Jonathon, I would really like to hear about the techniques you used to drill the bar on the lathe if you care to take the time. Also how long was the drilled blank? What was the pressure used on the drill?You must have altered the drive to the leadscrew to get the feed way down.
The runout you mentioned was really good.
Regards,
Ed
Ed,
Kind of embarrassed to describe the process.. Cause in no way should it of worked as well as it did
surprisingly enough, the improvised boring bit was made using a carbide cement drill. reading an old popular mechanics "shop Notes ", they mentioned using cement drills for hardened steel and referenced drilling through a file.. Sure enough there is an old file in my shop that has a hole through the outer end!
they mentioned running it fast and with firm feed. a junk masonry drill did just what they claimed. I bought a Green wheel and a new masonry bit. I ground my Jack-leg version, of an off center gundrill point..as I saw it in my head.. The pressure side was ( where the bit wants to deflect towards ) was rounded and smoothed with a diamond file.. this to me, in my ignorant way of thinking.. would limit how much it would "push"
I went to the airparts store and bought a section of chomoly that would slip over the fluting of the bit.. and using a dremel I cut in some fluid grooves. Marlow then fixtured it up and brazed the bit into the tube.. Toothpicks in the oil grooves kept them clear. Once that was ready we added the cutting fluid line on the Howa to the back of the tube and we could flow coolant/cutting fluid towards the tip. The bar of steel was placed in the lathe and steady and centered/indicated as best I/we could.. using a starting bit and a regular drill, just a bit over the 'gun drills' size we made a starter hole.. about 5/8th deep. We mounted up the drilling bit, eased it in and stared the cutting fluid.. Running max out for spindle speed.. I
hand fed in the bit.. baking it out every now and then to flush chip and make sure the swarf wasn't building up.. while the push tubing was smaller in diameter than the cutting face, it didn't allow for fully discharging the chips..
there was a lot of oohing and awing and stupid grins going on..
but the bit cut a reverse pip, just like a true gundrill does.. and if you eased it in carefully to re-engage the drilling it'd center itself back into place and restart the cut right where you left off.. we were in the High school's shop, where he taught and by the time we got everything going and got any depth.. we were nearing 10-10:30pm and had to stop.. Think we went about 20-22".. this left us room to square up/crown the muzzle and clean up breech the back end, after rifling. < there is a slight wear in and drop out at each end of the barrel, during rifling process> measuring end to end we had .008 runout and I thought that was fairly decent? We used my armoury reamer to ream and polish the bore before the rifling process..
Lessons learned.. #1 while it shouldn't have worked as well as it did.. we got lucky! I repeat, we got lucky!!!!
#2 I'd need to remake how I used the carbide of the masonry bit.. removing the carbide from the original shank and attaching it it directly to the driving tube,,The driving tube also needs
flutes pressed in to allow for free flow of the cutting fluid and swarf removal.
#3 more fluid pressure.. we were just using the Lathes coolant feed.. true gundrills operate above 200 psi for the oil flow!
# 4 higher spindle speed would have been better
# 5 true power feed, with constant swarf removal.. at a rate of .00025-.00035 per revolution/min is ( I think ) the recommended rate of feed .. giving about 0.4-.5" of cut per minute at 2000rpm
Good things.. the bit cut a reverse pip, and followed that down the bore! the cutter produced chips very similar to what a gundrill produced.. <and it (the chips) were like what was described to me by the barrel maker who got in touch with me.. and offered his advise..>
there was not all that much to lose in trying??? I figured I'd of know rather quickly if the bit was drifting, as I rested one hand on the push tube and used the other to feed in the carriage. excess vibration or any whipping would have indicated we were screwed.. ( and I cud have stopped and saved he bar stock.. ) as it was, it ran steady ??
that's about it.. nothing fancy or
proper about how we did it..but it did do the job.
Respect Always
Metalshaper/Jonathan