Double Tilt Incident

[DLB]

Yesterday I had one end of my Double Tilt (DT) machine elevated to re-install the headstock. The headstock was light, a PowerPro with no motor, but I had to stop at the ends of the tubes because the wedge locks were out of adjustment. With the weight of the headstock in this location the DT Base Arm pulled past the hinge pin and the entire upper assembly (headstock, way tubes, carriage, table, base arm) came free from the Bench/Base assembly. In my opinion this operation created sufficient leverage on the locking bolt to hinge pin interface resulting in flex of the hinge pin and/or base arm casting wall allowing the locking bolt to slide past the hinge pin.

Possible factors:
1) I had the carriage (fulcrum, in this case) very close to the base arm. This would add a lot of leverage compared to placing it farther away. In hindsight this was a bad idea for the DT.

2) My DT Base Arms are of an unusual design as noted in this post: viewtopic.php?p=292346#p292346 I'm sure mine are not unique, but it seems that base arms produced both earlier and later than mine are structurally different.

3) On my DT Bases: The locking bolt is a cone shape (point) on the end that engages the hinge pin. Engagement is way less than half the diameter of the pin. The angled part of the cone engages the pin at a tangent that is angled relative to the upward force that caused dis-engagement of the Base Arm. I've previously assumed the bolt portion of the assembly and its engagement with the hinge pin is the same as other versions. I realize now that I am not certain of this.

I'm submitting this primarily for awareness: Don't replicate this at home. It was mostly or entirely my fault for placing the carriage where I did. The open question would be what is the safe minimum distance for this operation considering a full weight headstock? I've described this incident to the mothership and intend to look closer. Will include some pics with any follow up.

- David

[dusty]

I do hope that there were no personal injuries incurred and that the hardware was not damaged.

I am unaware of any changes that were made to the DT. That being the case, I assume that have have the same version as you do so I am very interested in learning what the differences are AND more importantly what the weakness are.

I seldom tilt mine other than for vertical drill press so I never experience this. I'll be waiting to see the pictures.

[DLB]

I did a bit of experimentation with no headstock involved so I could better control the outcome. I confirmed that I had not left the lock knob loose. Also that both the casting and hinge pin deflect as this is coming loose. Here is a pic of my DT Base Arm with a hinge pin for reference and shown upside down in the 'locked' position:

Base Arm

DT Base Arm.JPG (194.85 KiB) Viewed 2771 times

Things to note:
The aluminum bar that the lock screw threads into is unique to this configuration. In the picture I referenced in the other thread you can see that there are two visible machine screws, one of either side of the operating knob. There may be other features unique to this configuration, this one is visible externally.
Note that the end of the lock bolt is a cone or pointed shape and how it engages the hinge pin.
Referring to force directions as it is mounted: the angled interface here redirects the upward force of what I was doing into a left force acting on the casting and a right force acting on the hinge pin, in both cases deflecting them like compressing a spring. In the incident, the forces and deflections were sufficient for the lock bolt to pop over the hinge pin, freeing the base arm and everything attached to it.

- David

[dusty]

20220216_131434.jpg (33.21 KiB) Viewed 2755 times

Mine is different but I don't see how therse changes could cause or prevent your incident. Casting is thicker around the bolt hole and lock bo;t may be different.

[Moparfonix]

That is scary, thanks for the heads up!

[DLB]

20220216_131434.jpgMine is different but I don't see how therse changes could cause or prevent your incident. Casting is thicker around the bolt hole and lock bo;t may be different.

Thanks. It looks like the lock bolt is different only in material and/or coating. Mine is black oxide, yours appears to be stainless or plated. Both are double start thread and have that O-ring groove. Both have the cone/point implying similar engagement with the hinge pin. From your description it sounds like the casting is thicker for the threads, not for mechanical strength.(?) Mine is only around 1/8" thick on that wall, not enough for threads.

My theory for now is that my version DT Base Arm is a factory reworked single tilt Base Arm, and that the more common version is made from a different casting. Reason for a reworked version is unknown, both older and newer DTs are different than mine. Both type locks would likely yield/release under force, but the amount of force needed might vary between them. And the force isn't typical, it is an upward force when the ways are horizontal so the only example that comes to mind is the operation I was doing, mounting a headstock with one end raised.

I have access to the other casting type, assuming there is only one. I'll have a look at that and report back. I think I can set up an experiment to compare strength of the casting wall, which directly relates to the amount of force that will cause the lock to release.

- David

[DLB]

I set up to measure deflection of the castings as representative of the force acting to unlock the locking mechanism of two variants of DT Base Arm. The force involved deflects both the locking bolt (with casting) and the hinge pin, but it was impractical to measure hinge pin deflection. As shown, I measured locking bolt movement relative to the way tube:

Setup

DT Base Flex.JPG (283.31 KiB) Viewed 2633 times

I had the headstock about two inches from the left end and the carriage about 8" from the right, did not move either for the two test samples. My original DT Base Arm deflected 0.022" under this load. The other type Base Arm deflected 0.035" under the same load. It was difficult to get a good reading, but my estimate is 60 to 70 thou of deflection would disengage the lock and free the Base Arm and everything attached. Pictures attached of the second Base Arm I tested, and presume is representative of the majority:

Other type Base Arm

DT Base T2 Front.JPG (280 KiB) Viewed 2633 times

Other type Base Arm

DT Base T2.JPG (243.51 KiB) Viewed 2633 times

Conclusion - If you compare the two types, original from earlier in the thread, you see that what re-enforcing there is on my original is horizontal while on the 'standard' version it is vertical. The DT Base Arms that I believe to be factory reworked single tilt Base Arms are more resistant to the force overcoming the lock than the cast version. Both versions are subject to the problem I described at the beginning of this thread.

- David