Lathe cutting tools

Posted by on September 21, 2008

It should be obvious that good work can’t be expected of a lathe whose bearings or slideways are well worn, or that is not set up straight in all respects. “Straight” means particularly no twist in the bed and tailstock dead on centre. Once those factors are taken care of the other vital factor is properly ground and sharpened tools. A blunt or improperly ground or sharpened tool cannot cut smoothly and accurately. I mention this because of the boast I hear from tool makers and fitters and turners that they are able to grind tools accurately by hand. These are the guys that claim to be able to grind drill bits by hand. Well, it is not impossible to grind a drill bit by hand sufficiently to make it cut. Making it cut accurately and without wandering is another matter. Fact is, it is not possible to grind the angle exactly the same on both sides of the point or get the point dead to centre by hand and anyone who says he can is lying.Lathe bits can also be ground by hand sufficiently to cut, but having tried it often I can tell you that each time you present the bit to the grinder the angle will be slightly different and you end up with a dozen different facets on the ground surface. Why anyone would want to do that when you can get it perfect with a simple jig beats me. Note that while you can get by hand grinding most tools the exception is threading tools. There are various thread included angles. BA is 47.5 deg, Whitworth 55, metric and US 60. If the tool is not accurately ground the thread it cuts will not be a proper fit to the female. Next time somebody boasts that he can grind 60 deg exactly ask him to draw a 60 deg angle on a sheet of paper freehand. The guy has yet to be born who can look at an angle whether on a sheet of paper or a lathe cutting tool and tell you for sure whether it is 59, 60 or 61 degrees.I have just ground a 60 degree threading tool with a simple jig I made. I can’t guarantee that it is dead on with zero error if measured in a lab, but I am satisfied that is within a quarter degree each way because of the way I set it up. How you set things up is the secret of accurate work. For example, use templates. When I set the compound slide to cut a metric thread I set it with a template to feed 29.50 degrees in accordance with standard thread cutting practice. But I also set the tool in the toolpost to exactly 90 degrees to centre line with a template or engineer’s square, having first ensured that the point has been ground exactly 30 degrees each side not 29 and 31.None of this is rocket science, in fact it couldn’t be simpler. Most amateur lathe users are like me, not into complex work like model steam locomotives. Most of us need to do simpler things like tools for gunsmithing or something of the sort, but still need to do accurate work. Why accept inaccurately ground tools out of sheer bloodymindedness when you can make them accurate with simple methods ?Another simple thing to fix is those awful English and American tool posts that come with most lathes, but I’ll make that the topic of another posting.

via email :Every time I try to reply it cuts me off. haha. At any rate here is the source for finding the book by Cleeve._http://www.bookfinder.com/search/?ac=sl&st=sl&qi=7Gk5T.k80bwS,4aAcXbPqsdAmsk_4812493540_1:38:1546&bq=author%3Dmartin%2520cleeve%26title%3Dscrew%2Dcutting%2520in%2520the%2520lathe%2520workshop%2520practice_(http://www.bookfinder.com/search/?ac=sl&st=sl&qi=7Gk5T.k80bwS,4aAcXbPqsdAmsk_4812493540_1:38:1546&bq=author=martin%20cleeve&title=screw-cutting%20in%20the%20lathe%20workshop%20practice)I just ordered mine and it is on the way.Humpy

You found one quickly, eh ! It is a British publication, one of the “Workshop Practice” series of books by Argus Books. It is detailed in the extreme and one could almost call it academic. Much of the content will not be of use to the average hobbyist. Cleeve earned his living making parts including lots of screws. It was making special screws in large batches that forced him to develop sophisticated techniques to speed and simplify the work. One of those was an automatic clutch that disengaged the lead screw at a predetermined position. That allowed cutting threads at 500RPM without running the tool into the shoulder. That is of no value to most of us but it saved him days of work when making a batch of two or three hundred screws six inches long. The real value of his book to we hobbyists is the detailed explanation of exactly how to best cut threads, an explanation I have seen nowhere else.Perhaps the most interesting point is his condemnation of gearboxes. He says there are simpler ways of building rapid feed and threading settings into a lathe without the limitations imposed by the gearbox. Gearboxes are great, seductively so, for quick feed changes, as anyone who has battled with change wheels will know. But they actually limit the variety of threads that can be cut. Furthermore most (but not all) will cut threads only in the language of the leadscrew, ie either inch or metric but not both unless an expensive translation gear set is purchased. A simple change wheel set up if properly designed permits almost unlimited choice. My old Myford will cut every imperial thread pitch plus all the BA pitches, and practically every metric pitch to an error seldom worse than 1 in 1000 and mostly a lot better. For all practical purposes I can cut any thread.Anyhow, it is a fabulous little book and essential reading for anyone who wants to cut threads properly. The other books in the series are all pretty good. I will list them if anyone is interested. One that is particularly good is “Drills Taps & Dies” by Tubal Cain. His “Work Holding in the Lathe” and Hardening Tempering & Heat Treatment” are also well worth having.

Comments

Respond | Trackback

  1. patrickr November 6, 2008 8:06 pm

    Hello there Richard Boothroyd,I presume you live in South Africa? Anyway, I have recently inherited my late grandfathers’ Unimat 3. It is a great little tool, except for one thing. Whenever I plug it in, and touch any metal part of the lathe, it shocks the living #$%^ out of me. Do you know where I can have the motor checked out, and where can I but accessories for my Unimat in South Africa?Kind Regards,Patrick Rimbault

  2. Richard July 19, 2011 4:31 pm

    Was inactive for some time because of my lack of IT literacy. There is an explanatory posting in the last few days. It will be active in future. How active will depend on whether it interests people or not.

Comments

Comments: