Troubleshooting Accuracy Problems on a Tenoner

      Here's a long story of a woodworking shop's difficulties in getting a tenoner set up with split tooling to work to the desired level of precision, accuracy, and repeatability. October 26, 2011

We have a single end tenoning machine which has two 600mm stacks. These stacks are raised and lowered via a powered re-circulating ball screw thread as in the first picture below. We have split tools between them. The problem we are having with these stacks is that they have a vertical tolerance of +/- 0.15mm. This means that when we are making a counter profile using both stacks, the profile can be as much as 0.3mm out (+0.15mm out on one stack and -0.15mm out on the other makes 0.3mm).

In joinery, 0.3mm might not sound a lot but when splitting tools, like what we are doing, it causes a significant problem of inaccuracy. Can anyone give any suggestions on what we can do to improve the accuracy? We have tried different lubricants on the dove tail slides and on the screw thread itself to no avail.

Can anyone tell by looking at my photos why the stacks are inaccurate? Is it something to do with the screw thread? Is there a more accurate system other than the ball screw thread?

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Forum Responses
(Solid Wood Machining Forum)
From contributor K:
All machines have backlash, some more than others. When I check out a digital readout against the machines mechanical dials on the hand wheels I always approach the index point from the same direction. In your case, when going down I would set it up to drop a little below the index point then bring the stack up to where it needs to be. When going up just go up to the index point, donít allow ďovershootĒ. If your machine can do that, your errors should go away.

From contributor O:
My Colombo pretty much holds to .1mm or less for height variation. My spindle is a lot shorter at 300mm but the same ball screw and drive setup as yours. When mine positions down, it goes down past the setting and then back up a little. If it is going up it just goes and then stops at the setting. It does this very quickly. I do remember reading something in the manual about possible programming for a slower, more accurate setting. The software is confusing on these and not very intuitive at all. I think they are decent machines but there is not much support outside of Italy. Have you considered bringing in a tech from the factory?

You have probably checked but it could be a tooling issue. I see what you are saying about the tool splitting compounding the error. Did you stack and shim the spindles yourself? We are not splitting on ours but using stacked tools on sleeves. We had issues with the tenon fit when setting the machine up. We discovered the cutters on sleeve were not stacked properly and had to do some adjusting and re-shimming. I will be curious how it works out. I am interested in toolsplitting but can see where a super accurate machine is required.

From contributor R:
I agree with previous responders that spindles should always move to final position in upward motion; this eliminates backlash. Looking at your tool picture I would also suspect the spacers are not very precise or clean. A very small variance close to the spindle will cause a major tolerance problem out at the tip. I suggest you check each tool set individually on a setting stand, and then try a test cut between tools to confirm stack settings.

From the original questioner:
I don't think backlash is an issue because my machine works like Contributor O's. The stacks will overshoot by about 6mm and then the drive motor will give a short burst to drive the stack upwards to the required measurement. It is this short burst that is problematic because the machine doesn't know exactly where the stack is going to land. It just hopes that the stack will fall in the correct place. This is why I'm trying out different lubricants. My theory is that if the stack puts up exactly the same resistance to the drive motor every time it gives a short burst then the error should be zero. I'm trying a teflon based lubricant product at the moment which dries so dust doesn't stick to it.

I would get a tech person from the factory but I have told the manufacturer about the problem and they say that the error is -/+ 0.15mm and there is nothing they can do to improve it. The machine manufacturer installed the tooling for me at their factory. The tooling is made by Oertly which is made in Switzerland, and I believe they are one of the best tooling manufacturers around. I very much doubt there is anything wrong with the tooling and we have explored this possibility by changing cutters and inspecting the shims to no avail.

On the first stack I have our scribe cutters and then on the second stack we have our combing tools so, as the tool set is made up over two stacks and the stacks are out by up to 0.3mm, I don't think changing shims will improve this. In effect, this inaccuracy between the two stacks is like having a shim that is randomly changing its thickness from zero to 0.3mm. I would like to get this error down to 0.1mm which is -/+0.05mm. There are other machine manufacturers who quote this accuracy of -/+0.05mm and I would like to know how they achieve this. I'm going over to Germany in May next year to see the big woodworking machinery exhibition in Hannover and hopefully will see how other manufacturers address this problem of inaccuracy.

Below is a screen print of the control panel which shows the inaccuracies of the two stacks. The first column (final Q) is where the user enters the required measurement and the third column (Real Q) is where the stack actually lands. As you can see Stack one is supposed to be at a measurement of 400mm but it has landed at 400.11mm giving an inaccuracy of +0.11mm. This stack could have landed anywhere between 399.85mm and 400.15mm given it's tolerance of -/+0.15mm.

