On Monday 08 February 2016 08:39:10 Marcus Bowman wrote: > On 8 Feb 2016, at 12:49, Peter Blodow wrote: > > In other words, you pretend to cut a left-hand thread but have the > > machine run in reverse so it turns out right hand? > > Peter Blodow > > Yes; kind of.... I prefer to think of it as cutting a right hand > thread upside down (or is that inside out?). There are four factors: > spindle speed, direction of feed, tool upright or inverted, and tool > facing the front wall or the rear wall. Where the tool must traverse > out of the hole and the blind end is at the spindle end, the ones that > work for me are: > spindle reversed and feed towards the tailstock; and > [tool upright, facing the front] or [tool inverted, facing the rear] > > If you use a thread mill it just takes the place of the single point > tool, but mills (or flycuts, essentially) rather than forming the > flanks with a stationary tool. Unless you have the kind of giant tool > arrangement mentioned in John Thornton's post. The mind boggles. > > Marcus
Mine did, looking at that pix with the stepped thread diameters. Where the rotation to disengage was like 45 degrees to gain removal clearance, but obviously had nearly 100% thread engagement when inserted and turned to hit the stops. The only way I can visualize that would be by broaching the stop shoulders, then cutting in reverse, with the tool facing to the back wall and both the broach width and the tools vertical measurement such that it could be brought into contact with the bottom of the broach cut. But unless all the various sizes and depths of broaching was done prior to starting the threading, I haven't a clue what would be done with the chip at the end of each segment cut. I seems to me the best way to do the broaching and boring would be on a shaper with a rotating workpiece holder. By that means it seems to me one could get an 85 to 90% thread engagement. But even then I'm not convinced I'd want the job of writing the gcode for either machine. The spindle it seems must have both a super low gear ratio, and an encoder measuring its rotation in arc-seconds. One question remains: Does the thread even have to have a spiral component, eg does it need to actually advance into the hole as it turns? Common sense says it should, in order to achieve a gas tight seal against a shoulder at the bottom of the bore. OTOH, with a shell casing supplying the breach seal as modern bolt rifles do, if the headspace when closed is tight enough to prevent a casing head separation, the need for the spiraling thread is removed. I have rifles in the cabinet of either persuasion, and most of the more modern rifles have zero spiral to the lugs as they rotate into engagement with the action. I changed a remi 788 in 22-250 action out for a Howa 1500 a few years ago, and there was just enough of a headspace problem that feeding it shells loaded for the remi that I was forced to stone a ramp of about 5 thou on the leading edge of the lugs so that a case whose shoulder had been stretched forward to fit the remi's extremely sloppy chambering, could with quite a bit of effort on the bolt handle, be crushed a thou or so as the bolt was closed. It could then be fired normally and IIRC I turned the die in another 2 degrees to push the shoulder back where it belonged for the next reload. Provided I couldn't detect a groove from previous stretching in the remi's action with a bent paper clip inserted. Case to scrap bin in that event of course. That stretching, and the short case life of only 3 to 4 reloads was the reason the remi action was removed from my cabinet. With the locking lugs at the rear of the bolt and in the back ring of the action, case stretch was nominally 13 thou from pulling the trigger, and because the remi chamber was so fat & sloppy, a std Bonanza Bench Rest Coax die, stretched it another 18 thou in bringing it back down in diameter to fit a std SAAMI chamber. The brass has to go someplace... OTOH, a 50 year older P17 can happily crush a case thats 10 thou long as it closes, there is that much of a ramp on the lugs. I haven't pushed a shoulder back it the about 10k rounds I've fed it in the last 50 years. And cases last until the primer pockets get sloppy, 20 to 40 reloads per case, whats not to love about that? Since its chambered for the 40 degree shoulder angle of the P.O.Ackley chamber, you KNOW from the presses complaints when you are pushing the shoulder back. That experince with the 788 was the end of me being a fan of remi, that was handed back to remi with a fat chamber complaint when it had less than 40 rounds of factory ammo fired on it, and remi said it was within specs & refused to rebarrel it. It was an 8 or 12 groove "micro-groove" barrel, and with reloads going 200 fps slower than factory, it put the first bullet thru the target sideways at about 350 rounds. remi said reloads, no warranty, so I had a Shilen barrel put in it, but even Shilens legendary accuracy wasn't to be found. Best group ever was over 3/4" with most in the 1.5" range. So I finally threw money at it and reworked the wood for a Howa action. First group was 3/4". Nuff said. I'll have to admit, this sure seems like a brain exerciser. And I am not convinced I've solved it, as I didn't see any signs of broaching in the one pix supplied of the multi-diameter thread. And that IS a puzzle. EDM perhaps? > > Am 08.02.2016 11:42, schrieb Marcus Bowman: > >> On 8 Feb 2016, at 10:04, andy pugh wrote: > >>> On 7 February 2016 at 23:37, Marcus Bowman > >>> > >>> <marcus.bow...@visible.eclipse.co.uk> wrote: > >>>> Easy. mount the tool upside down, and start at the blind bottom. > >>> > >>> Sorry, I am not quite understanding your description? > >> > >> Yes; apologies; my rather quick response was a bit cryptic... > >> > >> For an internal right-hand thread, the problem is often that the > >> tool feeds into the hole from the right, and bumps into the bottom > >> of the hole. It would be the same for an external thread bumping > >> into a shoulder on the left. Machine the hole to the ID for the > >> minor diameter of the internal thread, then create a clearance > >> groove of 1 pitch width, and just deeper than the thread height, at > >> the bottom of the hole. That blind bottom still a problem if your > >> reactions are as slow as mine, so, unless you have > >> auto-disengagement for the leadscrew, avoid the problem by cutting > >> the thread from left to right, starting with the tool in the > >> clearance groove, applying a cut, and feeding outwards into fresh > >> air. Run the spindle in reverse. > >> The tool can either be mounted against the front wall, upright; or > >> can be mounted against the rear wall, but inverted. (I think I have > >> those the right way around...) > >> I've used this method on both internal and external threads, with > >> front and rear toolposts and with the tools the right way up or > >> upside down, to suit the directions of cut,and it works a treat. > >> The key is to cut away from the blind end or the shoulder (and to > >> take care with spindle direction and tool orientation, both of > >> which can cause local overheating of the brain's visualisation > >> processing area). All much easier with CNC I dare say... > >> > >> Marcus > >> > >>> -- > >>> atp > >>> If you can't fix it, you don't own it. > >>> http://www.ifixit.com/Manifesto > >>> > >>> ------------------------------------------------------------------ > >>>------------ Site24x7 APM Insight: Get Deep Visibility into > >>> Application Performance APM + Mobile APM + RUM: Monitor 3 App > >>> instances at just $35/Month Monitor end-to-end web transactions > >>> and take corrective actions now Troubleshoot faster and improve > >>> end-user experience. 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