Friday, May 31, 2013

Mill: Machining an Arc on a Manual Mill

I don't have a rotary table for the mill (yet) but wanted the ability to make an arc if I needed. The easiest, thought tedious, way is breaking down it into tiny segments with a discrete X and Y movement. If it's broken down small enough, then you can approximate a smooth arc very closely.

To find what the movements were, I created a spreadsheet which computed the ratio of X and Y to the length of the arc at every half of a degree. Therefore, if you know the circumference of the circle that arc you want is a segment of, then you can calculate how far you need to move X and Y for that half degree.

To use the chart I calculate the circumference of the circle (2r*pi) the arc is a segment of and divide it by 720 to get the length of each half degree of circumference. I then imagine a circle on the table, with angle 0* parallel with the Y axis, and find what angle relative to 0* I want to start the arc at. I'll first multiply the X ratio by the length of the half degree and move my X that amount. Then I multiply the Y ratio by the length of the half degree and move my Y that amount. I then step to the next half degree and repeat the process until I've reached the angle I want to finish my arc at.

As you probably noticed the chart doesn't tell you which direction to move, just the amount needed to move.

The spreadsheet I use:
Circle.xlsx


Angle Ratio X Ratio Y
0
0.5 87 0
1 76.39 0.01
1.5 45.83 0.02
2 32.73 0.03
2.5 25.45 0.04
3 20.82 0.05
3.5 17.61 0.06
4 15.26 0.07
4.5 13.46 0.07
5 12.03 0.08
5.5 10.88 0.09
6 9.93 0.1
6.5 9.13 0.11
7 8.45 0.12
7.5 7.86 0.13
8 7.35 0.14
8.5 6.9 0.14
9 6.5 0.15
9.5 6.14 0.16
10 5.82 0.17
10.5 5.53 0.18
11 5.27 0.19
11.5 5.03 0.2
12 4.81 0.21
12.5 4.61 0.22
13 4.42 0.23
13.5 4.25 0.24
14 4.09 0.24
14.5 3.94 0.25
15 3.8 0.26
15.5 3.67 0.27
16 3.55 0.28
16.5 3.43 0.29
17 3.32 0.3
17.5 3.22 0.31
18 3.12 0.32
18.5 3.03 0.33
19 2.95 0.34
19.5 2.86 0.35
20 2.79 0.36
20.5 2.71 0.37
21 2.64 0.38
21.5 2.57 0.39
22 2.51 0.4
22.5 2.44 0.41
23 2.38 0.42
23.5 2.33 0.43
24 2.27 0.44
24.5 2.22 0.45
25 2.17 0.46
25.5 2.12 0.47
26 2.07 0.48
26.5 2.03 0.49
27 1.98 0.5
27.5 1.94 0.52
28 1.9 0.53
28.5 1.86 0.54
29 1.82 0.55
29.5 1.79 0.56
30 1.75 0.57
30.5 1.71 0.58
31 1.68 0.59
31.5 1.65 0.61
32 1.62 0.62
32.5 1.58 0.63
33 1.55 0.64
33.5 1.53 0.66
34 1.5 0.67
34.5 1.47 0.68
35 1.44 0.69
35.5 1.41 0.71
36 1.39 0.72
36.5 1.36 0.73
37 1.34 0.75
37.5 1.32 0.76
38 1.29 0.77
38.5 1.27 0.79
39 1.25 0.8
39.5 1.22 0.82
40 1.2 0.83
40.5 1.18 0.85
41 1.16 0.86
41.5 1.14 0.88
42 1.12 0.89
42.5 1.1 0.91
43 1.08 0.92
43.5 1.06 0.94
44 1.04 0.96
44.5 1.03 0.97
45 1.01 0.99
45.5 0.99 1.01
46 0.97 1.03
46.5 0.96 1.04
47 0.94 1.06
47.5 0.92 1.08
48 0.91 1.1
48.5 0.89 1.12
49 0.88 1.14
49.5 0.86 1.16
50 0.85 1.18
50.5 0.83 1.2
51 0.82 1.22
51.5 0.8 1.25
52 0.79 1.27
52.5 0.77 1.29
53 0.76 1.32
53.5 0.75 1.34
54 0.73 1.36
54.5 0.72 1.39
55 0.71 1.41
55.5 0.69 1.44
56 0.68 1.47
56.5 0.67 1.5
57 0.66 1.53
57.5 0.64 1.55
58 0.63 1.58
58.5 0.62 1.62
59 0.61 1.65
59.5 0.59 1.68
60 0.58 1.71
60.5 0.57 1.75
61 0.56 1.79
61.5 0.55 1.82
62 0.54 1.86
62.5 0.53 1.9
63 0.52 1.94
63.5 0.5 1.98
64 0.49 2.03
64.5 0.48 2.07
65 0.47 2.12
65.5 0.46 2.17
66 0.45 2.22
66.5 0.44 2.27
67 0.43 2.33
67.5 0.42 2.38
68 0.41 2.44
68.5 0.4 2.51
69 0.39 2.57
69.5 0.38 2.64
70 0.37 2.71
70.5 0.36 2.79
71 0.35 2.86
71.5 0.34 2.95
72 0.33 3.03
72.5 0.32 3.12
73 0.31 3.22
73.5 0.3 3.32
74 0.29 3.43
74.5 0.28 3.55
75 0.27 3.67
75.5 0.26 3.8
76 0.25 3.94
76.5 0.24 4.09
77 0.24 4.25
77.5 0.23 4.42
78 0.22 4.61
78.5 0.21 4.81
79 0.2 5.03
79.5 0.19 5.27
80 0.18 5.53
80.5 0.17 5.82
81 0.16 6.14
81.5 0.15 6.5
82 0.14 6.9
82.5 0.14 7.35
83 0.13 7.86
83.5 0.12 8.45
84 0.11 9.13
84.5 0.1 9.93
85 0.09 10.88
85.5 0.08 12.03
86 0.07 13.46
86.5 0.07 15.26
87 0.06 17.61
87.5 0.05 20.82
88 0.04 25.45
88.5 0.03 32.73
89 0.02 45.83
89.5 0.01 76.39
90 0 1

