How to: LINEAR STRETCH with a Power Hammer / Pullmax / Planishing Hammer The reason for this post is to teach some techniques when working with sheet metal. Most of these techniques are basic but a lot of people do not understand the operation of a linear stretch die in a power tool. So 90% of this post will concentrate on LINEAR STRETCH. This post could also be called: How to: build a Transmission tunnel from scratch. Before I start I would like to state that you will notice all of my work is done on a Large Baileigh Multi-Hammer (MH-19). This procedure will work just as good on a Planishing Hammer but it will just take several more passes at a slower speed. You will see it takes about 3min in the large hammer. It will probobly take about 10 min in a planishing hammer with the same finished effect. Lets get started: Linear stretch is used to stretch metal in only 1 direction. It can be used in many situations when working with sheet metal. A very common use is to make reverse curves at the bottom lip of a fender ( duck tail ). Another common use is when you need to turn an edge or return on a panel that already has a crown to it. If you are working with a flat piece of metal, then you would simply place your sheet into a sheet metal brake and bend your return to the desirable angle. When working with a curved panel this in not possible without ruining the curve you worked so hard to get. In most situations a person making a small patch panel would take his flat peice of metal, brake a return the correct degree (usually a 90deg) and then use a shrinker / stretcher on the lip to creat the arch in the panel. This only works if your panel has a crown in 1 direction only. If your panel is complex and not only curves around the flange, but also slopes away in 2 different directions, it will be very difficult (not impossible) to accomplish the desired effect with a shrinker / stretcher. This is when you have 2 good options. Some people use a "Tipping wheel" in their power bead roller. This is a sharp pointed upper wheel with a lower soft wheel (skateboard wheel). Your other option is a linear stretch die in a power tool (yoder, pullmax, planishing hammer, power hammer). I choose the second option: Here is how to do it… First I had to make a linear stretch die set-up for my Baileigh Power Hammer: I started off by using the 3/4" universal tool holder that Baileigh sells. It is meant for adapting old pullmax tooling to their hammer. I then purchased a adapter meant to adapt vintage CP (Chicago Pneumatic) planishing hammer dies into a pullmax machine. I Cut the shank down on the CP adaptor to fit as low as possible in the Bailigh 3/4" tool holder. Then the last step is to purchase a set of CP linear stretch dies. Once it is all put together I use electrical tape to hold the linear die from spinning. This makes sure all your stretch is only done in 1 direction. You will see I have 3 linear stretch dies. They all move the metal at different speeds. The wider the flat spot in center of the die, the slower the metal will move. The thin top will move metal faster but is more likely to leave marks in the metal. You need to decide which die to use depending on the Gauge of your metal and the power of your tool. See pictures of set up. Wether you are using a power hammer style machine or a planishing hammer you will always use a flat upper die. I use the Baileigh 2" radius master flat die. Here is how the set up looks in the machine: Quick side note: The Baileigh power hammer uses a very nice quick releae T-handle pin to hold the lower die post in place. This is great for 99% of the operations you will use the power hammer for. In this situation it gets in the way when trying to turn down a panel a full 90deg. To overcome this issue I machined my own pin: I took some round stock off the shelf. Turned it down to the correct diameter. Then I machined a groove on the back side for a trailer hitch pin that I had laying around the shop. I finished it off by TIG welding a fender washer to the front. Here is how it looks: For the Bailigh Power hammer here is my Machine set-up: I have the power set on 50%. I don't want a long throw. I want just enough power to move the metal very fast, but I don't want to mark the panel up and leave tooling marks. A long throw will also make the panel jump around and make it hard to follow a straight line accurately. I also left the machine in "Yoder" mode (No Pin) A.K.A. "Throw the Die" Now that you understand the tooling and set-up lets see what it does. I am building a firewall and trans tunnel for a Vintage Gasser. I moved the engine back 1 foot so I had to cut a large hole through the factory firewall. Once I fabricated the new firewall we had to make a trans tunnel to match it. We had the arch cut in the new firewall to clear the transmission and we had the new floors welded in. Time to fill in the large hole in the middle. We are going to make the trans tunnel in 2 halfs. Always start by making a template with paper or chip board. We layed paper in and traced the shape and curves of the floor onto it. We also had it stick through the front of the fire wall and traced the opening. The important part is using the factory straight cut of the paper and keep it at Top Dead Center of the trans tunnel front to back. This will help you later. Keep that edge running straight and let the paper pass out through the firewall and down bellow the floor. Hold it at the arch and tapper you want. (it slopes down and tapers in while being curved all at the same time) Have a friend use a Sharpie and trace all your cuts. Cut it out and test fit. Make sure it is 100% right before cutting out the metal. It may take a few tries to get it right. Paper is