Make holes slightly smaller?

wfrancis · Apr 18, 2014

I drilled some 3/8" mild steel with a .5" drill bit in a (I now think) worn drill chuck and it drilled the holes closer to .508". I bought and planned to press in some .503" bearing bronze bushings. I tried the usual tricks with the bushings to make them slightly bigger - straight knurl on the lathe and a few punches with my center punch. Alas, it seems the bronze is soft enough (also impregnated with oil) that this actually doesn't seem to make them any larger.

I'm considering buying some 5/8" bearing bronze and just machining my own to size but given that I already bought a few dozen bushings ready to go I'm hoping there is a trick I haven't thought of.

next time drill bit straight in the collet ...

ideas?

Thanks!

Will

tnmgcarbide · Apr 18, 2014

a drill is a roughing tool . not for precision fits. get yourself a reamer or boring head.

Joe Rogers · Apr 18, 2014

How about a series of stake marks around the perimiter of the oversize holes? How precision is the bushing location requirement? Even a couple of hammer
blows around the holes with a ball pein could shrink it enough to grab the bushing.
joe

Spyderedge · Apr 18, 2014

Have someone TIG it up and start over with a smaller bit and BORE to size.

matt_isserstedt · Apr 18, 2014

I would hold on to the plain bearings you have. Eventually you'll find a use for them. My advice is to proceed with Plan B and buy a solid or cored bronze rod...my personal preference is the solid CDA 932 = SAE 660 bronze. Despite the ability to hold oil and hold off frequent maintenance, the porous "oilite" bronze isn't a miracle cure. It can fracture on an overload, and its machinability is somewhat lesser.

dian · Apr 18, 2014

why dont the bushings "knurl up"? being soft they should. maybe try deeper.

gbent · Apr 18, 2014

If you want to blacksmith it, place about a 1-1 1/2" diameter bearing ball on the hole. Hit it hard with a sledge, 8# at least, 20# even better. Turn the part over and do the same thing to the other side. Now ream it with a 1/2" reamer. The upset portions will probably meet in the center.

Scruffy887 · Apr 18, 2014

This pic is here on a locked thread, bad title. I took a piece of Thomson rod and peened the end and it mushroomed just enough to expand some sleeves that I had, about 500 of them. .625 rod to be exact. Just kept pushing them on and unloading when 3-4 sleeves were below.

Metalcutter · Apr 18, 2014

Trying to Drill a Hole

First, drill a hole near the drill size you want the hole. Next, feed the 1/2" drill in afterward.

Explanation:
If a drill's point is off center, the drill will "crankshaft" around the point and make the hole larger. Hence, the pilot, so the point can't influence the size the drill makes.

After using the pilot, the drill still made the hole larger than I wanted. I needed a bit smaller hole than the drill would drill. After some thought, I picked up the micrometer and measured the drill's back taper toward the shank. When I came to the diameter I wanted, I cut the end off and repointed it.

With proper feed it's pretty easy for a drill to cut the size it is. Again, drill a pilot hole near the drill size.

Additional.. For accurate hole location. The center drill needs to "pull" a chip gently. If the following drill goes in the material without a chatter, you're on location. If it chatters going in, STOP! You are off center, and you will have to relocate the hole. Use a larger center drill, and run it slowly and feed it steady until there is no left over of the original start. Start your pilot hole, then check it's location, before you complete the hole feature.

Regards,

Stan-

tnmgcarbide · Apr 18, 2014

depends on the drill's mfg , but tolerances on a 1/2 " are +.006 ,-.001 . a ground PTD or greenfield will likely be closer to .500 than a
sloppy , as- forged horror freight or ch-enco . carbide drills are usually dead -on , but get a damned 31/64 drill and a .500 reamer .

DMF_TomB · Apr 18, 2014

drill hole size

wfrancis said:
I drilled some 3/8" mild steel with a .5" drill bit in a (I now think) worn drill chuck and it drilled the holes closer to .508". I bought and planned to press in some .503" bearing bronze bushings. I tried the usual tricks with the bushings to make them slightly bigger - straight knurl on the lathe and a few punches with my center punch. Alas, it seems the bronze is soft enough (also impregnated with oil) that this actually doesn't seem to make them any larger.

I'm considering buying some 5/8" bearing bronze and just machining my own to size but given that I already bought a few dozen bushings ready to go I'm hoping there is a trick I haven't thought of.

next time drill bit straight in the collet ...

ideas?

