Gib missalignment

Hello.

I have made a new brass gib for my SX1 mini mill.

They look bad and contact only on two lines and not 2 surfaces. Why did this happen?

Perhaps some more info re how you went about it and pictures if possible might get some folk on here to have ideas about what went wrong.
I see there is a thread about these here

https://www.model-engineer.co.uk/forums/postings.asp?th=100234

pgk

Impossible to say without pictures.

can't really expect hand scraped and fitted parts at that sort of price.

Warping during machining process.

Igor's image seems not to be showing up in the forum software. The link is here: https://ibb.co/ZHYZ9yY

Blimey! Where do you begin with dovetails cut like that. I'd be doubtful if you'd get any improvement without re-machining the sliding surfaces. Sorry to be so negative, but I think you've got a bit of a Friday afternoon mill there.

John

The roughness is probably just the very end of the channel. Worth noting that the gib screws on these are often the cause of problems, forcing the gib at an odd angle.

I second what MikeK has said. I've just been through this with my X2. I machined the gib screws to a point which improved things a lot.

Picture of old gib.

Picture of new gib

Picture of new gib at an angle

I have no idea why pictures do not show.

Edited By Igor Petejan on 23/03/2022 18:56:16

Edited By Igor Petejan on 23/03/2022 19:09:46

Will give it a try. Some prefer pointed gib screws, some say flat, I checked mine and they are flat. What about rigidity? Increasing it would be my goal. Also reduced friction, but that I already got due material combination.

I also have distinct feeling this machine was made Friday afternoon just before Chinese new year after a few glasses already.

Edited By Igor Petejan on 23/03/2022 19:10:20

I machined a 60° angle, just found out today it is supposed to be 55°.

Thanks MikeK!

I am currently working on an article for Neil on Gib Strips.

One thing I have pointed out at the beginning of the article is to check the angle of the Dovetail. This is something that varies from 45 to 60 degrees. Each manufacturer having their own personal favourite.

Regards

Gray,

If you use flat end gib screws you'll need to machine pockets into the gibs, at the appropriate angle. Otherwise the screws hit the gibs at the top (and only the top) and try to rotate it over. To be honest, I don't remember what I did for my X2 mill, but it's been okay...I do have a bar of cast iron to make new strips, though. (Lubed cast iron against cast iron has very low friction.) Have you marked up the gib with bluing to see where it actually contacts? Installing the gib with the screws backed out and watching it as you tighten them could be useful.

Getting an extra set of screws and putting, say, a point on them to experiment wouldn't be a waste of time, in my opinion.

As an aside, I put medium threadlock compound on the gib screw threads to avoid the need for the lock nuts. A piece of thin plastic may also work?

Mike

Godawful manufacturing all around there. None of the machined faces are particularly well done and the relief grooves are miles too big. It doesn't look like the angle of the moving dovetail matches the angle of the static one and there are is not enough bearing on the top and bottom faces to support the gib in any way.

I think that your best bet is to back off the adjuster screws and use feeler gauges down the side of that gib to gauge how much thicker it needs to be to fill the gap completely. Make a new gib from steel or cast iron (brass is a terrible choice for a sliding way) and machine it for the closest fit you can manage that allows movement. Spot it though the gib adjuster screw holes and drill the spots for pointed-end adjusters (or dog-end if you like). If you do a good job of it then the gib will have no choice but to sit parallel in the gap. The extra thickness will give better support and better lubrication retention and the table will sit much more rigidly on it's ways. When you machine the gib put the un-machined side towards the slide and the machined side towards the adjusters. If you can make a HSS scraper (or grind an old file) put scraping marks all along the working face for oil retention.

Edited By Pete Rimmer on 24/03/2022 00:45:23

Hi Igor, maybe your gib is a tad too tall. I doubt the angle you've cut it will make much difference and pointed screws should help clamping on the centreline of the gib, but might be a good idea to inspect the dove tail slides.

Regards Nick.

Some observations from reconditioning more machine than I care to remember.

If there are dimples in the Gig strip then spherical ended screws are best. Pointed screws will quickly loose there adjustment, they dub over like a bent nail. This is due to the too and fro movement of the Gib as the feed changes direction. This is one of the reasons for doweling the Gib.

Grub screws with a cone to match the angle of the Gib, avoid the necessity to dimple the Gib and are best used with a flat surface on the Gib, but the Gib must be doweled. Or some other form of constraint used to stop endwise movement. They also give line contact rather than point contact.

Regards

Gray,

The problem with my gib with spherical end screws arose because the dimples are too shallow and the contact was assymetric, forcing the gib into two lines of contact. The points are a bodge of course, a smaller radius hardened tip would be better but I just wanted to get the job done in a hurry. A better solution would be to drill the holes deeper to ensure symetrical contact with the original spherical tip, but setting up to drill accurately might be a pain.

Roy