Lathes

Posted by on November 11, 2005

A couple of tips about lathes that might be worth mentioning.  For handgun work and the related tool making they don’t need to be very big.  About the size of a Myford or Emco Compact 8 is good.  The Myford is 19″ ( 480mm ) between centres, overall length just over one metre, weight about 100 kg.Used lathes can be bought quite cheaply if you are prepared to make an effort looking, plus a bit more effort refurbishing.  A buddy of mine recently bought a South Bend of 4.5″ centre height x 33″ between centres for only R1500.  It needs some work but is in decent enough condition that it will be nicely serviceable when the work is done.  The work is within the ability of a hobbyist.  That is also quite big for a hobbyist, long enough to profile and chamber rifle barrels.There seems to be a fixation with high speed, but it is less important than low speed.  My little Unimat 3 runs up to 4000RPM.  I have never needed more than 1500 even for polishing.  1500RPM is useful for getting a good turned finish on firing pin noses which are about 2mm diameter.  Few things in gun work are smaller than that.  My old Myford has three direct speeds, 200, 350 and 640RPM.  It has three indirect speeds all slower than that, the slowest being 35RPM.  I seldom need more than 350 even for pins, screws, action screws, and lots of things around 5 or 6MM diameter.  The highest speed of 640 is slow for firing pins but enough for most anything else.  I use 200 RPM more than any other.   The low speeds are needed for large diameter work.I’m planning to make a little roller bearing headstock for mounting on the bed ahead of the main headstock.  It will be driven by the main headstock spindle but a pulley arrangement will allow up to 2000RPM for small parts.  It will use the existing saddle and tailstock.  It will be a cheap and easy way to get the high speeds needed occasionally for very thin work.I use free cutting steel for most of my work.  It has slightly more lead and sulphur and machines faster and to a better finish than ordinary mild steel.  It is also kinder to the lathe.  My lathe must see me through my time, so I must care for it, and free cutting steel reduces the load on the lathe and thus the wear and tear.  Don’t use construction reinforcing steel.  It is mostly high tensile ( heat treated ).  It is hard and tough, is difficult to machine and to get a good finish.  I wouldn’t touch it.  The apparently high cost of free cutting is nothing when looked at in terms of the cost per component or tool made.  6mm Costs about R5 per metre and 20mm about R30 per metre.If you find a lathe in respectable condition without a chuck or with a clapped out three jaw chuck don’t worry about it.  Buy a new four jaw chuck and use it for everything.  Three jaw chucks are nothing more than convenience tools.  Four jaw is hugely more versatile and permits precision work that can’t be approached by a three jaw.  Good Taiwanese four jaw chucks up to 150mm were available for about R1000 last time I looked.  It will mean you’ll have to make a backplate or get one made, but that’s neither difficult nor costly, and backplate mounting is the most accurate anyway.Lathes must be set up carefully, or they won’t turn parallel work.  That’s because the beds twist under their own weight, so the feet must be shimmed to get them level.  Mine turned way out of parallel before I set it up.  Can’t remember how much but it was a lot, somewhere around three thou in three inches ( 75 microns in 75mm ).  That would have been five or six times as much over it’s full turning length.  It isn’t noticeable over short lengths, but sooner or later you’ll need to turn something long and precise.  Longest thing I’ve made so far is a lapping rod for scope rings.  It’s 280mm long because that was the length of a piece of free cutting I had.  It tapers one half of one thousandth ( 12.5 microns ) from end to centre.  That’s diameter, so the lathe is out 6.25 microns in 280mm or 2.5 microns in 100mm.  I suppose some lathes are more accurate than that, but it’s more accurate than any gun work I can think of.  It took all day adjusting and testing to get it that good.Stand a lathe on a steel stand not a wood bench.  Wood benches move with the weather so you’ll be continually adjusting it.  Steel cabinet stands are not too hard to make.  Best is probably to get a sheet metal company to make one.  Don’t think it would be outrageously expensive.  Metal is not too thick, haven’t measured mine, but probably only one millimetre or so.  The drip tray should be thicker, 2.5 should be enough.  Then you need two mounting blocks, bright drawn would be OK, for mounting the lathe feet.  The feet and blocks should be bolted through the tray, and the whole stand bolted to the floor.  This makes excellent stability, so once levelled the lathe will stay level.[Originally posted to SATalkGuns -- Admin]

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