Press fit woes

[benmychree]

The silver solder would be the best cure, one thing with machine reamers is not to baby them with a fine feed, if you are agressive with feed, it will center them and end with a nominal size hole, another approach is "over and under" reamers; with a standard dowel pin it will bring a fit with press fit on one part and a slip fit on the mating part.

 

[projectnut]

So I attempted to make this spanner wrench, based on @darkzero's thread.
View attachment 324184
Everything went relatively well... Except at the end. It's ok for a first tool, save for the pin. There's plenty of little things wrong with it, but it would be serviceable.

The hole for the pin ended up too large, so the 0.1875" pin falls out. Now, I drilled the hole with a #14 drill, followed by a #13 drill, followed by a 0.1870" reamer. The idea was to have 0.0005" interference fit. What I didn't account for was the runout of my old drill press. I miked the drills and the reamer, and they check out ok. Then I measured the chuck runout using the reamer as the reference. It was a little larger than 0.005". Yeah, more than 10x greater than what I was trying to hit. That's not going to work.

So I need a long and short term solution. Short term: What tool is used to measure the ID of the hole accurately? (0.1800 -0.2000) I can turn a pin to the right size, but knowing the diameter might help! Might take a few goes at it, but that's ok. Or, I could drill the hole out, and machine a screw. The screw would thread in from the back. I could machine off the threads that protrude to form the pin. Longer term: Is upgrading my drill press with good bearings worth the effort, or should that money (plus extra I'd presume) be better spent on a real mill? Doing anything precision on this current setup right now is a waste of effort. So what to do? Seeking some practical advice. Temporarily have lost the wind in my sails.

I doubt replacing bearings on the drill press is going to result in better accuracy. Drill presses aren't precision machines. Drills are flexible and if not sharpened exactly on center will cut oversize or misshapen holes. Unless the drill is a high end brand that has been tested for accuracy it could be slightly oversize or not sharpened correctly. In my opinion it's almost impossible to sharpen a bit by hand to get the accuracy needed in this application. I have seen new out of the box low end drills so poorly sized and so badly sharpened there is no chance they could drill an accurate hole.

The best solution for accuracy would be to use a professional grade mill with a slightly undersized end mill. Then use a chucking reamer to get the hole to the exact size you need it. In your case silver soldering would be a cost effective viable alternative. Another option would be to check the hole size in the retainer you intend to use the wrench on. Often times they are oversize by several thousandths. If that's the case you can make or purchase an oversize pin to press in the hole. McMaster sells dowel pins .001" - .002" oversize. There are other vendors that sell dowel pins up to .003" oversize.

 

[macardoso]

I own a set of Mitutoyo telescoping gauges and Shars import brand. I hate to say it but I like the Shars better. They are more accurate and smoother to operate. The locking thimble has broken on most of the Mitutoyo ones that I own and they are unwilling to take my money to get replacements (also the price per thimble is more than an entire import set). I guess what I'm saying is don't write off brands other than the big names. For some stuff the name brand quality is not amazing.

I too own a set of split ball gages as @Ulma Doctor suggested above. I'd prefer gage pins over them any day but they work fine. Import quality is good enough for what I do at home. I'd trust them within a thou.

 

[Mitch Alsup]

Can I suggest silver brazing ? Maybe even silver soldering.

 

[homebrewed]

For the odd job where I need an accurate ID (and just having the standard caliper/mike stuiff), I've turned a taper plug to help with the measurements. The problem with calipers is that their inside-measuring blades aren't knife edges, so they don't actually contact the true diameter of the hole. Instead, drop the taper plug into the hole and measure its outside diameter right where it enters the hole.

 

[WobblyHand]

I doubt replacing bearings on the drill press is going to result in better accuracy. Drill presses aren't precision machines. Drills are flexible and if not sharpened exactly on center will cut oversize or misshapen holes. Unless the drill is a high end brand that has been tested for accuracy it could be slightly oversize or not sharpened correctly. In my opinion it's almost impossible to sharpen a bit by hand to get the accuracy needed in this application. I have seen new out of the box low end drills so poorly sized and so badly sharpened there is no chance they could drill an accurate hole.

The best solution for accuracy would be to use a professional grade mill with a slightly undersized end mill. Then use a chucking reamer to get the hole to the exact size you need it. In your case silver soldering would be a cost effective viable alternative. Another option would be to check the hole size in the retainer you intend to use the wrench on. Often times they are oversize by several thousandths. If that's the case you can make or purchase an oversize pin to press in the hole. McMaster sells dowel pins .001" - .002" oversize. There are other vendors that sell dowel pins up to .003" oversize.

I knew this was the real answer... Short term, I'll silver solder the pin. I found some bare mystery brazing rod in my garage and no flux. Not going to use that. The right alloy seems to be the high silver stuff like Harris 56%, as it has a lower melting point and good fill. Guess I'll order the kit with Safety-Silv 56 + flux.

Long term, I need some help spending money. You guys seem to be pretty good at that. What kind of new machine could I get that would be adequate for relatively small scale work? What can be done with a Sherline, vs say a PM25? I have no feeling for this at all. Just not part of my life experience yet.