Metric sizing on an imperial lathe

I have been turning some metric sized parts on an imperial lathe (S7) with the attendant difficulty in converting imperial to metric when putting on the feed.

In a Eureka moment I thought it may be possible to angle the top slide such that when I put on say 0.1inch feed (one turn of the handle on the top slide) it would reduce the work dia by 1mm. (0.5mm infeed) Some trigonometry showed that if I angled the top slide by 11.35 degrees from parallel the desired result should be achieved, and it was!

For those like me without digital readout this may be a help.

Alan

Correction.

I think I should have used the term "cut" rather than "feed"

Alan

For roughing out I use 40 thou per mm (20 thou on a direct reading S7 dial) to get close - 1mm is 0.03937". When close to size, it is easy enough to do the calculations for the final finishing cut.

My FB2 clone milling machine is a composite of an imperial base with a metric column, so I am always having to change one set of dials from "native" dimensions redardless !

Nigel B

Easiest way is to set the size on your digital callipers, lets say you want 20mm so zero when closed, open up until they read 20.00mm then zero again. Press the inch/mm button and then measure the part which will tell you how many thou to come off and just use your imperial handwheels.

Used this method for all the turning on the Firefly I just finished with no problem and it still lets you use the topslide to cut a measured amount which can just be punched into a calculator.

J

Hi Alan, just convert each metric dimension to imperial, it's much the simplest method.

Tony

A great help with depth of cut calculations et al is a cheap magnetic whiteboard mounted on the wall at one end of the lathe. Mine is the "bit bigger than A3" version from Staples. Fitted at the tail stock end. Stick the drawing up with the magnets and scribble on the rest e.g. size now - size to finish gives twice cut to be taken. Where practical I set the dials so the finish cut is on zero, alternatively I list the various finish cut readings in a different colour to that used for general calculations. (Its hard to be a whole turn out.) Technique works well with my 6 position turret bed stop.

Quick'n dirty jobs get drawn direct on the board.

Clive

With one evenings work all these issues disappear forever with the simplest most useful mod you will ever make to your old lathe

Fit a $10 digital caliper to the cross slide

It also eliminates measurement backlash issues, good to around 100th of a mm, simply press a button to do metric or imperial

Edited By Ady1 on 25/08/2012 07:48:06

Turning metric with imperial dials is really quite straight forward for everyday applications. Treat 0.040'' as 1.00mm. so by taking a one thou cut on the dials (two thou on the diameter) you should remove 0.025mm. So a 0.010'' cut will equal 0.25mm. 0.020'' = 0.5mm 0.030'' = 0.75mm. and so. I know it isn't correct to four place of decimals, but its near enough to get a turned diameter to size.

Alan,

Several "alternatives" have been offered, but no-one has yet thanked you for your original suggestion.

Well done, Sir ... it's simple and effective; and that is true Engineering.

MichaelG.

Alan's suggestion was indeed ingenious, but it does have one drawback. If you are turning to a shoulder, it means the end position of the cut (as measured by the leadscrew handwheel, the bed stop, or the DRO, whichever method you use) will change at each move of the topslide. Not insuperable, you should be stopping a few thou (sorry, a hundred microns or so!) short, by eye, and then finishing the shoulder face later; it's just something else to have to think about when doing a repetitive cut.

Personally I'll stick to the 40 thou per mm until there's a gnats todger left, then by measurement. The table inside the front cover of the Zeuss booklet is good for this.

David

Edited By David Littlewood on 25/08/2012 13:31:04

I think that the 40 thou per millimetre approximation is good enough at all levels. Let us assume that the gnat is unusually large, and we need to make a final reduction of 0.2mm on the diameter. If we set 8 thou on the cross slide dial, we'll be 0.0032mm undersize on the diameter. The 1 thou divisions on my lathe cross slide dial are about 40 thou apart; so I doubt I could set it more accurately, at 7.874 thou, anyway.

Regards,

Andrew