Table jumps around when drilling with an end mill

I don't yet have a lathe, so it would be difficult for me to make the bushings. So, oddly, it's easier for me to make the whole thing of steel. The overall dimensions are 1.00 by 3.50 by 0.250".

I'm going to try this, being all fired up after reading Tubal Cain's book Hardening and Tempering.

With a big propane-air torch (fed from a picnic bottle) and a pile of firebricks made into a little muffle, it's easy to achieve red heat (enough for O1), and I will try for orange-white heat (enough for A2). I've already hardened smaller pieces of A2 that way, and it was easy. Actually, I held the A2 too long at temperature, and got too much scale. Not that the scale hurt anything, although there was also a thin decarburized layer as well.

A hardening furnace would be nice, but a torch will work for this, and there is no requirement for a long hold time at orange-white temperature.

This jig plate will be used perhaps 200 times a year, and an unhardened predecessor (mild steel, perhaps 0.0625" thick) lasted almost a year.

Joe Gwinn

This sounds more likely than my chip-welding theory. It turns out that all the holes were misplaced, even if there was no drama, and having a wad of chips jamming under one flute could certainly achieve that.

I didn't want to use a reamer because I wanted the hole placement to be correct, and the theory was that the endmill would not react to errors in the hole, and cut where desired. Didn't work.

OK. The end mill has no visible wear or damage.

In my next test piece, I will rough all holes with a twist drill, and finish some holes with the end mill, and some by boring, for the experience.

I think you've put your finger on the root cause, chip packing. This theory explains all observations so far.

If I make the roughing hole big enough, say 1/4 or 9/32", it ought to work OK. But I will also try boring some of the holes, which is slow but sure to work. The objective is partly practical, partly educational.

Joe Gwinn

So make the bushings square if you want to make them yourself, but you do not need to make the jig out of tool steel. Nothing says you have to make the outside of the bushing round either. Only the hole needs to be round. The off the shelf bushings have the advantage of being a standard manufactured product and they are properly hardedned and ground.

To do a proper job on the jig with tool steel you will have to either allow more slop than you really want in a drill guide, or lap the holes to size and finish after you heat treat the plate.

Still sounds to me like a whole lot of work for a little value. If the jig is only going to be used 200 times a year, I would knock it out of plexiglass. Lexan also works good, and is less prone to shatter if tossed around. Another idea is to make the jig so you drill every other hole, toss in a couple of pins to index the jig and then drill the holes you missed on the first pass. This would cut the cost of the bushings in half.

Good luck what ever you decide to do.

You did lock the x and y gibbs?

Wes

Put a dial indicator against the side of the spindle (not the quill) and apply force to the spindle, possibly by a solid bar in a collet. apply about 50 Lbs force radially both toward and away from the indicator. A movement of .001" or so is not a surprise as the non-rotating bearings have clearance that is supposed to be filled by the bearing lube. This much radial play will not happen when the spindle is running.

If the movement is more than .001" you can then move up the assembly, first to the quill, then to the main spindle housing, and wee how much movement you get at each part. If this is a lighter machine, you may have to mount the indicator support to the head to separate flex in the vertical column from the quill and spindle.

If the play is in the spindle, you may be able to tighten a nut on the spindle to take up the slack in the bearings. Depending on the bearing type and lube used, you may want to go to zero clearance, or leave some clearance there to be taken up by the lube. Without specific instructions, you may need to creep up on it until the results improve. Or, leave well enough alone unless you are sure there is a problem here.

Jon

You can use standard bushings and grind a flat or groove on 1 or 2 sides to make them fit closer together.

On Sun, 25 Feb 2007 07:14:40 -0500, with neither quill nor qualm, Nick Hull quickly quoth:

.3125 holes on .4037 centers leave just .091 (less than 3/32") between them. That doesn't give enough space for a single standard bushing, let alone side-by sides. The key is to drill and insert a single bushing, then use a raisable bottom plug in the jig for the rest of the holes. It will position the 2nd (3rd, etc.) holes. He'd always be drilling through the single bushing. Jig for initial positioning and go from there. I'd use steel or aluminum. Plexi can degrade over time, scratch, crack, etc.

========================================================== CAUTION: Do NOT look directly into laser with remaining eyeball! ==========================================================

If the bushings are square, then so are the holes they go into. I don't see how this is easier than making a plate with a bunch of round holes in it.

