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lynnl
01-03-2003, 03:17 PM
Short of buying an oversize 1/4" reamer, what's a good way to get a nice slip fit hole to accept 1/4" dowel pins (which are about 3 tenths oversized for press fit). BTW the holes are in 1/4" flat bar stock.

gvasale
01-03-2003, 03:46 PM
Put one pin of suitable diameter in an electric drill coated with lapping compound and size the hole that way.

Big Dipper
01-03-2003, 03:50 PM
It might take a little experimentation. Try using cutting/tapping oil or dry. Leave more stock for reaming or less. There are some oddball tricks that might work if you have only a couple holes...run a piece of hard steel, like the back end of a HSS lathe tool up one or more of the flutes to create a slight burr. A wooden toothpick or two strategicly placed in a flute might help...all this assuming you want to make the reamed hole slightly larger. Judicous use of a stone on the flutes can make a slightly smaller hole...but pretty much trial and error.

SGW
01-03-2003, 04:20 PM
I think gvasale has the right idea with lapping, but I'm afraid doing it with an electric drill would likely leave you with a slightly cockeyed hole. I'm pretty sure it would if I tried it. If you can drill, ream, and then in the same setup do the lapping using a suitable rod in the milling machine or drill press or whatever you used to drill/ream the hole with, then you ought to be okay.
As far as "suitable rod" goes, you can buy expanding laps, but the idea here is not to buy anything...else you'd buy the oversize reamer. Try about a 3/16" dia. aluminum rod, slit through lengthwise for a couple of inches but not to the end. (If the rod is 6" overall, slit it from about 1/4" from one end to 2 1/4" from that end.) Expand the rod by putting a wedge in the slit, a screwdriver will do, and bending the slit open until it's a slight rubbing fit in the hole you want to lap. Then apply a small amount of lapping compound -- #240, maybe -- turn on the spindle at a couple hundred rpm, and run the rod back and forth through the hole. Expand the rod as it wears.
There's a temptation to use a lot of lapping compound to speed things up, but go easy. If you get too much, it tends to produce a bell-mouthed hole because the lapping compound collects at the edges.


[This message has been edited by SGW (edited 01-03-2003).]

docsteve66
01-03-2003, 04:35 PM
Lynn: I don't believe a reamer will help much cause those pins (as you mentioned) are intended to drive fit. Never tried to do what you want with a pin, but the "lapping" (valve grinding) compound should work well. done it many times. I suspect the pin will wear first becasue the iron is the softer and the compound will embed in the iron and work on the pin. BTW my favorite compound is Clover. Just be sure to wash with soap and water the pin and the hole.

For your job, I'd first try chucking the pin in drill press, some compound on the lower end of the pin and work in down. If pin and hole wear, a second pin will probably make your fit. I assume its a small run of holes you need.

You have a fair amount of hole metal to remove if the hole is on size from a 1/4 inch bit. You might grind the drill tip off center to make the hole larger (assuming you are not step drilling the hole), and use a brass lap to finish the job. IMHO best sliding fits arrive when I makethe parts to fit togehter (not trying to interchange).

Be sure to de burr your hole (little chamfer at the entry and exit). makes it less apt to bind (when a burr bends over and grabs the pin) and the chamfer holds some compound. Use lots of up and down motion on the quill. Keep it all cool.
Steve

(edit comment: While I was loafing and composing, every one else was posting. Sorry I duplicated things others had said already. But seems we are all in agreement. If you can vote on the laws of the machine gods, Lynn has his problem answered (IN DEPTH http://bbs.homeshopmachinist.net//smile.gif )


[This message has been edited by docsteve66 (edited 01-03-2003).]

yf
01-03-2003, 04:36 PM
Try the toothpicks as mentioned above or you can try putting a wire size drill that when put in one of the flutes of your reamer will give you about .001" oversized hole. Try it in a scrap piece first to make sure its the fit you want. This makes the reamer cut off center so it cuts slightly oversize. With the wire drills you can get a definite oversize, it wont squash down like a toothpick.
Hope this helps.

[This message has been edited by yf (edited 01-03-2003).]

lynnl
01-03-2003, 05:51 PM
Thanks to all for your inputs. I've already match drilled the holes (one in each end of 4 pcs about 3.5" long)....clamped together and drilled with a 'D' drill, then reamed with a .250 reamer. They're still clamped together, but I've already taken them out of the mill table vise. I was aware of the lapping process, but haven't ever done it and wasn't sure of the particulars. But I don't think I'd heard of using a 'filler' in one of the flutes to offset the reamer. That sounds interesting. I may try that on some trial basis and see how close I can get. If it doesn't look too promising I do have some Clover compound on hand.

BTW these holes are for the axles for the knurl wheels on Rudy Kouhoupt's design for a pinch knurling tool. I'm probably taking more pain than necessary, but I'd like it to be something to take pride in.

gizmo2
01-03-2003, 09:00 PM
Lynnl, you are NOT waisting your time going for a tight fit. I built a clamp type knurl tool from Lautard's Bedside, and got one of the slots in the arms too wide. It's ok for aluminum and brass, but wants to wander in tougher circumstances. Nice tight fit in the slot and nice tight axles should improve the appearance/success rate of your k-nurls. I'd even leave it a little on the tight side and let them work in.

lynnl
01-04-2003, 12:15 PM
Thnx for the input Gizmo. I've left the slots in the block that holds the arms as tight as possible yet permit getting the arms in. That was kinda my thinking on the holes for the axles too.
Had a pleasant surprise last nite while rummaging around in my box of reamers, on most of which the markings are no longer legible. Found a reamer that opened up the holes to exactly the right size! Proves once again that dumb blind luck will conquer skill, guile, and cunning every time. http://bbs.homeshopmachinist.net//smile.gif