How to get perfect hole alignment

[Tommy B]

I'm starting my CNC project real soon and I need a small bit of help. On my previous machine, which was a wooden one, it was real difficult to get the round linear bearing shafts perfectly aligned with one another (parallelism) and aligned with the moving carriage at the same time.

This is really the core of an accurate machine... the carriage to move without any binding areas. It was painful to watch the motors turn and see the frame flex into a high-stress position. I don't want that with my new metal machine.

On the machine I'm building now, the bearings will be 3/4" hardened steel rod, fixed permanently into 1" square tubing. In order to get things lined up perfectly, I don't really know what to do.

Essentially, the assembly has 4 components that has the steel rods either going through it, or just fixed to it: the two end supports and the two pieces of 1" square tubing. This is a very basic screw-to-linear motion design.

One method that I tried and got horrible results was to drill all 4 pieces of 1" square tubing at once, like a sandwich stack. I was amazed to find out how much a 3/4" drill bit can be off at the end of the stack of 1" sq tubing. The bit made a perfectly centered hole on the top piece, but started chewing into the side at the bottom piece. So, this method isn't going to work.

One method I'm thinking of now is to make a small fixture and use a drill guide to start the holes, one piece at a time. This may work, I don't know.

I don't have a Bridgeport, or anything fancy like that, just a regular old drill press and a combination square. (I'll probably get a dial indicator from Harbor Freight tomorrow).

So, what I'm asking is: How do I make perfectly aligned holes with only garage tools? After drilling the holes, I'm then going to put in some plain bearings, so I'm not asking about run-out, only alignment.

Can someone point me in the right direction?

[RICHARD ZASTROW]

First, a drill doesn't make round holes. Second, drills tend to "walk", not travel in a straight line unless forced to do so by a bushing or some other guiding device.

You will probably need to have bored holes to be in line and round.

Dick Z

[Tommy B]

Originally Posted by RICHARD ZASTROW First, a drill doesn't make round holes. Second, drills tend to "walk", not travel in a straight line unless forced to do so by a bushing or some other guiding device. You will probably need to have bored holes to be in line and round. Dick Z

OK, so how can I do this without any fancy schmancy tools? I'm looking for a procedure that can give me some accuracy down to 0.001.

Anyone have a procedure?

Somehow, someone made the first accurate tools, and they didn't have accurate tools themselves. Otherwise, how was the first one made?

There was a sign in one of my shop classes, "Those that say it can't be done should stay out of the way of those that are doing it."

Well, I want to get some advice from those that have built an accurate slide. I'm willing to buy a reamer if that's what it takes.

Some people on the forum have access to expensive jig boring machines. That's not me folks, I have to make up for my lack of expensive equipment with some clever techniques.

[djr76]

Dick Z was just giving you an opinion from the perspective of a machinist who has been in the business for many years. He gave you some sound advice being, boring a hole or using a bushing to keep the drill in line. Thats true the the first machine wasn't so accurate, but back then, they weren't working with .00000 tolerances either. Its going to be very difficult to get the tolerance you need with a cheap drill press and a combo square. Even if you get a reamer, its just going to follow the path of your pre-drilled hole. Only thing I could suggest is taking very light pecks to minimize any walking of the drill, then ream it, or build a fixture with drill bushings, or go to a local machine shop and have them do the tight tolerance machining you need.

[Tommy B]

Originally Posted by djr76 Dick Z was just giving you an opinion from the perspective of a machinist who has been in the business for many years. He gave you some sound advice being, boring a hole or using a bushing to keep the drill in line. Thats true the the first machine wasn't so accurate, but back then, they weren't working with .00000 tolerances either. Its going to be very difficult to get the tolerance you need with a cheap drill press and a combo square. Even if you get a reamer, its just going to follow the path of your pre-drilled hole. Only thing I could suggest is taking very light pecks to minimize any walking of the drill, then ream it, or build a fixture with drill bushings, or go to a local machine shop and have them do the tight tolerance machining you need.

I've gotten very good results using a razor knife as a scratch awl, and then a prick-punch followed by a 1/16" bit followed by step drilling. I'm confident I can hold about .020 with standard practices. I'm just wondering how do I correct for that last 0.020, and get it down to 0.001?

Perhaps there's a method out there using a dial indicator and some sort of special round file that would allow me to enlarge the hole until everything is perfect, and then use some sort of thread locking material to hold the bearing in the right place.

