Toolholding tips

After a few decades of machining in several industries, I have been afforded many opportunities to learn things the hard way. Numerous machine crashes, countless broken tools and lots of parts coming out of vises have taught me much about what not to do.

Some lessons were one- or two-minute events in which I walked away saying, “I won’t do that again.” Others took more than one iteration for the education to sink in and usually ended with me asking, “Why did I do that again?” In most instances, these were inexpensive mistakes that caused more frustration than damage. But in several cases, I really wrecked some stuff and the lessons were expensive or painful. Most teachings were learned through a series of events spread out over time, which we commonly just refer to as experience.

In time, experiences teach machinists to adopt their own general rules that are honed by successes and failures at the machine shop. Toolholding is a subject on which I have formulated opinions based on experiences that are worth sharing.

Total indicator runout of a toolholder needs to be only good enough. Buy a toolholder that does the job, but don’t overbuy. If you are drilling loose-tolerance clearance holes, a decent collet chuck often suffices. Likewise, if you need to ream holes with a 0.0005″-dia. tolerance, then a toolholder with very good TIR specs probably is warranted. Don’t let a cutting tool applications engineer scare you into expensive toolholders. Make suppliers provide tools aligned to your needs, not the marketing strategies of the suppliers.

Always use floating tapholders, even if a machine has rigid tapping capabilities. Modern machines have rigid tapping that synchronizes the spindle and tap axis, so special tapholders theoretically should not be necessary. However, it is easy to introduce small errors in programming and setups that can result in broken taps even when rigid tapping. A floating tapholder absorbs most minor errors, giving some insurance against broken taps, and a floating tapholder also improves the quality of a tapped hole.

Never use endmill holders with a setscrew. Go ahead and throw them out; they never have been any good. TIR is bad, grip on the tool is bad, the setscrews like to back out, and endmill holders are suitable for just one size shank. Setscrew-style endmill holders are good for only very large high-speed steel endmills, which have become all but extinct at modern shops. HSS endmills are obsolete technology.