Again another Marui project : Super Wheelie and Big Bear!

[silvertriple]

Makes you think all metal transmission for vintage cars being within reach , to my mind the gears are the biggest bottle neck to really driving it like its the 80's

Thinking the same - lot of ideas in

[Dadio]

Refreshing to see it driven hard , you see so many YouTube films of people really driving so slow and careful , just seems a shame to see such dynamic machines reduced to crawling about .

[silvertriple]

I'm quite confident that if there is a weak point it is the diff, I needed to test it . So running hard is the way

[silvertriple]

A bit of disappointment today after yesterday's success...

I took the Super Wheelie and mounted the SLM printed idle gear...


Shaft hole is perfect, and the movement seems fine until I close the gearbox. When the gearbox is closed, the shaft is in the right position and it's too tight. I suspect the root diameter is too large of .1 or .2 mm.... It would be still in the specs of the printer, but too big for the use... It worked perfectly for the Big Bear this is not the case for the Super Wheelie, they were in the same print batch, and the conclusion is that the result might be unpredictable...

This need more work again

[Dadio]

I'd try swapping them to see if it's a difference in the gears or gearboxes first , there's a lot of possibilities , I found quite a spread of sizes with genuine yokomo 834b gears of different generations .

[silvertriple]

I'd try swapping them to see if it's a difference in the gears or gearboxes first , there's a lot of possibilities , I found quite a spread of sizes with genuine yokomo 834b gears of different generations .

Well, I measured the gears and external diameter of the SLM printed gear is definitely larger and it gives an outside diameter of 19.4mm instead of 19.2 mm for the outside diameter reference as per a gear calculator. MJF PA12 gear was 19.2... This confirms the print predictability issue, as likely the inside diameter and tooth profile are offseted on this gear. And yes, it is still in the printing specification.

Spoiler : it's not the same situation on the Big Bear, where the gear drops in perfectly.

In fine more tests and prints are required, but i'm not sure what I can do to mitigate this : If i apply an offset to give more space to the gear, the profile of the tooth wont be the same, and if it prints more accureatly it would be wrong as well...

[Dadio]

Tolerance problems suck , I'd guess that if you had a complete set of steel gears then you could accept printing them at a -% of full scale and they would still be strong enough , I've played with scaling gears to get them to be the same size as the original, as I mentioned with the Yokomo 834b they are meant to be 0.8 mod but they measure smaller than a gear generated in Fusion with the same tooth count so I scaled them down fractionally , it's not so demonic to good mesh as you may imagine , I think the issue with the 834b gears was probably from contraction when cooling after injection molding, but they ran against metal pinion gears that will have been correctly machined to spec without issue .

[silvertriple]

Well, I'm still puzzled as the Big Bear idle gear which is 9T-25T mod 0.8 is right while the 9T-22T mod 0.8 is too big while printed at the same time in the same printer: not sure how to scale to get it right...

[Dadio]

Do they list their printing tolerances for printing ?

[silvertriple]

Do they list their printing tolerances for printing ?

yes, and it's in the printing tolerances, as mentionned previously...

EDIT : I initiated discussion with the provider to recheck this point. Tolerance is announced as +/- 0.3mm or within 0.4%. And 0.2mm on 19.2 makes it for about 1% so more than 0.4%. The or is the question. I'm getting confirmation with the support and I'm likely to file a quality complaint based on the result.

[silvertriple]

The printing specifications are the following for SLM


I had quick discussion with the support and it was dissapointing... The support guy just killed the principle of the rapid prototyping using 3D printing in one single statement.

The way the tolerances are stated "±0.3mm OR within 0.4%" makes this clearly not suitable for gears.

I'll check with other providers for SLM...

[silvertriple]

That being said, i'm tempted to order a few to see what is the deviation and assess how I could design a part which fits the requirements with a higher percentage of chances...

[Dadio]

I guess if you were printing mod 1 gears you could easily allow for the tolerance but with mod 0.8 it's harder .

[silvertriple]

Yes, but there is absolutely no room to make this in mod 1 in this gearbox... I had a quick look at cnc and cost would be crazy high... I need to think more...

[Dadio]

Yes, but there is absolutely no room to make this in mod 1 in this gearbox... I had a quick look at cnc and cost would be crazy high... I need to think more...

The honest way forward may be to publish or sell the file and let people decide for themselves how to proceed .