Correct bore size for shaft

Christopher Tidy wrote in news: snipped-for-privacy@cantabgold.net:

H7 fit, 4.751-4.761 as the shaft is most likely h6 (-0.001/-0.011) This gives you a min clearance in a MMC condition of .002. But a more likely clearance of 0.01 or so.

Hi all,

This might seem like a silly question. I need to get a brass gear made. It will fit a 4.75 mm diameter precision ground steel shaft and be secured by a grubscrew. What diameter should I specify for the bore of the gear? I don't want it to be a tight fit on the shaft, but I don't want it to be so loose that the grubscrew causes the gear to become eccentric. When I make parts like this myself I normally just remove the material a little at a time until I achieve the desired fit, but what do I tell someone else to do? My gut reaction is to go with exactly 4.75 mm, but I'd be glad to hear what the experienced machinists have to say.

Best wishes,

Chris

Ok. 4.75 mm is about 4.75/25.4 = .194 inches. hmm... small shaft. For this size shaft I would specify 0.0 to 0.001 inch clearance (or 0 to 0.0245 mm) clearance on the bore. This represents a reasonable manufacturing clearance, but if you are willing to pay you can get half that. If gear performance is critical then you might want to rethink the whole problem.

Don't forget to put a little flat on the shaft so marring from the grubscrew won't make the gear impossible to remove.

Hey Kelly,

My calculators suggest that 4.75 mm is 4.75/25.4 = .187007874", while

4.75/24.5 = .193877551. I think you WROTE us the correct "formula" (above), but you had a typo entering the mm to inch conversion figure.

In any event, for Chris' purpose, this 4.75 mm is VERY close to 3/16". I think I'd settle for ordering gears with a 3/16" bore and ream to whatever others here determine to be the "fit" size, if necessary. Personally, I really expect I'd try just a little warming of the gear in hot thin oil and "shrink" it on the shaft.

Take care.

Brian Lawson, Bothwell, Ontario.

An interferrence fit is suggested for this application. Bore should be 4.75mm H7 and shaft be 4.75 j6

4.75H7 is +12 / 0 µm 4.75j6 is +6 / -2 µm

HTH, Nick

Darn calculator, anyway.

thanks

Thanks for all the advice. I'm inclined to order a gear with a 4.75 mm bore and see if it fits. If not, warm it as Brian suggests, and if it still doesn't fit ream it out to 3/16" (4.7625 mm). I'm also wondering about cheating. I have some brass gears which fit the 4.75 mm diameter shaft perfectly, but they're too small for what I want to do. I'm tempted to order a standard gear, bore out the centre to accept the boss from one of the gears I have, then soft solder the boss in place (the loading is negligible). This saves the expense of getting a gear specially manufactured.

Thanks for the suggestions.

Best wishes,

Chris