Banding on finish when using powerfeed on G0602

[Ken I]

Joe, On my lathe the apron "hangs" under the carriage on 2 x M8 cap screws - the clearance holes are elongated in the X (crossfeed) axis - so some adjustment front to back is possible.

The half nuts run in a dovetail and are opened and closed by a rotating cam so the up down position is not adjustable.

You could change it down by putting shims between the apron and the carriage (between the joint held by the M8 caps) - which I have seen on this model lathe.

Mine needed to go up.

One method would be to rebore / bush the hole that the cam runs in to raise it - but in my case I noticed my rack & pinnion handweel drive had a lot of lash - enough to allow me to skim the top of the apron to raise both the half shells the amount required and improve on the rack / pinnion lash.

I measured the error by dial indicator on the screw - by closing the half shells and measuring the deflection - repeated at 180° from high / low leadscrew positions (bow in leadscrew) and averaged it out.

I also checked at the headstock, centre and tailstock extents of travel to make sure the leadscrew was paralell to the bed - which it was. (If that was out you would have to rebush the tailstock end leadscrew bush to correct whatever the X & Z errors measured out to be.)

Another problem that developed as the machine "aged" - the lever/cam mechanism that operates the split nuts has no detent and relies on the cam going overcentre - this developed a habit of coming out of engagement - a PITB when turning but a disaster when screw cutting. I installed a "friction washer" under the lever - but with this teardown I am going to look at adding a spring loaded detent ball / detent for the cam.

Since your lathe is new - if there are "fundamental" errors in its construction maybe you should make it your suppliers's problem.

Hope this helps.

Ken

 

[bob ward]

I had a similar surface finish problem on a new generic 14 x 40 lathe. If I hand cranked the carriage I got a nice smooth finish on the job, using the power feed the finish looked very much like that in the OP's pictures. I couldn't figure out the problem so I got the lathe suppliers out to have a look.

I forget the details now, this was 2 years ago, and the solution is quite counter intuitive, but what they did was replace the cross feed screw and nut and the problem no longer existed. YMMV

 

[wannasteam]

Hello all

Ken: Thanks for all the info. It sounds like you've done a lot of work tweaking your lathe. I guess I am fortunate enough to not have had those problems on my 7 x 12.

Apart from the standard mods, mine worked pretty well out of the box.

Bob: Replacing the cross feed nut and screw was definitely counter-intuitive.
I would have thought that the problem was with the main power feed screw.

Here a few pix of how I solved my particular banding problem.

Firstly, the picture is not flipped. The lathe has the leadcrew lever on the left side of the saddle and the wheel on the right. The model sold by Grizzly (my original pix) is the other way around. Hmmmm...I wonder if this is because we drive on the left side of the road here. scratch.gif

First thing I did was to remove the lower part of the saddle to see what was going on inside there. Nothing unusual so I nipped up a couple of gibs and oiled everything.

However, I noticed a hole kinda in between the 2 gib adjusters. (see pix) and noticed that it was tapped 6mm. I put a 6mm setscrew in it and lo and behold, it was a stop screw for the cam lever! Adjusting this screw allowed me to adjust the "tightness" of the half nuts. Kinda the same effect as slightly lifting up the cam lever. After a bit of trial and error, success! See pix of final result.

This seems to have solved the banding. Yay!

Another thing: I had to elongate the holes fixing the bearing block at the tailstock end of the leadscrew to relieve the tension on the leadscrew when the half nuts were tightened when the saddle was over at that end. The end of the leadscrew wanted to lift up about another 2mm.

On a different note, I also added one of those racheting handles to operate the carriage lock.

I hope this helps somebody else out.

Cheers

Joe