Thirty-something years ago, I developed and brought to market a friction feed adapter for Gerber 15" plotters we named "Scrappy". It replaced the existing feed out bail roller with two rollers that were closely in parallel and made complete contact with the drum. In addition we added a spring loaded clamp on each side to apply sufficient force to successfully feed scraps of vinyl through the plotter for cutting.
At the time, Gerber was selling so much vinyl that they had become 3M's largest customer. It was estimated that a regularly used Scrappy would save an owner about 20% on vinyl usage. As such they weren't very happy with our add-on device and came out against it on the grounds that it would damage the bronze oil-lite bushings as well as the stainless steel drum support shaft. It is true ... even without the Scrappy, that the contact between the shaft and the bushings is subject to wearing out.
Our response after consulting with some bearing experts was to supply replacement bearings with each Scrappy that were made from a self lubricating material named
Rulon J. The replacement was simple enough and the drum support shaft can be reversed so that the contact points afterwards are fresh and unworn. We were, in fact, complimented by two Gerber engineers who looked over the change at a trade show. They indicated that they had made similar recommendations to management even before we came along.
Gerber soon and quietly changed their drum support bearings over to roller bearings on all 15" plotters. Frankly, I'm surprised that oil-lite bushings were ever used in GS-15 plotters. As far as the Scrappy was concerned though, Gerber upped the ante by voiding warranties and withholding all service to plotter owners if a Scrappy was ever installed. That cut heavily into sales and after five years on the market we sold our last Scrappy and decided not to manufacture another batch.
I still have a few Rulon J bearings on hand. I would be happy to send you a set for only a small postage and handling fee.
Contact me by private conversation for details and to supply me your mailing address if interested.
A few points to consider, however:
1. 45 and 60 degree blades are not the recommended blades for a GS-15 unless you are cutting thicker materials like sandblast or glass etch resist. The recommended angle to use is a 30 degree blade for most films including most polyester films.
2. The difference in depth of cut is the direct result of the film lifting off the drum when the direction of feed is from left to right when it will cut deeper than when it moves from right to left and tends to cling to the drum. As such, using a 30 degree blade reduces that difference. If you look at the design of most plotters, including Gerber's Envision series, you will see that the stationary plastic cutting strip has replaced the rotating rubber coated drum. It not only costs less but also eliminates the vinyl lifting issue.
3. It is doubtful in my mind that worn drum bushings are the cause of the uneven cutting. If it were, it would likely occur in both directions as the drum support shaft would shift with any motion regardless of direction. It is also possible that when you installed your new drum that you left a little slop that allows the drum to move slightly when pushed back and forth. A proper drum install will have the drum free to rotate but tight enough not to wiggle side to side.
4. It is also quite possible that the tool holder lift fork may be worn or misadjusted. Check how much the tool holder lifts off of the fork when the blade is lowered into full contact with the drum. Adjust it to only a hair width and replace the lift fork if it is noticeably worn.
5. Finally, check to see if your 45 and 60 degree blades are longer than a 30 degree blade. I think most are but uneven cutting isn't an issue because the materials intended for it to cut usually come on heavy plastic release liners. Any film with a paper liner is much more likely to have problems with depth of cut, especially with higher angle blades that are inherently going to cut deeper than lower angle blades. If you have ever used X-Acto blades, the #11 blade has a higher angle than a #16 blade which is shaped at a lower angle and has a reduced tendency to cut too deep. That lower angle is a standard feature of any swivel knife I've ever used where control of the depth of cut is a primary concern. The more pointed the blade, the more readily it will stab deeper than a less pointed blade and, therefore will tend to cut more deeply more easily. Combine that with the plotter tendency for the vinyl to lift off the drum when moving towards the feed roll and you probably know enough to fix your uneven cutting without replacing your drum bushings.
