View Full Version : Cutting circles

03-24-2003, 04:12 PM
What is the easiest way to cut circles out of plate? I had to cut some 5 3/4" circles out of a piece of 6" square, 1/4" thick plate. I chucked it in a mandrel by a center hole, cut a ditch most of the way through, then trimmed off corners and turned the periphery smooth. Surely not the best way. I have no bandsaw, and I don't want to use the torch or a jigsaw. Experts, show me your brilliance!

Dr. Rob
03-24-2003, 04:21 PM

03-24-2003, 04:49 PM
Thanks, but I can't justify the expense of a 5 3/4" holesaw. I was hoping there was a safer/easier way of doing this, hopefully on the lathe.

03-24-2003, 04:57 PM
I don't do production work and I don't know the quantity that you are looking for or the quality. One thought I had for you was getting some 5 3/4" steel bar stock and a metal cutting bandsaw. Nice thing about these is you can set it cutting and let it go on it's own. They aren't very expensive machines either. I have not cut this size stock with one though either. On my lathe I might try parting off 1/4" slices. How about setting up lots of 6" squares on a threaded rod and turning it all down to the circles you need in one operation? Another method would be to shear them out of sheet stock with a hydraulic press. This you could farm out. I've seen lots of circles cut with CNC controlled plasma cutters. If you ask me that is the slickest way to do it. I'm trying to get my first CNC tool project built and then I'll be able to tell you how easy it is.

[This message has been edited by SJorgensen (edited 03-24-2003).]

03-24-2003, 05:58 PM
Holesaw! http://bbs.homeshopmachinist.net//smile.gif

But if that's out...mount the plate on a rotary table, with spacers under it, and mill out the circle, mebbe?

03-24-2003, 06:54 PM
Use tripanning tool on lathe. You can hand grind the tool bit yourself.


03-24-2003, 10:16 PM
set up a rotary table with a clock drive, use an EDM to vaporize out the circle. Book a holiday for 2 weeks, and have someone check your house for fire while you;re gone. Ok, I couldn't resist, somebody had to suggest a completely useless means of doing that. Maybe it was something I ate for lunch. Hm. Anyway, that has been a stumbling area for me, as well. I often wish to cut something out of sheet stock, and frequently don't do the project because of the hassle of extracting a piece the req'd size and shape from the blank. I can do circles in plastic well enough with a glorified circle cutter, but aluminium is harder, and steel is a no-no, with my jig. I guess it's time I went down to the dollar store and picked up a laser cutting table.

Dave Opincarne
03-24-2003, 11:23 PM
Make a hole saw. Press and weld a shank into a piece of 1/4" plate (you might even have some laying around http://bbs.homeshopmachinist.net//wink.gif)Turn the plate just smaller than the diameter you need with the shank held in the chuck. Tack weld a bimetal saw blade to the circumfrence. Use a hose clamp to hold it in place while you adjust the blade to run true.

03-25-2003, 12:09 AM
Find a local fab shop that has a cnc driven plasma arc, I have found that if I am not in to big a hurry I can get what I want for a contibution to the beer fund.

03-25-2003, 04:42 AM
I cut out a blank for a chuck backplate from HR steel with a trepanning cutter in the horizontal mill. Like a heavy duty circle cutter with a lathe bit for the cutter. You can make one.

With the plate stationary, circles can be cut from any area on the plate, and no center hole either, if not wanted. Can also be done on the lathe, use an angle to hold the plate on the cross slide.

If you're spinning the plate in the lathe... good luck.

03-25-2003, 09:02 AM
Joel, I would do it exactly like you did it. All of the other ways may work too, but none of them are easy and you will probably have to true up the edges in the end anyway.

steve schaeffer
03-25-2003, 10:08 AM
program a canned drill cycle to circumscribe the hole with a lot of little holes, then knock the slug out with a hammer and true up on the lathe (if you are using the circle)or finish mill out the lole (if you are using that)hahahahahahahahahah

you left out some info, do you need a lot of these? are they a job, or just something you need? i agree you can farm that out if its a job, plasma cut or laser cut machines are everywhere. if not how you did it is good, also you could torch it then true it up on the lathe, or if you are good and have the right set up you can torch it and run it as is. you can make a circle cutting attachment for your torch very easy, even something crude for a one off job. the setup is important, if you are cutting a1/4" plate you would want a very small tip size. oxygen pressure should be high on cut to keep kerf clean of plate. it should look like a sheared edge when you are done. i used to sell automated welding and cutting systems, before, and also worked in a steel mill for several years. you would be amazed at how clean and straight a torch cut can be if done properly. we could cut a 9 inch slab and it looked like a machined edge. as far as you go, you are no machine, so cut the parts before you have your morning coffee. a steady hand is needed to make a steady cut.
good luck.

03-25-2003, 07:10 PM
Thanks for the good ideas.

Steve, I only need a few, and they are for me. I have had mediocre results with a torch, but will apply your tips and see if I can do better.

03-25-2003, 10:38 PM
Did you use a circle cutting atachment on your torch? With one of them and a clean tip and "steady hand" you can get surprisingly good results.

