Oversized hinge pin holes, what to do?

[XLR8]

I've noticed that many of the hinge pin holes in Niki's '91 Stealth "racer" parts that came from iMaterize are printed oversize (a standard 1/8" pin is way too loose).
A #30 drill bit which is 0.1285" diameter fits almost perfectly. I've found reamer blanks in this size (#30 x 2-3/4" long) posted on ebay. Although I've never done this before, I think I can cut these blanks to length and grind the e-clip grooves using a thin Dremel cut-off wheel. Is this the best solution or is there a better way to solve the problem?

[jwscab]

did you run a 1/8" bit through the parts? If they are oversize, I think it warrants a call to imaterialize to discuss the issue. they would have been manufactured incorrectly.

otherwise, sure, you could make any size pin you wanted as long as it fit the holes. the biggest issue is if the blanks are already hardened, you are going to have a hell of a time making e-clip grooves.

[XLR8]

Hmmm okay, thanks..
The problem is that an 1/8" pin or bit literally falls through the hole so drilling isn't possible. I hadn't consider this before but you're right, I probably should contact imaterialize to make them aware of the problem. The parts seemed to be fine otherwise but the sloppy pin fit makes them useless.
So, do "reamer blanks" typically arrive hardened? Won't they require some flutes to be added to turn them into a reamer and will these flutes be ground into the blank? I'd assumed the blanks were hardened after the grinding process. If I can't grind the grooves then I'll need to scrap the reamer blank idea. I could fill the holes with epoxy and re-drill them to the correct size but what a pita.
Meanwhile, I'll send a note to imaterialize and wait to see what they have to say.
Here's a pic from ebay...

[jwscab]

They are usually ground in the hardened state.

Another option which I have done in the past is use brass tube with a 1/8" Id and make the od a light press fit and glue in place.

[XLR8]

The brass tube idea is a good one.
I've actually done that before as well with a Tamiya Sand Scorcher, the very first RC car I've ever owned. That car's suspension was (is) entirely metal and I was worried about all of those metal-on-metal hinge points wearing out over time so I drilled and fitted them with brass tubes. I guess I thought of them as sacrificial and replaceable bearing inserts. Since brass is softer than the mating steel pins, the inserts absorb all of the wear.
Tinkering with these parts again this afternoon, I've notice another thing; the holes are somewhat oval shaped. So, while there may be slop along one axis, the pin may be tight along the perpendicular axis. I'm guessing this is related to the printing process and how the parts were oriented in the machine.
Anyway, I've sent a message to iMaterialize requesting some help.
Thanks for your help sir.

[XLR8]

Reply from iMaterize:

"Thank you for sending us your feedback and we’re very sorry that the models are not what you expected them to be.

Please be aware that each material has accuraccy pecularities, since high precision can't always be reached. As you can see in our material overview on https://i.materialise.com/en/3d-printing-materials/polyamide, the accuracy for MJF Polyamide is 0,3% (with a lower limit of 0.2 mm).

In addition, you filed your complaint more than a month after the delivery of your products and unfortunately we can’t give you a solution anymore because in our terms and conditions (https://i.materialise.com/en/legal/terms) is clearly mentioned that upon delivery of the product, i.materialise expects that you submit the product to a thorough inspection. If, upon inspection, you feel that the product is not in conformity with the order, you should contact us as soon as possible and at the latest 14 days after your receipt of the product.

Kind regards,
Lizy Janssen
Customer Support Officer"

So, the important take-away here is to inspect your parts early and make any complaints known to the printer within 14 days.

I'll take a moment here to play the role of Monday morning quarterback... Given the importance of the fit of the hinge pins, I think it would have been better to print the holes under-size and let the customer drill or ream them to size. Then, not only could the holes be sized correctly, they would have also been round. I can appreciate the desire to reduce pre-assembly prep work and perhaps it makes sense to print the holes to size for replica parts where hinge pin fit isn't important but for "racer" parts, I think some prep work could be tolerated to ensure the best possible fit. What do you think?

So, the solution of choice here is to use 3.5mm rod (0.138") with the hinge pin holes that over-sized. I've found some 3.5mm stock online and expect it to arrive next week.
All things considered, this is a minor bump in the road and there will surely be more to come. Overcoming obstacles is just part of the fun.

[jwscab]

Ah that sucks. Shame it was too long beyond their inspection period.

Yeah the holes are definitely more like an oval. But the ones I've used seemed ok. I think they are designed right to spec but who knows. You would have to ask niki.

[XLR8]

I think you're right because some of the holes are absolutely spot-on and the pin fit is perfect.
It's odd.. The 4 arms are printed grouped together side-by-side and the two arms on the outside of the group (a rear and a front) have hinge pin holes that are sloppy while the two inner arms (also a rear and a front) have holes that are perfect. So this suggests that the variability results from equipment capability limitations and not from design error.

[mk-Zero]

Unfortunately the parts meet their spec. 3% on a .125 hole is about .004, and they are hitting that. Their lower limit is 0.2 mm, which is more like .008. So in reality, they could be much worse and still meet their stated tolerances. Nature of the beast when 3D printing. One strategy I have used is to design holes undersize and ream as necessary. I pretty much run a drill through every 3D printed hole anyway, so it's not even any extra work.

The sleeve idea is probably your best bet. You can probably find some SS hypotube at McMaster that will work well.

[XLR8]

Thanks Brian, I agree completely. It's better to print holes that require a good fit under-size then cut them to the correct diameter.
I have some 3.5mm rod stock headed my way.

[XLR8]

It suddenly occurred to me tonight that the ProSC 4x4 uses large hinge pins. I happened to have a spare set, pull them out of the bag and measured them; 3.5mm D'oh!
Oh well, I guess I'll just have lots of spares.