[brian257]

I have always liked Sherline machines and have had a manual one for a long time so I bought a CNC Sherline mill to do some heavier duty work that my K2 2514 router can not handle. I am disappointed to say that the Sherline can not do the job either, not that it is not a great machine for lighter duty work.

What I am trying to do is machine some 6061 aluminum blocks. I need to do a pocket about 2.5 X 4" and 3/4" deep with odd radii and straights on the sides. I am using an Onsrud 63-624 single flute cutter that is 1/4" diameter with a 1.25" length. Using the recommended 14 IPM at 2800 RPM. Onsrud says I should be able to do full width cuts 1/4" deep at this feed and speed, but I have found that on the Sherline that full width and only .1" deep gives a bunch of chatter and rough edge. .2" deep and taking off only .020" on the side is no better and even if I spend forever making this pocket with light cuts I can't get full depth and take a .005" finish cut around the perimeter without chatter.

I have put the bit in my manual Bridgeport and it can make a full width cut at .25" deep at 14 IPM like it was cutting butter so I am convinced that the Sherline just does not have the rigidity in the column for these cuts. The part is fairly low so the head is not cranked up very high. Machine is new and backlash and gibs are adjusted correctly and there is no play or flex that can be discerned.

With the ton of passes it will take to make this pocket it will take me hours to cut this. I was looking to do parts in 5-10 minutes. Using a smaller cutter will require less rigidity, but still take even longer.

So two questions. Is there something wrong here or are others with the Sherline machines getting the same results?

Next question is what machine would be recommended if I want to spend under 4K and get something rigid enough and a fast enough spindle to make a 1/4 X 1/4 cut at least 15 IPM?

[rpovey]

Your not going to be making 1/4inch deep cuts in AL with a sherline, it only weighs like 30 lbs. I usually keep to the 0.015 inch DOC range in 6061, much more than that and at that I can see the column flex. They're just not designed for heavy cuts.

You need more mass and rigidity.

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[TXFred]

I'm not aware of any CNC machine costing less than $4000 that can do that job. The closest is the Little Machine Shop CNC mill, selling for $4495.

There are smaller, cheaper manual machines, but you would have to convert them yourself.

Frederic

[DMF_TomB]

[QUOTE=brian257;1085206
What I am trying to do is machine some 6061 aluminum blocks. I need to do a pocket about 2.5 X 4" and 3/4" deep with odd radii and straights on the sides. I am using an Onsrud 63-624 single flute cutter that is 1/4" diameter with a 1.25" length. Using the recommended 14 IPM at 2800 RPM. Onsrud says I should be able to do full width cuts 1/4" deep at this feed and speed, but I have found that on the Sherline that full width and only .1" deep gives a bunch of chatter and rough edge. .2" deep and taking off only .020" on the side is no better and even if I spend forever making this pocket with light cuts I can't get full depth and take a .005" finish cut around the perimeter without chatter.
I have put the bit in my manual Bridgeport and it can make a full width cut at .25" deep at 14 IPM like it was cutting butter so I am convinced that the Sherline just does not have the rigidity in the column for these cuts. The part is fairly low so the head is not cranked up very high. Machine is new and backlash and gibs are adjusted correctly and there is no play or flex that can be discerned.
[/QUOTE]
0.25" dia 1 flute stickout 1.25"
rpm 2800 at 14ipm
DOC 0.250"
calculations show 0.22hp and 38 lbs cutting force at end mill tip
chip thickness 0.0048"
1) in my opinion your rpm is too slow, feed too high, depth of cut too much for machine
2) a Sherline using 0.22hp at 38 lbs cutting force...... you have got to be kidding. thats at least 10x what a Sherline can handle. Spend about $5000 for a machine weighing over a ton and maybe you can cut at that rate.

[brian257]

Kind of what I was figuring.

Tom, can you post or give a link to those formulas.