cheap. If your not happy with it, throw it away and start over. There is no rushing in Fabrication! Your product will not come out well and there is a good chance you will get hurt in the process. Take your time. Once your paper fits… add 3/4" to your template anywhere you want to make a return in the metal for plug welding. Then cut it out of Metal. I always use 18g for a firewall and 20g for a trans tunnel and driveshaft tunnel. Now that your metal is cut out, its time to get the roll you want. I use a Baileigh roller. There are many other ways to do it. Bend over a pipe in a vise, Bend over your knee, manipulate it by hand, around a tree or phone poll. Just remember it has to be rolled at an angle so it has the tapper you want. I use a circle scribe set at 3/4" apart to scribe my lines where I want all of my returns made. Run it along the edge of the panel and the inner side of the scribe will make a perfect line down the panel at your desired 3/4". I then trace it with a sharpie so I can see it better when feeding it through the Power hammer. Now for the fun part. Time to move some metal. The reason we need to Stretch the metal is because it has to turn down around the outside of a arch. If you take a piece of paper and bend the last inch over on a 90deg you can not bend that edge around a arch without tearing the flange every 1/2 inch or so. If you do this you will see how much the cuts open up. Well, we don't want to cut a bunch of slits in our metal so we literally need to make the metal Grow or Stretch. The linear stretch die hits the metal in 1 direction - front to back. The smashing force of the hit with a flat upper die, squishes the metal out "side to side" making the metal longer in 1 direction only. Thousands of hits along the length of the panel, backed up with a little hand manipulation pulling the panel down, starts to create the flange. It will take several passes through the machine to get enough stretch to make the full 90deg turn. If your using a planishing hammer, it will take many passes but it will work just as well. The Planishing hammer does not have the POWER to move metal as fast. But it will move the metal and do the job.. If you take your time and do not force the metal to pull over any faster then the machine wants to stretch, then you will not effect the radius of your panel when turning the edge over. If you try to pull it over faster then the machine is stretching, you will end up removing the crown from your panel. Bellow I have posted a few still shoots and a video of the operation. Here is my attempt at embedding a video. If it does not work the Link is Bellow: Here is the LINK Below... http://youtu.be/TmIN5Vjjwcg See how well that works? Now its time to "fine tune" the panel to fit the car 100% for welding. I saved the cut-out from making the firewall. Its a perfect template for the arch I need the trans tunnel to follow. We can use this template with a shrinker / stretcher to make small adjustments in the arch. This will give us a 100% fitment to the car without walking back and forth to the car 50times. Use very light pressure to make very large adjustments. The metal will move a TON. Now that your arch is perfect you can use a metal brake to turn up all your straight returns for the sides. Lets see how it fits: Fits perfect. You will notice that I also rolled a step in the top seam so the right side panel with over lap the seam for plug welding together: Time to make the other side and fit it all together: I hope everyone learned something from this Post. If you already knew all of the above, then I hope you at leased found some inspiration or motivation to go out in the garage and work on your project. Remember: You DO NOT need a ton of expensive equipment to do this. A cheap planishing hammer will work but it will not stretch that edge in 3min. Remember to take your time. There is no rushing in a shop with machinery or tools of any kind. Take your time. Do it right. And stay safe. Bellow I will leave you with a few links to other peoples videos on linear stretch: Fay Butler demonstrates how to use linear stretch in a planishing hammer to turn a edge over when going around a corner. (not a straight line which you would just use a metal break for) http://www.youtube.com/watch?v=mQoOVNFWe3s Kent White demonstrates another use for linear stretch. He has a straight panel that was broke to a 90deg edge. He then uses a linear stretch die in a planishing hammer to curve the panel. He likes to use this method instead of a mechanical shrinker / stretcher because it does not leave marks in the metal. http://www.youtube.com/watch?v=WdPmp0ZYPAg Thanks again for reading my post. It was much longer with 28 more pictures but I did not realize we are limited to 20 pics per post. I hope it was an easy read. I am not English major by any means. It all sounds good in my head, but I am the one who wrote it. Thanks, MotoMike43
^^ Really, I thought I would have won your vote w/ my metal shaping skills using my knee, HAha Moto mike, nicejob! jeff
WOW, Nice metal work period, Thank you .couldn't have at a better time, honest, (( I just took the firewall out of my hj project ) now I know it was the right thing to do , Excellent , and again Thanks JT
If you have any questions during your metal work project please do not hesitate to ask me. I am no vintage car genius. I come here for answers about the particulars on vintage cars. You guys know your stuff. I am a fabricator / welder / designer/ engineer/ builder/ machinist.... I cut things up, modify them and put them back together. I am always looking to share my knowledge and pay back for the help the forum gives me about old cars. We all share the same passion. Its always fun to share.
