Myford Backplate

Hello,

Can anyone explain to me why I'm getting run out on a cast iron backplate. I have machined the face whilst on the spindle, I then put a dti onto the first half of the plate (the side nearest to me) and recorded no run out from the edge to the centre. When I check the furthest away side I recorded approximately 0.0015" run out.

I have uploaded a movie to YouTube as I fear this may not make sense.

**LINK**

Your cross slide gibs are worn?

I would look at the end float of your spindle. Can you easily move the spindle back and fourth by that amount you are showing as an error? Did you have the saddle in a locked position on the bed? Did you take a skim cut for the final cut if it was running in a swash situation before trimming? Lots of questions I know, but these are all reasons for the error you are seeing. Excessive wear in the cross slide gibs and compound gibs can also allow movement , especially if there was a reasonable amount of an uneven cut thickness while the part is turning. Often excess wear in the cross slide can show as a taper on the face, but that is not obvious in your video.

Neil

OMG is that lathe running while you are dragging the pointer of the dial indicator across the backplate? If so, please stop it at once. It does horrible things to your nice piece of precision measuring equipment.

The opposite sideways forces on the plunger as you move across the centre point are most likely the cause of the needle fluctuations.

Also, your dial indicator needs to be much more solidly mounted for precision testing like this. It looks like you have a full sized dial indicator on one of those little hydraulic arm bases made for a small 1" face finger-type dial test indicator. Flex in the mounting set up, combined with the sideways forces on the dial gauge from the moving backplate mean your gauge is all over the place like a rash.

You need to mount the dial indicator on a solid piece of square bar clamped in the toolpost, so its as rigidly mounted as a cutting tool. Keep overhang to a bare bare minimum. Or at least  use a full sized magnetic base, set up with minimum overhang from base to gauge. Best to mount magnetic base solid on the bed way, not the carriage too.

Then without the lathe running, put the plunger on the face of the backplate at about the 9 o'clock position and slowly rotate the spindle BY HAND. The reading you get on the dial gauge is teh amount of axial runout you have. Repeat process with the dial gauge plunger running on the outside diameter of the backplate to check radial runout, or concentricity.

A further check for flatness of the machined face would be to check it with a straight edge, or with bearing blue on a surface plate (or piece of flat plate glass at a pinch). It should be either dead flat, or slightly concave so the chuck seats on the outer area of the face and will not rock. If the face is convex, adjust up the gibs in your carriage and cross slide as best you can and take another fine cut, with carriage firmly locked. If still convex, you probably have a worn carriage ways on the bottom of the carriage that are allowing the carriage to cant at an angle, or possibly worn cross slide ways. Not much you can do about that in a hurry. Instead, turn a relief of a few thou in the centre section of the face so the chuck bears on a ring around the outer portion of the face.

Running the dial indicator across the face as you did in the video, even with the lathe not running, tells you nothing much really, other than the dial indicator is following the same path the cutting tool did. The face could be concave or convex to billy-o and the dial guage follwing the same path as the toolbit did will read no variation. So don't even bother with that test.

Edited By Hopper on 05/05/2017 23:29:54

Any progress?

Gentlemen,

Thank you very much for your input. Sorry for the delay... I have recently had problems with my internet connectivity and have just received a new router from Sky. The new router appears to have done the job!

I am a mere Driving Examiner and have no engineering qualifications. I do not make a habit of using a DTI in that manor but I felt that I was struggling with my description so that was my reasoning for using the DTI.

I have made very subtle alterations to the gib screws which has made a big difference. I have checked the flatness use Marking Blue and a Surface Plate. I now happy with the tolerance that I have.

Once again thank you for all your help.

Regards, Tom

Great stuff. Glad to hear you got a result. Happy latheing!