Went and did some more experimenting and seems that the major rigidity problem with the Sherline is a twisting of the vertical column. With the headstock high I think you will get more of a problem with the column bending. You can actually put your hand on the spindle and push left and right and see the deflection. Hmmm, maybe a big thick solid steel column? Wonder of the A2Z columns are better.

Been thinking a long time about converting my Bridgeport to CNC. I can buy a set of ballscrews and make my own motor mounts and controller and do it well under $5K. I just hate to do it because it makes manual use of the machine harder unless if I do more work and have some kind of engage and disengage for the motors.

[LUCKY13]

I would say the GO705 Grizzly and doing the CNC change over your self if your budget is what you say. You would also need to modify the spindle to get more RPM if goign this route. You just dont get much for 4 G's these days and without changing a machine over yourself your going to have to spent something more close to 7 or 8 G's.


The Tormach 770 will run you about 7 G's but it does have a 10,000rpm spindle which will run these small cutters very good.



Jess

[Fastest1]

Give up on the Sherline for this job. It doesnt matter if you use A2Z parts or not. I have the complete Monster Mill and I basically just kept replacing parts on my Sherline til I got there. A G0704 would make a good mill for your job. I would convert the BP, you would never regret that! Btw I am with rpovey on the doc's. I was lucky to get .002 at any feed or speed. Tryally seems to do it but people of his skills are few and far in between.

[Winnfield]

Definitely a no-brainer if you have a full-size machine. Convert it and put some magnetic particle brakes on the end of your screws.

[brian257]

What do you mean by magnetic particle brakes? Are you referring to a magnetic clutch between the motor and lead screw?

[Winnfield]

Definitely not between the motor and leadscrew. You put them on the opposite end - the end that floats. They are adjustable, and will give you the friction you need to do manual machining without back-driving.

[brian257]

I seem to not be following here. Why would you want friction during manual machining? I was thinking some kind of clutch to disengage the motor and not back drive it when machining manually.

[Winnfield]

You want the friction to keep the machining forces from moving the table by turning the ballscrew. The threads of an acme screw keep this from happening, and when you use a ballscrew, you lose it. Most hardware can tolerate whatever current you feed back into it by turning handles - using a powerfeed, though, I don't know. I have run a lathe like this for years, but not a mill, so what I have described might not be a suitable solution.

[judleroy]

Stepper motors can be spun while Manuel machining but servos can't. Also you want friction while Manuel machining with ballscrews because they backdrive. They wouldn't stay put when you let go off the handles.
Judleroy

[foam27]

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Sorry if this is the wrong place, but, the G0704 did come up in the thread.

[brian257]

Ah ha! Now I get it. That is something that I had not considered. I was thinking of converting the machine to ball screws a while back so it would be ready for CNC sometime in the future and was wondering why they did not just build them will ball screws in the first place. Obviously price is a reason also, but would not have cost Bridgeport that much more over what the acme screws cost.

I guess you could get good results in manual as long as you always remembered to lock the tables or did not take your hands off the wheel.

My concern was back driving the motors in manual for two reasons. One was the extra friction and the other was that when unpowered steppers are turned they kind of ratchet from step to step which makes it hard to do precise slow turns and stop exactly where you want. Also kind of gives it a built in vibration as you crank the handle which can give a bad part finish.

The extra friction might negate the need for the brake if it is just the right amount.

The ratchety turning might not be as much a problem as I am expecting with the bigger hand wheels on the BP.

Thoughts?

[Fastest1]

Disconnect all leads on the steppers (make sure they are not touching each other) and see about that rachet feeling. Should be much smoother. You wouldnt feel it on a BP anyway if using them manually. I cant imagine why anyone would go manual after the refitting. Just use an MPG/Shuttle Pro and do anything you want quickly and smoothly.

[brian257]

Looked at the Shuttle and also the VistaCNC pendants and I see what you mean. Having something like that probably would end my hand cranking days. Thanks.

[LUCKY13]

Yea you willl throw handwheels out the door when you find how easy & good pushing a button works.



Jess

[Fastest1]

or using the MDI line for a smooth feedrate