Hole Size and Strength

[Beltzington]

This section should be named welding and fabrication so forgive me if I am off topic.

A very smart manager once told me never side track a project trying to achieve perfection which is something I struggle with at all levels. That being said I am not building a satellite and am wondering when talking farm attachments does oversize holes significantly weaken a product. Specifically would drilling a 3/4" hole for a 5/8" bolt even matter once it is securely tightened. The load would only be in one direction so even if it move 1/8" inch it would stay there. Trying to decide if I need to buy a $20 bit, also the extra wiggle room will allow for small fitting errors.

 

[Streetcar]

On a bridge ,the holes are typically drilled 1/16 oversized.
For an 1/8 oversized hole, we would just use washer on both bolt and nut side of the connection

 

[Shield Arc]

Personally I wouldn't do it. I like to drill 1/32 of an inch over size. But I guess it depends on the application.

 

[Big Barn]

Personally I wouldn't do it. I like to drill 1/32 of an inch over size. But I guess it depends on the application.

I think we all aspire for this ideal scenario.

Personally I think you'll be fine with the oversize hole. Oftentimes our layout is 'off' and for "some" reason, things just don't fit right. (Molecules move etc)

Heck. Back in the old days (before walking six miles to school in the snow) on some projects I had to torch out the holes and touch 'em up with the die grinder..think I learned that from watching the Flintstones.

Didn't have Jody and YouTube back then......

Terry

 

[LD1]

The size I drill depends on a number of things. Connection type, load directions, overall total load, how well I am able to lay it out, etc.

Sometimes I drill 1/8" over and still needs a little love from the die grinder and carbide. (I dont own a set of threaded transfer punches).

Othertimes, where I can clamp two parts together and get exact, Or drilling a clearance hole that will later thread into something that is not yet drilled and tapped, 1/32" is more than enough. Bolts are clearanced anyway. So simply using the nominal size and cleaning the bur off will clear a bolt without issue.

 

[yomax4]

Does depend on the application but if it's as close as you can get, That's as close as you can get. As stated above, I use the torch or plasma to drill holes at times. Just did so on my plow cutting edge. Not too critical.

 

[Gary Fowler]

If I am drilling a hole, there is no reason to drill anything larger than 1/16" if you have a complete drill bit set.
As Yomax said, I sometimes use a torch like I did when remounting my driver side step on my RV. The builder used self tapping screws to mount it which sheared off. I torched in new holes and put in 3/8" bolts with double nuts compared to the original 3/16" ST'ing screw. NO WAY was I able to reach up inside the tire well and drill out new holes which still included the sheared off ST screws.

 

[hosspuller]

Application is key. How much material is there to the edge ? If your 3/4 inch hole leaves 1/16 to the edge... Then no, use the $20 proper bit. If there are 2 inches to the edge ... Sure use the 3/4 drill and washers to adapt.

 

[bigtiller]

I had a helper like that but someone stole it.

 

[Big Barn]

That's when I use my helper!

Yup. Bigger er..uh.. hammers works for me too!

 

[Big Barn]

I had a helper like that but someone stole it.

She's bunking down in this guy's shop.

 

[MMagis]

As mentioned, application makes difference, but it sounds very unlikely a hole oversized 1/8” is going to hurt anything. If for some reason in the future you decided it was too sloppy, you could just make up a bushing to fix it.
But my guess is, you’ll forget all about it.

 

[dragoneggs]

As others and especially Hoss has mentioned the application and loading is key. Edge distance is a consideration for a couple reasons. One is tear out... if your hole is close to the edge of the structure and the load is generally in that direction then you want the hole to have at least 2D edge distance (measured from the center of the hole). Bolt fit (clearance vs. tight or interference fit) generally is only an issue in fatigue situations where the bolt can help create a little compression zone around the hole. Holes and notches are where cracks cracks start. A loose fit bolt not clamped/bolted down tight enough for operating loads will also produce excessive wear and maybe eventual failure of that joint or inadvertently overload somewhere else. Another preventive but probably way overboard step would be to ream your hole so it is smooth. Smooth surfaces wear better and slow corrosion and crack propagation.