I have thought about asking the machine manufacturer to alter the software so the machine has several attempts at getting the stack at the correct measurement instead of just one. There must be a reason why they haven't done this already. Maybe it's because they think the drive motor may burn out, I don't know.

Click here for higher quality, full size image

From contributor R:
Thanks for a good explanation. You are right Oertli is one of the best and can be trusted (although I would still make sure inserts and tools are referenced correctly after any tool/insert change using a precise height caliper on the bench).

If your tools are good then I would suggest the following checks to better define problem:

First clean and lubricate slide perfectly while adjusting it through its full height range. Next make six repeat settings to two different tool heights and compare screen measurement with a height caliper to confirm if the variation is electronic or mechanical.

A. If the repeated measurements (screen to actual caliper) are varying at all I would look for something loose in the drive/spindle assembly. You could try levering up the whole spindle assembly to see what it is solid or loose (often a loose re-circulating ball race is the culprit)!

B. If the screen to caliper measurements do match (which it should with a ballscrew) I would suspect that something is binding on the slide causing the override variation (dry or dirty slide/overtightened gib, etc.) Hopefully this process will help you find the exact cause, then you only need to find the solution!

From contributor D:
Most ballscrews are lubricated with grease, and usually some pretty finicky stuff at that. I've gone through all kinds of problems using so called "compatible" greases with the required Kluber brand in a CNC machine. Seems it reacted with the new grease already in the system, causing hardened, waxy deposits which gummed up the sensors all over the machine, telling me there was no grease even though the canister was full and pumping. Techs have warned me not to mix lubricants in the past, and I didn't listen.

Our old CNC seized an x-axis screw one day. I disassembled and repacked it, and I can assure you it was like surgery for eight hours, and I'm a very impatient guy. The techs wanted four thousand bucks to rebuild it and it would have been down several days - and they told me I had beginner's luck because very often the whole ballscrew cannot be salvaged and has to be replaced.

From the original questioner:
Luckily I was warned about different greases reacting with each other. Now, I always do my best to make sure that greases are compatible. I was also told by the machine manufacturer not to use grease containing additives like the one I'm using now with a teflon additive. I thought I would take the risk with this product as I'm running out of ideas and I've been assured by Interflon (the grease manufacturer) that it will be ok for this application.

Six repeat settings to two different tool heights. At the moment I have been running timber through the tooling and taking measurements from it using a vernier gauge to compare with the screen readings. I have found this to be very unreliable when getting down to accuracies of around 0.1mm with the timber moving, rough surfaces, rough edges etc. so I have just relied upon the screen readings. I probably need to check this using some piece of apparatus to measure vertically from the bed to a point on the tooling.

The thing is that whatever the problem is, it is applicable to both stacks because they are both showing the same errors. I consider it unlikely that both stacks have the same malfunction to give the same errors but I will investigate anyway. Let's not forget that, according to the machine manufacturer, there is no fault as it is operating within its tolerance and this is just the way the machine is.

But do all machines of this type have this error? Is it inherent in the machines nature or are there more accurate rise and fall mechanisms? Even if there are, could they be retrofitted to my machine or is it the case that either I have to live with the problem or buy another machine?

From contributor R:
After nearly 40 years working for European machine manufacturers I would make these comments:

There are better manufacturers, but you get what you pay for. The best manufacturers will have their own technicians available to the customer, that is how they know how to build it right for the country/methods/application which do vary enormously around the world. I don't believe that measuring from the wood will be accurate enough for you to define the problem. Variables in wood clamping, feed, flatness, straightness, etc. need to be eliminated.

A typical encoder has measuring increments of 0.01mm or better. With normal machine variances effecting load/acceleration/drag (including lubricant viscosity, temperature, weight, vibration, etc.) the spindle should stopping at final position within three increments of the encoder, or +/- 0.03mm.

Find a way to measure the actual spindle setting position to confirm this precision, and then define why you have the variances you have. Did you look for any looseness in drive, or binding on slides/gibs?

From contributor R:
I forgot to add: "That is within normal machine tolerance" is the standard answer #1 for companies that do not have their own technicians on the ground! Look at their sales documentation. Does it define accuracy? If it is sold for an application, it should be accurate enough for that application, with plenty in reserve to compensate normal variables of wood!

