Maintaining hole true position

[chevynut]

I'm looking for some tips on how to keep oversized holes centered where they're supposed to be when building welded assemblies.

I have run across several projects where I wanted holes to be slightly oversize to account for tolderances, so bolts go in freely. What I typically do is drill the hole the nominal size, use a jig to locate the parts, weld them together, then ream the holes oversize.

Do you guys have any tips for making this easier? A bolt with an oversized shank would be great, but I don't know where to get them. I could build a sleeve for the bolts, but I'd probably lose them. :)

Any other ideas?

[NickP]

What tolerance value are you trying to maintain? Is the mating part that accurate? How many sub-assemblies form the completed assy.?

[chevynut]

Nick, the problem is that if I use an oversize hole in the part, and a standard size bolt in the jig, the part can easily be skewed so the bolt touches the side of the hole. I want to try to keep the hole centered where it belongs. That way I can account for tolerances on the mating part.

A tapered bolt or nut would do what I want to do, I'm just looking for some ideas how to do this. I run into this problem all the time. For example, let's say I want to weld a flange nut onto a part with a hole in it. If the hole is oversized, the flange nut may not be centered. So now I drill the hole nominal size, hold the nut in place with a bolt, weld the nut, then drill the hole oversized. If you don't do that, the bolt may bind as the welding pulls the nut off center even with the bolt in it.

[NickP]

OK, what size hole? Drill bushings can be ordered in any size you need. Look at Carr Lane (http://www.carrlane.com/). A plethera of what you seek!

[chevynut]

Nick, I've used drill bushings a lot when I worked as an aircraft mechanic. We used them to locate the center of an existing hole that was over a new panel with no hole. How does a drill bushing help me do what I want to do? Both of the holes I'm trying to line up are existing holes.

As for sizes, I usually need typical 1/4, 5/16, and 3/8" holes.

[NickP]

Well, I am at a bit of a disadvantage not being able to seeor touch what it is you want to do. CarrLane has an enormous selection of alignment pins, plates, clamps and otherassorted items. I was so in hopes ofsomething being there that might spark the idea/solution. Can you post pictures of the items and showwhat it is you want to do? If proprietary,email it to me with a non-disclosure and I'll work through it with you.

[chevynut]

Nick, there's nothing proprietary about this. Let me explain better what the problem is.

I am assembling my seat mounts that have two plates attached to a tube. The two plates have holes in them that match the holes in the tri5 floor, which are nominally 3 1/8" apart. Each plate has two holes, and they get welded to the tube. I want the holes to line up on any car, which has some tolerances.

The holes on each plate are accurately located. However, I also need to locate the plates accurately relative to each other. I built a jig that has holes in it located precisely where I want each hole to be. I made the holes in the plates 5/16" in diameter, and the holes in the jig at 5/16" diameter. So if I bolt the plates to the jig, weld them to the tubes, everything is located where it's supposed to be. Once the welding is done, I ream the holes out larger, and they're concentric with the jig holes, and supposedly with the holes in the floor of the car.

I was trying to think of a way to eliminate the reaming operation, and still have the plates located correctly. If I oversize them first, then use a 5/16" bolt to attach the plates to the jig, they can be off-center. As I mentioned, an oversize bolt with 5/16" threads would be one way to do it. I could make a bushing to fit in the hole to locate the parts.

As I mentioned, I have a similar issue when welding nuts to the backside of a hole. If I oversize the hole, I can't get the nut centered. So I typically make the hole nominal size, hold the nut in place with a bolt, weld the nut, remove the bolt, and ream the hole oversize. If you've ever tried to do this, you know that welding can pull the nut off-center slightly, enough to bind the bolt.

Just looking for some ideas, or off-the-shelf solutions.

[NickP]

3/8" Shoulder Screws - http://www.carrlane.com/catalog/index.cfm/27225071F0B221118070C1C512A0A1F0900101B030010543C1 C0C16190D172D252A5E415B59 Change the jig holes to 5/16 - 18 - determing the shaft length needed - assemble and weld.

[Rick_L]

That's a pretty drastic oversize on the hole - but it's a thought in the right direction.

There are other things you can do.

1. Weld up the brackets but don't drill the holes. Or maybe do one hole, or two holes on one end only. Use a threaded transfer punch screwed into the threaded holes to locate the clearance holes. Then center drill and drill to size.
2. If possible, design your brackets so that there's no welds perpendicular to the length (minimizes shrink). Drill your holes to close fit for the welding. Then open them up to 1/32" oversize or something close, which is a pretty standard clearance hole.

When I had race cars, the best made ones had close fit clearance holes, the builders were mostly using one of the two above procedures.

[Run-em]

Hey, Laszlo, you want me to ask Ted? (See shade tree mechanics tips).

[chevynut]

Nick, yes that's the right idea, but like Rick said it's a pretty big oversize on the hole. Not sure that really matters though. If that's all it took, I'd just make the holes 3/8" and use a 3/8" bolt in the jig, then use 5/16" bolts for the final assembly. Maybe that's how they do it in the "real world". ;)


Rick, I want to laser cut the holes in the plates and NOT have to re-drill them. So #1 doesn't really buy me anything. #2 is pretty much what I'm doing now.

I woke up this morning and had a revelation....funny how that happens during sleep. :)

Something like this would work great, if the OD was a little smaller.

http://www.mcmaster.com/#cadinlnord/90596a025/=lxklpi

http://www.mcmaster.com/#cadinlnord/90975a036/=lxkq6l

You would put a bolt in it, slip it into the oversize hole, put the threaded end of the bolt in the jig, and lock it down with a nut. It could also be used to weld nuts on the backside of a plate.

