Is my cross slide gib strip not ground properly?

S

stioc

Registered
Registered
Joined
Feb 22, 2017
Messages
596
This is driving me insane. If I run the cross slide all the way out (towards the operator) it gets tighter and tighter to turn. I've tried adjusting the gib screws but even with them all the way out there's a lot more friction as I move the compound towards the beginning. So tonight I decided to remove the gib strip to take a closer look.

There are two sides to the gib strip, the polished side and the non polished side (this side has a notch for one of the gib screws to catch). With the polished side down on the surface plate, there's no bow in the strip but when I flip it over there's a bow. Also the thickness on one end of the gib strip is about 15 thou less than the other end. If I try to take the bow out by bending the gib, it shows up on the other side. So I'm guessing the bow is because the grinding on the non-polished side is uneven and ultimately this bow is what's causing my issue with more friction as the compound travels outward. Any thoughts, suggestions?

298065


298064
 
Your gib adjustment sounds like the type done by adjusting several setscrews along the length of the gib rather than a tapered gib. Since you have a bow on the back side but not the front, it would indicate that the gib is not of uniform thickness. I suspect that a previous owner experienced the same problem and tried to modify the gib.

The bow in the gib's back side shouldn't be a factor as long as there is clearance. My guess is that it has something to do with the lead screw. There are two possible causes for your problem. The backlash adjustment may be overcompensating for wear in the central region of travel causing it to be too tight at the extremes where wear is less. Or there may be a bias in the mounting of the lead screw thrust bearing. If the lead screw isn't concentric with the nut, it will bind more as the nut gets closer to the thrust bearing.

To determine which, does the cross slide still bind with the gib completely removed? Is there play at the front of the cross slide with the gib removed and the cross slide all the way to the front? Do you have the same problem when the cross slide is all the way to the rear? If you remove the lead screw, can you easily slide the cross slide along its full length of travel? If you loosen the end cap retaining screws, does the problem go away? Answers to these questions will help you isolate the problem.

On my lathes, the end cap containing the thrust bearing isn't pinned in place. The proper way to fasten the end cap is to bring the cross slide to the front as far as possible and then tighten the retaining screws. This will create the least amount of bias.

If the cause is over tightening of the backlash compensation on the lead crew nut due to wear in the central region, then the solutions are replace the lead screw or loosen the backlash compensation and live with the increased backlash. A third solution which I used on my Atlas/Craftsman 6x18 was to have a second nut which was floating and coupled to the primary nut with a spring. This has the effect of pushing the cross slide away from the work and reducing backlash but since it is coupled via a spring, it is self-adjusting, compensating for different amounts of wear.
 
I had a similar problem and on my 9x20 the dovetails were not parallel. It took some scraping to get them right along with scraping the gib. My compound was even worse. Most of it was in the top/female and I ended up getting a new one from Grizzley. It could use a little work too but is way better than it was. Since truing up the dovetails in the crosslide and compound my chatter and finish problems went away too. I've found there are few things you can point to on the 9x20 and say this is always the problem because even though symptom may be the same, often the cause is different. Wear and poor QC can skew things further. In my case it was worth the effort.
 
@RJSakowski - thanks for the detailed response. Some answers below:
- I'm the original owner bought it about 2 yrs ago but since I rarely back the cross slide out all the way I didn't care too much about it as I had plenty of real things to learn but I think the time has come to address the issue now :)

- Yes the gib has three small gib adjustment screws on the side of the cross slide, as you tighten them the gib pushes inward into the dove tail.

- Even with the leadscrew removed sliding the cross slide by hand with the gib in place it tightens up as I move it towards me. The last half inch to an inch feels like it's stuck and if I pull it hard it lets go suddenly. I'm thinking either the gib is tapered or the dove tail isn't parallel as @C-Bag mentioned. If I leave the compound slide more towards the center and with the gib screws backed out I can push and pull the gib freely. However, as I the compound slide is pulled towards me the gib strip feels wedged.

@C-Bag - I think you're on to something. Now I did measure one end of the gib-strip to be about 15thou thicker so it could be that and I can try to grind it by hand with a sandpaper on the surface plate. However, how do you check the dove tails (on the carriage or the cross slide?) and how did you grind those parallel?
 
Dove tails are usually checked by using two equal-sized rods that are tucked into the dove tails. A micrometer is used to measure the rod-to-rod spacing as they are slid down the dove tail. I did that to check the dove tails on my mini mill table because I was seeing a notable increase in friction as I moved the table to one side.

My problem turned out to be mostly due to misalignment of the feed screw bearing block relative to the feed screw nut, something much more easily addressed than non parallel dove tails. To address that, you have to loosen the nut, run it up to the bearing block and tighten it back down. On these machines there is just one bearing for the feed screw so that's all you need to do.
 
I'm hopeful that if I sand down the back part of the gib strip the problem will be fixed. Not sure hand sanding will work or if there's a better way to do it. May be if I mount the strip on the mill using an angle plate or something and then running a drum sander disc across it. Or lay it flat on the table using mitee bites or something and then use an endmill/cutter to skim the surface.

But if I do need to make the dove tails parallel I'm guessing the easiest way for me would be to do it on my mill using a dove tail cutter. Question is what size/angle cutter will I need, is there a fairly standard size that's used by these smaller lathes?
 
The thickness of the gib is not the problem. The three adjustment screws eliminate any problem of taper in the gib. If the male and female dovetails are not parallel they will cause what you are experiencing. Two years of hobby wear will not cause this, only poor manufacturing practices will.

Edit: correct Typo
 
Last edited:
The reason I suspect flattening the gib will fix the problem is because even with the screws fully backed off the gib sticks in the dovetail in the last 1 inch of travel (towards the operator).
 
It should be obvious that a gib that is set up with setscrews will not be effected by taper; from reading this discourse, it seems to me that the problem is with taper in the dovetails on the cross slide bridge. Re cutting it on the mill is a bad idea, you may not be able to find the proper angle cutter, the angles are all over the map, as in non standard angles, and it is all too easy to remove too much material, effecting alignment. It would be a much better procedure to re scrape the gib side of the bridge to remove possible taper, also check the gib for flatness on the surface plate with Prussian blue or other marking medium for full bearing, it can then be used (after possible necessary scraping) to scrape in the dovetail.
 
stioc said:
The reason I suspect flattening the gib will fix the problem is because even with the screws fully backed off the gib sticks in the dovetail in the last 1 inch of travel (towards the operator).
Click to expand...
Still, if the dovetail was not tapered, that would not cause binding.
 
You must log in or register to reply here.
Back
Top