From the original questioner:
No, it wasn't stated in the contract. When I was drawing up the specification in the contract I was warned about accuracy by another salesman but he was very unspecific. I questioned my salesman on this issue of accuracy and his response was: "load of bollocks. One hundredth of a millimeter? I will ask the question - the controller will work to that tolerance, but ask him to prove it to you on a machine! The timber moves more than the tolerance of the machine." This was in an email he sent me. Now, he tells me that all these machines have the same accuracy.

I think that this machine would have been ok if I wasn't splitting the tooling. What we do to get over this problem of the tenon counter profile not being exactly correct is to alter the profile tool sets on our spindle moulder. We split the tooling on this machine as well. We have to shim up or shim down on this profile tooling to counteract the inaccuracy on the tenoning side. I know we shouldn't have to do this but at least we can still use the machine.

I agree that in this case, I certainly got what I paid for. I paid around 70K for this machine. I had a quote from a Dutch firm of 100K for a machine with the same specification. The 30K difference for the amount of problems and hassle I've had with this machine is simply not worth it. I've tugged and pushed on all the components and they seem as solid as a rock. I cannot see any looseness anywhere. This machine is built like a tank.

From contributor R:
If you are sure the spindles are solid I suggest:

1. Get a digital height caliper that will allow you to measure setting precision on these two spindles in multiple test settings, as advised earlier without cutting wood.

2. If they are both varying excessively (which would indicate it is a machine design/component/software fault) then this problem should be addressed by the manufacturer. In this case I suggest you write directly to the manufacturer, demanding technical correction, or they should take back the machine and refund your money! (To support your claim you may wish to separately get advice from Oertli, and other manufacturers, on the tolerances expected from tenoners with combined stacked tools - there are many of them).

3. If your multiple tests prove the tolerance problem is more on one spindle than the other (I noted earlier that one spindle was set within 0.03mm), then I would report this directly to the manufacturer and demand their tech comes to correct the fault in the one bad spindle.

From the original questioner:
Oertly didn't specify a machine tolerance that was needed when their tooling was being split. They told me that they haven't had this problem before or certainly not to this extent. I'm inclined to believe them. Even though they suspected it was the machine at fault they still did all they could to try and remedy the problem with several alternate cutters. They worked on the problem over a period of several months and they also made many site visits. I don't blame Oertly and I'm very happy with what they have done.

From contributor R:
Sorry, you missed my point. I realize that Oertli is not responsible for the problem, but their advice could be useful to argue your case with the machine manufacturer.

From the original questioner:
I have spoken to my Oertly salesman about what an acceptable tolerance is when a machine is splitting tools. He basically reiterated what he has already told me. They don't really issue such figures because it has never been necessary and they haven't had this problem before. I can see why a tooling company wouldn't issue such figures because when designing tooling they have to presume that the error in the machine is zero. Anything else would alter the profile.

I suppose the tolerance that is needed in a machine depends on the application. My application requires excellent accuracy otherwise the customer will see gaps in their window joints. I have drawn out diagrams to illustrate the problem more clearly below. The diagrams are cross sections of where our counter profile meets a profile. The first top diagram is where the error is zero. The counter profile fits perfectly around the profile. This is what I would like to have. The bottom diagram shows what my machine produces. As can be seen, there is a gap. Because my machine has a tolerance of +/-0.15mm, the error or gap can be anything up to 0.3mm which is noticeable. A gap of 0.1mm, which a tolerance of +/-0.05mm would give, would be far less noticeable. I'm under the impression that a tolerance of +/-0.05mm is an industry acceptable level of error for my application.

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From contributor R:
I had a chance to discuss this with an old colleague in electrical engineering (Weinig)

His opinion is:

- Multiple attempts at reaching a setting position tolerance is not necessary (and not possible with standard control logic/software).

- If you pulse the adjusting motor on/off repeatedly it should not burn it out, but this will probably heat up and trip out overload circuit, requiring reset.

- Suitable encoder and control should readily position the spindle within +/- 0.03mm (wood tolerance will be more, depending on various factors).

From contributor O:
I have been checking cuts on my tenoner for the last few days to see what I come up with. You must have paid the big money to have a machine that reads out to the hundredth of mm. The tenoner and all the other machines in my shop with electronic shafts only read to .1mm. I think that is plenty accurate for wood but can sure see your issue using the tool splitting method. They do a lot of tool splitting on the Weinig Unicontrol machines. It would be interesting to see how they hold accuracy. We have several electronic shafts in the shop and by far the most accurate is on the Martin shaper and S4S Machine. The Martinís will jog up or down by push of a button in .1mm increments. None of the other machines will do this.

