D1 Camlock Issues

[BFHammer]

I just purchased a Shars 'Adjust-Tru' type ER40 chuck combo with a D1-4 backplate for my PM1236.

It seems that one of the holes for the camlock studs is threaded such that whichever stud I place in that hole it is +/- 180 degrees off from the other two studs Therefore, once rotated in or out to align with the lock screws the stud (and notch) in question is higher or lower than the other two. With the backplate boss side down on the surface plate this translates to about .005 - .006 difference in the height of this stud, either higher or lower than the other two.

With the backplate installed on the lathe the stud in question either barely reaches the 3 o'clock arrow on the spindle locks or almost past the 6 o'clock arrow. With a DTI the backplate shows a high or low spot on the face or edge of the boss of about .004, where as the other two register the same.

My thinking is that this won't be an issue once I face the back plate and turn the boss; however it's aggravating that it seems to prevent it from seating fully on the spindle. Any remaining variance should be able to be adjusted out with the chuck, but I wonder about repeatability since the chuck may move on and off the lathe frequently.

Am I missing some simple solution here?

 

[mikey]

Since the start of the thread in the hole "clocks" the bolt position, could you grind some of the lead thread in the hole to allow the bolt to start in a different position?

 

[Ken226]

Mikey is usually spot-on, and I very rarely find myself disagreeing with him, but in this case, I would very respectfully disagree.

True, the clock position of the lead of the female thread will determine the clock position of the screw/stud, when it starts. But will not change it's position relative to the backplates rear face, when threaded to the correct depth.


Changing the clock position that the screw starts in, by removing material from the lead, wont change depth to face clock position relationship, of where it ends up.

For example:
Let's say, for the sake of argument, that when threaded into the hole 1/2" deep, your camlock studs engages properly, and the D shaped camlock cut center is 1" from the back face. Since the thread pitch is 24tpi, The screw has been rotated 12 turns into the hole to reach 1/2" depth.

Now let's say, for the sake of argument, you increase the counter-bore depth by 1/4". The screw will start 1/4" deeper than before, but will now only rotate 6 turns before the D cut is the same 1” above the chucks rear face. The camlock stud will now be in the same position relative to the chucks rear face as before.

(Don't take any of the dimensions I mentioned to be factual, I just picked them randomly to make the math easy)




If you think in terms of the thread helix position / backplate face relationship, you'll see that removing material from the threads start, won't affect the thread helix position to face relationship.



As long as the locks are between the alignment marks, your good-to-go. It doesn't have to be right in the middle. If you have to choose between just past the 1st mark, or just shy of the 2nd mark, I would choose 2nd mark. The cam will have more mechanical advantage.

Or, you could move the studs around to different holes, perhaps youll find an arrangement that gets you closer to where you want.



Unless I'm misunderstanding or reading your problem wrong.