Amateur Machinists' Tools, Techniques and Materials

Archive for August, 2011

The 30 Degree Thing.

Posted by hammerscale on August 22, 2011

American standard threads are cut with 30 degree angles on both sides, (adding up to 60 degrees total)

The first, and most obvious approach, is grind a piece of HSS tooling to a 60 degree angle and plow directly into the workpiece with the cutting tool perpendicular to the axis of rotation.

Cross feed example

Cross feed example

This uses the cross feed alone to advance and retract the cutter. The position of the compound is not relevant in this operation as it is not moved during threading.

This works, and works pretty well, but also has two negatives to consider.  First, it cuts on both sides of the tool piece.  This can lead to a rougher cut.  Second, the compound, and the advantages it offers, will not be used in this instance.

The second approach.  Mount the same HSS tooling with the compound set for 30 degrees off that perpendicular line.  The cutter again is perpendicular to the axis of rotation, but will be traveling in at an angle of 30 degrees. Advance the cross feed handle until it just touches the workpiece.  Set the cross feed micrometer to zero.  Now move the cutter into the workpiece with the compound handle.  By backing out of the thread using the cross feed, you can always return to exactly where you were referencing the zero mark on the dial.
(On my particular lathe there is a built in stop you can use for quick return to the zero point on the cross feed.)

Compound example

Compound example

Setting to 29 or 29.5 degrees provides just a little clearance on the back side of the cutting edge, theoretically creating a cleaner cut.

The compound provides for a bit more control and finer adjustment for tooling advance.  However, since you’re going in at 30 degrees the micrometer reading is skewed (precise, but not accurate)

Also, many smaller lathes, especially older models, have compounds that are not really compatible with modern QC tool posts and you may not have enough room to rotate to 30 degrees.


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Lady Chatter-ley

Posted by hammerscale on August 18, 2011

Nobody loves chatter. Chatter concerns a number of elements at the lathe. Work piece flex, tooling flex, tool dullness, or just plain bad luck. Most important, however, is torque. Torque, or lack of it, at the spindle will explain many chatter problems. This is an issue of an irresistible force meeting an immovable mass. That force is the rotating spindle and that mass is the piece of tooling. This is the ideal; that the force moves smoothly and uninterrupted when it contacts the stationary mass. What occurs with chatter is not the ideal, the spindle is being stalled momentarily as the tooling is pressed into it. This stalling may be a microsecond as the spindle slows a bit then returns to full speed. You can sometimes actually feel/see/hear this as you feed the cutter into the work.

Chatter occurs most often with broad radial cuts in stock. Think of the worst case scenario; a large HSS cutter brought into contact with a relatively hard material on a small lathe, maybe a ½ hp Atlas, for example. The cut will chatter, squeal, and you may even stall the lathe. The finish on the material will show this. Change the cutter to a small cutoff blade, chatter will be reduced or go away, change the material to soft brass, again, less resistance, finally, change the lathe to a 5 hp 3000 lb monster and once more the problem goes away. The diameter of the workpiece also plays a role. The further from the axis the cutter contacts the workpiece, the more mechanical advantage it has over the lathe motor.

To increase torque, run the lathe in backgear and/or reduce the speed. Reducing speed with a VFD won’t increase torque, however, you have to reduce speed mechanically. Finally, if attempting a deep broad cut on small lathe, you simple may be asking the machine to do too much.

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VFD Caution

Posted by hammerscale on August 17, 2011

A recent thread in Home Shop Machinist BBS tempers the blessings of a VFD with a caution about using it to excess…

I’m replacing the Timken roller bearings on my Rockwell 11 lathe as it’s never been right since I oversped it after fitting a vfd to it.

Considering my Mitsubishi VFD goes up to 400 hertz, the temptation is real, and pulling/replacing bearings in the lathe is something we all might want to avoid unless we absolutely have to.

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Lathe maintenance

Posted by hammerscale on August 15, 2011

This past weekend I finally got to replacing the vari-speed belts on my lathe. I had previously replaced one, a couple years ago when an original belt broke, but put off replacing the other until now.

The factory belts must have been well over 40 years old, so you can imagine it runs alot quieter now. The original parts only have catalog part numbers, no industry standard measurements, plus they are the older/much harder to find 16/16th’s (1 inch wide) vari-speed belts. I think Gates still makes them, but I haven’t found any from the usual suppliers, so instead I used standard size 19/16’s belts slashed down to 1 inch width with a utility knife.

The following are the sizes I used, but double check yours;

1922V338 Upper left belt

1922V298 Bottom right belt

Vari-speed belts Rockwell 11"

Vari-speed belts Rockwell 11"

It ran so quiet that I could now be fully irritated by the click-clunk of the dogs in the headstock. I packed some grease in next to the left of the spindle gear where it meets the clutch. Makes a difference.

Headstock Rockwell 11"

Headstock Rockwell 11"

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