I am starting to notice the time it takes to square blocks for some of the larger parts that I makes. Large seems to be 2x8 inches.
Clearly I can save time by making more accurate rough cuts. My Delta bandsaw is working out well for both acrylic and Aluminum. My layout technique could be better, and I end up leaving larger margins than necessary as a result.
So far, I have been using mostly a combination square for layout, but that does not work well for irregular stock. Are there any particularly good ways to draw rectangles on stock with no clean edges?
For reasons unknown, I have not yet purchased layout fluid - that will get fixed in my next order.
Just for laughs, suppose you had to clean up a 2x8x1 piece of acrylic. The thickness varies from 0.94 to 0.98 inches with some cupping. It is to be milled down to 0.900 inches. To keep things simple, suppose the final dimensions are to be 2.000x8.000 inches. How big would your rough cut part be? Would you start with an endmill and then switch to a flycutter? How long would it take you to get it square and to thickness?
Strive for accuracy on every thing you do, even if it is not a critical part. Roughing techniques can be learned any time. If you haven't yet used layout fluid, you're already roughing it!
Feed and speed as fast as possible, with these exceptions:
Don't wreck the part.
Don't break the cutter.
Don't stall the spindle.
Use the flycutter for squaring/sizing, not the endmill. DOC will be limited by the length of the toolbit's cutting edge, feed will be limited by the rigidity and power of your machine, RPM will be limited by the cutter material.
It's important to experiment. Keep feeding harder and running faster until you have some type of failure.
Also, keep the sawed edges as close to finished size as possible. Ideally you want one finishing cut on the mill and that's it, but in reality one roughing and one finishing cut is more likely.
So far, wrecking the part seems to be the limiting factor, at least with acrylic.
Interesting. I will check some things and follow up with some dumb questions. Sadly, I probably can't do so today given a hard deadline.
It must be true that I can cut faster than I am at present. Your point on the cutting edge might be an important key. The grumbling (didn't push it to failure) might have been less about DOC and more about the bit.
What might be the max DOC you would expect with an ideal flycutter in Al and acrylic, respectively?
I would expect to make a separate finishing cut, if only because it simplfies getting a good measurement to know how much to remove to get to size.
There is no doubt that I need to start sawing closer to final size. Better layout will help me do it reliable - I hate to make something enough under size that it ends up as scrap before the tricky stuff. Of course the good news is that scrapping early leaves a rectangular block that will be good for something in the future.
I assumed it would be. I worked with it in high-school and I remember it cracking and chipping a lot.
AFAIK plastics suffer from issues relating to heat and pressure. You can run into trouble melting your part, cracking it from localized heating, or just placing too much pressure on it because of your feed/DOC.
Grinding your toolbit correctly is very important. I believe you want agressive positive rake, a *very* sharp edge, and lots of clearance on the front face. Consider carefully honing the edge with an oil stone. I believe the sharper the edge, the less heat generated.
Others will have much more experience than I.
Assuming a Bridgeport mill (or similar) in aluminum you can get away with .2 - .4" DOC. Perhaps deeper, but you'll find the motor will have trouble keeping up as the DOC increases. I'm really not sure about acrylic. You may be able to look up some values in Machinery's Handbook.
Very true. If you're making more than one of the same part, you can get away with setting the Z height (using the knee) and just using one pass to get to size. Depends on your tolerance and how much material you have to remove. You're right about one-offs however. You pretty much always need a roughing and a finishing cut.
With experience I've found you tend to trust your own eyes and judgement more. On a properly setup bandsaw, I'd cut within 1mm (.04") of the line. You have to adjust this figure if your bandsaw doesn't cut squarely, or if you have trouble keeping the blade on the correct side of the line.
Just as I've been getting better with the machines, I now spend a lot of time with disk grinders, die grinders and polishing stones. I think I'm going to be in *heavy* learning mode for another five years. I know it should never entirely stop, but I'm pretty sure that feeling of ignorance which all apprentices should have eventually goes away...
For layout, start with a surface plate or something similar. (I have a chunk of flame cut 3/4 thick plate which was blanchard ground both sides. Not perfect, but very usable) Size of plate should be large enough to put your work piece on and then some room to use a surface gage, or whatever.
To layout your 2 inch x 8 inch rectangle from stock with no regular edges, the objective will be to hold it stationary in reference to your surface plate (on edge). Basically, the surface plate becomes your known straight edge.
Use the surface gage to layout two parallel lines (2 inches apart) and parallel to the surface plate. No surface gage? Use a HS tool bit sharpened on end for a clean scribe mark for the first layout line. Then set this same bit onto the 2 inch side of a 1-2-3 block and scribe the second line.
Use an accurate square ( I often end up using my 1-2-3 block) to layout a vertical line. Then measure 8 inches from the first vertical line, mark it, and use the square again to layout the final 8 inch length.
Finally, perfect your skill at band sawing to line, but actually leaving the line. A bit of practice and you'll be able to clean up in one 'finish' pass.
Of course!!!!!!! :) Would a square mounted over some 3/4 inch plywood and some type of clamp work? The idea would be to use the square as a reference surface for a comination square that sets the directions, and use some notched rods or something to get point contact of the work against the square.
Do you visually align the block with the first line, or is it pressed against something?
Something else I have considered is to scribe a couple of circles and the scribe tangent lines. But having an external square reference should be easier if not more accurate.
There's that word "skill" again :) Humor aside, that is clearly essential. The other thing that bothers me is that if I cut too closely and the rough part does not sit well in the vice, it could tilt such that it ends up being under sized. A visual pass of an endmill next to the top line would reveal big errors. Is there a better way?