A2 (air-hardening) steel doesn't distort much, as no quench is involved. A little post-hardening lapping with a 5/16 brass or soft steel rod charged with carborundum dust in grease is probably a good idea anyway, to polish the ring that will touch the outside of the drill bit.

Great accuracy isn't required. The desire is for appearance and consistency .

The steel plate is to be screwed and glued to a far larger jig made of a phenolic-linen laminate. This jig also acts as a router template.

And slow production down, and make for easily-lost pieces.

Thanks. As I've said elsewhere, this project is partly educational for me, although the result will be used, not just admired.

Joe Gwinn

Joe,

First forgive me for reading too fast, I see now your problem.

Here is what I would do to make the jig.

On the part that fits into the phenolic section drill 6 1/2 inch holes .4037 centers. On holes 3 and 4 you should make them just a little larger. This will give you a scalloped edged slot.

On a separate piece of 1/4" drill and ream three holes to 1/2" spaced .8146 apart. Set into this separate piece three bushings (McMaster P/N 8493A131)

In production, the operator inserts jig 2 into jig 1 and it indexes on the lower part of the bushing setting into partial holes 1 and 5. He then drills 3 holes and then resets jig 2 into jig 1 this time indexing bushing 1 and 3 on partial holes 2 and 6. 3 more holes are drilled. The result is 6 holes spaced just the way you want them, and no grief to machine the jig, no heat treating and it should last a lifetime.

Some hints. When drilling the holes in Jig 1, you should countersink the holes a little to make insertion of the jug easier. Also since it is a whole lot easier to get two pins to align in two holes rather than 3 pins to align in 3 holes this is why I suggested making holes 3 and 4 a little bigger. In the alternative, you can just make a slot for holes 3 and 4.

You should consider, if feasible, some provision to attach jig 2 to jig 1 when it is in storage.

I think this design will serve you better than your original plan for several reasons. First all of the hard work is done for you in using off the shelf bushings, no heat treat, no grinding or polishing.

If, for what ever reason, they change the design on subsequent production runs, It is easy to pop the old bushings out and pop then into a new one.

I suspect it will be faster and cheaper to make it this way.

Good luck.

I see your plan now. The scalloped-edge line of holes would be best made by boring, as this handles interrupted cuts best.

One twist is that the router shoe rides on the top of the jig plate while routing out a nearby cavity, so nothing can be sticking up from the 1st jig plate, and the 2nd jig plate would need to be removed.

It's possible, but as i said the purpose is partly practical partly educational. Nor do I think that hardening is all that much trouble, even if there is some warping. We shall see.

Thanks,

Joe Gwinn

Yes. It turned out to be chip packing that was causing the problems.

Joe Gwinn

Thanks. I'll try this.

The bearings do sound OK.

Joe Gwinn

As I pictured it, Jig 1 would have nothing protruding from it's surface so this would not be a problem. Jig 2 has protrusions from both sides but the guides protruding from the bottom stick out 1/4 inch and set flush into Jig

1. Jig 2 could be left off while routing so this is not a problem.

Yep!! I agree.

That's true! End mills are not intended to be used as a drill-----there are far better ways to remove the metal. The fit may be tight, but the hole may or may not be round, and likely has a pattern, developed by the end mill walking about. If you don't have a reamer, and don't wish to bore the holes, start with a center drill, followed by an undersized drill, then open the hole to the desired drill size. Common practice is to drill the hole 1/64" undersized, then open it. If your drills don't cut size, drill the hole 1/32" undersized.

. Also, a mill is not

That's not true. An end mill, typically, is short and rigid and intended to cut where you aim it. Even when drilling. You get in trouble with drilling with an end mill because there's no provision for discharging the volume and shape of chip generated, so you get chip loading of the flutes, resulting in a winging end mill. It's not uncommon to get a hole that's generously oversized as a result, assuming all flutes aren't loaded equally. If they are, the end mill more or less refuses to cut.

That's not true----although I fully agree with starting holes with either a center drill or spotting drill. Twist drills are notorious for not drilling straight, round or on size holes. The slightest thing will deflect the point, resulting in a hole that runs off in any direction. You see evidence of this when you drill a long object in the lathe. Rarely does the hole meet at the center when drilled from both ends. If you want holes located precisely, the best way to achieve that is to rough drill, then bore on location. While I don't endorse the use of end mills, they will provide a hole on location, but you risk the oversized condition so common with any tool that is side cutting.

