Machine Shop Tips and advice From Members

                                  

A two part tip for making your threading life easier

 

Here is a method for threading I have been using for 30 years.  I call it the infeed for 30 degrees. After you have set your top compound over to 30 degrees for threading zero its dial and get your touch with the cross slide.  Now divide .750 by the number of threads you are cutting.  Lets say we are threading a 1/4 inch 20 thread  .750 divided by 20 equals .0375,(.750 divided by the number of threads equals Infeed at 30 degrees.) This is the depth you must cut to with the top compound for the finished thread. No need to even check for a fit until you are within .002 or .003 of this number. This works regardless of the size part you are doing a 1 inch 20 thread bolt would still have an infeed of .0375. If you do not want to remember this formula you could make of a chart of the most used thread pitches.

 

Now that we know exactly how deep we are going for any number of threads we may be doing, lets try to do it a little quicker.  Using the 1/4 20 thread example, lets germ the bulk of the job done in the first 3 or 4 cuts. As long as you have a good sharp tool, on center, and good cutting fluid, things can go very quickly. On this thread with its .0375 depth I will take cuts of .015 first pass,  .010 second pass,  .005 third pass, .0025 fourth pass, and you can see how quickly we have closed in on finishing the thread. Of course if you are threading a bolt this small of any length you should have it center drilled and supported with the tailstock.  When I worked for a large Steel Mill in a Maintenance Shop, and they wanted everything yesterday, we did many bolts in the 6 and 8 inch  diameter size with 4 threads to the inch. This requires an infeed of .187.  I would start this thread with a cut of .050 or .060 in depth. This may sound scary but it worked, besides these are the kind of things you do when someone is standing there looking over your shoulder waiting for a part.

Speeds and Feeds for the Novice   

I realize that most of us come to building small engines as a hobby, not as a race or a job, but I believe seeing our parts take shape in a timely fashion can better hold our interest in seeing the job through. From my experience through the years with novice machinist they are seldom turning or milling a part anywhere near what they should. Without getting into the Science of cutting, mainly because I am not that smart, maybe we can cut your time to do a part in half or better.

 

What I am writing here will be based on using a good sharp free cutting high-speed steel tool bit. Why high-speed steel? Why not carbide tipped or insert tools? Well take a look at your insert tool; it is not as sharp as a butter knife and it requires much more horsepower to cut efficiently, unless you are blessed with a small lathe with big horsepower you will most likely get more done with high- speed steel.

 

Most carbon steels can be cut with a high-speed steel tool bit at 100 surface feet per minute, a little less for alloy steels and considerably less for stainless steels which create a lot of heat when machined. Most brass and aluminums can be cut at 300 and 400 surface feet per minute with ease. What are surface feet?  It is the amount of steel you can pass by the tip of your cutting tool in one minute without heating it to point of failure due to heat softening. So now all we have to do is unwrap that round bar in our lathe and see how many feet are going by per revolution, you already know how to do this from school using Pi, 3.1416 X diameter gives you the distance around your piece. For ease of use in the shop I just think of a 4-inch diameter piece being close enough to 1 foot around for setting up to make a cut. This makes it a slam-dunk for figuring the spindle speed, 100rpm gives us our 100 surface feet, a 2 inch piece would be 200rpm and a 1inch piece 400rpm. Always try to start your cutting at these speeds and work downward if you must and things will go quicker.

 

Roughing feeds should be in the .015 to .020 range and depth of cut as deep as the horsepower of your machine will allow. Try taking .125 depth of cut and see what happens you will know right away if your machine is going to pull the load. My little 11-inch lathe is 2 horsepower and will handle a .250 deep cut in cold rolled steels rather easy. In closing all I am trying to say is that working from close the maximum capabilities of a tool will produce parts much quicker and leave time for more projects for the little women of the house.