Parallel Extension Tables

[JPG]

20201213_062730.jpgThis picture was taken this morning after I read the posts here about twisting the Way Tubes. I felt very, very little (if any) indication of twisting when I attempted to impose that twist action on the tie bar. I don't employ the "screw screw". Never have.

OK try this:

Put a 1/4" shim between the headrest and one end of the tiebar.

Lower the tiebar/tubes on to the shim.

Note the clearance at the other end of the tiebar.

Tighten the clamp(it's motion will be restricted by the shim, but try to duplicate typical clamping force(temporarily adjusting the clamp will make that easier).

Now note the resulting clearance. I believe the tie bar will have rotated.

[dusty]

After posting this next set of pictures I must take a break from this effort. I have to get the shop back into a condition where it is safe to walk in.

After my efforts yesterday I was very disappointed on the swaps of my "Swap the Tables Test". I decided to go back to square one and do both tables over again. I am now ready to do the swap but I wanted to show a copy only vaquely related pictures.

I start by removing the Extension Table Legs from the table and put the table on table tubes mounted to the Main Table. The carriage is then slid toward the Table legs to achieve vertical alignment. Then I lay washers in place and raise the Table Legs to engage the studs on the table. Add the other washers and nuts and finger tighten starting with the upper four nuts. Then finger tighten the lower nuts then gingerly tighten the nuts with a wrench.

If the Extension Table was square with the Main Table to start with, I should now have a perfectly matched set of tables. Check the rails to see if they slide smoothly and move to the other end.

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[algale]

Do you think this approach give you a different result than Nick's approach? He loosen all the nuts on the extension substantially, brings the main table over and then puts the tubes through the rails and tightens them to achieve alignment to the main table before carefully snugging up the nuts. You go the reverse order. But I don't see any difference in the end result since both rely on the tubes to make the alignment.

[dusty]

Do you think this approach give you a different result than Nick's approach? He loosen all the nuts on the extension substantially, brings the main table over and then puts the tubes through the rails and tightens them to achieve alignment to the main table before carefully snugging up the nuts. You go the reverse order. But I don't see any difference in the end result since both rely on the tubes to make the alignment.

algale, No there is really no difference other than both Extension Tables are physically in contact with the Main Table. Diagonal Measurements of the two can be compared where as the other way that can not be done. At this point nothing new has been done toward achieving the objective of understanding.

I drug my feet today and have not yet done a swap of the tables.

I did use the AngleCube today to check all of the vertical involved and the Table tops and a number of different locations. All observations were as expected except the vertical measurements of the extension table legs. They were not quite the same as when the table tunes were not involved.

I need to record the various measurements with and without the tubes to see if this is meaningful.

When working as Quality Test Inspector I put every thing in a log book. That was a good habit - actually mandated - but I lost that trait when I retired.

[DLB]

I finished aligning my Main and Extensions tables following the methods in the above Sawdust Session with mixed results. This 520 is new to me, so I first aligned the rails on their tables with the jigs. I was pleased with the results of using the Wixie to set the table to 90 degrees, which I confirmed with an engineering square, and his method for aligning the Extension Table on the right. Moved to the left and adjusted the big setscrew, which took quite a few iterations. To my surprise I did get similar results in terms of easy-sliding table tubes. However, the Extension table surface did not meet my expectations for being coplanar to the Main. Measuring with a straightedge I found the results unacceptable with a visible gap at the inside edge both front and rear. Confirming that with a Wixie, the table is off about 0.2 degrees from the Main and from its reading when installed in the Base. Also the front edge of the extrusion was not as well aligned to the main as I would expect it to be for 'maintaining alignment.'

Part of the process in the video was to identify the curve in the table tubes and mark the top. Nick indicated that the two screws in the main would then straighten the tube against the inside of the extrusion. I don't know that this is enough pressure to completely straighten the tube. But I do know that when you move to the left, assuming you have a headstock in there, the standard tubes are not long enough to reach both screws in the main and go through the extension table. So I conclude there is no way that the table tubes are straight when you move to the left unless they were straight to start with (unlikely).

Answering an earlier question asked by Al Gale, I found that making the big setscrew adjustment did change the relationship between Main and right Extension a bit. Mine was pretty far off, ~5/8 of a turn, and I realigned the table on the right when I was getting close. At the end state it worked the way Nick described/implied, but again I would not call it aligned.

Whether unique to this particular Mark or not, I found inconsistency every time I raised and lowered the tilt while adjusting that setscrew. The tie bar would land either farther forward or rearward and the lock did not consistently center it. This is no good for alignment consistency.

- David

[algale]

Very interesting observations, David, especially about the impact on the right side table when the "secret" adjustment screw was used to fix the left side table.

[JPG]

I would expect the set screw to affect the right aux table to main table as the set screw changes the way tube twist thus moving the main table relative to the right aux table. That effect will be greater the further to the left the carriage/main table is moved.

Assuming one has a long enough straight edge, a comparison of main/right aux table coplaner with the carriage full right and full left will indicate twist if present.

The tie bar clamp is critical to all this. It MUST force the tiebar tight to the headrest at both ends of the tiebar(including resting on the set screw).

[DLB]

I would expect the set screw to affect the right aux table to main table as the set screw changes the way tube twist thus moving the main table relative to the right aux table. That effect will be greater the further to the left the carriage/main table is moved.

That was my observation exactly. So 1/4 turn, for example, had a relatively large impact at the left table mount and a much smaller impact at the right. So the process may be iterative depending on how far off the setscrew is to start with. But the extension table alignment procedure Nick presented made it relatively painless to re-align the table on the right.

- David

[dusty]

OKAY. I have read the entire discussion (and watched the video again) to make certain that what I have been doing does not conflict with Nicks recommendations. I am relatively close. However one might make that determination, the tubes must be straight or this won't work.

Now to the part where I am lost. What exactly is expected to happen when the "secret screw" (which I have never used" is adjusted. I understand the obvious. When the screw is raised the tie bar is raised at the point of contact. How does that effect the extension table at the far end of the Mark.

Please don't tell me that the assembly consisting of the Tie bar, Way Tubes and Base Arm is twisted.

Also, I am curious about why there isn't a set screw on both sides of the tie bar. What if this assembly needs to be twisted the other direction?

Can the set screw be adjusted from the bottom side to eliminate the need to go back and forth to drill press mode?

Great observations from all. This gives me a lot to think about including how to do all of this without relying on straight table tubes. A laser comes to mind. The laser dot from might laser pen may be too large to be useful for this task.

[dusty]

After completing my previous post I watched Nick's video again. This time concentrating on the "secret screw".

I may have come away with an understanding that I never had before. First he made a comment about the table normally being canted to the front. Very important statement though it is an assumption. Normally canter to the front.

To correct this he raises the "secret screw" and then locks the Headrest Lock. This is when the twist occurs. The Headrest Lock actually induces the twist because the set screw holds up one side while the Headset Lock pulls down on the Tie Bar.

The rest of you apparently caught on to that earlier but I did not. I also now understand the recommendation to start the alignment procedure on the right end of the machine (the correction is provided at the left end and is intended to make the left end the same. DUH

I still have the question about what happens if the table is canted the other way