03-25-2003, 11:06 PM
Ultimate cheap circle cutting technique; Nail a square piece of plate aluminum or steel to the lower end of a 2x4 and use duct tape to tie it to the frame of the rear car door. Drive around with the piece in contact with the pavement. Remove when the piece is the appropriate size.
(As an alternative buy a hole saw or even better a trepanning setup (not the skull version.))


03-26-2003, 02:59 AM
Simplest cheapest circle cutting attachment for the torch is, electrical "bug nut" (splicing bolt) with nail brazed on head.

Bug nut stradles torch tube and nail rides in center punch mark. Adjust radius by sliding bug nut on torch tube.

Could also hose clamp, bent nail to torch tube but its not too solid.

It helps to use some kind of turntable, so torch remains stationary, prevents fouling hoses.

L Webb
03-27-2003, 12:38 PM
Easiest way is to blank them out with a punch press. But if you don't happen to have at least a 125 ton punch press and the tooling, then you can trepan the circle easy enough on a lathe. The biggest mistake people make when trepanning is too fast of speeds. The plate must be held in the middle securely and plenty of coolant.
I'd go into more detail but I can't give away our secrets.

03-30-2003, 04:10 AM
I like water jet and laser cutting. Laser cutting does not warp the material at all (a good thing to remember). Water jet leaves a frosted surface on thick aluminum (the cuts) and slightly distorts the Aluminum from the pressures involved.

Blanking on a punch press distorts the edge of the metal considerably.

Cutting by torch or plasma can change the material properties in the heat affected zone.

Doc Nickel
03-30-2003, 04:36 AM
Actually, the waterjet creates very little distortion, and even then only on thinner stock. Usually if I get a part "distorted" out of the waterjet, it's a factor of inherent stress in the rolled or extruded aluminum billet. I used to have a problem with the blanks cut from the ends of sheared barstock "cupping" a little, but the blanks from the center of the bar were dead flat.

The 'jet actually applies very little force aginst the material- it's basically just a vehicle to drag the abrasive past the kerf.

Laser cutting typically doesn't distort the thinner materials, as there's less loitering time. Cutting the thicker materials usually does induce some heat distortion, but usually a little less than a CNC plasma cutter.

03-30-2003, 05:52 PM
That does it! I'm off to purchase a 20,000 psi water pump and some cheap towels. I can probably EDM some tiny holes in my carbide boring bars to use as nozzles, hmm. Kidding aside, I've heard that water-jet cutting is noisier than stink, is that true? Is it practical for an HSM guy to build his own w-jet system?

Doc Nickel
03-30-2003, 06:32 PM
It's pretty noisy all right. Most cutting actually takes place under water- the cutting table (at least on the unit I'm familiar with) has a variable level tank. You set the parts up "dry", then flood it. That keeps the spraying and the noise down.

And only 20K? I'm told this one starts at 50K psi, and if it drops down to less than 42K, they pull the pump down and rebuild it.

If you can make a pump that can push water to 50,000 psi, and find Sapphire nozzles with an .015" orifice, the rest of it- the CNC 2D positioning, the cutting table, and the grit applicator- is relatively easy if you want to build one. http://bbs.homeshopmachinist.net//biggrin.gif

03-31-2003, 12:24 AM
How about using a Tesla pump to get the pressure for water jet cutting?
A lot simpler than the hydraulic piston pumps.

Someday when I have the time (never http://bbs.homeshopmachinist.net//biggrin.gif) I have to try that. http://bbs.homeshopmachinist.net//smile.gif

03-31-2003, 12:54 AM
I wish Tesla and Einstein were both alive today, we could pose the pressure requirement to Tesla, then ask Einstein if theoretically, the water jet could exceed the speed of light. hmm. 50,000lbs pressure is a tad more than my best water pump can do.

03-31-2003, 02:43 AM
I did some installation work for a guy who once ran a waterjet cutting firm. He mentioned to me that there were some pressure washer pumps on the market capable of 40KPSI that he thought would work great for water jet cutting.
They were a lot cheaper than the dedicated waterjet machines available then (8 years ago)
The machines he was using went up to 80 KPSI if I remember correctly. One of the big expenses was the grating that the workpieces laid on.
They got eaten up pretty quick. http://bbs.homeshopmachinist.net//smile.gif

03-31-2003, 02:45 AM
I'm pretty sure that the first guy to ever build one of these didn't have a million dollars to spend. It really isn't fair of Thrud to suggest laser and water jet cutting without telling us how to build one in our HOME WORKSHOPS. I'm going to throw some numbers out and suggest a way to do it (just speculating.) Suppose you had an 8 ton double acting ram and the rest of the stuff to run it. If this modestly sized ram where used to drive a 1/2" rod in a long cylinder the pressure could reach 80,000 psi. Two rods could work alternately for a continuous output. I don't know if check balls work at those pressures without special provisions but the pressures are achievable. Making the orifice inexpensive and easily replacable could be a challenge. I shy away from sapphires. It would be cool if someone could design one for the home builder.

03-31-2003, 03:25 AM
Pressure washer pumps at 80 kpsi. Sounds like a way to cut door and window openings through concrete. How would you clean siding with it? Scare the dirt off?

03-31-2003, 03:23 PM

What you described is pretty much how waterjet pumps are built.


The pressure washers were run at 40 KPSI and with a fan spray nozzle.
It takes an amazing amount of pressure to remove some types of grime from New York's buildings.