Technical How to Drill/Tap DOM Tubing for Ladder Bars?

I posted this question last week buried in my build thread and didn’t get any replies. I’ll try here for more exposure.

Right now, I'm trying to figure out ladder bars for the rear of my Model A. It appears 7/8 OD DOM .156 wall tube using 5/8 inch clevis's (sp?) is a popular way to go. However, this tube has an ID of .562 which is about 1/64 smaller than the recommenced drill size for a 5/8-18 tap of 37/64 or .578 inches. How do you drill a 4 foot tube? I have a floor standing drill press, but no lathe.

Or do you ignore the extra .016 (1/64)?

My build thread:
http://www.jalopyjournal.com/forum/threads/kurt’s-puddle-jumper-1930-model-a-coupe-build.1029160/

 

I would consider welding in bungs that are made on a lathe and threaded. Something along the line that Corey T sells

 

You only need to drill it a bit more than you need (depth, not width)for the clevis to screw in fully,once tapped. The whole tube doesn't need to be drilled out, just the tap area.

 

It's pretty common to direct tap 0.156" wall 7/8" DOM. Due to the minor ID variances, I usually buy 0.188" wall and drill and tap. Plus I like the extra thickness. So I don't think you'll have an issue just direct tapping your DOM. The important thing is the tap alignment due to the depth you need to tap. I have a lathe, so it's not an issue. I would try to find a buddy with a lathe who might do it for a couple beers.

 

If its possible to ream or drill , do it. There is kind of a mill scale that is very hard and the extra material makes the job more force than cut.

 

I don't know how common it is to drill and tap DOM, jmountainjr is sure correct that is much easier to do with a lathe, if you can find someone with one to help.

By far and FAR by a long ways is to use "Weld In Bungs" as suggested by Mike. Simple weld job, looks great when finished and I believe gives you a much better solution. You should at least take a look at doing it this way. A simple search will bring up Bungs of the correct size and taped and prepared for what ever Clevis's you are planning on using.

 

Pretty much every ladder bar, or radius rod made aftermarket.

 

True DOM is cold drawn, there won't be any scale. HFS (Hot Finished Seamless) can have scale, but DOM won't.

I always tap it in a lathe, not that hard, we even do Chromoly this way on off road race cars.

 

In my early years I tried doing just that by hand and wound up buying a lot of drill bits and taps. You need a lathe and just 1 drill and tap set to do a nice strait cut

 

If you only need 1/64th you could use a hand reamer. It will follow the hole and a starter tap and a little care should result in an excellent job.

 

And spend $60 for it

 

Not a big deal to do this with simple equipment. I used to just clamp the tube down to the table and drill it with a decent hand drill. Made a tap guide from a short piece of DOM (drilled to 5/8" and honed for tap clearance) and chunk of angle to make it easier to get started straight. Also made another guide for an 11/16" tap for tie rods and drag links (takes a little more torque to drill the tube to 5/8" tho).

 

rgclouse, fyi: http://www.welderseries.com/Bars-7-8-OD-p51072224. We do custom length bars fairly regularly.

Thanks
dw

 

Thanks Rich,

Your photos gave me a great idea for drilling the tubes. I figured out I only need 43-44 inch bars, and my drill press can handle 43" (or 45" if I drill a center hole in the base). I can install a piece of angle from the base to the table and make some clamps like your jig.

By the way, I lived in Burnsville for 35 years befoe retiring and moving south.

 

I prefer reamers to drills for doing this.

 

Actually it's more like $38.25. Or if you wanted a machine reamer $22.88. Don't need the gold plated cutters for a deal like this.

 

I did this simple tech a few years back, it worked for me
http://www.jalopyjournal.com/forum/threads/straight-tap-in-tube-starter-for-tech-week.437420/

 

If you drill / ream the tubing free hand, with out some sort of centering device you could end up with very little wall thickness. A, 5/8" thread crest diameter in 7/8" OD tubing gives you a wall thickness of 1/16", not enough in my book for ladder bars. And that is only if the work was done as carefully as possible. Use .188 wall thickness, or use weld in bungs.

 

Is that a weld in bung? Or a weld on bung? I can't picture how you could have a bung drilled and tapped for 5/8-18 that you could fit inside a 7/8 tube. I drilled and tapped my four bar tubes. Had a pretty heavy wall even at the ends.

