Milling a Circle

[rcazwillis]

Let's start with "I am new to this!". Holy cow, milling a circle without CNC is crazy. I know my skills are new, but can anyone do this accurately? Just curious.

I am converting a HF Mill/Drill to CNC and was making brackets for the motors today. I had to cut two circles out of 2024. Ended up finishing them with a rotary tool in the vice. First pieces I have machined. Turned out acceptable. Soon as the CNC is done, I am cutting better circles.

[2muchstuff]

Use a fly cutter in the spindle. It will allow you to cut just about any size you need.

[rcazwillis]

Originally Posted by 2muchstuff Use a fly cutter in the spindle.

I see. That would work nice. Is the fly cutter used to enlarge an existing hole? It does not appear that it will start from scratch.

Also, just using ebay as a quick research point, would most fly cutters just use tooling (bits? not sure of the correct word) for a lathe or is there tooling specific for the fly cutter. It appears that width is important for the tooling to fit each fly cutter.

Thanks for the info.

[Halfnutz]

If you can afford the 75.00 get a cheap boring head set. You will not regret it. You will be able to dial in holes to within .001 " accuracy with it.

[2muchstuff]

Fly cutters use standard size lathe bits and do need to have their holes started by some other method. There are some other fly cutters out there that have two bits on them, one on either side, that will allow you to rough bore straight down. Then clean them up with a fly cutter. I had the right idea in my head but the wrong part name, Halfnutz had the right name for it, an indexable boring head. With this, you still need to prestart your hole by some other means, drilling. Any machine tool supply house will have them. Let the chips fly.

[jkeyser14]

What size holes are you doing? You can drill a small hole where the center of larger cirle will be. Then use a dial indicator to center the hole on a rotary table (it will take a lot of fine adjustments and can be a real PITA). Then just use a mill bit and the rotary table to make a perfect circle.

[rcazwillis]

Thanks for the replys. Three solid options!

[lerman]

Originally Posted by jkeyser14 What size holes are you doing? You can drill a small hole where the center of larger cirle will be. Then use a dial indicator to center the hole on a rotary table (it will take a lot of fine adjustments and can be a real PITA). Then just use a mill bit and the rotary table to make a perfect circle.

I recently had exactly that problem (well, almost exactly). I had to make some plates to adapt a NEMA 34 motor to a NEMA 42 mounting. It had a 1" hole in the center and a 2-7/8 diameter recess 1/8 inch deep. It also had 4 holes in a square and a second set inside the first set.

The easy alignment solution was to put screws (or pins) in the corner holes and use them to center the plate on the rotary table's T-slots. By using the pins in the T-slots, I didn't have to bother with an indicator.

Of course, as soon as I have the CNC machine I'm building the plates for, the problem becomes trivial.

Ken

[Mcgyver]

Ouch! you guys talking like cnc is a replacement for general machining are killin' me

"Soon as the CNC is done, I am cutting better circles" - not necessarily. you will not achieve the level of accuracy (in the home shop) with x-y coordinate cnc milling that you will with a single point tool when it comes to boring a hole. A full cnc shop would do critical jobs like boring a cylinder on a lathe or with a boring head.

Not a rant, just want to make the point that the tooling and skills you acquire in machining doesn’t become redundant when the cnc machine is finished.

I had to bore some largish holes tonight so snapped some pictures. This is a 3.625 bore in 2 pieces .5 AL plate. They are seperated by parallels and on the rotary table because I’m going to do some profiling (now that’s why I wish I had cnc!) afterwards.

I also took a shot of some tooling. Three fly cutters are on the right. They are used for facing cuts and are very handy, and cheap because they take hss tools. The large one never gets used and is just for bragging rights. There is a boring head to the left and the foreground is trepanning tool. Thesev (trepanning) can be hairy to use but basically makes a large bore by cutting out a disk. The advantage of the boring head is its micrometer adjustment.

[rcazwillis]

Originally Posted by Mcgyver I had bore some largish holes tonight so snapped some pictures. This is a 3.625 bore in 2 pieces .5 AL plate.

I don't know squat and I freely admit it. LOL! But I am learning. From the pictures (worth a thousand words) it appears you used a large bit for a starting hole. Then the indexible boring head was used to cut material out until it was the correct size. When doing the boring head step, I am guessing that you have to take multiple passes through each piece until you get the desired diameter. Is this correct?

[ViperTX]

I have to agree with Mcgyver.....boring head is the way to go for an extremely accurate round hole within the dimension of whatever boring head you have....I use the brazed carbide bits on mine and am quite impressed with the accuracy you can dial in.

[Mcgyver]

thats exactly the process. you take small cuts towards the end to let the spring in the tool work itself out. The other thing you can save some time is cut it out will a hole saw (i've done 1" in AL that way) but its a little nerve racking, then finish with the boring bar.

for me the learning is the fun part and shows no signs abating anytime soon