Step by Step Hole Reaming Instructions

Technical Technical Discussion
1

In order to machine our own gearbox side plates or chassis rails, we’re going to need to ream some 1.125" and .875" bearing holes. We can do this in house, vs having to go out of house to CNC machine.

What is the sequence of events that takes place en route to reaming the hole? In particular, how many intermediate drill sizes are we going to need? Currently we don’t own any drills larger than 1/2", so in order to ream holes we’re looking at investing in both a reamer and the intermediate drills that will be necessary. What size would you drill the final hole before reaming? Would you recommend a 1.125" and .875" reamer, or something slightly undersized? The bearings will either be pressing into a sideplate (3/16 or 1/4) or a nano-tube style chassis rail/gearbox of 1/8" wall, 6061 or 6063 aluminum.

Thanks.

2

I’d recommend a drill that’s 1/64th under, and when you run the reamer, go slow RPM. To get a press fit, you’ll need a reamer that’s about a half-thou under size. Or, don’t rely on press fits.

One trouble you’ll have is finding reamers in those sizes with a 1/2" shank.

You might also consider looking into TCT hole cutters in those sizes.

3

I’ve heard that it can be quite a pain to machine 6063, so watch out for that. I know that some tubing sizes are really hard to get in 6061, but, well, watch out.

If you can’t dependably do press fits, then design around it. Make sure that all bearings are retained from both sides by either their flanges or snap rings/shaft collars. All the press fit is doing in this case is making sure the bearings don’t fall out.

4

What I did when cutting 1.125 bearing holes in 1/8 wall box beam. Started with quarter, then straight up to 1 inch, the largest drill we have. The pilot should be just big enough for the center part of the 1" drill which is not center cutting. Then I bored the hole to just under, and reamed it by hand to the final size.

5

To this point we haven’t needed to rely on a press fit for bearing retention. We can design in flanges on the bearings, spacers, e-clips, etc to keep the bearings in place.

6

I don’t have any experience with these. Do they hold tolerances well enough for this application?

7

I’m looking to doing the same, machining 1-1/8 inch holes to a tight tolerance. I’ve been looking in to either a “sheet metal cutter” which can do 1/4" (e.g. http://www.trick-tools.com/Sheet_Metal_Cutters_81) or an annular cutter with a weldon shank adapter for our R8 mill.

I’d like to know if anyone with machining experience could comment on the above?

8

Out of curiosity, why did you ream the hole if you were already boring it? Boring should have given you superior dimensional accuracy.

Neither of those is going to give you a press fit. Possibly not even a very close hole to what you want.

The only good ways to get a dimensionally accurate hole to tight tolerances with manual equipment are reaming and boring. Boring is better as it will give you both a straight and dimensionally/positionally accurate hole while reaming will follow the existing hole. If it’s not straight, or on location, your reamed hole won’t be either.

9

We don’t own any boring equipment. Any specific recommendations on what we would need?

10

You’re going to need a mill and a boring head (top left). You could run a boring head in a drill press, but you’d have no way of accurately locating the hole in the part. I suppose a drill press with x-y table would work. I would much rather use a reamer or some other means-boring is a much more involved process that is going to take awhile to setup and run.

It’s possible you could use an annular cutter or a hole saw if you were willing to live with a loose bearing fit. The annular cutter is probably going to do a better job. You could also buy an end mill in the size you want, which would do a pretty good job. That would run you about 60 bucks for a 1.125".

11

I’ve always known we had boring tools for the mill, but have only ever used a boring bar in the lathe. Is the procedure to set the boring bar to the appropriate radius above the part, and then cut, or to somehow adjust it while in the part? I ask because on the lathe, I often adjust the radius of the cut while inside the part (so long as the part isn’t too thick).

12

I don’t know what a machinist would tell you, or an instructor if you took a class, but the way I’ve always done it is to visually adjust the boring head above the part so that you’ll take a small cut. Make that cut and then measure the diameter of the hole. Once you know the diameter of the hole you know how much you need to adjust the boring head. Odds are that unless you need to remove a very small amount, you can’t go immediately to this number, but you at least know your starting point. In a production environment I’m guessing you would have some kind of offline fixture that would allow you to adjust the boring head to the right dimension with no trial and error.

If I’m understanding your question correctly you’re asking if you can change the cutting diameter of the boring head without retracting it from the hole?

