Boring Hole

[Little-Eagle]

Hi All,

Let me start by saying I'm still new to machining and I try to research this stuff myself but thought it might be good to post here to get people in the know to answer this question.

I've bought a boring head kit and I'm not getting good results as far as finish goes.

I'm trying to bore a 40mm diameter hole in mild steel on the milling machine and the finish I'm getting is quite rough, not quite like a thread but nowhere near a smooth finish.

I'm currently running the machine (Hafco HM-50 turret mill) at its lowest speed which I think is 220rpm, not sure of the feed speed, feeding by hand.

Any advice would be appreciated.

Damo

 

[aliva]

I think your speed is sufficient, the issue I see is the feed, the slower you feed the better the finish. if you can't feed automatically then try increasing your RPM to compensate for your feed rate. My mill has 3 feed settings per RPM , .003, .006 , .0015.
Also depending on how good your cutting edge is will determine how much of a cut you can take, try reducing the depth of cut. since its mild steel .030 should work fine. Again finish is a product of feed rate , RPM, and cutter condition. It may take a little experimentation to get the desired combination

 

[Kevin J]

Hand feeding is tough, power feed will show a vast improvement in finish. Honing the cutters will help. Someone with more experience can help with speeds and feeds.
Regards, Kevin J.

 

[Technical Ted]

A few thoughts... They look like carbide cutters, so you can probably increase speed and see what happens. Also, a radius on your cutter will help get a better finish, but keep in mind both speed and a radius could lead to chatter, so it's a trade off and you'll have to experiment.

If the bars can be rotated in the holder make sure the cutting edge is on center. Using cutting oil can help in mild steel as well.

Use as steady of hand feed as you can. You can get decent finishes with hand feed, but it takes practice.

Good luck,
Ted

 

[Bob Korves]

Boring bars as delivered, especially Chinese import ones, often do not have enough relief to the grind and/or enough shank clearance built in, and the tool then rubs on the work below the cutting edge, especially in smaller holes. Check the tool for rubbing below the cutting edge. If that is happening and you have a way to grind carbide, then correct the problem. If not, a quick and simple fix is to just rotate the cutting edge downward a bit to a negative rake angle with the work, which adds clearance to the tool geometry. Readjust the tool to center line of the work. Amazingly, boring bars can cut quite well with a negative cutting angle, and it can also help to reduce chatter. Chinese boring bars and other brazed carbide tools are often roughly made "projects", not ready to use tools.

 

[Technical Ted]

Also, use the shortest and stoutest bar you can with a corrected/adjusted grind as Bob suggested. If you still can't get the results you're looking for, you can make or buy a bar to hold HSS tool bits. These may prove to work better for certain applications. I personally am a big proponent of HSS tool bits when you have less than an ideal setup, especially rigidity wise, and can't run fast enough speed to get good finishes with carbide.

YMMV,
Ted

 

[mikey]

Please correct me if I'm wrong but you seem to be backing the sliding part of the boring head out to make your cut. The tip of the bar should be facing 180 degrees the other way and the lead screw to set depth of cut should be turned clockwise to advance the cutter. As it stands in your photo, the head and screw are unsupported so chatter is inevitable.

Try this. Put the bit in the tool holder hole closest to the center of the head and turn the cutter 180 degrees. Be sure the face of the cutter is parallel with the center line of the head and then lock it down. (Edit: With the mill off,) turn the depth of cut lead screw CW until you contact the bore, retract the quill and set your depth of cut. Lock the center gib screw to lock the head in place.

I would use a higher speed - try 1200 rpm and see if it runs smoothly with the extension you dialed into the head. If not, back off the speed. Use cutting fluid if you can and feed steadily and it should cut clean.

These Chinese cutters will work. They will not cut as smoothly or as accurately as a sharp cobalt bar but they will cut. Sharpening them is a good idea but I don't use a nose radius on these bars - they deflect too much. Carbide cutters deflect much more than a sharp HSS cutter, especially at the low speeds a boring head works at, so if you require accuracy then consider investing in a good set of cobalt bars.

EDIT: be sure your gib screws are set properly - excessive play will cause chatter. On these heads, the two outer gib screws are used to adjust play. Tighten them gently until the head moves with slight resistance as you turn the depth of cut lead screw; once there, leave it alone. Use the center gib screw to lock your depth of cut once you set it; it is not used to adjust play in the gibs, only to lock your depth of cut adjustments.

 

[Ken from ontario]

Please correct me if I'm wrong but you seem to be backing the sliding part of the boring head out to make your cut. The tip of the bar should be facing 180 degrees the other way and the lead screw to set depth of cut should be turned clockwise to advance the cutter. As it stands in your photo, the head and screw are unsupported so chatter is inevitable.

+1 to what Mikey said.
The exact same thing happen to me . a dry test run would have made it clear which way the sliding part should turn.

 

[Silverbullet]

Do you have a quill lock , put a bit of pressure to tighten the quill , not to stop it but to take up excess wiggle room. And stone the cutters slightly putting a rounded cutting edge. Keep plenty of cutting fluid on there too. Lock the boring head itself . All the wobbling has to cause trouble somewhere.

 

[T Bredehoft]

I removed my quill lock, drilled a hole in it, inserted a spring and brass plug. Now I just snug down the quill lock and it keeps the quill from vibrating/wobbling.