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kenrinc
01-19-2009, 05:03 PM
Hope you can all educate me. I've been through all my documentation and although I do have definitions here or there, I can't find any detailed drawings or data other than, "gib is tapered, screw on each end for adjustment" etc, etc...

Is the dovetail it's mated to actually tapered too or is it just the gib? I could, just taking a wild guess, assume that both surfaces are tapered and opposed of each other. I've never seen one in person so it's difficult to imagine without some better graphic data. I'd like to try making one of these but need a clearer understanding. Is there a specific spec for the taper itself?

Thanks

ken-

Teenage_Machinist
01-19-2009, 05:29 PM
Yes, both are tapered.

Spin Doctor
01-19-2009, 05:37 PM
The dovetail in the sliding element is tapered while the dovetail on the ways is straight. Normally around 1/8 per foot. When the gib is installed in the slide the combination of the two tapers cancel each other out and ideally the dovetail is straight. Adjusting the gib opens or closes the dovetail in the slide to allow for proper running clearance. Also the gib surface that mates to the slide should be scraped to fit. Adjustment is normally accomplished either with opposing screws on each end who's head bear against a partial pocket cut in the end of the gib or the gib will have a tail that a threaded stud passes through. The gib is then adjusted with two nuts. One on each side of the tail

kmccubbin
01-19-2009, 05:43 PM
A timely subject for me, thanks. I recently received a new MLA t-slot cross slide which I will be fitting to a Clausing lathe. The factory slide has a tapered gib. I'm trying to decide whether to copy it, or just use several gib screws. Is there a clear-cut advantage to the tapered gib?

Thanks,
Kerry

DENedbalek
01-19-2009, 05:56 PM
Take a look at Vikki's site (http://homepage.ntlworld.com/v.ford/tsg.htm) for one example of tapered gibs.

Dwayne

dp
01-19-2009, 05:57 PM
Perhaps everything that can be known about sliding contact bearings (gibs):

http://books.google.com/books?id=uG7aqgal65YC&printsec=frontcover&source=gbs_summary_r&cad=0#PPA425,M1

Spin Doctor
01-19-2009, 06:09 PM
. Is there a clear-cut advantage to the tapered gib?

Thanks,
Kerry

Yes there is. With a tapered gib ideally the gib has bearing surface on the whole length. With a screw adjusted gib it has the potential to be bearing just in the areas where the screws are. Also a tapered gib is much easier to adjust in fine amounts than a screw adjustment.

kenrinc
01-19-2009, 08:07 PM
Excellent stuff guys. Thanks.

Ken

Andrew_D
01-19-2009, 10:20 PM
I've been considering tapered gibs for some of my machines as well. I've found a few references to tapered gibs on the home hobbyist sites, including the one mentioned above.

Does anybody have more examples of tapered gibs? links? photos? threads?

Andrew

dp
01-19-2009, 10:44 PM
They're pretty simple devices - a screw-adjustable wedge, but they all appear to have in common a means to lock the gib in place at both ends so any slop is prevented from allowing the gib to slide. Manuals for machines that use them would probably have the best info.

Forrest Addy
01-20-2009, 12:40 AM
The usual gib taper is about 1/4" per foot not 1/8". There are gib tapers that are shallower or steeper of course but 1/4:" per foot is typical (1:50 in the metric world). The gib seat has the same taper as the gib. When it can be managed the gib is finish machined when jigged to the gib seat. Dove tail gibs have the 1/4" per foot taper but since the gib is inclined by the amount of the dovetail angle there's a solid trig problem to solve when getting the taper right.

It makes sense to machine the gib bevels after the taper is cut. Make the gib extra long so you will have stock for fitting and scraping. When all slide features are finally fitted and the gib seats firmly with full bearing only then is the length marked ahd the gib cut to length.

While its handy to machine dovetail slides using the so-called "dovetail cutters" they only come in 45 and 60 degree angles. Most machine tools featuring covetail slides use 50 or 55 degrees dovetail angles. Think is 45 is a bit too steep and 60 degrees a bit too shallow. Dovetail angles are duct soupl on a planer or a shaper where the slide make be tippted to any desitred angle and broadnosing technique may be used. Dovetails thus generated may be made more accurately and to better finishes than readily acheivable on a milling machine. This is one of the few areas where shapers and planers are superior to milling machines.

carlquib
01-20-2009, 01:09 AM
Hey Forest, care to elaborate or point me in the right direction to find out more about broadnosing technique?

-brian

oldtiffie
01-20-2009, 04:08 AM
There are a lot of assumptions - some right, some quiestionable and some invalid - as regards re-fitting or re-manufacturing tapered gibs.

Its a big ask even under manufacturing conditions to get a gib at the correct taper - and flat within its two "rubbing" mating planes to a very high order of accuracy at least.

Getting a good/new tapered gib to be a first-class fit on a used or worn machine dove-tail (male and female) is pretty much an exercise somewhere between pure stupidity and sheer frustration and perhaps futility.

