Jib Screws

[Paul Alciatore]

I am in the process of cleaning up and restoring a double dovetail slide assembly I recently bought on E-Bay. There is a pivot between the two slides and I suspect it was used as a lathe cross slide/compound. It appears to be very well made but shows signs of many years of wear and abuse. Both dovetails are cut at a 45* angle.

My question: when I disassembled the slides for cleaning, I noticed that one of the four jig screws on the lower slide and one of the two screws on the upper slide had a truncated conical tip. The other screws all had completely conical tips. All appear to be originals. Is there a reason for the different kinds of screws?

[Rotate]

First of all, I think you mean gib, instead of "jib". If you look at the gib, you'll notice that there is/are dimple(s) where the screws are suppose to seat. This is so that the gib does not slide when the stage is moved. Study it carefully and you'll see which screw tip goes where.

Albert

[CCWKen]

Yea, I was about to get out my sail boat reference. Is this a welding jig or a sail?

[lynnl]

I've always been uncertain of the pronunciation of this word. Is the 'g' pronounced as in "get" or as in 'gee' like the word jib. (I'd hate to embarass myself at a mechanics' convention someday.)

[Rotate]

Press here for proper American pronunciation

[Paul Alciatore]

OK, OK, OK! I used the wrong word. I'll sleep on a bed of swarf tonight and wash my mouth out with cutting fluid. Ain't like the first time there's been a grammatical (had to look that one up) or spelling error on this board.

Can we get back to the original question. Yes Albert, I have already looked for the tell-tale marks to see where each type Gib screw was originally and, after noticing the difference on the upper ones I even took the precaution of removing the four lower ones one at a time for cleaning. I'm a better mechanic than author. And, this appears to be a well designed and built slide as in addition to the screws, there are two locating pins for each gib so the screws do not have to prevent slideing.

But, what I was really asking was what would be the reason for using different styles of screws? Since it's a 45* dovetail and the cones on the fully pointed screws are approximately 90*, it would seem that the contact would be along the full length of the cone. With the truncated ones, it would be along the remaining conical length, about half as much. The dimples bear this out. Does this accomplish something? For instance, is one better for setting drag and the other better for locking? Or was it just assembled with randomly selected screws from two slightly different batches? Or what? I ask this because several of the screws are in bad shape and I will be looking for replacements. If there's a serious reason for using different kinds of screws, I'd like to know.

[lynnl]

I'm no expert in setscrew-ology but as I understand it, there are different designs for different applications. But in this case my first guess would be they ran out of one kind and just used what they had available. ...or the setscrew bin just happened to contains different types.

BTW Albert, I got an error on that pronunciation link.

[This message has been edited by lynnl (edited 05-30-2003).]

[Rotate]

Paul, I would think that a conical tipped setscrew would mate with the dimpled part of the gib and the others would have the truncated tip. I have noticed that "made in China" often means "we use the cheapest fastener possible". I have replaced all my gib setscrews with good quality domestic ones and I find it much easier to adjust it to the right pressure.

Albert

[SGW]

George H. Thomas, who wrote for Model Engineer magazine, had Opinions about gib screws. (He had Opinions on a lot of things technical!) Anyway, he was very big on arranging to have good, full contact between screw tip and gib such that the gib wouldn't be cocked by the screw pressure. He also suggested pinning the gib strip in place at one end so it couldn't move longitudinally.

He describes all this in his Model Engineer's Workshop Handbook (or some similar title).

[Thrud]

Paul
If the Gib is pinned use a flat tipped screw (brass tipped works best).

I think someone got lazy.

[Paul Alciatore]

Thrud,

Yea, not one but two locating pins.

But with the 45* dovetail, wouldn't the original conical (pointed) style screw provide a better area of contact while a flat tipped one would dig in and perhaps tend to cock it? Presently there are no deliberate indentations for the gib screws, just small "pressure" dimples.

Or should I attempt to cut flats for the new screws to set into? I could use a 1/8" or 3/16" milling cutter and cut at a 45* angle. Could probablly just cut where the dimples are. I'm sure flat tipped screws would be the easiest to buy or make.