Harold

I did a trial piece with the 1/4" roughing twist drill followed by either the same 5/16" endmill or by boring. What a difference! That endmill really needed the hole to be roughed out first. It was far faster to rough drill then use the endmill, rather than to push through with the endmill alone. The holes were quite tight on a 5/16 drill blank, especially the last holes drilled, as I gained experience.

Boring also worked well, but it was slow and fiddly to hit the desired diameter accurately. Next time, I would use a 9/32" roughing drill, to reduce the number of passes required. The advantage of boring would be that I can then easily make the holes slightly larger than 5/16", useful so that the jig doesn't require too much precision (and tramming of floppy drill presses) to be practical. An alternative is to lap the holes with a brass rod, after the plate has been hardened.

So, it has to have been chip packing between the face of the endmill and the bottom of the hole that was causing the problem.

The holes are still not quite where they belong, especially in the X direction. My suspicion is that the X table lock isn't tight enough, allowing some movement. The little cast iron knob on that lock is not original, and is far too small for the purpose. It looks like one I've seen in the MSC catalog. I have not seen anything suitable in the MSC catalog: the knob requires a deep 3/8-16 thread and a nose not exceeding (or machinable to) 3/4" in diameter, to fit into a spotfaced ~0.100" recess in the bottom of the saddle. I already have a brass washer down there, to reduce friction between knob and saddle, so more of the tightening effort goes into clamping force, and less into hurting my hand.

I had to turn the tip of the knob I have down to fit into the recess to get even semi-solid clamping action. Not having a lathe, I used the mill (running in reverse - no, the boring head cannot unscrew) with a backwards boring bar to bore an inverse ~0.750" hole in the tip of the knob. Champing that odd-shaped knob to the table with hub straight up was quite the trick. I used a clamping ring from a 5C collet holder as a rest (to clear the knob's hub) and clamped the knob's arms to the table through the ring.

Joe Gwinn

It is true. :-) It is not *self*centering. Self, that's the difference. It sure is rigid. But it also doesn't cut where it was aimed to. That's why final cuts are done. If you have made a pilot hole and move the table a tad and then drill, the drill will bend and start where you centered. We both know where it will

Not my observation. Even if the chips come out freely (or you wash them out with coolant). If only one edge on the face has a bit more to cut (or is standing out just a tad more) the troubles beginn. I don't think it is chiploading. But we both agree on what happens.

Talking of mills? ACK.

They do that (with a little trick). And if they aren't hand-ground by me. :-) The trick is: If the work is in a vise and do it on a drill press, rotate the vise every once and a while for a quarter.

The tailstock is nearly never absolutely on center and dead parallel to the spindle. Much better in the mill.

ACK

ACK. Hard for small holes.

I never checked how much off-center the holes were when drilling with mills. Because when I did, I never cared for the quality.

Nick

Ah! I see we're talking about two different things. Yes, I agree-----a drill is self centering in that it will go to a pre-drilled location even if the location is not in line with the spindle. That, indeed, is self centering. I should have said that an end mill will cut where you want it to cut----or----said another way-----it centers where it is located-----but that isn't the same thing you said. I was wrong.

Depends on the rigidity of the machine. Spindly machines would have some difficulty with such an operation, but they'd have difficulty with milling, too. A rigid machine won't give a damn if there's dissimilar amounts of metal to cut, not as long as the chips can be easily discharged. That's a tall order when drilling with an end mill unless the size of the chip is smaller than the width of the flute.

I've used end mills to properly locate holes that are in the wrong place enough times to know it works fine----especially if you're opening the hole by a small amount, so the chip is insignificant. I think I can honestly say that such an operation is the only time I endorse the use of end mills for drilling.

I've drilled more than my fair share of holes in a mill that was dialed in within limits to find the hole coming out in the wrong place. It's the nature of twist drills to drift, often for what appears to be no apparent reason. That's not to say that they can't drill straight holes, but it's not guaranteed. Chances are better that a hole will drift, regardless of how it's drilled, than it won't. The amount of drift is the only question I'd have, not that it would, or wouldn't.

It isn't attributed to that alone. I've drilled holes in one piece that come together beautifully, followed by one that is off a half drill diameter. Twist drills simply do not drill straight holes reliably. If they did, there'd be little need for gun drills.

Harold