Like I said, I have to make up for lack of a jig borer with some sort of clever technique. I know it can be done. Going to a machine shop is out of the question. One time I needed a lead screwturned at the end to accept a coupling. I called up Kerk Motion, and they wanted $400.00 just to put the lead screw back on the lathe and turn it down. Ummm, I'm a hobbyist and don't have $400.00 just to take a little metal off the end. The joy is in building it myself anyway.


Building a fixture is probably going to be my starting point, like you advised, from there though, how do I correct for say the 0.010 that I'll be off? If a reamer is just going to enlarge the existing hole, but not correct for mis-alignment with the other parts, that still means I have mis-alignment, but larger holes with less run-out.

Any additional help would be appreciated. Getting the terminology right is help all by itself. I'm going to look up drill bushings on McMaster right now. Please keep the advise coming.

[Tommy B]

I just looked up drill bushings on McMaster. If I were to make a jig and press two of these into it at the right spots, I'd have a nice jig that would align the bits for all 4 parts. If my spacing were supposed to be 3" and I made it "3.005" that really wouldn't be a problem, because all of the holes would be aligned, and that's far more important than the actual distance apart for this mechanism.

This is a real good starting point. BTW, anyone know how accurate these are? How much correction after this will I have to do, and what would be the proper method to correct the small amount of mis-alignment?

[Nails]

I'd mount your bearings in a separate piece of aluminum, not directly onto the frame. Then bolt this "bearing block" onto your frame. Make the mounting holes oversized enough in the bearing block to allow enough adjustment for perfect alignment before you bolt it onto your frame.

If you want to do things the way your are thinking, you will need a fixture attached to the table of your drill press to clamp the square tubes onto. This fixture must have edges, or stops of some kind to locate the side and end of the sqare tubing. Instead of using just a drill, or some kind of bushing, start the hole with a center drill. McMaster-Carr

As you can see, they are really short, so the hole it makes won't walk off of its intended center. Then follow by the normal drill, or if your hole is really big, start with a smaller intermediate drill and work up. The initial hole created by the center drill will act to center your drill bit and not allow it to "walk", as it normally would when drilling into a flat piece.

Separate bearing blocks is the way to go, IMHO.

[Nails]

I just re-read your initial post. I see you want to parallel mount shafts, but same methods work. Mount them into an adjustable blocks

[thedave]

Here is a few steps to get you where you want to be.

1. Get a dial indicator and see what the runout is in your drill press. This will tell you how accurate you can get. Test the runout on both the chuck and the 3/4" drill bit. I know you said you are not concerned with runout, but that is where alignment starts. If you drill 2 parallel holes and you have .002 runout, you could be out of alignment .004 before you drill a hole.

2. Get a decent center finder. Get some Dykem marking fluid and a sharp scribe. This will allow you to start the hole where you want it.

3. Buy a decent center drill and use it to start the hole. This will keep the larger drills from walking.

4. Use stub drills. they are shorter so flex less.

5. Start with a 1/4" drill and work your way to 3/4" in 1/8" or 1/16" steps if you have the drills for it.

6. Let the drill do the work and use something like WD-40 to lubricate as you drill.

If you want to use a reamer, make the last hole 47/64 ths.

The above steps will also help to a fairly accurate drill jig. I use Carr Lane SF type bushings. the SF type bushings allow you to get away with a less accurate drilled hole for the bushing.

[M250cnc]

Originally Posted by Tommy B OK, so how can I do this without any fancy schmancy tools? I'm looking for a procedure that can give me some accuracy down to 0.001. Anyone have a procedure? Somehow, someone made the first accurate tools, and they didn't have accurate tools themselves. Otherwise, how was the first one made?

Your right in the beginning there were no machines. But to get to a modern precision machine, the machines went through many evolutions.

So your only option is to forget it, or pay someone to do it for you.

Phil

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[Tommy B]

Originally Posted by M250cnc Your right in the beginning there were no machines. But to get to a modern precision machine, the machines went through many evolutions. So your only option is to forget it, or pay someone to do it for you. Phil Sent from my HTC Desire using Tapatalk

There was a poster in my high school machine shop that said, "Those that say it can't be done should stay out of the way of those that are doing it."



And, I'll have you know that I am an Amer-I-CAN. See that little "I can" at the end? Yup, I can do it, if I just find someone that is willing to point me in the right direction.

[M250cnc]

Originally Posted by Tommy B And, I'll have you know that I am an Amer-I-CAN. See that little "I can" at the end? Yup, I can do it, if I just find someone that is willing to point me in the right direction.

Yeah but you cant/don't know how to do it that's why your here asking.

Amer-I-CAN'T more like

Phil


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