gib strips

In lathe and mill slides there is often a tendency to get slightly tight and loose spots which must be due to the normal geometric inaccuracies of the mating parts. Adjustment of the gib strips doesnt really solve this. Removing any slackness just results in increased friction elsewhere because there is no compliance (or very little) in the structure. So Im wondering, why aren't the gibstrip adjustment screws made with some built in compliance to help them ride over the 'tight spots' ? I was thinking that the grub-screws could be drilled and fitted with short length of plastic rod to create a kind of spring preload effect ? Any thoughts on this ?

The cutting forces are likely to be more than the "springs" so you will get movement under load.

J

The cutting forces are likely to be more than the "springs" so you will get movement under load.

J

The usual procedure with a parallel gib strip is to make it flat and then put a very slight curve into it, so that the middle of the strip makes contact with the dovetails before the ends do and this does give the spring effect you're talking about.

Once that's done, the adjusting screws are set working from the centre outwards, until smooth, even movement is achieved.

The bend serves to bias the slide towards the fixed dovetail as a little wear develops in use.

Martin.

If you are seeing tight spots and loose spots in the travel of a slide then the gib or slide is wrong. trying to design a solution to something that is wrong is not going to improve the basic fault. There should be no tight spots and if there is then you need to find out where they are and which surface of the mating sliding parts are in error. Gib strips are there to enable the mating of parts to slide easily and to some extent adjust the degree of the 'slip' and basically must be flat. The mating part to the gib must equally be flat but must allow a very thin film of oil to prevent 'sticking' or tight spots. This oil retention is normally provided by one or both sliding surfaces having very small hollows usually provided by scraping or in some cases a rough grind. Scraped surfaces are the perfect answer but like most things require a lot of skill and time to make well. Then comes the fixing of the gib which can be, as seen on many imports, by a screw at each end of a taper gib or possibly by a simple flat gib of constant thickness which is held in position by a series of screws (Myford). Personally, I prefer the Myford approach but often after removing either style of gib it is not always straight which is due to the stresses inherent in the gib material and manufacturing process. This does not impede the function of the gib because the screws 'bend' the gib flat to its mating slide.

The problem you describ is only cured by examining the flatness and ability for oil retention to all the slide mating surfaces. The difference between a quality precision made slide comes down to the accuracy of these surfaces and often requires a lot of fitting time or expensive machinery to accomplish this. As an owner of both a Myford and import machines the latter all required remedial work to achieve a silk-like sliding motion and thankfully, the use of a gib strip provides the ability to scrape or re-work the surfaces without losing accuracy to the functional slide.

Hope this helps - you might find it interesting to research the various past discussions about the design of the gib screws and gibs, I think the dome end shaped screws provided the best result in the case of dovetail slides to make very fine adjustments to the gib pressure. These solutions are not likely to be found on import machines but can be introduced to give top quality results.

Do not and I will repeat NOT put any springs into you jib strip settings I do not know what you have equipment wise but whatever you do this is a big no no. are you a member of a local SMEE club if so ask someone to help you set up your equipment. Now from an friendly old engineer. firmly grab hold of your bit of equipment that has 2 gib strios in it put a bit of wellie into it and push it backwardans sideways, and if you can feel any movement forward or left or light then your jib strips need adjusting. you will never get an accurate cut untill you do so.

good luck
Bob

Recognising the limitations with flat gibs, some higher end lathes were given tapered gib strips

Quite true the better lathes had tapered gib strips, both my Harrisons had.

The M300 cross slide same as many other 600 group lathes ie numerous Colchesters has tapered gib with a tapered dovetail that doesnt run parallel with leading dovetail.

Used to seize up and wedge in due to a bow at one end.

One very effective answer to the problem of tight and loose spots on slides was provided by the late George Thomas in his book The Model Engineer's Workshop Manual chapter 16 page 181 entitled Top-slide Locks. don't be put off by the title as in the article GT describes in detail how to dowel the gib strip to prevent it moving back and forth very slightly as the slide is advanced or retracted. The tight spots are caused by the gib strip tending to climb the points of the adjusting screws thus moving it into closer than required contact with the fixed part of the slide.The procedure is very simple and can be applied to most slides not just Myford top slides. i have doweled the slide on my Abwood milling vice that has transformed it's operation making a joy to use.

I'm with Eric Clark. My Mini lathe has just had the compound and cross slide gibs dowelled and a locking screw added pretty much i.a.w. GHT's book. These simple mods made a huge difference, well worth the 60 minutes or so that it took!!

cheers

Bill Pudney

Agree with Eric Clark (and Bill Pudney) - my Mini Lathe gib strips were driving me potty until I did that mod - like night n day.

A small P.S. if I may: Exactly how does one pronounce the word gib in this context? Is it a hard G (as in the Bee Gees or Guarantee)? Or is it soft (like a J, as in the Gib sheet on a yacht)?

I bet we'll have quite a discussion on this here - several different people have each separately told me both of the above are correct (and the "only" way to pronounce the word).

Krgds

AES

Soft, as in 'Gibraltar'.

Neil

Hmm... Is Bee Gees really a hard G as in Guarantee?

Thanks Neil.

For Sid: Well spotted. Notice to self "Brain must remain engaged while operating keyboard." - I intended my post to read " .... Barry Gibb, as in the Bee Gees ..... ".

AES