[Newbie] Lathe Bed Wear ?

[Mike Lutes]

Hi
I'm new here and I've got a question pertaining to setting up and evaluating my new to me late 40s vintage SB 14.5x36"

I've levelled it and turned a 24" x2" CR test piece. I turned it with a dog between centres after grabbing and truing a short straight shank centre in the 4 jaw.
Light cuts and adjusted the tail stock until it was cutting the same size on each end.
It cuts true for the first 16" from the face of the Chuck then tapers to .005" undersized in the next 5 inches and back up to full size in the next 3".

My question is why ? I thought the wear on the bed might make it drop the tool and therefore make the piece oversize but the opposite appears to be happening.

Any thoughts ?
Thanks
Mike

 

[4GSR]

One thing I've seen on SBL's is if it is equipped with a taper attachment, that extra weight hanging off the back side will cause more wear on the back ways verses the front ways. Of course, you have just about the same amount hanging off the front on the larger SBL's too. I wonder if the rear hold down is too tight causing the back side of the carriage/saddle to bear hard on the ways? I wonder if the cutting pattern would change if you shorten to a 12" piece instead of a 24" piece?

 

[Mike Lutes]

Hi
That makes sense to me
It does have a taper attachment. You are likely onto something with the back side hold down because I had tightened it slightly.
I'll check it out when I get back out to the shop, likely after Christmas.
I'll also try turning a 1' bar if playing with the backside hold down doesn't help.
Mike

 

[epanzella]

Did you check the alignment of your headstock before you started? If your headstock is out of alignment due to a bed twist and you adjust the TS to counteract it, the bed is still twisted. You'll end up with the turn good in some places along the bed and bad elsewhere.

 

[4GSR]

I highly doubt there is an problem with headstock alignment being an SBL. Unless someone has tampered with the alignment over the years.

 

[epanzella]

I highly doubt there is an problem with headstock alignment being an SBL. Unless someone has tampered with the alignment over the years.

Bed twist will affect headstock alignment with the bed. This should be sorted out before moving the tailstock. A piece unsupported by the tailstock at least 1.5 inches in diameter and 8 inches long should be turned lightly to verify a consistent diameter. There is info on the web on how to correct a problem here by adjusting the lathe feet.

 

[4GSR]

Bed twist will affect headstock alignment with the bed. This should be sorted out before moving the tailstock. A piece unsupported by the tailstock at least 1.5 inches in diameter and 8 inches long should be turned lightly to verify a consistent diameter. There is info on the web on how to correct a problem here by adjusting the lathe feet.

Not with the cutting pattern he's getting. Nothing to do with bed twist. If you re-read his first post, he leveled the lathe, which would removed any twist or would have indicated twist in the leveling process. Apparently, there was none. In short bed lathes like a SBL, they are less likely to twist as would be seen in a longer bed lathe.

 

[epanzella]

According to the original post, he never checked for taper when turning an unsupported piece in the headstock. This should be step #1.

 

[Mike Lutes]

I got a chance tonight to play a bit and I found by slightly loosening the rear carrage hold down it now cuts easily within .002" for the length of the 2' piece ( it's about .001 large in the middle ). Good point about turning a piece unsupported and that will now be my next step in evaluating and tuning this old girl for possible service doing mainly gunsmith work.
The half nuts look ok as far as I can see without dissambly and the thread dial appears in good condition so I expect it will thread ok but time will tell.

Thanks for the help
Best of the Season
Mike

 

[4GSR]

According to the original post, he never checked for taper when turning an unsupported piece in the headstock. This should be step #1.

Problem solved. Regardless, the problem was not with the headstock. I've been doing this for over 40 years and learned lots over the years with alignment of the lathe and other machine tools. Not saying I'm an expert, but very knowledgable with alignment issues when they do come up.