I could also turn then down on my lathe a little so they' the right OD. I could also have some of these made on a CNC lathe or screw machine. The shoulder wouldn't have to be so large. I have wondered why nobody offers anything like this on the market. Seems like this would be a common problem, unless they really do go one full size larger on holes.

[NickP]

I really wish you would post a picture of your task. A visual aid is an extreemly good thought generator. I have built so many different jigs and fixtures in my past lives that were I to see it, I might be able to assist better than this 20 question barage. The shoulder screws I mentioned also come in 5/16" shoulder BTW. Also, a set of locating pins machined to a size needed inserted in the holes for the purpose of location along with a pair of vertical toggle clamps for retention may suffice to the need.

http://www.carrlane.com/SiteData/FeatureImages/019600vtct_p.gif

Again, a picture is worth a thousand words!

[chevynut]

Nick, I thought I described an example of one of the issues I run across. There are many, and I run into this problem all the time. I weld a lot of flange nuts on the backside of holes, so that's a problem I frequently have. Yes, a machined pin would work, but so would a machined bushing. I was just looking for obvious solutions. I'm sure lots of manufacturers run into this problem. Maybe they just oversize the hole to the next larger size and call it good.

Here's a pic of the seat mounts similar to what I'm building. I have them finished, but I used a 5/16" bolt in a 5/16" hole to mount them onto my assembly jig. Then after welding I reamed the holes oversize. I think the example of welding nuts to a plate is self-explanatory and there are nuts inside the tube in the pic.

[chevynut]

I think I've pretty much concluded that manufacturers simply go to the next size up on their clearance holes. I have noticed that 3/8" washers are WAY too big for a 3/8" bolt, so I use 5/16" washers on them and they still fit. Next time I think I'm going to cut the 5/16" bolt holes to 3/8". But that doesn't solve the problem with the nuts. I guess I'll have to use one of the weld nuts I linked, and just drill 3/8" holes. I'll have to give it a try soon.

[Rick_L]

Standard clearance holes for most manufuacturing is based on tolerance of the hole location. It's also usually assumed that the clearance holes are never undersize, they are nominal to slightly oversize (that's what a typical drilled hole is).

This method works for both 1 dimensional and 2 dimensional hole patterns.

Holes in parts manufactured on a milling machine are usually spec'd at +/-.010". With that tolerance you need a hole that's .020" oversize. So you pick the next drill size that's a minimum of .020" oversize. Once you get over 1/4", that's usually 1/32" oversize with standard drill bits.

Punched holes in sheet metal +/-.020". Parts that are drilled by hand depend on the equipment and the layout method. Those are going to need .040" oversize holes. Again, once you get over 1/4", drill sizes are limited, you may end up with 1/16" oversize.

A big hole in a flat washer is a waste, there it's more about a lack of precision in making the flat washer, and maybe quick assembly. When I buy cheap flat washers, I usually buy the next smaller size if the holes are real big.

[Custer55]

The last time I welded nuts to a plate with oversized holes I used making tape wrapped around the bolt to keep the nut centered. I worked ok but you have to rewrap the bolt for each hole.
Brian

[Bluegrass Trifive]

Nick, there's nothing proprietary about this. Let me explain better what the problem is.

I am assembling my seat mounts that have two plates attached to a tube. The two plates have holes in them that match the holes in the tri5 floor, which are nominally 3 1/8" apart. Each plate has two holes, and they get welded to the tube. I want the holes to line up on any car, which has some tolerances.

The holes on each plate are accurately located. However, I also need to locate the plates accurately relative to each other. I built a jig that has holes in it located precisely where I want each hole to be. I made the holes in the plates 5/16" in diameter, and the holes in the jig at 5/16" diameter. So if I bolt the plates to the jig, weld them to the tubes, everything is located where it's supposed to be. Once the welding is done, I ream the holes out larger, and they're concentric with the jig holes, and supposedly with the holes in the floor of the car.

I was trying to think of a way to eliminate the reaming operation, and still have the plates located correctly. If I oversize them first, then use a 5/16" bolt to attach the plates to the jig, they can be off-center. As I mentioned, an oversize bolt with 5/16" threads would be one way to do it. I could make a bushing to fit in the hole to locate the parts.

As I mentioned, I have a similar issue when welding nuts to the backside of a hole. If I oversize the hole, I can't get the nut centered. So I typically make the hole nominal size, hold the nut in place with a bolt, weld the nut, remove the bolt, and ream the hole oversize. If you've ever tried to do this, you know that welding can pull the nut off-center slightly, enough to bind the bolt.

Just looking for some ideas, or off-the-shelf solutions.
Take a look at stripper bolts. Misumi and McMaster Carr (and probably others) sell them. You might have to have them made but they're not too expensive. They might solve your problem.

[chevynut]

Geez guys, this thread is 3 years old :). I solved the problem with locating the plates by slotting the holes a little. I have to line up the slots so they're centered on my jig, but it works fine for the few parts I make per batch. So far I've just been welding the nuts on the backside then I ream the holes out with a slightly larger drill bit. It takes a little time, but it's not that bad since the nuts are pretty hard and I can tell where to stop. I changed the design so I use fewer parts now and it's a lot easier to weld the nuts on.

I still run into instances where it would be nice to have a bolt with a slightly oversized shank. The shoulder (stripper) bolts go up one whole size so if I wanted to weld a 1/2" nut the hole would have to be 5/8" which is way too big. If I had a way to grind the shanks down they would work. When I was an aircraft mechanic we used over-sized bolts all the time to replace standard size bolts in holes that had signs of metal fatigue. We'd ream the hole out 1/64" to 1/32" oversize and drive the new bolt in. The threads were still standard size..wish I'd kept some of those. ;)

[NickP]

Geez guys, this thread is 3 years old :). Someone must have been looking for some content.