To adjust the cut diameter you have to turn a screw that is mounted radially on the body. The machine has to be stopped to do this, but if you’re inside the hole and you try to increase the cut diameter you will push the boring head against the wall of the hole (see below).

http://www.criterionmachineworks.com/Graphics/borehead.gif

You could do it in the hole if you moved one axis of the machine away from the hole, but you will still have to retract the tool from the hole and come back to the zero position to re-start your cut . This is common on CNC mills, so that they can retract the boring bar from the bore without scraping the wall.

13

I reamed the hole because the boring head we have isn’t very good, it’s metric and has a lot of backlash in the adjustment screw so I wasn’t comfortable trying to get the exact size without going over.

14

Using a drill press to get a located hole I suppose one could use a height gage to scribe a x y on to the piece. Then use a center punch and a center drill to get a good start on the hole. Then I would take a .257 drill bit to drill a pilot hole. Once you got the pilot hole one would use a ground c’bore to the press fit size for the bearing with a .250 pilot to guide the cbore.

See this link to see what I’m talking about.

15

Lowes sells Ideal brand hole cutters in 7/8 and 1 1/8 sizes. We have the 7/8 one we use on occasion for prototypes. It works well. I don’t recall the exact hole size it comes out with, but in a drill press I recall it being .005 bigger or less. It would be better in a mill. It makes a nice clean round hole that works fine with flanged bearings.

16

I’ve encountered the same problem with our team. We’re working in my basement where we have a 1940’s Boice-Crane drill press and several other machines handed down to me from my grandfather. It has a 1/2" chuck and the speed can be varied over a wide range.

Here’s what’s worked for me…

We have two adjustable reamers – size “G” for the .875" holes, and size “I” for the 1.125"s. Along with those we have two reduced-shank drill bits - 55/64" and 1 7/16". You can pay $$$ and get these at someplace like McMasters or you can do what I did and pay a lot less on ebay or grizzly.com. So far the imported tools are working out OK.

Clamp the workpiece very securely and - as was recommended above - drill a 1/64 undersize hole. You’ll need to set your drill press to turn at two or three hundred RPM and you may want to start with a pilot hole. Go slowly when drilling the hole.

The reamer will be too big to chuck in the drill press, but you don’t want to do that anyway. Manual reaming is the way to go here. Normally you’d use a large - and expensive - tap wrench to turn the reamer after inserting it in the drilled hole. But, you’ve already got a handle in the form of the rail you’re working on, so why not use that?

Clamp the square end of the reamer in a vise so the reamer points up. Now you can easily adjust the reamer so the cutting diameter is a few thousands less than the bearing diameter - use a micrometer or caliper to check the adjustment. Then with the reamer still clamped in the vise lower the rail onto the reamer turning the rail in the cutting direction only. This will only work, of course if you’re making two holes all the way through both sides of the rail. The advantage is that the two holes will help you keep the rail perpendicular to the reamer. Unclamp the reamer from the vise to remove the rail from the bottom when you’re done reaming.

I found that one reamer pass with the 1/64 undersized hole seemed to work well.

It’s a trial and error process to get the best reamer adjustment. Test by installing the bearing, then tweak the reamer adjustment and start over with a newly-drilled hole until you’ve got it right. If you go to a different material - polycarbonate, for example - you’ll need to re-adjust the reamer for that material.

Note that I’m a novice machinist and I’m no mechanical engineer, so take my advice with a grain of salt. :slight_smile:

Another option I like that I haven’t seen mentioned is a Greenlee chassis punch. They cost $$$, though. Also, like the hole cutters already mentioned, you end up with a loose bearing fit. If you want an interference fit you need a reamer (or one of the other options that require more sophisticated machinery).

17

We don’t bore a ton of holes in production, but we set up all our tools offline with four of these:

http://www.zoller.info/usa/products/presetting%20&%20measuring/vertical%20presetting%20and%20measuring%20machines/»venturion«

Accurate to micron levels.

18

Found this:
http://www.hougen.com/tech_tips/cutter_info/hole_size.html

According to Hougen, if you have a rigid setup there is a high probability that your hole will end up somewhere between +0.0005 and +0.0035 from the nominal drill diameter. Sounds pretty good for a flanged bearing looking for a slip fit.

19

We’ve been doing exactly this. We use a 20mm chassis punch and then ream the hole to 22mm and deburr until we get to the proper press fit tolerance. It works very well and the bearings haven’t come out yet. It takes around 10-15 mins per press fit.

No machinery required.