How to mill a square recess?

[davec43]

How would I make a square hole in a piece of aluminum while dealing with backlash? I am talking about a 3/4"x1" hole through a piece of 7075 2" x 3" x 1/2" using a 3/8" endmill. I am thinking I could either measure the backlash and add it in, or I could rough ( I would rough the hole in either case, of course, Harold) the hole and then plunge the endmill in the corners for the last 10 or 15 thousandths and approach my final dimensions from the same direction for each side with the cutter out of the hole. Backlash is my constant companion. I got rid of most of it in my X-axis one day, and then discovered I had 6" less travel on each end.

Is there a better way to do this?

[Harold_V]

Because backlash is a function of clearance, in part due to wear, I think you'll find that working with a constant as a backlash figure may or may not yield the degree of precision you desire. For a new machine, it's likely OK, but, as you discovered, wear is not consistent on old machines. The amount of backlash is most likely to be different, depending on the location on the screw.

You're already on the right track by roughing. That permits measuring and making determinations as to how your dial reading equates to actual location, regardless of the direction one turns the dial. In essence, you then work with the dials with direct readings in all directions, because backlash for the given location on the screw has already been determined by measurement of the location of the cut.

What I do for cutting pockets is to rough, as you've reported, then take a light finishing cut (on all four surfaces), which then permits measurement to determine the location of all four surfaces. You can then alter your target dial settings so the finish cuts take the pocket to size. I report them on a piece of paper, using a + sign, at the end of each arm writing down my target dial reading.

If more than one part is to be machines, marking the locations of each cut on the saddle and table with a grease pencil aid in knowing which turn is correct when roughing. I almost never make a layout of a part when machining. It's wasteful of time, and not necessary. Above the marks, I also note the dial setting.

Does any of this make sense?

Harold

[davec43]

So, assuming a 1" sq hole for simplicity I would rough a through opening in 6 passes or so to say .950 on each side. Then I would make a cleanup pass on all 4 sides moving .970 on my dials on each sides, making careful note of the dial positions. I would then measure all 4 sides and calculate how much more each would need taken off, and add that to my dial position on each side.

I am only making one part for a vise stop.

I am not sure what you mean by marking each location for roughing. Do you mean marking on the dial where to stop for each side? I can see that being useful, even for roughing one cut when making 6 passes.

Thanks, Harold

[Harold_V]

So, assuming a 1" sq hole for simplicity I would rough a through opening in 6 passes or so to say .950 on each side. Then I would make a cleanup pass on all 4 sides moving .970 on my dials on each sides, making careful note of the dial positions. I would then measure all 4 sides and calculate how much more each would need taken off, and add that to my dial position on each side.

I generally work from a pair of points, usually the left side of a part, and the side nearest the column. That way both handles (saddle and table) read in the proper direction, or right handed. As a result, the window I choose to cut may not land on the numbers you specified. From this you can deduce that the numbers aren't important, but how you use them is.

Considering what I said, above, it's clear that if you have picked up the two reference points I mentioned, and you consider the size of the end mill you'll use, you can rely on the right hand side of a window to be in keeping with a dial setting, because you're turning the dial in the proper direction. Same thing applies to the side nearest you (the side opposite the column). It's the opposite faces that won't be able to rely on a calculated dial setting. Fortunately, those two sides can be measured directly to the reference points, so you can rough them, than take a light finishing cut with the dial turning in the CCW direction (reading backwards, which, now, has lost register with the reference point). Once you've measured, you can then calculate the dial setting to achieve the desired size.

When roughing, assuming normal conditions, and no risk of over-cutting, I like to leave .030" for finishing. I'm generally more interested in a nice finish and a precise size than I am in speed, so I take three finishing passes, the last one only a thou, and I climb mill. It's a little tricky going in to corners, but if you've established the corner locations within a thou, stopping a couple thou short of hitting the corner leaves no undercut, and the corner radius isn't materially altered. I suggest you play with cutting a few windows and discover what works best for you. I'm quite sure there's more than one way a guy may prefer to cut a window. I do what I do because it's easy (for me) and yields great results.

I am only making one part for a vise stop.

No need for marking the table and saddle in that case.

I am not sure what you mean by marking each location for roughing. Do you mean marking on the dial where to stop for each side? I can see that being useful, even for roughing one cut when making 6 passes.

It does no harm to mark the dial, and I generally do, but the piece of paper I mentioned will discern which mark goes for which position. Rarely will you ever find the same mark serves for two sides of a window.

