spiral taps

[Dunc1]

I needed to tap several holes 10x32. A neighbor said that the easy way was to use a spiral (gun?) tap. He brought one of his to my workshop & proceeded to show me their merit.

Anyway, got me thinking. I now have a good selection of taps: some high carbon (labelled tungsten) and some high speed steel.

I looked online at one of the suppliers catalogs and found a wide selection. What gave me pause tho was the "limit" designation: most commonly, H1 or H2 and infrequently, H3. I have a vague idea that this somehow involves the tolerance of the finished thread.

For all-round, general tapping (mild steel, brass, aluminum), which one should I order? And, to satisfy my curiosity, on what occasion(s) would I want to use the other limits?

Thanks

 

[bret4]

[FONT=Arial, Helvetica, sans-serif][FONT=Arial, Helvetica, sans-serif]For a all around tap I would go with Plug taps in a H3 size. This is a really common type of tap that works in many different materials.

A Spiral point tap pushes metal chips ahead of the threads and is good for through holes. In blind holes it could tend to break easy because of the chips building up in the hole in front of the tap.

Bottom taps are good for finishing a thread in a blind hole. I tend to like using a bottom tap after having run a plug tap in first. Bottom taps are harder to start because they do not have much lead ground on the first threads. This lets you screw a screw in close to the bottom of the hole.

[/FONT][/FONT]

 

[kvom]

Here's a good reference on class of fit and H limits fcor taps: http://www.cutting-tool-supply.com/TechTips/Tapping/HLimitsClass/HLimitsClass.htm