Deformed plastic gear on milling machine

[Bill PhinnParticipant @billphinn90025]

A couple of edits:

The diameter of the big cog is more like 87.6mm, rather than 88mm. Definitely not as small as 87mm, though, as Howard suggests.

The observant may have noticed that the total thickness given in my previous post of 28.65mm is less than the sum of the two cog thicknesses and the width of the connecting/separating shaft by 0.4mm. I haven’t quite worked out why this is, but it may be something to do with the slightly raised area on the big cog, both on the upper surface (visible in my photo) and on the underside. Possibly this raised area is there to reduce friction against the gear shift paddle that sits between the two cogs.

[Michael GilliganParticipant @michaelgilligan61133]

[JasonBModerator @jasonb]

Based on tooth count and it being a MOD gear diameters are likely to be (NoT + 2) x MOD

Big gear 56 +2 x 1.5 = 87

Small Gear 31 + 2 x 1.5 = 49.5

[Michael GilliganParticipant @michaelgilligan61133]

… which makes ‘about 0.6mm’ on 87mm a reasonable amount of growth for a hygroscopic material, methinks.

Perhaps more relevant though, is the fact that the small gear appears to have been only meshed to less than half its width.

.

.

 

MichaelG.

[JasonBModerator @jasonb]

Yes that is what I said in my first reply

[JasonBModerator @jasonb]

11.3 + 11.3 + 6.45 = 29.05, in my book that is more than 28.65. I’ll draw it for you but need numbers that work.

[DC31kParticipant @dc31k]

[RobCoxParticipant @robcox]

I’ve only just come across this thread, being one of those who visit less frequently since the site update.

Bill, have you sorted out getting your gear made? If not, I think I may be able to help you out here. I’ve made an attachment for my Elliott 10M shaper to cut gears, any DP, MOD and tooth count and it should be perfect for cutting the two gears as a single unit.

If you want to see more, (a bit of shameless self promotion here), have a look on youtube for ThePottingShedWorkshop, where I’ve posted some videos about it.

If you want some help, PM me and we can go from there.

Rob

[Michael GilliganParticipant @michaelgilligan61133]

[Bill PhinnParticipant @billphinn90025]

[JasonBModerator @jasonb]

Bill, the post I took the picture from in this reply of mine went on to show a clamp that the guy made to lock his selector know in the fully engaged position as he too found that the selector and therefore gear worked the it way down. It was a bit rough but did the job. I will see if I can find it again.

If the existing plate is just held by those two screws you could just rotate it 180deg and drill some new detent holes.

 

If 29.45 Overall is OK I can easily put a 0.2mm raised area either side of the large gear or could reduce the central bos sto keep OA as 28.64 with the 0.2mm bearing. then you have the option to print or go elsewhere.

[Dave HalfordParticipant @davehalford22513]

On the detent plate, should it be a large diameter plate to give more authority. The detent lever on a Centec is much bigger.

The 2nd U tube link posted by Pete Greene from 6.00 to 6.04, where the guy actually tests the shift, the gear with the bad one that you can barely see, seems to move down as the gear is selected and up when not selected.

If the gear below is moving away under load and the detent is bad as well.??

[JasonBModerator @jasonb]

Link to the thread I mentioned above with the locking lever.

[DC31kParticipant @dc31k]

[Dave HalfordParticipant @davehalford22513]

Don’t think a ball will work in a hole deeper than half the ball diameter.

Thats a mans locking lever on Jasons link.

[Bill PhinnParticipant @billphinn90025]

Thanks a lot for the further replies.

Jason, the overall thickness is 29.05, though as we’ve said the sum of the cog thicknesses and the connecting boss comes to 28.65. I don’t think adding a 0.2mm boss either side of the big cog to make up the difference would do much harm.

Countersinking three new detentes on the opposite side to the present ones certainly seems like a time saver compared with making an entirely new plate, not least because it’s not just a round plate but a top-hat shape with an integral spigot that passes through the sidewall of the mill head.

The thread about the Craftex mill makes for interesting reading. I’m not sure I’d opt for Brian Rupnow’s lever lock solution, though; it seems a bit cumbersome and, if I’ve understood things correctly, the locking pin/screw locates against the back of the motor cover, which is only plastic.

Dave and DC31k, re your comments about the detente plate, I would say the plate is large enough for the purpose and the detente divots about the right depth (I’ve never known the mill to actually jump out of low or high gear); the problem with the gear selection, I suspect, if there really is one, is that the low gear detente isn’t sufficiently distant from the centre detente to hold the low gear in its correct position at the very top of the gear shaft; instead it appears to slip back down the shaft slightly as the ball locates in a poorly positioned detente.

DC31k, a pin or dog point set screw in a hole rather than a ball bearing in a detente certainly seems like a more secure locating/retaining solution. I may well resort to this if I feel even a repositioned ball detente isn’t getting the low gear to stay where it belongs.

[DC31kParticipant @dc31k]

[JasonBModerator @jasonb]

[Bill PhinnParticipant @billphinn90025]

Thanks, Jason. I’ve clearly got things the wrong way round there. Sorry.

Let’s assume the overall thickness is 29.05, consisting of a gap of 6.45 separating two gears that are 11.3mm thick just inside the point where the teeth begin, whilst the large gear has a raised area only on its inner surface of 0.2mm (extending to around half the diameter from the centre? – based on the picture of my gear and Brian’s gear).This makes the large gear both 11.5mm thick and 11.3mm depending on where you caliper it.

If there is anything still wonky about these figures, do let me know.

Edit: Just realised there is at least one thing wonky: to preserve the gap of 6.45 and have a thickened area of 0.2mm at the centre of the bigger gear, the overall thickness will be 29.25, not 29.05.

DC31k, thanks for the further information. My initial idea was to mark the positions for the new detentes on a piece of card using a pair of compasses, using the locations of the high gear and central detentes as the guide for the new low gear detente position, and then prick punch through the card on to the plate. It may be safer though to do as you suggest and use the actual position of the gear knob to determine the correct location of the detentes for the three gear states. I’ve got transfer punches to go down the hole.

[JasonBModerator @jasonb]

Still can’t make it work out to 29.05

If the top gear is a constant 11.3

The Gap a constant 6.45

And the large gear 11.5 when measured towards it’s middle

Then total is 29.25

Or

If top is 11.3

Gap 6.25

Raised area 0.2

And gear at it’s edge 11.3

Total is still 29.25

Neither is 29.05 or 28.64

This is it dimensioned either way but total stays the same.

 

[Bill PhinnParticipant @billphinn90025]

[JasonBModerator @jasonb]

OK, Message me an email address and I’ll send a file that can be used for printing if you decide to go down that route.

[Pete RimmerParticipant @peterimmer30576]

[DiogenesParticipant @diogenes]

Edit – redundant post – !

[Bill PhinnParticipant @billphinn90025]

Just to update, I’ve received a newly made 3D printed gear from the print shop.

Apparently the material used is PA12. This is a form of nylon, I believe. I’m not sure how strong this is relative to other nylons or whether it’s the optimal plastic for a gear for this purpose. Unfortunately, I don’t know what percentage of infill has been used on my gear.

I’ve just watched a video of a test done by Youtuber Clough42. His results suggest that plastics such as ABS and certain nylons that people assume are a stronger choice for gears than the most commonly used printing plastic PLA may not actually be stronger than it and may in fact be weaker.

Anyway, I’ll have to fit the gear first and use it for a while before I can vouch for its durability.

Many thanks to everyone who generously offered me their help and advice.

[Author]

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