Over a few days cutting I am within .1mm accuracy. When off, itís usually to the + side. I am measuring wood and as pointed out that is not the most accurate. The first half year we used the machine it would come up .1mm off on the readout like yours does, but seems like it rarely does that anymore. We use the machine more in the manual setting mode than the automatic settings but that doesnít seem to make any difference.

There are settings in the parameters for inertia (how many mm are necessary to stop the positioning motor). Recovery (how many mm the shaft goes down before going up to correct position). Mine is set to 10mm from the factory. Num impulses encoder (number of pulses to the encoder, mine is set at 250). Screw step (the step of the screw installed, mine is set at 5).

I donít know if this is useful information but if some of the parameters are off it might affect accuracy. Your machine is also more complicated than mine because of the two shafts and moving table. Do the two shafts hold about the same or is one more accurate than the other? I am curious how you do tool splitting on the spindle moulder but will save that for another post.

From contributor V:
I don't believe grease selection is part of your problem. What kind of feedback does your machine get on the positioning height? If it reads a rotary encoder on the ball screw, that's a poor method of determining height. Instead, there should be a vertical distance indicator that measures vertical movement directly.

Also, a good method of accurately holding the proper position is to have a position clamp strong enough to hold during cutting operations and engage the clamp when the position reaches about 1/8" before the final position. The positioning motor must then drive the ball screw the remaining distance against clamping pressure. (This may require a stronger drive mechanism than you have currently.)

From the original questioner:
I don't know whether there is a clamp as that area is all enclosed but having one makes a lot of sense. I believe my machine uses a rotary encoder on the ball screw to measure the stack's height. Maybe this can be seen from one of my photos. A vertical distance indicator, this makes a lot of sense as well. It would be independent of wear on the ball screw and probably a lot of other factors that I don't about yet.

From contributor R:
Your machine picture shows a rotary encoder. Unless you are going to re-engineer the machine I would suggest you let the manufacturer figure how to get acceptable accuracy.

From the original questioner:
Finally, something has been done. It has been about seven months since I originally asked if there was a parameter that determined the number of attempts the stacks had to get within a certain tolerance. Now, I have my answer. We found and altered this parameter along with the tolerance parameter. We have used the machine and initial trials suggest that this may have resolved the problem.

Most of the time, the machine gets within +/-0.05mm on the first attempt and for the rest of the time the maximum number of attempts needed seems to be three. If this solution doesn't prove to be satisfactory there is another option of using a two speed motor. The idea being that when the motor is running at its slower speed it will be far more accurate. This sounds like a good idea and if I had been offered it at the time of manufacture I would have had it as the cost would have been relatively small. If I wanted these two speed motors retrofitting now, they are saying that I will have to pay.

From the original questioner:
I'm still having trouble with my accuracy or inaccuracy. I'm not happy at all. My salesman wants to test my machine one day for a couple of hours to check the accuracy. I have told him that the problem is intermittent and might not occur while he is testing it. The machine sometimes works fine and achieves its desired position within a specified number of attempts other times it doesn't. When it doesn't reach the desired position it generates an error message which doesn't mean anything to me or my operators.

See my Ďprint screení below where I have circled the error message in blue. I have only just been told about this error message so my operators have never known to look out for it. The other thing about the machine failing to reach the desired target is that it doesn't pause to wait for an input from the operator, as what to do about this problem; it just carries on machining regardless of the inaccuracy. So, if the error was 10mm, the machine doesn't care, it will just keep going. The error message should pause the machine and wait for a decision from the operator whether to run the attempts again or stop the program so the operator can inspect the machine for possible causes.

As my salesman wants evidence of my problem I have told him that because it is intermittent it might not be possible to detect it over just a period of hours. So, what I suggested to my salesman was to ask Colombo for all the recorded actual values that the machine has achieved in getting the stacks to the correct position. This would show all the inaccuracies and everything my salesman would need to satisfy himself that I have a genuine complaint. To my surprise, my salesman told me that Colombo doesnít record the actual values achieved by the stacks. He then went on to tell me that it wasnít possible to do this. This came as a surprise to me as well. Fancy that, a computer that canít record values, especially when it records all program and parameter changes which is no different to recording the actual values. I find this all terribly convenient for a salesman wishing to deny my complaint.

