Spindle Speed Calculator

The Spindle Speed Calculator app helps engineers to select the right set of parameters for profiling providing spindle speed and depth of cut combinations to achieve stable milling processes.

The web app generates a Stability Lobe which recommends spindle speed and depth of cut to safely apply avoiding chatter vibrations. Machinists using the app benefit from increased productivity, saved time, energy & materials and improved surface finish.

Chatter-free spindle speed and depth of cuts can also be input into our feed rate calculator to optimize feed rates taking into account cutting forces.

The Spindle Speed Calculator app requires Frequency Response Function (FRF) obtained by tap testing. Check out our tap testing solutions here.


  • FRF data
  • Tool geometrical features
  • Milling Parameters
  • Workpiece Material


  • Stability lobe

Why use tap testing in combination with Productive Machines web apps?

  • To predict and identify the stabilityof machining operations against chatter vibration before cutting any metal
  • To assess theprocess transferabilitybetween the machine tools in the shop floor
  • To support the machine tool acceptance tests

Tap testing alone will not enable the user to identify safe spindle speeds for machining. For that you need to understand the behaviour of the cutting tool as installed on the machine tool. For more info about our tat testing solutions click here

Plans and Pricing for our Apps

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Tap Starter - Launch Bundle

Get one year free Essential Optimization and tap test training when you buy DigiTap standalone software and our tap test hardware kit.


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Predictive Maintenance

Inspect the health of your spindle to extend tool life and prevent downtime (requires FRF data)

Free for charge

(first year)

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Feed Rate Calculator

Apply optimal feed rates to optimize the cutting forces of your machinning process to increase speed.


(when billed annually)

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Essential Optimization

Apply optimal Feed Rates and Spindle Speeds to achieve the fastest chatter-free results (requires FRF data).


(when billed annually)

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Tap testing solutions

Gain insights into your machine's dynamics by collecting and analysing FRF datawith our tap test hardware kit and DigiTap standalone software application.

Tap test hardware kit


DigiTap standalone software


Tap Testing is enabling Productive Machines Optimization

Tap testing, enhanced by Productive Machines technology, brings precision and efficiency to manufacturing. It empowers operators to optimize machine tool performance, reduce waste, and enhance product quality.

What is tap testing in CNC machining?

Tap testing, also known as impact testing, identifies structural dynamics by impacting a structure and measuring its response, in other words, tap testing tells us how flexible a machine tool assembly is.

In CNC milling, the tip of the milling tool is excited with a hammer to induce vibrations while collecting the measurement with an accelerometer..

The relationship between the force (input) and acceleration data (output) is the transfer function of the structure which is also called Frequency Response Function (FRF).

FRF represents the dynamic stiffness of the structure across the frequency spectrum and is used as an input for spindle speed calculator to calculate recommended spindle speed and depth of cut.

How to perform a tap test?

Productive Machines has partnered with Kistler, a lead supplier of a Tap Test equipment, to offer a complete hardware tap test kit. We have also partnered with Research Centres to provide tap test services or on-site/remote training for companies unfamiliar with the tap testing method.

What do we do with FRF data?

DigTap, our tap test standalone software, calculates the FRF data from the tap test. FRF data includes natural frequencies, stiffness and damping ratios of the machine tool assembly.

FRF data feeds into the Spindle Speed Calculator to generate a stability lobe diagram. The user then knows to select a stable spindle speed and depth of cut under the stability boundary.

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