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CAD - Engineering - Technical > Machining/ design CAD question...
 
 
cmsilvoy
Veteran
Location: Durham, NC

Hi guys,
Not sure if my topic will attract the right crowd, but hopefully so!
I am working on a project to make a SWEET helicopter even sweeter by customizing some frames in CF material.
I routed a set of frames today to check tolerances and noted an important effect:
The 3mm holes did not accept 3mm hex screws. I know, I drilled the holes EXACTLY to 3mm. The e holes required a little "loosening" before a screw would slip in.
My question:
To what size should I drill a hole to hold a corresponding bolt to tight tolerance?
ie, 3mm = 3.05mm?
4mm = 4.05, etc???
The same also goes for tightly fitting bearings into the side frames.
This is probably something that I should be able to find in a machinery handbook, but maybe somebody could spare me a little effort and spell it out here
Thanks everyone!

Chris
10-12-2004 Over year old.
 
 
Hockeytown
Veteran
Location: Pontiac, Mi. U.S.A.

Here is what SolidWorks says.........

Using the Hole Wizard in SolidWorks I have options for Close fit, Normal Fit and Loose Fit.

With an M3 bolt, the diameter of the hole is 3.2, 3.4 and 3.6 respectively. These values are taken from the ANSI Metric Standards.

If the hole is exactly the same diameter as the bolt, you will definately have a near 'press fit' situation.

Have fun!!

Darin[color=red]
Hockeytown[/color]
10-12-2004 Over year old.
 
 
CK_
Senior Heliman
Location: Redondo Beach, CA

I use 3.1 for an M3 screw as a minimum. If you are making side frames you might want to go a little looser than this as this will allow everything to float and line up better before you torque down on the screws.

I'm not sure about the bearings. You could just make a series of holes on some scrap at dia+0, dia+.001, dia+.002..... and find out which one gives you the press fit you want.

Chris
10-12-2004 Over year old.
 
 
Peefor
Veteran
Location: St Albans UK

It depends a very great deal on what sort of set-up you have. How rigid the material is held and how flexible the tool is and any 'free-play' and tolerances in the machine.

If you drill, (with a drill bit) a 3mm hole a 3mm bolt will enter easily. If you mill a 3mm hole with say a 1.60mm dia bit it is very unlikely the bolt will enter, if the tool is offset 0.80mm. Depending on the application I may reduce the offset to 0.70mm.

I would suggest some test holes in some scrap material to work out how much allowance you need with your variables. What does help too, if you do not cut two sides together, is to cut them in the same place on the machine bed, so any pitch errors are the same.

Pete
10-12-2004 Over year old.
 
 
Pole
Senior Heliman
Location: Norway

fore the bearings it depends on the brand, but from 0 to + 0.015 I think is ok... but take a look at this link.. SKF


And btw. try to get as smoooooth surfase as posible...

Stay tuned fore more happy days
10-13-2004 Over year old.
HOMEPAGE  
 
 
OT45
Senior Heliman
Location: Kingston, NY

I suggest reaming the holes for the bearing press fit. Mill the bearing pockets undersized by.005" and ream with .0005" bigger than the bearing od. You might have to go bigger or smaller depending on fit. It helps to have a few reamers on hand.

scratch building is not just for planks
10-16-2004 Over year old.
 
 
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