hello everybody.I have a problem with deep hollowing. the wood is green ash its 28% moisture content its 27" tall and 16" dia a vase.the cutter started to skip at 18" .i have tried all the carbide cutters 1/8' .3/8" square ,cupped and tear drop. I haven't tried the hogger cutter or pro forme cutter yet. I talked to my woodturner club got some ideas ,it still is a problem.. I have the keith clark hollowing system with folcrum tube.Thanks Al.
Hi Al. I also own Clark hollowing tools and have been able to achieve 25 inches of depth in a test with a 12 inch fulcrum but that was with straight grain cherry(knotless), not ash. For that test, My bar was about 60 inches connected to a D handle to prevent rotation of the bar. I had a second fulcrum about 3inches on a 2nd cross bar support followed by a trap for the Dhandle. My bar is not slotted so I may have more material on the bar and therefore possibly more strength with less tendency to flex. I don’t feel that you have reached the maximum depth at 18inches as there are utubes/videos that show up to 30 inches. For those unfamiliar with the system, the 1 1/4 boring bar rides inside a tube(like a piston inside a cylinder) whose outer diameter is about 1 3/4” effectively strengthening the boring bar reducing bar flex.
Nevertheless, after 15-18 inches deep I could not be as aggressive removing stock.
I will add to the other posters above with issues relating to our system. Perhaps, this may help you troubleshoot specific to your tooling.
The 3/4 cutter bar holder should be as short as possible. The one I used is about 6 inches long and sets into the boring bar about 5inches.
(I use the 3/16 HSS and the 6mm hunter/Jamieson cuter. Teardrop when I can control the depth of cut)
Check for play in the bar bearing and tighten if present.
Check that the fulcrum cross lower bar support has no play as well as fulcrum and boring bar play.
The joint that connects the upper and lower part of the fulcrum should be tight.
Any one of these conditions, can’t induce vibration which may reduce your depth.
One of my crossbars was a few thousands too small of 1 1/4 which caused this condition. Keith replaced it.
The Blank attached to 10inch faceplate with all screws holes used. I use 3inch lag bolts.
Speed of hollowing: Personally, with a 27x16: 350-500 rpm.
As you go deeper, the less aggressive you should be. Cuts should be lite and cutter angle should be skewed. Less is more.
That can be difficult because as turners we tend to push too hard.
On a metal lathe cuts are sometimes setup to take a few thousandth of an inch per pass making many cuts to accomplish a depth that could not be made on one pass. To emulate this concept of limiting the depth of cuts on the Clark system, you will need three 1 1/4 shaft collars and an $8 box of HFT body shims.
Install and tighten 2 shaft collars on the crossbar right and left of the fulcrum to restrict side to side movements.
Install the other shaft collar on the boring bar After the 12” fulcrum. Do not tighten. Move the boring bar cutter to the highest point not hollowed and move the shaft collar along the boring bar down to the fulcrum and tighten the shaft collar. Your initial cut depth is locked. Swing the boring bar from the center to the rim for endgrain to make sure it clears the surface. Turn lathe on and pivot the bar back and forth making your first cut. Turn lathe off. Without moving the boring bar, loosen the boring bar shaft collar and insert 1/64 shim between the fulcrum and the shaft collar. Tighten the shaft collar and remove the shim. Your now set up to cut an arc from the center to the rim of exactly 1/64”. Test with 1/32, 1/16 etc.
It goes rather quickly but I built a threaded advance out of a threaded rod which made the hollowing endgame(when your nearing maximum boring bar cut limits) a little more effective using the limit depth cut method. I know that the proforme, exocet and tools limit the depth no mater how hard you push, but at these distances you want to avoid pressure on the wood to prevent vibration. To repeat what everyone else said, you want to be on or slightly above center and the cuter setup angular to shear.
If you can’t get to 27 inches, perhaps you should consider the 18inch fulcrum.
Hope some of these ideas help you work through your problem. Let us know how you progressed.