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Metalworking (rec.crafts.metalworking) Discuss various aspects of working with metal, such as machining, welding, metal joining, screwing, casting, hardening/tempering, blacksmithing/forging, spinning and hammer work, sheet metal work. |
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Toolholder unseating in the Dickson style toolpost
In the continuing saga of repairing the Clausing 5914, tightening
dovetails, et al, another chapter: I am in the process of making the pivot axis upon which the Dickson-style toolpost will revolve (unless prevented by insertion of the 7mm diameter anti-rotation pin). This pivot axis is a thick steel cylinder 1.260" in diameter with a 11/32 hole down the middle (clearance for the 5/8-18 threaded rod). The current operation is to make a 1.263" diameter recess about 0.125" deep in the ridged spacer plate between tool post slide on the 5914 and the bottom of the toolpost. Never having made such a recess before, I started with a 3x3" piece of 0.5" thick scrap hot-rolled steel in the 4-jaw chuck. First step was to drill a 11/32" hole in the center, using drill bits held in the tailstock. After tightening the tailstock's bed clamp so the tailstock wouldn't slide away from the headstock under drilling pressure, no problem. Now for some boring, with tool held in the Dickson toolpost. I first used a stout boring bar with a 0.5" diameter round shank, held in a toolholder with a V-groove intended for holding round shanks. The boring bar was 3.25" overall length, with about 2.25" sticking out, with a brazed carbide insert tip. This kind-of worked, but left a very rough, chewed surface. Then I noticed something odd. When the tip of the bar hit the bottom of the recess, the whole holder+bar tilted, with the holder unseating visibly from the toolpost, and returning to normal when the tool was withdrawn, all with no apparent damage to anything no matter how many times this was tried. Nor did the crosslide or toolpost tilt. I had cranked the holder down to the toolpost quite strongly, and I was quite perplexed. Then I noticed that the clamp bolt was hitting its stop, rather than exerting full force on the toolholder, so the holder was not necessarily held all that strongly to the toolpost. Hmm. Maybe the clamp bolt is bent. Or something. Let's use a different face of the toolpost, and thus a different clamp bolt. Done. Made no difference whatsoever. Hmm. The holder is awfully high on the toolpost, with part of the holder sticking out above the top of the holder. How can a little overhang matter so much? It does seem to matter, as the same holder held lower doesn't cause the clamp bolt to hit the stop. No idea why. Disassembly required; later. Changed to an ordinary 3/8" shank brazed-carbide bit held in the same holders. The stick-out is about 0.75", and the holder is lower in the toolpost. This worked, yielded a smooth machined surface, and no tilting when the bottom of the recess is machined. The cutting edge in the boring bar is about 0.25" above the bottom of the tool slot in the toolholder, while the cutting edge in the 3/8" lathe bit is 0.375" above, a gain o 0.125". What a difference. Could not use a larger lathe tool bit without the heel of the bit skating on the walls of the recess. It would appear that the Dickson toolpost needs to be higher than it now is, with the current 0.7" spacer made perhaps 0.25" thicker. I'm assuming that the clamp bolts are not in fact bent, because they would all have to be bent the same. Nor do I like the damage inflicted by toolholder on the boring bar shank. I think I will want a holder with a tight-fitting hole that clamps the shank without setscrews clamping directly upon the shank. Joe Gwinn |
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