Thanks!

Will

.
i was taught as a 1st year apprentice drilled holes are typically 1 to 3% oversize
.500x.01= .005
.500x.03= .015
you got a .508 hole or .008 oversize, pretty much exactly as they teach apprentices what a typical hole size is expected to be

magneticanomaly · Apr 18, 2014

Knurl the bore and ream. Common trick for overhauling automotive valve guides Goodson should have the tool.
Next tme, do as everyoine says and drill undersize and ream.

wfrancis · Apr 18, 2014

Thanks for all the ideas everyone!

I have used this bit before and recall it only being a thou or two over but I probably used it directly in a collet. I did drill two other sizes (three if you count the spotting drill) and the last pass to .5" wasn't a big leap so I really think it's a worn keyless drill chuck. Pity, it's a real albrecht.

Scruffy, I am intrigued by your idea to flare the bushings slightly (if I understand you correctly). I'll make a small tapered shaft and slightly press a few on and see if that works. Otherwise, plan B and I'll just make my own about .510".

DMF_TomB · Apr 18, 2014

wfrancis said:
Thanks for all the ideas everyone!

I have used this bit before and recall it only being a thou or two over but I probably used it directly in a collet. I did drill two other sizes (three if you count the spotting drill) and the last pass to .5" wasn't a big leap so I really think it's a worn keyless drill chuck. Pity, it's a real albrecht.

Scruffy, I am intrigued by your idea to flare the bushings slightly (if I understand you correctly). I'll make a small tapered shaft and slightly press a few on and see if that works. Otherwise, plan B and I'll just make my own about .510".

.
as a professional machinist i find oversize holes are caused mostly by
1) poor sharpening, especially point off center
2) high feed rates tend to cause more oversize holes
3) no pilot hole, if pilot hole is under 1/64 then the chance of last drill drilling oversize is reduced
4) a drill tip that is wobbling because of an eccentric tool holder does not help but it is only one of many causes
....... i have seen a 1.500 drill bit, drill over .030" oversize far too many times and have had way too many parts scrapped. 99% of the time it is the programmer who did not allow for a drill to drill oversize that was the real problem.

mike 44 · Apr 18, 2014

Would Loctite solve the problem?
mike

Mcgyver · Apr 18, 2014

wfrancis said:
I drilled some 3/8" mild steel with a .5" drill bit in a (I now think) worn drill chuck and it drilled the holes closer to .508". I bought and planned to press in some .503" bearing bronze bushings. I tried the usual tricks with the bushings to make them slightly bigger - straight knurl on the lathe and a few punches with my center punch. Alas, it seems the bronze is soft enough (also impregnated with oil) that this actually doesn't seem to make them any larger.

I'm considering buying some 5/8" bearing bronze and just machining my own to size but given that I already bought a few dozen bushings ready to go I'm hoping there is a trick I haven't thought of.

next time drill bit straight in the collet ...

ideas?

Thanks!

Will

you are making two basic errors 1) as pointed out, you should never expect an accurate diameter from drilling and twist drills typical drill oversized and 2) you're thinking your problem is the drill chuck. its not a boring bar....being a little eccentric in the chuck doesn't matter....its the grind on the twist drill that matters. The closer to perfect it is, the closer on size the hole is (assuming reasonable conditions' feed/speed/coolant etc).

I would drill it oversize, weld/braze/loctite a plug in and do the job over, this time drill undersized 1/32 and ream.

Also be aware the hole diam for sinter bronze bearings matters. When pressed in the correct nominal hole, the ID also gets smaller creating the desired clearance for the shaft. iirc machinery's handbook discusses changing the press fit diameter to change the resulting clearance

jamie76x · Apr 18, 2014

Shrinking oil.

MetalCarnage · Apr 18, 2014

mike 44 said:
Would Loctite solve the problem?
mike

I was thinking something like Loctite 680

-Ron

jrmach · Apr 18, 2014

Sometimes I have to look at the address bar and make sure I am on P.M.

CalG · Apr 18, 2014

A half inch hole in 3/8ths thick steel? That hole can be closed up with a OA torch and a hammer with no problems at at all. The new thickness will be two hammer blows difference, one on each side.

Only you know how red the steel will be or how hard the hammer blow ;-) It won't take much to upset that stock! (I'ld do it cold if it was nothing important)

ps if youy want to keep things looking nice, Use a press instead of the hammer ;-)

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