Question
This is a question for those who run jointed moulders. Does an increase in spindle RPM affect the jointing process, for the good or bad? I'm looking to increase the RPM of our moulder spindles with the intention of increasing feed speeds with the same number of knives. It will work in theory, but I wondered if anyone had tried it in practice. I'm looking to go from 6000 RPM to 7200 RPM. Anyone jointing at 3 and 6 knife heads at 7200 RPM?
Forum Responses
(Solid Wood Machining Forum)
From contributor R:
Here are the hard numbers that you must weigh the cost against. You are currently at 6000 rpm, and let's say 3 knives finishing. We will be shooting for 16 knife marks per inch, so your feed speed is right at 90 feet per minute (180 at 6 knives). Changing your spindles and motor's pulleys gets you to 7500 rpm and will increase your feed speed to right around 110 fpm with 3 knives finishing. If you're going 6 wing cutterhead, then your feed speed is double at 220fpm. Is your machine capable of running this speed? If not, you will be "double striking" your knife marks and it will give you finish problems. Most jointed machines I have seen with exception of a small number max out at 200fpm or less. Increasing the rpm might work with the 3 knife scenario, but why not just run 6 knives instead? I have changed a few jointed machines to 8000 rpm, but the company that I did this for no longer joints the machine, so it is considered a single knife finish machine. There are moulders capable of jointing and rpm in the 10,000 range, but in hsk technology where the cutterhead is much smaller in diameter. I think the manufacturer has made your machine with the rpm and feed speed to work with each other and you will not beat the system by increasing rpm. My suggestion would be buy a machine with newer technology with more suitable rpm to fit your feed speeds. I have never jointed at 7500 rpm just for the reasons explained above. I don't even think it is possible with that diameter of cutterhead.
Only about half of the production runs will require the use of the jointers. The other half will only be a single knife finish, and that's where the increase in rpm will help a lot. The reason I am reluctant to just increase the number of knives is because, as you know, they aren't cheap (these are all backpack carbide knives) and I see a huge time difference in grinding up and sharpening 3 as opposed to 6 knives, so as a compromise I'd settle for an extra 60 fpm with the addition of 1 knife. I was just wondering if jointing at 7200rpm created any problems
1. Balance: at 6,000 RPM, an improperly balanced tool that has 1 gram of imbalance will create a 22 pound rotational force. At 8,000 RPM, the same 22 pound force is created by 1/4 gram.
2. Verify with the machine manufacturer if the spindles are capable of running at the higher RPM. The bearings and spindle design may be exceeded.
3. Verify that you have enough horsepower to run at the higher feed rate.
4. Verify that you have the required room in the guards for the larger pulleys required to run the higher RPM.
If the machine manufacturer provides positive answers to the above questions, then consider the purchase of a dynamic balance machine to be able to perfectly balance the tools for the higher RPM.
