newbie - setting up a tricky job

[BillWood]

Hello,

Am setting this job up - see linked photo - http://www.hobby-machinist.com/attachment.php?attachmentid=48616&d=1393202433&thumb=1 from this thread http://www.hobby-machinist.com/showthread.php/13041-Acetal-Delrin-cross-slide-lead-screw-nut

The plastic block deformed slightly during the moulding, the hole is no longer square to the sides or central and now I have to do everything using the hole as a reference rather than the sides of the plastic block.

I can see that the drill bit ensures axis of rotation is at 90 degrees to the hole in the workpiece, and that it may be necessary to nibble a few bits off the chucked end of the workpiece to get the drill bit sitting exactly on the jaws

How do I ensure that the axis of rotation pierces exactly through the diameter of the hole and is not slightly off to one side ?

What tools, accessories and jigs should I be reading about ? What books or web site would help me to better understand how to set up this job on the 4 jaw chuck.

I've not done anything with a 4 jaw chuck before, and very little with a faceplate.

Regards

Bill (Hercus 9" & 3 jaw, 4 jaw, jacobs chuck, no faceplate yet)

 

[DMS]

Just to make sure I understand what you have going on here..

You're trying to make a delrin nut similar to Danreb. You have molded the screw threads, and now you are trying to face the base parallel with the screw. You are using a lathe because you do not currently have a mill.

The following advice is going to based on the above assumptions.

A couple things to keep in mind. The plastic is pretty soft, so don't crank down too hard. It's also pretty slick (makes great bushings), so don't take too big of a bite when you cut. As far as alignment, it would probably be most accurate if you used the screw itself, but it may be hard to get in and out. If you can find a drill bit the right side as Danreb did, try that, but IMO there is not a lot of clean surface on a drill bit to get a measurement off. It may be better to turn up a small rod that just fits in the nut and use that. Assemble everything, and clamp it in the 4 jaw lightly, but tight enough so that it stays in place.

If you have a dial test indicator, you can sweep in the drill bit (or rod) that you used. Set the rod as close to horizontal as you can get it. put your DTI on your cross slide, with the tip on the rod, and run it back and forth, adjusting the chuck until the indicator doesn't move when you move the cross slide back and forth (you may not get it to 0 on the first pass). Then change the setup, and put the tip on the tailstock side of the rod. Repeat the same procedure as before, moving the cross slide back and forth. Tap the part with a small hammer to make adjustments. Once you are close to 0, go back to the original setup, and see if anything moved. Repeat this until you are satisfied everything is good. Once you are, tighten the chuck down a bit, and check everything again.

It's pretty tedious, but take your time, and you should get good results. Also remember to take light cuts. If you don't have a DTI, you can use a dial indicator. If you don't have either, you can try lining it up "by eye" using a parallel (which can be quite accurate). To do this, get a parallel that is just big enough to fit between the chuck face and the rod. Hold the parallel against the chuck face and adjust the gap between the parallel and the rod until the gap looks the same. The smaller the gap, the easier this is. It can also be helpful to shine a small flashlight at the part from the back.

 

[BillWood]

Thanks for your comments.

The assumptions you made were right.

A mill is on the wish list.

Job done. Nut works and is very tight but definitely no backlash.

Bill

 

[DMS]

Great news. Have you lubed the nut yet? Danreb indicated that things were decent dry, but improved a lot once he added some oil.