Make a rear mounted cutting off (parting) tool post with a metal lathe & milling attachment. (S3-E4)
2.1 هزار بار بازدید -
4 سال پیش
-
Cutting off a piece of
Cutting off a piece of metal in the engineer's lathe can be difficult, often causing the tool to chatter violently, dig in, jam and break the tool with damage to your work and the lathe. Using a solid rear-mounted tool post solves most of the problems. I used the newly mounted milling attachment to cut slots for mounting the tool.
In the Boxford users group we had extensive and interesting discussions about the best way to cut off a piece of metal after turning it - a process known as parting. I came to the conclusion that the best method would be to make a dedicated rigid tool post and bolt it to the back of the cross slide. It requires the cutting-off (parting) tool to be upside down and this appears to be the secret. The waste metal or "swarf" falls out of the slot by gravity instead of falling into the slot, jamming the tool.
With this design it is not necessary to rotate the chuck backwards (which is an alternative method when the parting tool is mounted upside down in the original tool post).
There is another reason why the rear-mounted tool post works so well. Look at the way the tool post responds to the heavy forces that occur due to the wide cut made during parting. With a standard front-mounted parting tool the force on the tip makes the whole tool post rotate slightly around its mounting point on the compound slide. This causes the tip of the tool to dip down and dig into the work making the cut too deeply - a process we refer to as "submarining". If you look at the rear-mounted tool the forces make it rotate around its base and the tip of the tool rises up and away from the slot, so it does not jam (a process we called "aeroplaning" in the Boxford users group. Of course the improved rigidity also reduces these effects.
Using a parting tool upside down in the normal tool holder has the same benefits except for improved rigidity. But in that case you have to run the motor in reverse (see the video on reversing switches). However, the chuck is mounted on a screw thread without a locking pin on the standard Boxford lathes and it tends to come unscrewed when heavy forces are applied during parting in reverse. I saw this happening and by the time I got the machine stopped the chuck had already come off completely! Not good for the bed or your head!
Placing this new tool holder on the back of the cross slide also eliminated a lot of joints that can cause movement in the system. By comparison, a tool mounted in the usual tool post is connected to the compound slide which is connected to the cross slide and each of these joints allow the tool to move slightly.
Whatever the reason, the tool post that I made from a 50mm square block of aluminum has been highly successful.
There is one minor disadvantage in this design. The height of the tool relative to the center line of the lathe is not adjustable, except by regrinding the tool to shift the cutting point. This should be taken into account when setting up the mounting slot. With the tool upside down we can move the slot slightly lower and then grind the tip slightly higher to result in a well centered tip.
If the tool post still moves we could think about adding a bed clamp to the read of the carriage in addition to the one at the front of the carriage.
I start the video by showing the finished product so that you can see the end goal.
Playlist:
https://www.youtube.com/evanecent/pla...
Season 3 Episode 4 of the playlist
"How to use a general engineering lathe" by Evan Lewis (evan-e-cent now evanecent).
Boxford Users Group:
https://groups.io/g/BoxfordLathe-User...
Patreon:
Consider supporting this channel on Patreon: Patreon: evanecent
In the Boxford users group we had extensive and interesting discussions about the best way to cut off a piece of metal after turning it - a process known as parting. I came to the conclusion that the best method would be to make a dedicated rigid tool post and bolt it to the back of the cross slide. It requires the cutting-off (parting) tool to be upside down and this appears to be the secret. The waste metal or "swarf" falls out of the slot by gravity instead of falling into the slot, jamming the tool.
With this design it is not necessary to rotate the chuck backwards (which is an alternative method when the parting tool is mounted upside down in the original tool post).
There is another reason why the rear-mounted tool post works so well. Look at the way the tool post responds to the heavy forces that occur due to the wide cut made during parting. With a standard front-mounted parting tool the force on the tip makes the whole tool post rotate slightly around its mounting point on the compound slide. This causes the tip of the tool to dip down and dig into the work making the cut too deeply - a process we refer to as "submarining". If you look at the rear-mounted tool the forces make it rotate around its base and the tip of the tool rises up and away from the slot, so it does not jam (a process we called "aeroplaning" in the Boxford users group. Of course the improved rigidity also reduces these effects.
Using a parting tool upside down in the normal tool holder has the same benefits except for improved rigidity. But in that case you have to run the motor in reverse (see the video on reversing switches). However, the chuck is mounted on a screw thread without a locking pin on the standard Boxford lathes and it tends to come unscrewed when heavy forces are applied during parting in reverse. I saw this happening and by the time I got the machine stopped the chuck had already come off completely! Not good for the bed or your head!
Placing this new tool holder on the back of the cross slide also eliminated a lot of joints that can cause movement in the system. By comparison, a tool mounted in the usual tool post is connected to the compound slide which is connected to the cross slide and each of these joints allow the tool to move slightly.
Whatever the reason, the tool post that I made from a 50mm square block of aluminum has been highly successful.
There is one minor disadvantage in this design. The height of the tool relative to the center line of the lathe is not adjustable, except by regrinding the tool to shift the cutting point. This should be taken into account when setting up the mounting slot. With the tool upside down we can move the slot slightly lower and then grind the tip slightly higher to result in a well centered tip.
If the tool post still moves we could think about adding a bed clamp to the read of the carriage in addition to the one at the front of the carriage.
I start the video by showing the finished product so that you can see the end goal.
Playlist:
https://www.youtube.com/evanecent/pla...
Season 3 Episode 4 of the playlist
"How to use a general engineering lathe" by Evan Lewis (evan-e-cent now evanecent).
Boxford Users Group:
https://groups.io/g/BoxfordLathe-User...
Patreon:
Consider supporting this channel on Patreon: Patreon: evanecent
4 سال پیش
در تاریخ 1399/02/22 منتشر شده
است.
2,179
بـار بازدید شده