Technical Blog excerpt courtesy of Techniks USA
Collets come in many different types and sizes. Here is an overview of three of the more popular types of collets, along with the pros and cons of each system.
The ER collet system has become very popular due to the flexibility of the system to hold a variety of cutting tool shank types including drills, end mills, and taps. Also, ER collets provide several solutions for increasingly popular coolant-through cutting tools.
Most standard ER collets have between a 0.020” and 0.040” holding range, making them a good choice when needing to hold odd-sized cutting tool shanks. This holding range also means fewer ER collets are required to hold a range of cutting tool shank diameters as opposed to other collet systems like TG.
The popularity of the ER collet system has led to several variations to hold a wide assortment of cutting tool shanks. Some ER collets have been modified with squares at the bottom to hold taps. Others have been modified to provide quick-change capabilities or compensation, also called “float”, for rigid tapping cycles as shown in the images below.
Specialized ER Collets
Other modifications include special slotting designs that seal around the cutting tool shank and force coolant through channels in coolant-through tooling, as well as modifications to include coolant ports in the collet that direct coolant to the cutting area.
TG collets have about the same accuracy as DA collets, but because there are more slots, and therefore more faces clamping on the cutting tool shank, they tend to deliver greater holding power.
TG can be a good solution for larger shank diameter cutting tools, but they generally limit how far down into a pocket you can reach due to interference with the collet nut, as TG collet nuts tend to be quite large.
TG collets are not as popular as ER collets for several reasons. Most notably, the larger diameter collet nuts can require the use of extended end mills to avoid interference from the collet nut when milling pockets.
Also, since TG collets have a very small collapse range, they are intended for use with one size cutting tool shank.
ER collets, by contrast, offer a large collapse range that can be helpful when clamping odd-shank diameter tools.
On the flip side, TG collets tend to have a bit more holding power than ER collets due to the collet base having a 4° taper as opposed to the 8° taper found in ER collets. This can make TG collets a good choice when machining with longer-length cutting tools.
Double-Angle (DA) Collets
Double-Angle (DA) collets have been around for a long time and continue to be used in the market. There are, however, many issues associated with DA collets of which users should be aware.
Let's just clear the air and say it: Don't use them. If you have them in your shop, replace them with ER Collets and ER Collet Chucks as soon as possible and you will recoup the cost of the new holders and collets in your tool life probably within a month or two.
One of the primary issues with DA collets is that they essentially clamp on the cutting tool shank with only two opposing faces in the I.D. bore.
DA collets have four slots in the front of the collet and four slots in the back of the collet creating four clamping faces.
However, when DA collets are tightened towards the lower end of their collapse range, two of the faces tend to be pushed out of the way so only two of the faces are clamping on the cutting tool shank. This may cause some runout at the nose when the tool is inspected in a presetter.
Additionally, when the tool begins cutting and side forces are applied to the cutting tool, the cutting tool tends to deflect into the area where the faces are not clamping on the tool shank.
This results in excessive chatter that dramatically reduces tool life and results in rough surface finishes. You will be hard-pressed to find a quality end mill holder manufacturer endorsing the performance of their tooling in DA collets.
We are very excited to announce that we are now able to offer on-site technical training to YOUR machinists at YOUR location! This is offered at no charge to customers who use any of the manufacturer's whom we represent in California and Nevada.
However, just because you don't purchase things from us, don't feel left out! We also offer on-site topic specter training on any of the following topics for $150/hour.
Each presentation lasts about 2 hours. The presentations last approximately 45-60 minutes with the remaining time for Q&A and discussion about unique applications in your facility.
Training Classes Available:
Advanced Part Manufacturing:
Next Generation Tooling is excited to offer some new services coming in 2015!
Below is a very fast video of our new training series on Tapping which we can present to your manufacturing team at your site.
