View Full Version : backlash in a Logan lathe
09-14-2001, 12:44 PM
I have recently acquired a Logan 10'' lathe,
but have noticed excessive backlash in crossfeed travel.I am new to metalworking,
so any help would appreciated .
There is ALWAYS backlash, unless you have special ball screws and nuts (as on CNC machines). It's just that some machines have more than others, but with conventional feed screws you always have to take it into account.
Basically, what you have to do it make sure you always read the graduations while turning the screw in the same direction -- generally in the direction of feed. That's pretty automatic with a lathe, not necessarily so with a milling machine. If so, you'll always "take out the backlash" before reading the graduations and you'll get consistent results.
The backlash doesn't necessarily hurt accuracy all that much if the wear on the screw is reasonably uniform. You may have to turn the handle half a turn before the cross slide begins to move, but once you've taken out the backlash, as long as you keep turning in the same direction the relative measurements you get off the dial graduations will be reasonably correct. So don't bee *too* concerned from an accuracy point of view.
At some point, of course, it just becomes a pain to have so much slop in the screw, and you may want to think about replacing the screw(s) and nut(s). But even if you put in pristine new screws and nuts, you'll still have backash to take into account; maybe only 3 or 4 thou, but it will be there, and you'll still have to make sure you always read the dials while turning the handles in the same direction. (Either direction is fine; it's just that once you start in a given direction and establish a reference point, you have to keep going in that direction for all readings based on that reference.)
Is that at all clear?
09-15-2001, 12:54 AM
clear as glass. just have never been on a lathe , or any other metal
machine tool before. thought i may have a lemon. i am just starting out , trying to teach myself the basic function of each cutter tool. I am buying hss bits, would it
benefit me to study up on grinding my own?,
or since i only spend a few hours a weekend stick to buying them?
09-15-2001, 01:36 AM
Any machine shop you visit will most likely tell you they still use HSS a great deal. They are great when you want to make a special profile - such as the end of ball handles.
Brazed Carbide tools are reasonable and can also be ground for special profiles but takes SiC, or Diamond wheels to do so.
I prefer insert tooling only because I get superior finishes. You machine needs to be fairly ridgid to take full advantage of their performance potentials.
If you already do woodworking, you way want to buy Leonard Lee's Tool Sharpening book or tape from Lee Valley Tools. It will teach you how to put a good edge on HSS tools of any type. Clearance angles are different for metal work and you can get the pertinent data from "Machinery's Handbook"
Make sure safety is your first concern with your machines: tie your hair back (if you still have some), short sleeve shirts, no rings, watches, pendants, or anything that could wrap around the work or chuck. All that stuff your three fingered Shop teacher tried to tell you in school! Protective eyeware is essential - prescription glasses are NOT safety glasses.
Enjoy a great hobby
09-15-2001, 03:55 AM
Welcome, a lot of people are overly concerned about cross slide leadscrew backlash. If it is less than 1/2 turn don't worry about it, I can hear people shaking their heads in disbelief right now, seriouly backlash doesn't hurt until the nut lets go, usually more play in the thrust bearing than in nut.
If you want to check it, it is located opposite of dial end on this machine, it is easy to adjust, but don't get it too tight. Things must move freely.
Thrud ad SWG have already given some good advice, I'll add some on tool bits. There are books with pictures on grinding tool bits, I think I might have ground one like one of them there pictures one time. Pictures are worth a thousand words but look at some toolbits that someone else who knew what they were doing ground, worth a thousand pictures? Some one on one training is worth a lot, find an old machinist whom has used high speed tooling to show you.
That Logan is a good machine, I have one, is handy as a pocket on a shirt. Have some larger machines but I think I use the little Logan the most.
Work safe, and keep your ways clean and well lubed.
You've got to learn to grind your own HSS toolbits. Halfnut is right on when he suggests finding somebody to show you. You can also study some commercial toolbits and copy them and read descriptions, but there is nothing like half an hour with somebody explaining it.
If you can find the book "Design and Use of Cutting Tools," by Leo J. St.Clair, it will give you a LOT of information. It's 300+ pages about, essentially, lathe toolbits. It is, unfortunately, out of print, but you may be able to find a used copy somewhere.
Get yourself a protractor with a pivoting arm so you can measure the tool bit angles. St.Clair found that even machinists with long experience couldn't reliably estimate tool bit angles closer than several degrees, and the angles matter. If you get the angles wrong, either the tool won't cut very well or the edge will break down far sooner than it should and you'll have to do a lot more sharpening than you would if the angles were correct.
As far as grinding wheels: I like using "green grit" silicon carbide griding wheels for my HSS griding, even though they're intended for grinding carbide. I find they run cooler and cut better. They also break down and wear away pretty fast, but for the amount of grinding I do that's unimportant.
09-15-2001, 11:42 PM
Halfnut, thanks for the insight on the takeup
helped tremendously.I know a friend who has
been around a machine shop some in his past.
I'll have to kindle his memory , but maybe
it will pay off enough to get me pointed the
This BBS is the stuff!Now when I;m in the dark, at last I can find the light switch.
THANKS for the info.