Author Topic: Easy question for a mechanical engineer - Stress  (Read 1202 times)

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Offline Jester

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Easy question for a mechanical engineer - Stress
« on: September 05, 2021, 09:27:52 pm »
A few days ago I picked up a 5x6 horizontal band saw, more or less identical to the infamous 4x6 band saw Harbor Freight etc. sell. The table for vertical use is pretty Mickey-mouse and one of the common mods is to make a better table for vertical use.

I'm going to make an easy to install & remove table out of aluminum based on this video:   jump to about 13:55 to see the block.

My question relates to mounting the mounting block (see below) to the cast iron frame. The frame where the aluminum block will mate to is fairly thin I'm guessing about 5/32" thick, I'm going to mount the block with 2 x 1/4" bolt. My question is to provide the least stress possible to the cast iron frame should I drill and tap the frame or should I use through-hole bolts with washers?  Or I could do both?

« Last Edit: September 05, 2021, 09:32:59 pm by Jester »
 

Offline james_s

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Re: Easy question for a mechanical engineer - Stress
« Reply #1 on: September 05, 2021, 09:45:34 pm »
I'm not a mechanical engineer, although my dad was and I have a fair amount of experience building/repairing/modifying mechanical things. If I'm understanding correctly what you're doing, it should not make any significant difference whether you drill plain holes for a clearance fit or drill slightly smaller holes and tap them. The important thing is to use a good sharp drill bit and drill nice clean holes. If you're going to tap them, use the correct size drill for the tap and use a bit of cutting/tapping oil and make sure you run the tap straight through the hole. I like to chuck them in my drill press and rotate the chuck by hand to thread, or if I'm feeling cocky I'll turn on the drill, line up the tap and then switch it off just as the tap contacts the hole. Learned that trick from my machinist friend.
 
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Offline 455Turbo

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Re: Easy question for a mechanical engineer - Stress
« Reply #2 on: September 06, 2021, 02:28:04 pm »
Normally one would consider the tensile strength of the screw and the the tensile strength of the 'nut' to determine the necessary thread engagemment.

These are considered so that you can ensure that the screw fails in tension before the thread fails i.e. leaving you with a hole and no threads left in it.  In your case, there's no way to know the tensile strength of the 'nut' which is the cast iron of the machine. So you can't do the math here.

But threads in cast iron ususally run pretty deep, four or more times the screw diameter. This is because the tensile strength of cast iron is low compared to that of the steel screw. Since you have only 5/32" of thickness, the thread won't even be 1x the screw diameter in length. If you thread the cast iron, it's going to be a pretty weak thread.

If you feel that you must drill holes in your saw then a nut seems to be the better choice.

But, either way creates a stress riser in a cast iron member that is supporting a tensioned band saw blade. This is sort of like drilling a hole in a c-clamp (although the forces are acting in opposite directions). Either way, if it were my saw, I wouldn't put any holes in it at all.
 
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Offline james_s

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Re: Easy question for a mechanical engineer - Stress
« Reply #3 on: September 08, 2021, 03:27:27 am »
I was thinking in terms of the strength of the casting once it has a hole in it for attaching something that will not be highly stressed rather than the strength of the attachment of whatever is being screwed on with the screw. I agree that you need to be careful with where the hole is drilled and be mindful of stresses on the part and removing too much material, though I generally wouldn't expect a single hole to be particularly risky in a light duty benchtop tool.
 

Offline Jester

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Re: Easy question for a mechanical engineer - Stress
« Reply #4 on: September 08, 2021, 07:40:06 pm »
I was thinking in terms of the strength of the casting once it has a hole in it for attaching something that will not be highly stressed rather than the strength of the attachment of whatever is being screwed on with the screw. I agree that you need to be careful with where the hole is drilled and be mindful of stresses on the part and removing too much material, though I generally wouldn't expect a single hole to be particularly risky in a light duty benchtop tool.

I'm not at all concerned about the screw or threads failing. I am concerned with creating a stress riser in the casting.

Well this is what I came up with, it takes about 5 seconds to install or remove.

The mounting block attaches to a portion of the frame that is actually more of a shroud for the blade so I don't think that area is heavily stressed.

The quick release clamp is in a high stress area, however I mounted the lever with 2 x M4 screws so the holes are pretty small and they are in the center of a fairly thick and webbed portion of the casting, hopefully the angle iron bracket in parallel with the casting will negate the effects of the small holes. Time will tell.


5s video here:  https://www.hobby-machinist.com/threads/4x6-quick-release-table.95026/
« Last Edit: September 10, 2021, 04:17:07 pm by Jester »
 

Offline EPAIII

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Re: Easy question for a mechanical engineer - Stress
« Reply #5 on: September 22, 2021, 06:53:19 am »
That is a nice table. I may have to do something like that with my Grizzly 4x6.

As for tapping those holes vs. using a nut and washer, I would bet that some Chinese engineer calculated that frame's shape and thickness so as to not waste a single ounce of CI. I would use the nuts with not one, but two washers. One standard sized washer and one about twice that size and thickness. That will spread the stress over a large area.
 


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