So... in the end... it is always a compromise as to the fit of the fastener and the ease of fabrication and install. My :2cents:

 

[CCWKen]

There is a standard for bolt hole clearances. Typically a close fit and a free fit.
Standard Tap Drill / Clearance Holes Table - Engineer's Handbook -

If you are having problems laying out the position of the hole(s), then that's another issue. Buy a set of transfer punches. I use these all the time.
Transfer Punch Set - 28 Piece

 

[Scrambler82]

IMHO... The bolt hole should be as close to the diameter as possible.
Once the bolt is in the properly sized hole and tightened to specifications, it becomes part of the metal piece.
There are applications where movement is needed, a bridge bolted too tight will fail sooner than one that can move in the wind and Chang weight. The same goes for each application, as stated, allowable tolerances are engineered in, not decided on when the holes are being made.

Another way to look at it is, if all the holes are 1/8 to 1/4 oversized, every time I you move the tractor with a load all of the bolts need to move in the hole to hit solid metal, add up all of the movement and the tractor or a piece of equipment is out of alignment and is not pulling properly or is wearing where it shouldn't be wearing.

I go back to the original statement... Make the hole as tight as possible !

 

[dragoneggs]

Another thing, is to get the nut and bolt real tight. These two things are a big help in doing that!

I like your tools SA. Im quite disappointed you didn't bring them out when I was over!

 

[Shield Arc]

I like your tools SA. Im quite disappointed you didn't bring them out when I was over!

I keep my helpers in the house when I have visitors!:laughing:

 

[Streetcar]

IMHO... The bolt hole should be as close to the diameter as possible.
Once the bolt is in the properly sized hole and tightened to specifications, it becomes part of the metal piece.
There are applications where movement is needed, a bridge bolted too tight will fail sooner than one that can move in the wind and Chang weight. The same goes for each application, as stated, allowable tolerances are engineered in, not decided on when the holes are being made.

Another way to look at it is, if all the holes are 1/8 to 1/4 oversized, every time I you move the tractor with a load all of the bolts need to move in the hole to hit solid metal, add up all of the movement and the tractor or a piece of equipment is out of alignment and is not pulling properly or is wearing where it shouldn't be wearing.

I go back to the original statement... Make the hole as tight as possible !

Every structural bolt that I have installed on a bridge is tighten so that there is no movement between plates.
Bolted Bridge connections are designed based on clamping force not shear of the bolt. Riveted structures used the shear strength of the rivet body at the connections

To op , why not use bigger bolt. What exactly are you building.

I looked at snow plows on the job today. They all had slotted holes for adjustment with bolt size 5/8 or larger. They had washers on both nut and bolt side.

 

[Streetcar]

As others and especially Hoss has mentioned the application and loading is key. Edge distance is a consideration for a couple reasons. One is tear out... if your hole is close to the edge of the structure and the load is generally in that direction then you want the hole to have at least 2D edge distance (measured from the center of the hole). Bolt fit (clearance vs. tight or interference fit) generally is only an issue in fatigue situations where the bolt can help create a little compression zone around the hole. Holes and notches are where cracks cracks start. A loose fit bolt not clamped/bolted down tight enough for operating loads will also produce excessive wear and maybe eventual failure of that joint or inadvertently overload somewhere else. Another preventive but probably way overboard step would be to ream your hole so it is smooth. Smooth surfaces wear better and slow corrosion and crack propagation.

So... in the end... it is always a compromise as to the fit of the fastener and the ease of fabrication and install. My :2cents:

Dragon
Why would you recommend reaming a drilled hole.
I agree that proper location ,(edge distance), spacing and tightening are important for proper performance of the connections
Irregularities (notches, cracks etc) are stress risers and can lead to failure.
Smooth Holes on the other hand are only a problems for the material is not enough section to handle the stress. Drilled holes are a standard method to repair cracks in bridges

 

[bdog]

Not a bolt but on my tandem disc which is a heavy duty 14' model the hole it the hitch is significantly smaller than the hole in my draw bar. There is maybe 1/2" difference is size maybe more. I have not had any problems or unusual wear with the loose pin bouncing around in there and I have pulled it as deep as it will go over about 80 acres.