Sheet Metal Guy Blog

30 Jul 2008   09:56:23 am
Calculating the k-factor
To my knowledge, there is not a formula for calculating the k-factor. Oh, I am certain somewhere some mathematical engineer has a formula. But it is most likely too complex for most of us to understand or be able to use. The k-factor is the percentage of the material thickness where there is no stretching or compressing of the material in the bend area. Thus, the neutral axis! The harder the material, the less compression there is on the inside of the bend. Therefore, more stretching on the outside and the neutral axis moves toward the inside of the bend. Softer materials allow more compression on the inside and the neutral axis remains closer to the center of the material thickness. Bend radius has a similar effect. The smaller the bend radius, the more need for compression and the neutral axis moves toward the inside of the bend. On a larger radius. the neutral axis remains near the center of the material thickness. The conclusion then is that the k-factor should always be a number greater than zero and less than or equal to .5 (50%). However, anyone who has worked with sheet metal knows you have to lie and cheat to get the right answer. This means the k-factor could be any number.
		Category : Sheet Metal | Posted By : SheetMetalGuy | Comments[35] | Trackbacks [2926]
09 Jun 2008   01:18:38 pm
Unfolding formulas
I was sent several pages of unfolding information last week. Since we do say, “If you have a k-factor chart you would like to share with others, email to me”. Well someone did. I sat down with all eight pages and reviewed the details. While several pages are dedicated to calculating the Back Gage distance for the press brake, buried throughout each page is the standard formula for Bend Allowance. He probably doesn’t recognize it. But when you check the math, that’s what it is. I see this a lot. People will tell me they have their own formula and it is different. Yet when you break it down, it is still the standard formula. They may have filled in the Angle as 90 or unknowingly set the k-factor to 1/3, but the formula is the same. Here it is for those who don’t know it. Bend Allowance = Angle * (PI / 180) * (Radius + K-factor * Thickness) Remember we have a page wih these formulas. Here is a link: http://www.sheetmetalguy.com/bending-formulas.htm There are also a few pages to determine the minimum die opening. I found these to be interesting and did not see anything to say they will not work. What I really liked, was the chart of Bend Deductions. It is a classic. In the title block, it was approved 10-15-35, so you can tell how old it is. It covers 90 degree bends for a number of different thicknesses and radii from 1/32 to ½. Yes, they are shown as fractions. People did not have computers or even calculators back then. I did take my calculator and reversed the numbers to see what the k-factor was that they used. I was very surprised. It is always in the neighborhood of .5, sometimes bigger and sometimes smaller. But since the bend deductions are written to 3 decimal places, I kind of expected a different range for the k-factor. Well it does say “For Rough Work Only – Accuracy Not Guaranteed”.
		Category : Sheet Metal | Posted By : SheetMetalGuy | Comments[148] | Trackbacks [2282]
28 Feb 2008   11:51:56 am
Is there a chart of k-factors I can use?
Every now and then I get email asking, “Where can I get a K Factor Chart? Original chart, not someone's calculations.” This one came from Noel this week. So I thought it might be a good time to answer it publicly. To the best of my knowledge this chart does not exist. You can find several technical manuals offering their theoretical chart. Back in the 70's I was taught a rule of thumb: Radius < Thickness, K=.25 Radius < 2 * Thickness, K=.33 Radius > 2 * Thickness, K=.5 While this is just a rule of thumb, try the math on a couple of your material thicknesses. The difference in the answer if you use a k-factor of .25 vs .33, it's only a couple of thousandths. Here a quick formula for Bend Allowance (BA) of a 90 degree bend is 1.57 * (radius + thickness * k factor). So for .048 thickness with a .0625 inside radius, with k = .25 the BA = .117, and with k = .33 the BA = .123, a difference of 0.006. The real issue is that the k-factor varies not only with the hardness of the material, but also with the tooling and method you use on your press brake. The k-factor which works for one shop does not necessarily work for the next shop. You may want to talk with people you know in other shops to see what they are using. Or perhaps a tech support rep for a press brake manufacturer. They should have a chart to get you started. From there, it is a matter of experience and adjustment. You must find out what works for you. If you have a k-factor chart you would like to share with others, email to me and I will post it.
		Category : Sheet Metal | Posted By : SheetMetalGuy | Comments[99] | Trackbacks [2536]
02 Oct 2007   03:08:09 pm
How accurate are your CNC programs?
Although CAM software has the ability to do so many things for you, some customers insist that the post processor output “canned cycles”. This makes the programs shorter is their believe. In some cases it may be true, but there are some coding methods for canned cycles which lead to inaccurate results. Let’s take a look at a couple of Weidepoint commands. First the ARC/ command. There are four parameters for this command: radius, start angle, incremental angle, and number of hits. The first hole should be very close to the correct location, but after that, each hole may be progressively farther out of position. The problem is the incremental angle. If your Arc pattern is 12 holes, 15 radius, and 8 degrees apart, no problem. But what happens when the pattern is 8.135 degrees? The last hole is .014 out of position since the angle is rounded either up or down to satisfy the 2 decimal place input requirement of the canned cycle. The result, a bad part. The LIN/ and GRD/ commands can have the same issue. The distance between holes may need to be rounded off for input purposes of the canned cycle and the hole pattern is now progressively off. If your post processor outputs canned cycles, take a close look a the output to see if this error is possible. You may want to take action to avoid using these “canned cycles’ in future programs.
		Category : Sheet Metal | Posted By : SheetMetalGuy | Comments[131] | Trackbacks [4160]
26 Sep 2007   02:37:25 pm
Think Backups Aren’t Important!
I received a call last week from a customer. He lost his hard drive! A worm or virus messed it up and the IT guy reformatted it. After installing his application, CAD/CAM, and other software, they tried to restore his data files from a backup. But it was corrupted and hey did not have another backup, not even an older one. There wasn’t much I could do for him. I only have the application program, not his data files. I did have one of his databases from a couple years ago when he emailed it to me with a question, but all his job and DNC files are gone. Typical of many small to medium companies, the CNC programmer saves all of his jobs and DNC files on his own hard drive and they rarely get backed up. Then when you least expect it, something happens, a virus, a stolen computer, anything and the files are gone. Make certain you are getting your files backed up. Create a folder on the network server and manually copy the files to it everyday if you have to, but get them backed up on a regular basis. I realize that some files must be maintained on your local drive, but even these can be copied elsewhere as a means of backup. I have a few customers whom I know use an old DOS batch file tied to an icon on their Desktop. All they have to do is click the icon and their files are copied to the network drive which the company backs up every night. Whatever you do, make certain you don’t get caught without a backup of your files.
		Category : Sheet Metal | Posted By : SheetMetalGuy | Comments[113] | Trackbacks [2558]
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