Automatic Tool Changer Problems. 2

[Eoghann]

We manufacture large grinding and cutting equipment. Recently we produced a grinder with an automatic tool changing system but have been experiencing problems with the tools sticking in the taper. We are using a SKF 50 style taper and an OTT Jacob clamp/push cylinder for holding/ejecting the tool in the grinder.
The OTT Jacob cylinder gives about 1000kg of push to eject the tool. The tools are made from EN19 and the tapers have been surface hardened (Nitrided). There is a problem with rust forming on the surface of the tool and the female taper of the grinder, but only where the tool/grinder actually touch.

http://www.blackisle.org.uk/spindle_rust.jpg

This pic shows that the thin end of the taper (in open air inside the grinder) has very little rust compared to the rest of the taper.

http://www.blackisle.org.uk/grinder_rust.jpg

shows the inside of the grinder.
The equipment is working in a fully covered building (main doors always wide open), about 500km from the nearest sea.

Does any know why the rust is building up in these areas? Is it due to fretting?
Is there any lubrication we should be looking at to coat the spindle?
Anyone else have problems with tools not ejecting?

Thank you for any help you can supply.

 

[EnglishMuffin]

Comments:
1. It certainly appears to be fretting - fretting usually looks like rust and takes a surprisingly short time to appear in rotating situations. I've never seen it quite that bad on a 50 taper though. There also appears to be "rust" on the tapered non contacting part of the grippers in the second photograph, which might be anomalous, although I could be wrong. In my view, there is no satisfactory "materials science" solution to fretting that I have ever come across, at least that would be applicable here, although I am sure many will take issue with this statement. There are theories involving stray electrical currents which can exacerbate the situation, and which may be worth checking out in this case. Lubrication can help, but the problem in this case is the difficulty of maintaining the lube film over the long term.
2. Any mismatch between the tapers should be such that they are tight at the front and loose at the back, which does not appear inconsistent with the photograph of the taper.
3. The standard 50 taper rotating Ott piston is known to be inadequate from the standpoint of knockout force, although Ott won't admit it. I think the 40 and 60 taper ones have proportionately more. Unfortunately, there is a hydraulic pressure limitation with those Ott cylinders, and you are probably fairly close to that now.
4. In developing a spindle with a 50 taper Ott drawbar, I ran into sticking problems. I solved them to some degree by obtaining a special version from Ott with a double wedge "intensifier', which I think they had originally developed for MAHO.
5. Your "fretting" problem, if indeed it is simply fretting, is so severe that I would check a number of things. Do you have the correct draw-in force ? There are special guages available from Ott for measuring this. To achieve the correct draw in force, it is also very important that the tool grippers are adjusted to the proper axial position, and that the internal profile that mates with the grippers is correct. Even an error of a few thousanths in this axial position can have a profound effect on gripping force. It also goes without saying that you should check the internal taper profile very carefully. This can only be done reliably with an air guage - blueing is too "hit or miss".
6. Is there any other information you can provide regarding exactly what tooling you are using in the 50 taper spindle, (presumably some type of grinding wheel), and what sort of operational conditions it experiences ?

 

[EdDanzer]

There is fretting showing on both parts. This is caused by the tool shank moving in the spindle taper. The sticking happens when the tool shank is moving from radial load and thrust which swells the taper in the spindle. Because of the difficulty in fitting the tapers we experimented with soft shank Cat 50 tool in a large HMC with good success. These shanks are CNC turned with .006 per rev feed with a .030 R insert to give a surface that can deform some to fit the taper better. The original hard holder with a 4 3/8” trepanning tool required a 3 pound hammer and the drawbar push to loosen it for removal, the soft shank popped out with the drawbar push only.
To determine if the spindle taper is swelling, measure the OD of the spindle at a marked location before the tool is installed, the check the spindle diameter in the same place before removing a stuck tool. Because of the small OD at the end of the spindle you may have to increase the diameter at the end.

 

[EnglishMuffin]

EdDanzer: Interesting - I've never heard of anyone doing that before. OK if it stands up over the long term - I guess you can't argue with success. Most people are stuck with standard tooling - the standard taper tolerances are so tight that meeting them is very difficult without grinding. Can you order coarse-turned soft shank tooling from most suppliers?
The spindle OD always increases to some extent when you clamp a tool - the only question is, how much. With the spindle shown in the picture, and with that particular drawbar, in my opinion it would be unlikely to be more than about .0001" and should be within the capabilities of a correctly designed drawbar to overcome. However, an even bigger effect that does occur is that if the spindle heats up, the taper expands, and the tool gets sucked in further. If you slow down and let it cool off, it can take a tremendous amount of force to remove. A similar thing can happen with high speeds, where the spindle expands because of centrifugal force, and again the tool gets sucked in and gets stuck. But the fact remains, usually all you can do is make sure you have enough knock out force, and that particular draw bar is deficient in that respect. OTT do have computer programs that are supposed to predict some of these expansion effects, for which they require drawings of the spindle, but the bottom line is that they need to provide enough knockout force. The ultimate solution is to go to HSK tooling, which should not suffer from any of these problems, but that may not be feasible here, and it's rather expensive.

 

[EdDanzer]

Because we only do small lot low volume parts I don’t know the life of soft shank taper tools. What I have observed is that hard taper against hard taper under heavy load doesn’t work well. Even the .0001 movement will cause unstability. The experiment we have thought about trying is to have 6 pad, 3 front and 3 rear on a hard tool, but the soft tool is easy to make. As to HSK this is similar in design to Kennametal KM shanks, our static stiffness tests showed non linear deflection greater than other tooling. We don’t have access to HSK for testing but consider it a hollow boring bar shank when used as a boring bar. There is no reason for face contact to increase stiffness of the round cross section at the face. We saw this with Komet ASB shank tools.
thread281-80549
goes into more detail of the testing we did out of frustration.
Other soft tools we made include a back face mill to do back face milling 6” from the Cat 50 flange, and several long shanks for boring heads.

 

[ornerynorsk]

I'd agree with the inadequate drawbar force, which ultimately causes fretting, and, in this case, is allowing swarf and coolant to migrate in as well, which only accelerates the negative effects.

 

[EnglishMuffin]

EdDanzer: HSK tooling is not quite the same as KM. According to test data I have seen, it is considerably stiffer than conventional taper tooling, up to a point, which depends heavily on draw-in force. It also depends on exactly which sizes you compare. In this case, I would consider the HSK100 size if I were going that route. You are not the only skeptic, but I believe that in Europe, HSK tooling will eventually totally supplant SK taper tooling. The wisdom of this is arguable, of course, and is partly driven by an "invented here" philosophy.

 

[EdDanzer]

Eoghann,
Have you checked the drawbar tension with a meter to know how much there is?

 

[EnglishMuffin]

With that drawbar, from memory I believe it should be around 4500 lbf (min), (assuming Eoghann's drawbar has the cylinder that I think it does). Hopefully, he has an OTT electronic force gage.

 

[Eoghann]

Sorry I haven't been in touch, I've been in Germany on a different job.
Thanks for your replies, we haven't yet checked the drawbar strength, I will talk with my bosses and get the OTT tool for my next trip out. We have been given a recomendation of 25kN from OTT.
I will post the results as soon as I get them.

Thanks for all your help, it is much appreciated!

 

[wedgee]

You might want to try black oxiding the tapered shank of the tool holder. It should prevent or at least delay the rusting problem. Tool holders for machining have had this done for quite a while with good success.