 

When I was building the ladder bars for my coupe, I decided that 7/8' tubing looks a little skinny for 45" long bars, so I decided to go with 1" DOM tubing and 3/4" clevises. To get the tap straight, I made a tap guide from some 1.25" round bar I had, drilling one end with a 1" drill (came out 1.010) in the lathe about 2" deep and the other end 3/4" (came out .760"). It lines the tap up straight with the bore and worked well. Get a quality new tap and use pipe thread cutting oil (has sulfur in it) to get a nice finish.I got the thread cutting oil at Ace Hardware.

 

Check your math. 7/8"-5/8"=1/4" so you have 1/8" wall remaining in the threaded area.
Plenty of ladder bars, 4 bars, and hairpins built from 7/8" tube. Even Henry's tie rods
are 7/8" OD with 11/16" threads.

 

Quick quote from McMaster Carr,(high speed steel) not every town has a machine tool dealer

 

Rich B is correct. My math was bad.

 

First I want to thank everyone for the lively discussion. Special thanks to RICH B who gave me great ideas for a jig to drill in my drill press and to SCOTT for the great tech thread on a tap guide.

WelderSeries, thanks for the offer, but I’m trying to do everything myself. I may buy some Urethane bushings and outer tubes from you.

The bottom line seems to be if I’m going to use threaded 7/8 inch DOM, to use a reamer. Since I have never used a reamer in my 73 years (other than a ridge reamer), I don’t think I’m going to start now. I will try jigging it up in my drill press and drill to 37/64.

I will let you know how it works out.

 

^^ make sure the set-up is anchored securely....danged drills seemed to want to grab when removing a small amount of material.
An old trick my boss told me for keeping a 2-flute drill from grabbing or wallowing....
Line up drill to hole as close as possible.
Place a 4- to 8-folded paper shop towel; preferably with nylon scrim (don't use cloth) on the hole.
Give it a few shots of cutting oil, bring the drill bit down to contact the material.
Start/run at slow rpm. ...works best with foo switch control.
The paper towel will help the drill enter the hole and reduce the tendency for it to grab.

 

Well I said I would report how I did it.

I built a jig to hold the ladder bar tubes to drill them in the drill press.


Worked good got all 4 tubes drilled. Only problem was I had to take it off the drill press each time to get the tube out of the jig and put in the next. Then reinstall it and adjust till it was plumb and centered. Took about 15 minutes to setup and 30 seconds to drill.

. (Note: picture was taken before on the first tube. I welded the jig at the bottom before the 2nd tube.)

Got 3 of 4 tubes tapped. (Got too hot in the garage). Had to build a tap wrench since I didn’t want to spend $80 for one. Every cheaper one had bad reviews like “it broke on the first use”. Half a HF socket adapter welded to 14 inches of ¾ inch rod. 3/8 socket drive fits perfect on the 5/8 inch tap.

I tapped about ¾ inch with a starter tap and finished with a plug tap.



I tried the thin wall conduit connector trick that SCOTT showed in his thread, but the connector fit too loose on my 7/8 inch tube.

So I just tapped them freehand.

 

I was rushed and forgot a couple of things I was going to relate in my previous post.

While building the jig to hold the tube for drilling, I broke my 3/8 inch tap preparing the bottom holding plate. Tried the 2 places in town I thought would have taps without luck (the ACE Hardware had taps, but the slot for 3/8 was empty). Put in a order to DrillsandCutters.

I figured it is only a jig so I welded a 3/8 nut on the inside of the plate. Now I find the tube will not fit. Should have done some math before hand. A squared + B squared = C squared. 2 x 1/8 angle iron plus nut doesn't give enough room. Had to drill out (5/8 inch) to get the nut out of the inside. Welded a nut on the outside.

Isn't that how is always goes when you are trying to do something.

When tapping the tubes, I had a hard time holding the tube in my vise. I used the drilling jig on the 2nd - 4th tubes and added the C clamps the first time it tried to slip.


I must say the tapping was easier than I expected, but gave my shoulders as much of a workout as when I had physical therapy for my shoulder.

I did get the 4th tube tapped this AM before taking the wife to physical therapy for her shoulder (she had it replaced).

 

Hell, R.G.!
You mention the 'workout', and how it reminds you of when you had P.T. for your shoulder...Now, taking the wife to P. T. after a shoulder replacement draws a visual picture, like the wife tapping the first three!

 

Much easier to find someone with a lathe ,tap drill and tap.

 

But not as much fun!