It's a comprehensive overview of screw thread terminology, thread forms, fundamentals of threads, classes of fit, Tap basics, types of chamfers, the tapping process,tap types, screw thread inserts, helix angles, core diameters, re an hook angles, thread reliefs, pitch tolerances, H limits, Tap substrates, Surface treatment and coatings, tapping speeds, tap drill sizes.
The ISO metric screw threads are the world-wide most commonly used type of general-purpose screw thread. They were one of the first international standards agreed when the International Organization for Standardization was set up in 1947.
The "M" designation for metric screws indicates the nominal outer diameter of the screw, in millimeters e.g., an M12 screw has a nominal outer diameter of 12 millimeters.
4H and 6G taps can be supplied upon request. 6G taps are Oversize. Please refer to the Au "G" values table below.
For the special tolerance "7G", an oversize of approx. 2 x Au "G" has been taken into account.
Tolerance Chart - Metric Coarse / Fine
Tolerance Chart - Oversize 6G Taps
Au "G" Values for Oversize Taps "6G Taps"
The tolerance of the tap should be manufactured as close as possible to the finished internal thread tolerance.
This practice ensures that the threads produced will comply to the gage tolerances providing that the working conditions such as machine, chucking tools, and workpiece match the application.
H LImits Explained
H limits are used to properly size a tap for the threaded hole to be produced. They are selected based upon the tolerance required for the part. These tolerances are defined by the symbols class 1B, 2B, or 3B.
The sizes of a 2B and a 3B tap are different. The 2B tap is smaller in size. It has an outside diameter of 0860 inches. The 3B tap is larger by .0130 inches, making its outside diameter .0990 inches. As the sizes progress up the tap scale, it increases in size by .0130 inches in outside diameter. Other types of taps, such as hand or fractional taps, increase in increments of 1/4 of an inch and referenced not as a number, but the size in inches
Selecting a H Limit on a tap
Once the class of thread and part tolerance has been defined, an H limit is selected to produce a thread that is within the minimum and maximum limits for that class if fit. These limits are the same as the Go and Not Go thread plug gage dimensions.
The goal is to select a tap with an H-limit that is near the middle of the part tolerance. For instance, if the total tolerance was .005", the tap should be approximately .0025" larger than the minimum limit of the part and .0025" smaller than the maximum. In order to handle the widest variety of tapping conditions, the "40% rule" is commonly applied.
Using this rule, the tap is selected at 40% of the part tolerance. For instance, if the part tolerance is .005", multiplying .005" by 0.40 equals .002". Thus, the tap would be .002" larger than the minimum limit of the part or Go thread gage.
With the position of the tap in relationship to the part tolerance established, the selection of an H limit number, such as H3, H4, H5, etc. is possible. H limits are a sequence of size "steps" in .0005" increments beginning at the minimum size limit of the part, starting with H1.
In other words, an H1 limit is one .0005" increment larger than the minimum limit or Go gage, an H2 is two .0005" increments (or .001) larger than the minimum limit, an H3 is three .0005" increments (or .0015") and so on.
In the example above, a tap that is .002" larger than minimum limit, is four .0005" increments larger, or an H4. This would be the tap H limit recommendation for this tolerance.
Taps are general marked with the appropriate tolerance class for their intended use. The U.S. GH thread class numbers are not marked on the tap.
Tolerances for the various GH numbers are shown in the chart below
Classification for the tolerance 1B can be provided upon request.
Taps for cast iron and titanium tapping are designed one GH class higher to provide better tool life.
Technical Support Blog
At Next Generation Tool we often run into many of the same technical questions from different customers. This section should answer many of your most common questions.
We set up this special blog for the most commonly asked questions and machinist data tables for your easy reference.
If you've got a question that's not answered here, then just send us a quick note via email or reach one of us on our CONTACTS page here on the website
Our technical section is written by several different people. Sometimes, it's from our team here at Next Generation Tooling & at other times it's by one of the innovative manufacturer's we represent in California and Nevada.
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