Help with horizontal milling of large surfaces with spiral cutters.

[Flynth]

What is the biggest conventional milling cuts you're comfortable taking on horizontal milling machines holding the part in a vice?

Since I got my horizontal milling machine I was interested in seeing some big material removal with big cutters.

I'm specially interested in using 3in diameter, 1in arbor, up to 4 in long, 10 teeth spiral cutters to mill surfaces quickly. My mill has no backlash removing device so climb milling is a bad idea.

Still I tried climb milling with a narrow (1in wide) piece of mild steel and 20 thou depth of cut. It left beautiful finish,but I could see the table being pulled into the work. So milling deeper is probably a recipe for broken cutter teeth.

So I understand I should be using those cutters with conventional milling only. The problem with that is that conventional milling with horizontal cutters generates a force that tries to lift the item off the table. I've managed to successfully remove lots of material with it once, but back then I had a part I could bolt directly to the table.

This is impossible with parts one wants to mill entire top surface of. For such parts I'd like to use a vice, but I'm worried the cutter will pull the part out of the vice and break stuff.

I tried today milling a similar 1in wide piece of mild steel with 20 thou cut and conventional milling held in a vice. It held up, but 20 thou depth is not a lot. I can achieve the same with a fly cutter on a vertical mill. Is there a rule of thumb formula for a "vice pull out force"? I can calculate the lifting force generated by the cutter, but I'm not sure what is the actual lifting force a typical 6in vice can withstand and still hold the part.

 

[MrWhoopee]

There isn't a lot of lifting force until you get to some serious depth of cut, like (just a guess) more than 10% of cutter diameter. Most of the force is pushing the part horizontally. Mount the vise so you're cutting towards the fixed jaw and go for it.

 

[markba633csi]

Sometimes it's helpful to add a piece of "crush" material like thin wood or copper wire between part and movable jaw to give extra grab

 

[Flynth]

There isn't a lot of lifting force until you get to some serious depth of cut, like (just a guess) more than 10% of cutter diameter. Most of the force is pushing the part horizontally. Mount the vise so you're cutting towards the fixed jaw and go for it

10% of a 3in cutter is quite a bit. (0.3in or 7.5mm).

Regarding aligning the vice I was told with a spiral cutter it pushes quite hard towards the direction of the spiral. So my vice jaws are parallel to the table long direction such that the unmovable jaw is blocking this side force. Perhaps adding a stop to prevent the item being pushed forward is also a good idea. Thanks.

I was thinking about what you wrote about the force and it does make a lot of sense. I have a book about milling written in Polish in 1960s that has lots of "rules of thumb" for estimating cutting forces etc. In there they say that the lifting force(in conventional milling) is from 0 to 0.3 of the total cutting force(from their tables) . They don't give further info how to estimate it, but also they say the forward force is from 0.1 to 1.2 times the total cutting force.

Unfortunately that book doesn't have any such rules of thumb for estimating how much force an item held in a vice (by friction) can withstand. There is a youtube video by NYC CNC channel where they measure actual force required to move a piece out of a vice with different torques applied to the handle and the forces were a lit less than I expected.

For an average clamping force of 3500lbs (1.5tons) they measured 350lbs(150kg) to move a well held piece of aluminium. Interestingly the force required to move it was a lot higher when they used superglue instead of a vice to hold the block... I'm mostly concerned with steel, but I often put a piece of aluminium tig wire along unmachined face of a part held in a vice.

Sometimes it's helpful to add a piece of "crush" material like thin wood or copper wire between part and movable jaw to give extra grab

Yes, IMO this is very important when holding items that have unmachined sides.