forging a post vise, improvements to the screwbox

[Domnu]

Hi, I am planning on forging an old school post vise from some 2" square bare i got lying around. when looking at old vises i always see the same points being severly worn, namely the nut and screw aswell as the contact point between the screw and the front arm. i was wondering what you guys think about making the nut aswell as a washer from silicon bronze. I was thinking about casting the nut, machining it out and threading it in the lathe. The idea being to reduce wear between the screw and screwbox as there is less wear between the bronze and steel vs steel on steel. I have personally never seen bronze parts used on postvises and don´t really understand why, in most other machines I have taken apart bronze washers, bushings and nuts are common. Any ideas?

[George N. M.]

OK, let's consider why bronze bearings and washers are used in machines which are mainly made of steel.  As I understand it they are sacrificial and can be replaced faster and easier than steel components.  Bronze is softer than steel and will wear faster.  Also, bronze bearings require some sort of external lubricant such as oil or grease.  Some bronze bearings are sintered, that is, made of small bits of bronze so that they will have pore spaces between the bits, and then are impregnated with oil into the pores so that they are self lubricating.

In a vise there are very strong torque and pressure forces on the nut and screw.  If they are made of bronze which is softer than steel they will wear out faster than if they were steel.  Also, the forces are mainly pressure, not the high speed friction of rotating parts.  With a softer material there would be more deformation and less wearing away as in a high speed environment.

So, bronze could work but I don't think that it would be a better material choice than steel for a vise.  There are applications, such as a corrosive environment or need for no sparks where bronze is a better choice than steel, but in this case it would be my second choice.

"By hammer and hand all arts do stand."

[Frosty]

Well said George, I call that a bingo. 

Were I to want to experiment with a bronze wear surface in a vise, I'd plate the screw generously and give it a few years of heavy use to see how it works.

A vise is a low speed friction joint but there is crazy high shear forces on threads. A heavy lube like grease catches particles, dust, scale, grinding debris so the grease becomes an abrasive. Light oil won't trap as much abrasive debris but doesn't have the high film strength you want in high pressure situations. 

Am I making sense so far? 

I've had fair luck with dry lubes, graphite or dry lithium works pretty well but doesn't last very long. This is where bronze plating the screw enters my thought processes. Leave the screw box steel but make it as smooth and polished as you reasonably can. The screw is easily accessed and can be inspected every time you open the vise. When it becomes worn, clean it and re-plate it.

Yeah, I've been kicking this idea around for some time.

Frosty The Lucky.

[ThomasPowers]

I've seen cast bronze screwboxes in replacement situations.  Also screwboxes with a long "cover" for the screw; so for use with stock 1" or under the screw is not exposed to pick up crud. (Most of the work I do on a vise meets that criteria.)

The mod that impressed me the most was a person who brazed a cap on the end of a Columbian open ended screwbox and mounted a zerk in it so when they lubed the screw it pushed any grit containing old grease *out*  of the screwbox!  I hope to do a modification of that to a couple of my vises.

I've seen several folks use thrust bearings in place of the washers on post vises. 

The different "metal on metal" is good to prevent galling where parts rub under pressure---which can be a form of solid phase welding!