Lathe bed badly worn

I have a 70 year old Harrison Cub lathe with a badly worn bed. The bed is prismatic (I think that what its called), two flat shears like a Myford with a triangular rib along the front edge of both flats. This is so the saddle runs along the front angled section, the tailstock on the rear.

The main wear is under the rear shear, where the clamping strip which runs under the shear is steel and does not cover the whole width, so the rearmost half is badly worn ~ .01" for half its width which tapers out to about half way down the bed , because the saddle is not used much down this end.

The problem is two fold, in there is a lot of cast iron to be removed ( too much for grinding?) and the lack of a good reference surface, because the top of the shears near the chuck will be worn on the top. So the thickness of the rear shear near the chuck is .015 less then at the tailstock end.

To ameliorate the situation I have re-engineered the rear jib to make it out of brass and deep enough to bear on the original thick part of the shear.

There is a similar problem at the front, but it is made worse because there is no surface to measure from other then the top of the triangular rib. But in general there is the same wear, because if the gib is tightened under the front shear, then trying to move the saddle to the right causes it to bind.

Any one got any ideas, I suspect to have it machined true would cost hundreds of pounds (and a lot of aggravation!), so a total replacement would seem to be the most logical course of action?

Frank

My first lathe was an old Grayson 3.5". It came to me pretty well worn but I was in the fortunate position at the time where for the price of a couple of pints I got the bed skimmed true. My mate did it with a fly cutter if I remember right. The point I am trying to make is that grinding is only necessary when the bed is hardened. My old lathes bed was just very nice free cutting cast iron - unhardened.

I realise that my experiences with an old lathe will not help you so I'd like to be MORE helpful. You write "so a total replacement would seem to be the most logical course of action?"

Sadly, I have to agree with you - enjoy your new lathe Frank.

Rik

Frank I bought a 1956 Chipmaster and when I got it in my workshop (with great difficulty) I found it had the same conditions you described. So I took a series of measurements along the be using the dismantled tailstock base as a carrier with a dial guage measuring various points at say 1" intervals along the bed. I then made up some sanding blocks by gluing a belt sander belt cut in strips to pieces of wood and literally sanded the bed where it was not worn to get it down to the worn level. This may sound like a crude break it or fix it solution but it worked well. The tailstock base was also worn lower at the front so I shimmed it at the base so that it was parallel it both directions which then made it too high I think. This was solved by making a new eccentric Morse taper 3 external Morse taper 2 internal adaptor which I rotated to get at centre height and the tailstock can then be adjusted to get on centre at the base as normal. I wanted a Morse No2 tailstock anyway. The final result was a lathe which can turn over 20" length to within .0005". Good enough for me. People have told me that Chipmasters had hardened beds but if mine has it did not stop the the process. In a way I had no other choice but it worked out very well. I will add some photos to my other pics.

Alan

Edited By Alan Jackson on 27/11/2014 18:42:12

The Chippies are induction hardened like all Colchesters and Harrisons of late. Sanding hard steels isn't a problem they all wear.

Familiar with the bed take my advice scrap it, looking £800 for a quick course regrind job that wont last, strip down, assembly, re setup etc. Then have the job of making new shims throughout since originals will be way under. Scrapped my 140 4 years ago that wore on the V visually. To rectify if take off one edge you have to take off 7 other edges, its not just a case of skimming over the top of the bed.

Had an ML7 that I took in for a regrind 22 years ago, going rate 10 years prior was £1000 done properly or around £300 pulling a fastun. Milled whilst I waited as a foreigner, gibs ground lasted 6 months tops with constant future scraping, filing and sanding to keep usable.