The marks I mentioned don't go on the dial. They go on the sliding surfaces. For example, when feeding the saddle, it moves in relation to the knee. I generally make a mark that is parallel with the front edge of the saddle (I mark the saddle way wiper, in fact), then transfer that mark to the knee, directly below. When the saddle is moved, the mark on the knee is no longer in alignment, but when you approach the line again, it tells you when to stop turning the handle---especially if you have written the dial setting below the line. So then, the line on the wiper doesn't get marked, but each line on the saddle does. It prevents you form going past the desired location, but you must keep in mind which way you're turning the handle (backlash). If you use the system I suggested (piece of paper), that is no longer true, because you have determined the dial setting, reflecting backlash.

I use this system when drilling holes. By keeping the backlash ALWAYS in the same direction, there is no confusion.

None of this is difficult. Once you've done it a time or two, it makes all kinds of sense, and is easy to remember. Best of all, you can turn out work that rivals that from a CNC, and nearly as fast.

Harold

[Mid Day Machining]

Does your machine have a digital readout on it? If not, you could use some 2 inch travel indicators as guides. It's not really that hard.

[davec43]

Does your machine have a digital readout on it? If not, you could use some 2 inch travel indicators as guides. It's not really that hard.

I do not have a DRO although it is on my list. I am not familiar with the 2" travel indicators.

[davec43]

The vise stop is finished and the window in the slider turned out OK, but not quite perfect (things seldom are).

Having been self employed most of my life, it occurs to me that I spent several hours making something I could have bought for $49. I think of it as education.

I read over the post on Harold's Haas today. Lot's of interesting comments. I have not shaved since 1973 but then I'm only 61.

Nice machine Harold, and thanks for the help.

[Harold_V]

The vise stop is finished and the window in the slider turned out OK, but not quite perfect (things seldom are).

Don't short-change yourself. It looks great! Well done.

Having been self employed most of my life, it occurs to me that I spent several hours making something I could have bought for $49. I think of it as education.

Exactly. If you don't use these functions, they will forever remain a mystery and be trying when they are required. Dues must be paid. You made an investment.

I have not shaved since 1973 but then I'm only 61.

Rarely have I encountered anyone who has not shaved as long as I haven't. From your comment, I gather that, to you, your beard is very much a part of who you are. That's true for me. I can't imagine a clean shaven face.

Nice machine Harold, and thanks for the help.

Thanks!
Don't know how much help I was, but one thing you can know---I was trained and worked all my life as a machinist by using dials. I have never, to this day, operated a manual machine with a DRO. That simple fact served to teach me to trust the dials, and to understand how to deal with backlash. I recommend it highly, even when a guy owns a DRO. Running manual machines without regard for backlash (easy to do with a DRO) can get you in trouble.

Harold

[Patio]

Running manual machines without regard for backlash (easy to do with a DRO) can get you in trouble.

Harold

I figured that one out last night. I was locating holes in a part, to make another. While doing so I hit the part with a hammer and watch my DRO change position. When I hit the part, I moved it in the direction of the back lash, with just dials I may not have been aware of what I had just done, but the DRO told the story. This mill is my first machine with a DRO and what happens is I wind the screw to move the table, miss my mark, back it up just a bit to get the table where I want it. The problem is that I am now left with a table that is not fully loaded up against the screw. That can be a problem.
I am going to have to pay more attention, and try to approach from the direction, that will load against the direction of my cut.

I am learning that I have no idea as to how many different thinks affect everything, which means I have no idea as to what I am doing.

[stevec]

and try to approach from the direction, that will load against the direction of my cut.

"Try" won't cut it (so to say), you HAVE TO do it every time, until it becomes the habit. Only then you can avoid it from time to time as long as you know you're taking a shortcut

Omigod, there I go trying to sound knowledgable again .

BTW I believe you really do have a VERY GOOD idea of what you are doing.

[Patio]

"Try" won't cut it (so to say), you HAVE TO do it every time, until it becomes the habit. Only then you can avoid it from time to time as long as you know you're taking a shortcut

Omigod, there I go trying to sound knowledgable again .

BTW I believe you really do have a VERY GOOD idea of what you are doing.

I am trying to create good habits, but that usually happens after the problem has arisen. These are the problems of one trying to self teach, and not having the benefit of an apprenticeship. Thanks for the vote of confidence though.

[oscer]

This is a good thread,hope I don't muddy it up too badly. I like using grease pencils to define "sizes" I would add various colors to the mix, ie. blk may represent left rear corner of window with marks on the saddle, table and also on the dials. red may represent right front corner in the same way. also a dry erase board nearby can be used to show what color goes with which detail on more complicated parts. When I used to run manual VTLs I had an assortment of about 8 different colors and sometimes that wasn't enough we would then use dots or tees instead of lines to differentiate other dimentions (those old Bullards had alot of room to write on the dials). I believe this grease pencil method is useful even if you have a DRO as it's easier to see the dials approaching size than the digits zooming by.
oscer