suffered galling to some extent.

[VicParticipant @vic]

I did think of Brass, I’ve seen it used before for gibs.

[Nicholas FarrParticipant @nicholasfarr14254]

Hi, I’ve used ground flat stock to make a new gib for a bent one for my Boxford and also for a friends Boxford. Like anything, keep them oiled and there shouldn’t be any problem.

Regards Nick.

[HOWARDTParticipant @howardt]

Cast iron, it remains straight after machining.  Steel and brass bar will distort as the stresses are released on machining and any bend will create points or lines of contact with uneven contact forces.

[PeteParticipant @pete41194]

For items like this I’ve learned after far longer than I’d like to admit to that the correct material selection for any use should be made using some rudimentary engineering data. As well as each materials known properties for whatever that expected use might be. So a gib is expected to have a sliding surface and be adjustable to take up future wear in either component. For a material to itself wear as little as possible in this application yet still provide that good low friction sliding surface, you first need to look at what’s called it’s coefficient of friction. That involves not only the material the gib is being made from, but the surface material type it’s being used against. This is only one website of many listing a large variety of materials with there known coefficients of friction. https://www.engineeringtoolbox.com/friction-coefficients-d_778.html Oddly and for cast iron, they only list the friction reduction using grease as the lubrication which would be wrong for our applications. Obviously a proper way oil would reduce that friction percentage even more. But cast iron against cast iron would be one of the better materials to use and why most industrial machine tools do use it for a gib.

There’s also no such thing as any perfect material for any use. It’s always a series of engineering trade offs with some pluses and minuses. Yes you gain with lowered friction and good wear properties, but cast iron is also fairly soft, so any contamination harder than the cast iron can and will embed into it’s surface. At that point it works just like a lap and will noticeably increase your slide wear. So one trade off is ensuring it has the correct and adequate lubrication and kept as clean as possible. You could also use a hardened steel gib in an effort to prevent that. But since the slide itself is still cast iron, it then becomes the lap with that same contamination. And that hardened steel would also have a higher coefficient of friction.

[old martParticipant @oldmart]

The last gib that I made was for the longer cross slide for the Atlas 12 x 24 lathe, about 2 1/2″ longer than the original cast iron gib. It uses 6 adjustment screws while the original uses 4 and I used brass bar which Mike had donated to the museum. Cast iron would be much harder to get hold of than brass and only marginally better. The brass rubs against the cast iron of the saddle (carriage).

As I posted, I remembered that I also made a brass gib for the base of the new tailstock for that lathe. That gib was slightly relieved in the centre 1/4 to help steady the tailstock.

[VicParticipant @vic]

Thanks Old Mart, I’ll try and get some brass.

[Pete RimmerParticipant @peterimmer30576]

Brass is a terrible choice unless it’s a particularly hard type. Cast iron, steel, bronze or a high density plastic are all better. Brass sticks and smears then wears quickly plus it picks up contaminants and acts as a lap.

[MikeKParticipant @mikek40713]

Looking up the coefficient of friction for various things will show that lubricated cast iron on cast iron has very low friction, much better than other metals.  Cast iron also has the benefit of being stable.  I have a bar of cast iron that I’ve been saving to make new gibs for my mini-lathe and mini-mill.

[SillyOldDufferModerator @sillyoldduffer]

If only the very best will do for your lathe, and money is no object, then the answer is Silver!

However, in my bodge workshop, where unloved machines are kept in working order, I note that the slides of a hobby lathe must be one of the least demanding bearing requirements in the world.   Within limits, almost anything will do.    As bearings go, hobby gibs have an extremely easy life; they don’t take high-pressure, get hot, or move rapidly.   More important, I feel, for my slides and gibs to be straight, polished, oiled and properly adjusted than made of anything special.

That said, one can do proper engineering on the slide problem.  The gib material has to be strong enough to resist wear and plastic deformation.  It ought to be markedly harder than what it bears against, and it must resist corrosion.   It mustn’t gall, should take a high polish, and have a low coefficient of friction.

Many metals and plastics disqualify themselves: pure aluminium is soft and sticky and it and stainless steel both gall.   Magnesium corrodes in water and air.  Many plastics are too soft and deform when stressed, a few swell when they absorb water, and some are attacked by oil.  Babbit metal is too soft and brittle metals should be avoided because working the adjusters might break them!

Other metals are reasonable and cheap, but may be mildly problematic: cast-iron is good unless it cracks,  steel might corrode, brass can act as a lap etc.

The best choices are a little exotic – bearing Bronzes, bearing plastics, bearing Aluminium,  leaded Copper, and Silver are all good, but too expensively over the top for my taste.

Fast moving heavily loaded slides need careful consideration, but unless the machine is special, I’d compromise on an easy to source, low cost, hard Brass or unhardened ground steel, and keep the slide well oiled.

Brass may be inferior to Bronze, but it has a long history as a bearing metal, so much so that old books call bearings ‘brasses’.   Although Brass bearings rarely used today because designer bearing materials are available,  Brass is still a reasonable choice for home-made gibs.   If still worried, or in need of something to brag about at the club, the Brass could be Silver-plated!

Dave

 

 

 

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