One cheat I've used is to rough a little large, clean up one edge, and then saw the other edge again before milling it. The idea is that the second saw line is easy to find thanks the clean edge opposite it.
As I expected, you guys saw closer and take deeper cuts than I do. Better layout will help. I'll probably have some more questions about tool bits in the next couple of days.
It sounds as if you are still wanting to work on a horizontal plane. What I have suggested is to work in a vertical plane. I guess I left out the part of holding your workpiece against a 90 degree angle plate (or a reasonaby clean & square piece of angle iron/alum/??)
If working in a vertical plane, you can use an two (2) items with a difference in dimension of 2 inches to layout exactly the 2 inches without any visual reference. Remember, you are working from your 'surface plate'.
Then for the horizontal dimension, (a vertical line) the line is simply on the work piece, but is square to the horizontal line due to the sqaureness of the 1-2-3 block, which by definition has all surfaces square to each other in addition to being 1x2x3 inches.
If your mill vise is of reasonable quality, the top of the solid jaw should be parallal to the bottom of the vise. As such, simply line up your part visually so your layout line is equal distance above the vise jaw. Use your 'scale' to measure it....
Good observations, Robin. One more thing that is very important is the direction of feed and cut. It's a good idea to always cut into an edge, never away from one, and to climb mill. That way you don't chip the material, particularly at the corners, which generally require particular care (brittle plastics).
More toy$ - I knew this was coming. Is this a job for a bench grinder and set of oil stones? Is there a better way?
I did a quick experiment last night with a 0.1 inch DOC. There was a little more vibration than I might have liked, but the machine handled it. It's not a bridgie [*]. On one such pass, I found cracks in the remaining material, so I don't know that I'll be repeating such a cut. Still, it is nice to see that some of the cuts I thought were too deep were probably ok. I can revisit it once I have a proper sharpening setup. However, better layout and sawing will probably save me more time than deep cuts.
Cutting into an edge makes sense, and I have been doing that with plastic whenever possible. I have not deliberately tried to climb with a flycutter, though thinking back I probably did it once on Al.
With Al, I simply back off if the machine seems not to be enjoying its work. Plastics are the problem, because an aggressive cut can turn into a broken part.
To avoid cutting away from an edge, are you suggesting that I take two climb passes for a deep cut? That still ends up "leaving an edge", but as somebody said a few weeks ago, to do damage, the bit has to take a divot out of the part (or crack it as I have just learned) instead of merely breaking away part of an outside edge. Fair?
I see what you are saying. We definitely were 90 degrees apart. One potential snag is that your method depends on having an object of a suitable thickness. A mix of parallels would probably cover most things I would need to do though.
A bench, or pedestal grinder, is all that is necessary, but it should have a good wheel---one made for grinding HSS-----aluminum oxide, and not too hard----something in the H-J range-----vitrified bond, and maybe a 60 grit. It should be dressed either with a star dresser (I don't like them) or a diamond to true it, then followed up with the sharp edge of a silicon carbide sintered dressing stick. The diamond isn't necessary, but makes dressing the wheel easier. Diamond dressed wheels don't make for a good off-hand grinding surface, however, -----thus my recommendation to follow up with a dressing stick. As far as stoning goes, the usual shop stone works fine, nothing special necessary. It's all in technique, however. You can destroy a tool's ability to cut by improper stoning. It's very important that you do not round the cutting edge, regardless of the material being machined.
The typical aluminum oxide grinding wheel that comes as standard equipment on small grinders tends to be bonded way too hard to successfully grind hard materials (think HSS). As I stated today when discussing this very issue, privately, by email, it can be challenging to find the right wheel for grinders that mount wheels with small fractional bores, 1/2",
5/8" or even 3/4". The greatest selection of wheels is available with
1-1/4" bore, wheels that are commonly used on cutter grinders and small surface grinders. A standard grinder can be easily converted to that wheel size by machining new wheel arbors, although you may have to remount the wheel guards because you extend the wheel. If you can work without a grinding rest, and I do, that isn't a problem. Just discard them. Wheel arbors should be properly configured to avoid breaking wheels, so don't jump in blindly if you decide to make the conversion.
Very unlikely the depth of cut was instrumental, although not impossible-----more so the tool design. A properly sharpened tool should be able to remove a huge amount without chipping when machining acrylic plastic. Speed and feed play a serious role, too, so you have to balance the tool configuration with the other variables for success. What's very important is that the tool be very sharp, and have ample clearance and rake, as Robin suggested. You don't want the tool to do any pushing, just shaving, so to speak.
In order for you to be successful in machining, it's quite important to learn proper tool configuration, and to be able to generate the forms as needed. That includes grinding chip breakers. At first it may appear to be a daunting task, but don't allow it to discourage you. Practice makes perfect, and once mastered, it's no big deal. As I've stated time and again, learning to grind proper toolbits will set you free.
When fly cutting, it's not a big problem like it can be with an end mill. By climb fly cutting, and walking around the perimeter of the part in question, you can always take cuts towards the inside, which will almost totally eliminate edge chipping. The only exception is outside corners, where you have to sort of back off as you approach the corner, diminish cutting feed rate, and gently round the tip of the corner. Again, piece of cake.
See above. It would be so much easier to demonstrate this instead of try to discuss it. More than one pass may or may not be necessary, depending on many variables. When you fly cut, it's usually more than desirable that the tool have not only decent rake, but have a lead angle on the tool as well, assuming you're not trying to establish a shoulder. That way the top of the cut is first to see the tool, and the bottom the last. You machine a bevel, in other words. That minimizes the chance of chipping. Does that make sense?
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