Nylon milling machine gears

[Cuttings]

I have the exact same mill and have not yet had a problem with that gear failing. I do flip the head over, and take the inspection plate off the back and make sure there is plenty of grease on those gears probably a couple of times a year. I found that the original grease easily got throw of the gears so I found a different grease with more adhesive qualities that clings to the gears better. Now if my memory is correct on this and it has been quite a while, I believe I made a new shaft to fit in there because the detent was not allowing the gears to be fully engaged in that low speed range. I probably still have the original part laying around if I dig a bit. I may have made a drawing for the new part

[J Tiers]

Perhaps this failure is an issue with gear engagement, in which case one would expect it to occur whenever the gear train is placed into the low speed range and is stressed, and not just with this one particular operation. That possibility, however, is directly contrary to what was reported, to wit

"This milling machine does everything I want, except drill 1" holes in aluminum. I don't do that very often, but when I do, it's mostly ornamental holes thru flywheel webs. And this is the point where my mill lets me down."

he's going to get better cutters, but that statement is not in contradiction about the problem...... The ENGAGEMENT problem only occurs when drilling large holes.... and elsewhere, as I quoted, he says it works fine unless it pops out of engagement.

So the engagement issue causes the damage (seems reasonable) and the engagement problem os caused by drilling large holes.

Also read statement by Cuttings (Larry) who has the same mill: "Now if my memory is correct on this and it has been quite a while, I believe I made a new shaft to fit in there because the detent was not allowing the gears to be fully engaged in that low speed range. I probably still have the original part laying around if I dig a bit. I may have made a drawing for the new part"

That seems to be confirmation that engagement can be an issue with that mill.

[Cuttings]

I just had a look around but could not find the old shaft. After looking at the parts exploded view on page 13 of the manual my memory is a little clearer. I believe I made a new #290 shaft so I could slightly change the position of the 292 lever which operates the 293 shift fork and lift the gear enough in the low range to completely engage it. Once in the correct position I drilled new dimples in the shaft for the 286 & 291 set screws to hold it there. I have had no trouble since.
My old memory just kicked in again. The 282 set screw in the 285 high low shift knob can be tightened up to put more tension on the 283 detent spring help to better hold it in position.

[strokersix]

Perhaps a converted metallic gear design to molded nylon (acetal?). Often less than satisfactory if simply change the material. Could be a little more molding shrink than anticipated. Now there is excessive backlash or lack of mesh engagement in addition to a lower strength gear. Just a guess. I have no knowledge of the specific application.

I am a huge fan of nylon gearing. But it has to be designed that way from the start for best results. Conversions can be problematic. Lower strength material, loses strength and stiffness at warm operating temperature, sensitive to moisture and contamination. But if you stay within limitations nylon gears are excellent.

[aostling]

. . . Can you imagine requiring such methods on a manual transmission in a car to keep the counter-shaft and the main-shaft in engagement under load?

Not only can I imagine such methods, I can remember mine. After 200,000 miles my 1982 Stanza would always pop out of 5th gear. I fashioned a giant rubber band from a bicycle innertube to keep it in place!

[Willy]

Not only can I imagine such methods, I can remember mine. After 200,000 miles my 1982 Stanza would always pop out of 5th gear. I fashioned a giant rubber band from a bicycle innertube to keep it in place!

Yes but that wasn't due to the mainshaft and countershaft being forced apart due to radial thrust loads trying to separate the two.

Your issue likely manifested itself when under load in 5th gear, fine when cruising in 5th, but jumping out when under load?

We have to remember that the gears in most automotive transmissions are helical and are subject to axial thrust forces as well as radial thrust.
One issue that would cause your problem could have been improperly adjusted shift linkage, but the source of the majority of the problems like this stem from worn synchronizers,especially when when one takes into account the mileage of the vehicle in question.

[loose nut]

On the issue of $70 gears, if you can create an .stl file of the gear from your cad program, you can have them commercially printed very inexpensively. There are any number of vendors, I am using Jawstec for glass-filled nylon gears for a project I'm working on and the printed parts are very affordable. (No personal connection to the vendor.) I'm using FreeCad because, well, it's free, and it has an excellent gear production module.

I can print you a gear like that out of PLA or PLA+ if you want to give it a try. They work OK as change gears.

I would just need the dims.

[Doozer]

It should be possible to add a spring to the end of that handle such that it will hold tension on the gears in both positions.

Putting a spring on the shift handle (I have seen bungie cords used)
is a terrible idea. It will wear out the shift fork. Seen it many times.
I do however question why the spring loaded ball detent is not holding
the shift knob in position. Perhaps the ball compression spring is too
weak or the ball has worn a path between the detent positions and
the end holes that detent the ball are not deep enough. The detent
mechanism is likely under the shift knob.

-Doozer

[drawfiler]

The plastic gears in this type of machine failing is quite common, there seems to be a design fault. I have hobbed pairs of gears in steel as replacements, these have been successful and long lasting.
if anyone would like a set for their machine, please make contact