Thread: Increasing the capacity of bowl jaws

I'm sure turners feel they will be able to use home made & after market extension jaws safely. If they are cautious & prudent they probably can do so for a while.

I would caution turners to seriously reconsider using extensions to bowl jaws.

Have a look at the internals of a Vicmarc 120 chuck or any 4 jaw scroll chuck. The Vicmarcs are robust well made chucks, & in my opinion rank among the best on the market. But like all engineered tools they are designed to perform within design limits with a responsible safety margin. Some chucks are definitely not as robust as the Vicmarcs.

The scroll is all that retains the backing slides in the chuck. When you open a scroll chuck to full extension only some of the annular tabs on the base of the backing jaw slides engage the scroll. The design is in its weakest orientation if slide #1 happens to align and is just engaged with the leading edge of the scroll in use at extension.

Vicmarc chucks are designed to limit the extension of the backing slide / jaw set with a stop pin. Other manufacturers use various means to achieve the same thing. This ensures there is sufficient engagement of the tabs in the scroll and sufficient strength for the chuck and bowl jaws to be operated up to the recommended maximum speed in normal usage. The chuck designer also takes into account stress from typical loadings and repeat usage. Now if the jaws are operated at maximum extension with an extension on to that then the original design safety margins may not be sufficient to guarantee your safety in repeat use (or perhaps single use).

The additional loading on the parts has the potential to cause metal fatigue on the few tabs that are engaged in the scroll or to the scroll itself. This is particularly hazardous if the slide stop pin is removed “to gain a little extra movement range.” Metal fatigue may never be discovered given the frequency of most turners chuck servicing schedules.

I have serviced a number of chucks for others that have had parts of the scroll missing particularly the leading edge of the scroll. So abuse does create metal fatigue and it is usually because the chuck was operated with the slide stop pin removed or if the chuck has been dropped with the jaws open significantly.

100_1531.jpgVM120 parts GW 1.jpgwwf vm90.jpg

Have a look at the backing jaw slides in my photo. The slides are arranged from 1 to 4 from the left. Now in your third photo it appears the slides are from top & clockwise 4,3,2 with #1 hidden behind the chuck body. It would appear that the stop pin is still fitted - excellent! #1 is the weak link – 2,3 & 4 will have two full tabs engaged. #1 will only have 1.5 tabs engaged. My second photo shows the slides going into a VM90 but it shows the scroll rings to illustrate how the tabs engage.