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.

Don’t fear collets. It is common for a cutting tool applications engineer or a salesperson to say he or she cannot guarantee performance if you don’t use a specific type of toolholder, such as hydraulic or shrink-fit styles. In these cases, please refer to my first general rule. A toolholder needs to be only good enough for the application. Collets like the ER style provide excellent TIR and gripping characteristics and can be used for milling, drilling and reaming in all but the most demanding situations. Collets are cheap, and — unlike shrink fit or hydraulic — most wrecks trash just the collet, not the toolholder.

Toolholders with spindle face contact are overrated and have a narrow application window. HSK and some of the CAT-style spindles that allow the toolholder flange to contact the spindle face are warranted when very heavy tools are used in the horizontal position or in high-spindle-speed applications. Most machining applications do not benefit from the additional cost of the spindle or toolholder. Significant testing is needed before investing in this technology.

HSK holders provide the best holder-to-spindle interface available on the market. If you are faced with high-volume applications or extremely close tolerances, HSK is the best choice. When spindle speeds reach the 12,000 rpm range, HSK is preferred. Not only does it have superior performance, but the spindles and toolholders have a superior cost. If a machining application requires hydraulic or shrink-fit holders for quality reasons, then an HSK interface is likely the best choice. Likewise, if machining can be done with ER collet chucks, then HSK is probably overkill.

Choose hydraulic toolholders before shrink fit. Hydraulic toolholders work by hydraulically deforming the inner diameter to grip the cutting tool, and they work very well. Shrink-fit holders are heated to expand, and when cooled they shrink to grip the tool. Shrink-fit holders grant the best TIR characteristics and have very narrow cross sections, allowing better clearance in tight areas. Hydraulic toolholders also have excellent TIR characteristics but are larger than shrink fit and have a limited reach. Hydraulic is the first choice because of safety and cost as shrink fit demands ancillary equipment to heat the holder and people are subject to burns when using shrink fit.

Buy a force gauge for the machine tool, and regularly check pull force. When a machine ages, the mechanisms that hold the tool in the spindle wear, causing pull force to degrade. The first indications of a problem are things like dropped tools, chatter and difficulty changing tools. By the time that a machinist recognizes these symptoms, the spindle may be damaged to the point that it must be removed from service. Each machine builder has a specification for pull force, and machines should be checked routinely as part of a preventive maintenance program. Spindle rebuilds are expensive and can result in significant downtime. The quality of a toolholder is irrelevant when the spindle taper is damaged.

Clean the spindle taper daily. Chips and debris from machining get between the toolholder and spindle. Cleaning tools that are made to fit the spindle taper are cheap and easy to use, and cleaning can be done in minutes. Like other preventive maintenance activities, cleaning the spindle averts costly failures.

Toolholding, like many subjects at a machine shop, can generate vigorous debates, and this list is probably no exception. These are simply the things that I have learned and that work for me.