I am a fabricator / welder / designer/ engineer/ builder/ machinist.... I cut things up, modify them and put them back together. I am always looking to share my knowledge and pay back for the help the forum gives me about old cars. We all share the same passion. Its always fun to share.[/QUOTE] Well said.
Thanks to everyone who likes my Tech Post. Today is the last day for Tech week Entry. Lots of great posts have been submitted. I know I have learned a few new things. Good luck to everyone who Posted.
Thanks Austin. I really enjoyed reading your tech post as well. You made a beautiful replacement panel. I have yet to dare making a panel that large. I still have much to learn on the hammer.
Here are a few of the pictures that I had to remove from the original post Due to the 20 pic per post limit: Here is a picture of the Baileigh 3/4" tool holder by itself. (ment for pullmax machine dies) Here is a good picture of the " air gap " A.K.A. the bottom of the stroke. This is set to about 1/4". Here are pictures of the machine settings. Power at Half. And NO pin. Sprung mode/ yoder mode/ throw the die.
Thanks for the compliment. I still have a lot to learn. I am only 31 years old and I have only been power hammering for 5 months. I went to the Baileigh Free class in Jan of this year and ended up buying a hammer for my shop. Before the class my only sheet metal work was all box and pan break kinda stuff. The hammer has opened whole new doors for my shop and the cars we produce. Some day I hope to be good enough to build entire car bodies.
I had the pleasure of using my new linear stretch set-up again today. I had to make small floor pan patch for a customer. The patch was 90% flat but on the inside edge it made a slight curve down and then immediately returned to flat agin. I made the curve in the break with a few continuous light bends. I could not flip the panel in the break and get the return bend close enough to the curve because the 18g steel would not flex enough to force it under the break fingers. After debating for awhile on how I was going to make the return without marking the crap out of the metal... I decided to try and pull the return down with the Power hammer and the linear stretch die. I was skeptical if it would work with the 18g steel. I made a machine adjustment by adding a little power. I also changed the linear head out for the more aggressive head (smaller contact patch). Sure enough...It worked. It took about 20 passes through the machine to get the full 90deg return to pull over (10minuets) But it worked and worked well. It did add a very slight crown to my panel which I easily hammered out with a few minuets in the hammer with a 36" radius lower die and a flat upper die. Hammering in the middle of the effected area with the highest point of the crown facing down. This raised the metal up making the panel 100% flat again. I didnt get a chance to snap any pictures ( totally wasn't thinking. Customer was in a hurry to get it back). Thought I would share. I really like this new process in the hammer. Makes me wonder what other uses I can get from this tool set-up...
Dude... another successful Baileigh workshop this weekend. I love those things, so many guys like us in one room. Good luck with the article... although id really rather win myself.
Austin I wont deny that you are a better Sheet metal guy then Me. I still have a lot to learn. Your post is great. I learned a lot from it. I wish I was not so far from the classes. It Cost me a lot to fly out there for a weekend. I hope to get to one more this year. Time and Money. 2 things I dont have. Good luck with Tech week.
That was really helpful! I just machined a linear stretch die for my Baileigh plannishing hammer. I hope I can turn some flanges like you did. Thanks!
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