Lathe power feed causing taper.

[JeffinNZ]

Hi team.

My mini lathe has a quirk. Was having fits yesterday trying to figure out why I was getting a .010 taper over 3 inches. Turned off the power feed and turned by hand and the taper went away. Any ideas?

[HollowPoint]

Back when I had a mini-lathe I used to get tapered cuts also.

On my lathe it turned out to be the lack of rigidity on my tool post. As the cutter made contact
with my part it would flex inward toward the part. By the time it got to the end of the cut, it had
flexed inward enough so that my part was larger at the beginning of the cut than it was at the end.

I also experienced a similar problem when I used the compound slide to make my cuts. In those instances
it was because the Gibs had loosened up on me; which had the same effect as the lack of rigidity on my tool post.

If you haven't already tried; try taking smaller bites. In leu of that, it may be worth while to invest in an after
market Cross-Slide Plate to add rigidity to the tool post.

HollowPoint

[oldred]

I think Hollowpoint has the problem figured out there and since it tapers under power but not when fed manually I am willing to bet you are feeding slower when doing it manually. This would create a lighter loading on the cutter point thus less flex would result.

[customcutter]

You didn't mention what diameter or material you were cutting either. Definitely sounds like the feed rate is involved and either causing deflection of the material or the tool post. Is it possible to set up with the tailstock and live center?

CC

[JeffinNZ]

I was turning mild steel of approx. .280 and taking .003-.004 at the time. Material was held on a live center.

[gzig5]

The drive for the power feed on the mini lathe comes off the lead screw and when fed by hand the drive is off the handle to the rack gear. There are several inches between the rack and the lead screw and I'm betting that that extra leverage coming off the lead screw is tilting your carriage a bit. Feed rate probably has something to do with it too. Try tightening the gib between the carriage and bed, if there is one. You have to remember, these little lathes are not that rigid and not built to very tight tolerances.

[jmorris]

I was turning mild steel of approx. .280 and taking .003-.004 at the time. Material was held on a live center.


Sounds to me like your tail stock is not centered. You can't get .010 taper making .003 cuts

[nhrifle]

My mini lathe has been a project to get accurate cuts, but it now cuts as well as I could ask from such a small machine. Several problems could be causing your issue.

First, check to make sure your bed is level in both the X and Y axes. Use an actual machinist level, not a handyman special. This has been known to cause turned parts to be tapered (probably not your problem but worth checking).

Next, align the pillow blocks. Run the carriage to the extreme end of travel, loosen the pillow block bolts, engage the half nut, and tighten the bolts. Repeat for the other side.

Now align the apron. Run the carriage to the center of travel. Loosen the bolts attaching the apron to the carriage about a turn. Engage the half nut and evenly tighten the bolts.

Eliminate as much play as possible in the carriage and cross slide. May as well lap the gib strips while you are doing this and give them a light lube.

Check the alignment of the headstock. Clamp a straight rod in the chuck and mount a dial indicator to the cross slide. Run the indicator down the rod, beginning at the chuck. If misalignment is indicated, shim the headstock as necessary.

Check the alignment of the tailstock. I use two small rods with ends turned to a point for this. Put one in the headstock chuck and one in the tailstock chuck. Lock the tailstock near the headstock and bring the two points together. If they do not meet, the tailstock is a two piece casting and can be shimmed to allow alignment.

These little lathes are fun to work with and a great learning experience. Detailed and illustrated instructions for the above repairs are available online. Hope this helps.

[JeffinNZ]

Ladies and gentlemen; we have a winner. jmorris hit the nail on the head. Tail stock out of alignment. Now cutting parallel.

[Char-Gar]

When I set up my 9 X 18 Logan and turned a piece of steel 12" long between centers I was getting a .010 taper also. I nudged the tailstock over a hair and I could cut the same 12 inch piece of steel with only a .0005 difference it's entire length. That is a good as I can expect from this machine. It has held that adjustment now for 5 years.

I fretted about the level of the lathe because I didn't have a good quality level. Some experienced machinists told me that on a short lathe bed like mine, level was not all that critical as it was ridged. With longer beds (over 28") they can get wonky out of level.

Do keep an eye on the gibs....

[oldred]

Ladies and gentlemen; we have a winner. jmorris hit the nail on the head. Tail stock out of alignment. Now cutting parallel.




If the tail stock had been the only problem the taper would have been present when fed by either hand or power feed! If the tail stock is off center then it makes absolutely NO difference how the feed is powered.

[W.R.Buchanan]

Morris beat me to it.

If you really want your tailstock centered up put an indicator in the chuck and sweep all around the center in the tailstock until it reads 0,0,0,0.

make sure to look at it up and down as well since the tailstock may need to be shimmed for elevation.

Randy

[oldred]

I think we all thought tail stock alignment as a first thought but he said the taper went away when he used manual feed and only happened under power feed, that simply does not sound like a tail stock alignment problem. If the tail stock is out of alignment the taper should be present regardless of method of feed.

[JeffinNZ]

Either way, adjusting the tail stock fixed the problem. Maybe the manual feed was not a good as I thought.

[oldred]

Either way, adjusting the tail stock fixed the problem.


And that's the important part! Adjusting the tail stock is a normal part of operating a lathe, just glad to hear it was something as simple as that and not a warped bed or something else extremely hard to fix. Some years ago I had a Jet 9x20 (The Jet version is supposed to be the top-of-the-line Chinese small lathe) that had a "bump" in one of the ways about 11" from the chuck that caused a .007 error for a couple of inches, this was a major PITA! A buddy of mine and I crudely scrapped the ways (ok we honed off the "bump" as best we could) and corrected most of the problem, I'm happy for you that it wasn't something like that.