Hi all, This seems like a rudimentary issue but it’s irking me so I thought I’d check if anyone else has this problem. I have the EB20 collet kit from Avid. Using a 1/4" compression bit from Whiteside and the 1/4" EB20 collet, the bit keeps dropping lower in the collet. This results in big gouges in the spoilboard. I’ve resorted to CRANKING the collet as tight as I can. I have never had to do this before with other collets and I’ve been doing this stuff for a long time. My experience has been that over tightening is not a good thing and that a snug tightening (like, tight yes, but not super cranked) should work best. Am I the only one that has this issue? I wonder if the collet is out of tolerance?
Has the issue with the collet been there since you started using it or did it just start happening? Generally I take extra care to clean the collet and nut every time I change bits. The seat in the nut can get stuff packed up in there as well as the slits in the collet. I have not seen any issues with the collets I got from AVID.
Also, so other bits have this issue in these collets or is it just these bits? Maybe they sent you a metric bit by accident, did you get a chance to measue the bit diameter?
I am assuming that you mean ER20.
Based on the recommended torque for a 1/4" shank should be 21 ft-lbf. See https://www.shars.com/media/manuals/clamping-torque.pdf
I have found the same as Erik that the collet, nut, and spindle need to be absolutely dust free. Often I find some dust in the slots of the collet that needs to be removed.
One of these days I need to break down and get a torque wrench for my machine. I think I tend to way over tighten…
I don’t use a tq wrench but would be interesting to see how close or far off I am from spec. Gennerally I just go by feel. Bring it down until it snugs up then an extra little nudge for good luck LOL
All good suggestions so far. Here are some others as well as support for what the others have stated.
Measure your cutting tool shank, if your shank is oversized by even .0001" it cannot be used in a collet if it exceeds the collets “Nominal size”. Likewise, make sure you’ve grade the correct collet size. If our collet says 1/2", then it’s nominal size s .5". An ER collet has a clamping range of minus 1mm. The smaller the shank to nominal, the more your clamping force degrades for traditional collets because of how the ID of the collet is ground. You will hear people claim ER maintains full clamping contact to the shank throughout the range, but this is a much deeper conversation.
As stated earlier, cleanliness is next to godliness. This includes an inspection of your collet slots. If they get full of chips and dust, the cannot clamp down or compress.
3). As stated earlier, you need to verify your clamping torque. The defacto standard of all things ER generally comes from REGO-FIX tool corp…the inventor or ER collets. They gave up the patent to make it a standard, and thus control the standard. Over torquing ER collets forces the collet harder into the collet taper, but the top of the collet tends to get twisted at the top from stressful interaction between the collet face and the nut. Image the effect that’s having on your collet bore (ID).
Here is the DIN/ISO Standard for ER20. The first number is shank size (mm), and the second is clamp torque in ft./lbs. Notice it officially changes with shank size which many people of not realize.
4). Make sure your shank is FULLY engaged in the collet. The shank must be fully supported.
You need to review your axial and radial DOC. You may be engaging the helix of the cutting tool past the collets physical capacity to clamp effectively. The helix can actually exert so much force that it walks itself out…especially if you are experiencing harmonics through chatter where the tool is essentially walking out through vibration.
You might want to look up a “Friction Bearing Nut”. These are highly effecent at transferring wrench torque into output torque…probably by a factor of 1.5-1.7x over your standard collet nut. Here s a brief video demonstrating output “break” torque for a constant input torque across different ER nut types.
I’ve worked in the OEM toolholder industry for 25 years on the metalworking side, but this issue is universal, regardless of material being machined/milled/routed.
Good luck and let me know if I can offer any more help or suggestions.
Thanks everyone for the great insight. The collet is brand new and very clean so that’s not the issue. According to my dial-type calipers, the bit diameter is .250. I really don’t think I was installing it with the helix past the bottom of the collet because I know not to do that, but I will pay special attention moving forward just in case. The discussion about harmonic vibration causing the bit to walk off the collet seems relevant because it makes a pretty distinct sound when it is happening. It also doesn’t seem like the collet is out of tolerance because the bit slides in perfectly. I guess I’m in the market for a torque wrench!