Hi all
I am desperately looking for technical information without success. Does anyone have any recommendations or an abacus on the minimum distance between the axis of a tap and the edge of the tapped part? If possible, depending on the materials being processed, in this case GS 500-7 cast iron.
Thanks for your help, Hugo
Hello
Have you looked, or even consulted a manufacturer or seller of taps?
Cdt
Fred
Hello Fred,
I consulted different documentation on the material, on tapping tools, specialized sites on machining as well but nothing.
Cdt, Hugo
It's more a problem of the strength of the materials than of manufacturing.
I was taught that the distance between a hole and a wall is equal to the diameter of the hole.
It's a basis, to go further, I think you have to go through calculations if you have particular constraints.
I think it's mostly the distance between the bottom of the net and the edge of the piece that counts.
It depends on a lot of parameters such as the degree of fixation desired according to the effort.
If it is to hold a piece that does not have any particular tensile effort, you can go up to 1mm between the bottom of the thread and the edge of the piece. Sachnat that it depends on the number of screws that hold the part. If it's a crown with 12 screws, it's not the same as a single screw at the edge.
We cannot give precise rules, which is why there is no documentation on the subject.
After that, it's experience that says that in some cases you can take this or that risk.
I think it is prudent to stick to Stefbeno's recommendation and go through the PEF simulation, especially for cast iron.
Post your piece this will give us a more precise idea.
Kind regards
It is a cast iron arm that is heavily stressed by the weight of the equipment it supports. The arm is fixed at one of its ends by means of 4 or 6 screws but the distance between the axis of the holes and the edge of the part is very small.
The 3D model has been calculated, but nothing guarantees me a good rigidity of the part at the periphery of the taps (this is only a simulation).
me personally if it's cast iron
proportionally via the type of room in sight
knowing in + that you're going to put a load on the arm of how much we don't know
I doubt the effectiveness of your fixations
if it's foundry, why not optimize the thickness at the right of the taps?
and move the taps a little to the outside
what is the rating of your half shell (diameter and wall thickness)
@+
FYI:
- The arm is 3.10 m long / Height of the tapped base 275 mm / Outer radius: 90 mm / Wall thickness: 20 mm
Optimization is still possible, but the constraints and customer requirements on the volume of the parts are severe. Otherwise, of course, I would have sized the arm differently.
Cdt, Hugo
How much fastening do you have?
Four if I read correctly but especially what diameter the taps.
since the depth is 20 mm it would mean that your bolts are between 10 and 12 Ø but give us the exact dimension :-)
It is mainly the two holes at the top that are stressed in tension, the others are only in compression with a little shear, knowing that the screws never work in shear.
Kind regards
Currently 4 fasteners, the possibility of increasing to 6 is greatly considered to reduce the stress on the anchor screws.
18 mm drilling and 15 mm deep tapping.
For your information there is about 28 kN of traction on the most stressed screws --> This forces me to switch to M12 with 4 fixing points and possibility to go down to M8 with 6 points. But my fear remains unanswered, what is the mini material to have on the tapping periphery for GS 500-7 cast iron.
Cdt
increase your wall thickness to 24
and make M8 x 6 tapping holes
you will have a diameter of thickness on each side
but not so great cast iron
To go via helicopters maybe?
@+
As GT22 said, it's not great to tap in cast iron, especially to work on tearing.
Do you have the possibility to make smooth holes in the cast iron and make "nut inserts" out of steel? These pads are just a piece of steel plate with 2 tapped holes.
As for the question of the real minimum of matter, given your constraints, I believe that there is hardly any calculation that can say so.
Good evening Hugo
Normally, if your simulation is done correctly, that is to say with the right connectors, indicating that it is tapped, it will tell if you are within the limits or not.
That said: back to basic
A Ø 12 screw quality 8.8 with a pull in the axis is 1,980 Kg or 19,800 N or 19.8 kN obviously more in quality 10.9 or 12.9.
Remember that the thread is 1.5 times the diameter so if it's M12 it's at least 18 mm of active threaded part.
So either you say 28 KN for the most stressed screws (which ones ??? and how much ??? the top two I guess) is it a value for two screws or one screw.
Since it seems to be a load at the end of the arm, you will have movements in the radial direction. Which means on the radial sides. So if you only take loads in vertical, it will be in the carrots, especially since cast iron is not known for its elasticity. Your screws too close to the edge are mainly stressed in radial and I hope not in shear.
Instead of having taps: you can replace it with screws washers bolts. There at least you will be sure of the absorbable constraints.
I am wary of cast iron, like the plague and the tax collectors, because the slightest defect in the casting, straw or air bubble is well its fart.
Given the effort I wouldn't have fun being closer to the edge than the radius of the puck adapted to your hole. You're not talking about the puck.
Where does your cast iron part come from, it's your company that makes them?
To everything'
Indeed, the simulation validates or not the forces in the threads, which has been done. But for example, I can put an M8 tap instead of the M10 currently on the model and the result is okay on the constraints and displacements --> only the analytics tell me that the M10 is not enough.
To bounce back on your Zozo writing: in the GS cast iron (500-7) in this case I am in CL 6.8 at the level of the screws, above it is useless and in any case it is the elastic limit of the cast iron that is more critical.
No shearing on the screws, only traction.
The arm wraps around a tube, and on the opposite side of the arm I have a shell which has laminated holes for the screw heads (customer design): nevertheless a proposal of laminated holes on the 2 parts is interesting!
Your approach to say that the mini would be the radius of a wide flat washer is a good first approach: I validate.
Machining in cast iron is not a problem, it is even controlled and repeatable in a sustainable way at my supplier. The arm is not made in-house but with one of our subcontractors.