Frequently Asked Question

Measurement of Slender End Mills with small diameters.
Last Updated 7 years ago

It is not trivial to measure long, slender end mills with an instrumented hammer. For example, an end mill with 0.25 inch (6.35mm) diameter and overhang of 2 inch (50.8mm). The natural frequencies will range from 500 Hz to 5000Hz. Hammers with 500 N or higher force delivery with plastic tips will have a spectrum of 2000Hz, and will not be able to excite modes between 2000Hz and 5000 Hz. The following techniques can be tried in measuring the FRF of such a slender tool.

Let’s consider that you have a small Kistler, PCB, Dytran or similar hammer with 500N or 2000N force range. If you use hard steel tip, the hammer may bounce on the tool with several contacts, which leads to unacceptable Frequency Response Function (FRF) measurement. CUTPRO MALTF will inform you automatically that the measurement is not accurate. If you use a soft tip, then you may not excite the natural frequencies of the tool. Harder the tip is, wider the frequency range of excitation will be, but at the expense of bouncing (i.e. multiple impacts). Softer the tool tip is, easier to create a single contact with the tool, but at the expense of longer contact time which will lower the frequency range of the hammer. Therefore, you may not be able to measure the machine correctly.

First try some tricks:

*Put a layer of a scotch tape on the hammer tip, and see whether it reduces the bouncing.

*Try bronze or Aluminium tip.

*Use the smallest hammer you have, i.e. PCB’s or Dytran’s miniature hammer.

If you are still unhappy with the bouncing or frequency spectrum range, do the following

1- Put the accelerometer at the tool tip, but hit the tool close to the chuck where it is more rigid. The bouncing will be reduced. Save the measurement as Cross FRF (i.e. frf_12). Make sure you measure the distance between the impact and vibration measurement point.

2- Move the accelerometer close to the chuck where you hit with the hammer. Hit the tool at the same location as in step 1. Save the measurement as direct FRF (frf_11).

3- Go to modal analysis module of CUTPRO. Select file -> select f FRF files and click on Flexible Tool FRF. Load both measurements collected in steps 1 and 2, and apply modal analysis. The program will estimate the correct FRF at the tool tip without having to hit the tool at its tip. It is not as accurate as measuring at the tool tip, but a lot better not being able to measure it at all.

An alternative method is to use two hammers:

Use PCB’s or Dytran’s miniature hammer to measure the high frequency range (2000Hz and up), and use larger hammer for lower frequency range. Apply modal analysis to the two measurements separately, and enter them as modal parameters in CUTPRO. You need to train your hands not to cause multiple impacts with the miniature hammer.

Please Wait!

Please wait... it will take a second!