Lathe source and resolution of taper ?

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dansawyer

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The lathe is a 1957 Clausing Colchester 13x36. The project is a 1/2 inch by 4 inch 'extension' to a worm gear. The first step is to create a cylinder to use as the blank for the part. I machined the cylinder blank and was surprised to find it had 5 thou taper. To check I placed a measured piece of drill rod in the lathe via a 5C collet and the tail stock. I then ran a dial indication along the horizontal axis from the head to the tail stock and measured both the rotaional and runout and the runout from moving the carrage. This showed about 5 tenths runout. Yet when I machined the part with the same setup there was 5 thou taper.
What is wrong with the measuring the technique?
If the technique is valid what is the source of the taper?
 
Dull toolbit? Slipping compound? .005" ain't much. Stick a mag base with indicator on the carriage, and put the indicator tip on one corner of your toolholder and zero it out. Repeat the cut to the same depth, with the same toolbit, and the same feed speed. Watch the indicator while it's cutting. See if there's movement. If there's no movement, at least you know it ain't that.
 
Did you check tail stock alignment?
 
Are you between centers? You could be deflecting due to wear anywhere along the way. I would look up "Rollie's Dad's Method" on this site for a starting point.
 
Aukai said:
Did you check tail stock alignment?
Click to expand...
Yes.
Also, the test piece is round as measured by a M. mic. The piece is centered in the spindle as measured by the dial indicator on the saddle. The test piece has a center hole, it to is centered on the tail stock as measured by the dial indicator on the saddle. I can find no runout beyond a couple of tenths in the machine alignment.
 
Sounds like wear in the bed. As you run the dial along the side of the test bar from tail stock to head stock try pulling and pushing on your compound and looking for movement. The dial will have no pressure and may not move on its own. When cutting the machining operation will be trying to push your tool away and towards any wear if present.

Edit: as you pull back and forth you should get some dial movement based on gib setting etc. it should be consistent if no wear and vary where there is wear.
 
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One of the advantages to learning on an old lathe is that we find these sorts of things, and learn how to compensate for them. My lathe is absolutely no exception, it takes more kinks than a New Year's swinger party to get a good part out of it- but I can get good parts out of it, because I've learned them.

I would encourage you to look at the squareness of the headstock by aligning solid centers. There also may be play in your headstock bearings that allows deflection away from the cutting tool when under load, but not necessarily at rest. It'a not common for a head axis to nod, but it is common to deflect. It's also easy to move a tailstock from alignment, or to find play in the tailstock quill. All worth checking to get you back on your way.
 
All the suggestions have been helpful. I can find no wear pattern other measurement that would account for the taper. The spindle turns true and has runout in the 2 tenths range. The apron travel on the ways measures parallel to the test bar, both horozontally and vertically.
 
It could also be bed twist, that was the problem with my lathe (Also a Clausing Colchester). I bolted it to the floor and shimmed the legs until it turned true.
 
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