Similarly, the gib has to be pretty close to a 100% contact along its both rubbing faces.

If the ways are worn there will be at least two (and maybe more) points along the slide travel where the gib fit will be "just right" and else-where it may vary from "too tight" and "almost right" to "quite slack" to "bl**dy useless".

Its a "big ask" to get - using the "X" slide as an example - the back dove-tail mating faces, the front of the table inner dovetail and the mating face of the sliding gib as well as the front faces of the gib and the front face of the static dove-tail all a perfect fit with perfect straightness of gib, table and saddle dove-tails all together.

A poorly made gib in a perfect mill will "feel right" even if it is bearing at only one or two points along its length.

All the gibs in my HF-45 mill are "tapered" and they "seem" OK so far as I can tell - so far - as I've had no real problems.

"Setting" or "bedding" a tapered gib is not as straight-forward as you might believe. It takes patience and can be both tedious and somewhat of an art in itself.

I have the multi-screw adjusted "flat" gibs on my 3-in-1 (lathe, mill, drill) as well as my "Sieg" X3 mill. I find them to be excellent as I know just how well and where they are tight, just right and binding and slack. They too are a bit of an art to get just right too - but quite easy after a bit of pratice.

My preferred gib adjusters are those that are on the ram of some shapers and slotters. They are a solid block and the whole of one side of the dove-tail is formed on them. They are easily removed for maintenance and are replaced and adjusted easily as well. They have both (vertical/over-head)"hold-down/clamping" bolts as well as (horizontal) "jacking/adjusting" screws. Shims can be used as or if required.

But given that we have to work with what we have, my preference in my shop as it is, is for straight gibs with adjuster screws.

On any slider the excess clearance on a gib will or may show up worst when machining at right angles to the slider. ie it will show up on the "X" slider/gib when machining in "Y".

As a matter of practice and habit, I try to make sure that the cutting load from the slide to the machine frame is taken on the side of the slider opposite the gib/s. It does not and cannot always apply - but that's life - so I make do.

I am not - nor can I - say that this is best for anyone else.

Everyone else will have their own preferences for their own reasons.

Spin Doctor
01-20-2009, 06:58 AM
The usual gib taper is about 1/4" per foot not 1/8". There are gib tapers that are shallower or steeper of course but 1/4:" per foot is typical (1:50 in the metric world). The gib seat has the same taper as the gib. When it can be managed the gib is finish machined when jigged to the gib seat. Dove tail gibs have the 1/4" per foot taper but since the gib is inclined by the amount of the dovetail angle there's a solid trig problem to solve when getting the taper right.

My bad Forrest. At my old job we had masters for grinding gibs of various tapers, 1/8th,1/4 and a special for on machine tool manufacurers slides on transfer lines. Some of the slides would have gibs on both ends of the slides while I have seen slides with tapered gibs bearing on both of the inside surfaces of box ways.

Teenage_Machinist
01-20-2009, 07:12 PM
Does it seem realistic to replace the standard gibs on my Chinese machines with tapered ones?

kenrinc
01-20-2009, 08:35 PM
That link on the 1st page had a link to a guy who did the saddle gibs on 7x10 although I was intrigued that the rear did not have a stop screw. He used brass. Bronze good for this? It wasn't until Forest mentioned it that I realized many are slanted rectangles in addition to being tapered (when in a dovetail).

Link was: http://warhammer.mcc.virginia.edu/ty/7x10/vault/Lathes/7x10-7x12-Projects/TaperedSaddleGibs/

Ken-

nheng
01-20-2009, 09:15 PM
The book "Machine Tool Reconditioning" by Edward Connelly, published today by http://www.machinetoolpublications.com/ has good coverage of gibs including the tapered type. The book ain't cheap but many members here have copies of it from a bulk purchase a few years ago. If you live near one, you might be able to pry it from their cold, greasy hands ;) for a brief loan.

Here's an image that shows an adjusting / retaining screw. This one's on a vertical mill. The slotted head protrudes from the larger diameter, short cylinder section just behind it, all part of the screw. This short cylindrical section rides in a slot in the gib, holding its location, as mentioned earlier.

The gib can jam without this action. On my Harrison lathe, the cross slide gib has a screw at both ends to capture the motion, preventing any play in the sliding direction. I like this arrangement a bit better than the single ended, which does have some play, but has worked well for years on Bridgeports and other machines. Den

http://its.fvtc.edu/machshop3/basicmill/maint/saddlgib.jpg

Teenage_Machinist
01-21-2009, 08:04 PM
Those are for the saddle. What about the dovetails such as cross slide, ect that would need re-milling the dove tail?

BadDog
01-21-2009, 08:12 PM
Absolutely. Tapers must match and oppose, otherwise opposing sides of the gib'd surface would not be parallel. Just sketch it out on paper and it should be pretty obvious...

Teenage_Machinist
01-21-2009, 08:36 PM
I know it would need re-milling, but does it sound feasable to do?
]
I can borrow a mill.