Clearance fit in a small gearbox

I am building a small gearbox using M0.5 gears with 2.5mm bores. There is going to be a clearance fit between the gears and the shafts. The gears will be sliding on the shafts like on the gearbox shown in the attached picture.

Before I get the gears made of ISO 45C steel I would like to get them 3D printed in resin for a prototype.

For shafts I am going to use 2.5mm h6 dowel pins.

My question is how do I create the right clearance between a shaft (dowel pin) and a 3D printed resin gear. Can I get the bore in a gear printed to 2.4mm and then ream it with H7 2.51mm reamer? That way the clearance would be similar as in H8/f7 or F8/h7 preferred fit.

The reamer: GARANT NC reamer H7 2,51 mm

In my limited experience with 3D printed holes I find them not to be very precise. I would recommend printing it as a solid then drill and ream.

I’d think you would be best served to make the hole undersize and open it up with a reamer this way the hole is printed on center or close to. I can’t see trying to run an indicator around a printed gear being the easiest thing to do.

no704 said:

In my limited experience with 3D printed holes I find them not to be very precise. I would recommend printing it as a solid then drill and ream.

Click to expand...

If you do that the area where the hole should be might just be that core filler stuff and not solid plastic.

In my experience, you'll spend more in labour hours farting around with 3D printed parts that are nowhere near the size of the real part than you'd spend to just machine them (even in a prototype material like aluminum), especially considering that because they'll be way off size, have a terrible finish, and break immediately, you won't get any useful information from them at all.

I see it a lot - people spend tons of time trying to fit the 3D printed prototype to whatever machined component already exists and you just get nothing from it because none of that effort would be necessary for the real product.

that size tooth is pretty darn small- I printed some concept gear racks recently and they were .6 mod. The tooth form was terrible and I used a pretty nice printer to print them- ended up using a pinion like a shaper cutter to get some shape to the teeth. Short version- printing is a waste of time for small teeth like that.

I think you all assumed that the gears will be printed with an FDM 3D printer. I would get them printed with a resin printer (SLA/DLP technology). From what I have seen very good accuracy and surface finish can be achieved with a resin printer. Correct me if I am wrong.

My question is more about what tools to use to make a hole the right size whether that's in plastic, brass or steel. I noticed that mostly H7 reamers are widely available. Can I ream a hole with a 2.51 H7 reamer and expect a fit with clearance similar to clearance in H8/f7 or F8/h7 preferred fit?

Preferred fit in H8/f7 or F8/h7 would have minimum clearance of 6μm and maximum clearance of 30μm.

2.5mm h6 (+0 -6) shaft and 2.51mm H7 (+0, +10) hole would have minimum clearance of 10μm and maximum clearance of 26μm. That clearance is within a range of H8/f7 or F8/h7 preferred fit.

So my question is if I can produice the right fit with a H7 reamer.