So, what do I do now? I canít prove the inaccuracies because I canít expect my staff to write down every single actual value and then my salesman could say that they were made up or false. Iím thoroughly disappointed. The other issue I have is how many attempts to reach a desired position are acceptable? Iím sure a machine with the worst accuracy could achieve its desired value if it were given enough attempts. I want my machine to reach my desired accuracy the first time. Iíve had to set my number of attempts to 10 to achieve an accuracy of +/- 0.04. Am I asking too much of my machine? I donít know. Colombo has stated it is only accurate to -/+ 0.15mm. So, I feel, it is really just luck when it achieves my desired accuracy.

Iím investigating another machine company called Vertongen. They are claiming that they design their machines far different to Colombo. Iím going to visit one in action. I hope the above is useful to anyone buying a new machine, i.e., make sure it records and saves all its actual values because if it doesnít you might end up in the same position as me.

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From contributor R:
Whoever you buy from, write down your required tolerances on first piece set-up, and get them to accept in writing and clarifying any variables before you sign on the dotted line.

From the original questioner:
I did talk to a Weinig representative when I was deciding which make to choose. He wanted to sell me a Duin. The problem was that I couldn't get a price out of him for the specification of machine that I wanted. The same went for a Saomad salesman. The only make that would give me a price was Colombo. So, I don't know how much cheaper the Colombo was in comparison to Duin or Saomad. I didn't even know that Vertongen existed at the time. Of course price was an issue but at the time I had no information about why different makes of machines have different accuracies. I didn't even know that there were inaccuracies; I thought that these machines were exact. I still don't why one make is more accurate than another but I'm in far better position to know what to look for.

From contributor R:
Why did the Weinig rep propose some alternative? Does Weinig not make what you need?

From the original questioner:
I can only speculate that my Weinig rep might have thought that a Weinig machine would have been too expensive for me, athough he never said this. Weinig must be more than capable of supplying this type of machine. Maybe Duin was a cheaper alternative for him although he wouldn't give me a price for my specification either. He did give me a price for a lesser specification Duin.

Prior to buying my Colombo I found that trying to get information and prices was like getting blood from a stone. This goes for all reps and all companies. Why this industry is like this I do not know but that is how I found it. This is why I value this forum so much. Vertongen may be an exception to this idea as they are now releasing technical information to me about servos and drives and software. I think this is a good sign.

From the original questioner:
When deciding what options to have on my machine, Colombo and Denwood supplied me with the document below. As you can see, there is no mention of accuracy and no mention of the different methods of driving 600mm tenoning stacks. It isnít exactly very clear in any case. It didnít mention accuracy in my contract either. Why, would they omit this vital information? If it had mentioned two speed motors to give a better accuracy, like what they are offering me now, it wouldnít have cost much over 1000 Euro (factory fitted) and I would have snapped their arm off for it. My colleague came up with the idea that Colombo didnít mention these things because they didnít want to raise this issue.

Why wouldnít they want to talk about accuracy? Surely they are capable of accuracies as good as anyone else. The hardware and software that the likes of Duin, Vertongen, Weinig etc. use must be available to Colombo as well. I donít understand Colomboís reasoning on this issue. What am I missing? In any case, I just want them to fix my machine. I canít see it costing Colombo and/or Denwood much more than 4000 euro.

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From contributor O:
I have not had the problems you are encountering with my Colombo but it is a lot simpler machine. I now realize that if you get into a CNC window machine it is important to have factory backup and service in your country. In the US for window machines that would be only Weinig, SCM and Biesse. Even at that I don't think these companies have many window machines in the U.S. or if Biesse even has a Uniwin over here. In the U.S. a router might be a better choice because service is more available. I donít know about the UK if you have factory support for many brands. I would suspect the company that is the dealer would not be able to even turn the machine on much less fix the problem.

Colombo does not list their options very well in the literature. A lot of things I would have done different if ordering again. I think a lot of the small Italian companies that make window and door machines are good in their own region but not for international sales.

From the original questioner:
Does anyone have any ideas on how I could record the actual values that the machine achieves through its subsequent attempts to reach a given target? These values are displayed on the machines PC monitor. The PC uses windows XP. I'm considering using software that just records what is dispalyed on the monitor. Does anyone know of such software?

From the original questioner:
I have found some software that records whatís happening on the PC monitor. I'm not sure how much memory it is going to need. I've reduced the size of the recording window and image quality to a minimum to save on memory. See below a snip of the video that I'm recording. At least now, when one of the operators has trouble with the accuracy, I can look at the video footage to see exactly what the stacks are doing. It's not ideal and will be very time consuming but it's better than nothing.

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