Speed and Feed Rates

This is a basic chart I keep for the speed and feed rates for the mill:

Aluminum Aluminum Steel Steel Bronze Bronze
End Mill RPM In/Min RPM In/Min RPM In/Min
1/8" 7640 107 2750 17 4584 64
3/16" 5093 71 1834 11 3056 43
1/4" 3820 53 1375 8 2292 32
5/16" 3056 43 1100 7 1834 26
3/8" 2547 36 917 6 1528 21
1/2" 1910 27 688 4 1146 16
*Brass same as aluminum

And speeds for the lathe:

Diameter Aluminum Steel Bronze
mm RPM RPM RPM
6.50 4000 1600 3200
12.50 2000 800 1600
19 1500 600 1200
25 1000 400 800
38 750 300 600
50 500 200 400
75 325 150 300
100 250 100 200
*1/3 RPM for knurling, threading, and parting

Mill: Vises

Please read the complete article:
http://benchtopmachineshop.blogspot.com/2017/04/mill-vises.html


While I'm happy to clamp directly to the table, I like vises because they're already trammed, so anything I clamp with it I know is already aligned. I primarily use two: a screwless precision vise and an angle drill press vise.

The 3" screwless precision vise fits beautifully on the mini mill and has more than enough clamping power for the machine. Be sure buy one with slots on the sides instead of just holes, since it makes clamping the vise at angles much easier. I purchased mine from Shars and paid about $54 shipped. As soon as it arrived I made two modifications. I removed the holder the bolt threads into, and removed the cross piece from it which hooks into the grooves on the bottom of the vise. I then turned a new cross piece on the lathe whose length just fits. This keeps the assembly from rotating when tightening or loosening the vise. Second, I put a spring the bolt. This keeps tension on the bolt at all times, making it much easier to move and hook the cross piece.  

Another reason I like the screwless vise is because I don't need a swivel base to rotate it. I can just loosen the clamps, rotate the vise on the table, check it with my protractor, and tighten the clamps back down. This provides more rigidity and more space between the head and table. Making the clamps is a good project for the mill.

The angle drill press vise isn't a milling vise, and has a whole set of issues because of it. The moving jaw will always want to lift when tightening, so I need to hit it with a hammer once it's tight and also make sure the work didn't move, thought using parallels helps since I can carefully tap the work back down until it's resting fully in the parallels. The vise also isn't super stiff, but for the power the X2 has it's adequate. Finally, it can be a bit of a chore setting the angle on it, and usually requires a bit of tapping it back and forth with a hammer. However, the angle adjustment is dovetailed, hardly has any play, can be locked very solidly, and has basic angles preset with the use of a pin. Some operations on the mill would be pretty difficult to set up without an angle vise, so I'm happy to deal with its shortcomings, especially since it only cost me $43 brand new from Sears.

I have since gotten a Kurt-style vise as well. You can see my thoughts on it here: http://benchtopmachineshop.blogspot.com/2015/02/mill-kurt-style-vise.html


Lathe: Quick Change Tool Post

Something which took me a year and a half to add, but now I wish I would have done immediately was add a quick change tool post (QCTP). Aside from making it super easy to switch bits, it also lets you dial in the tool height exactly without having to use shims, and will keep that height dialed in. It just makes the lathe so much nicer to use. I went with a A2Z CNC and TS Engineering clone for $60 off eBay. It mounts to where the old tool post mounted. I chose to use a threaded rod with the stock handle one it to mount mine instead of the included bolt, since this way I can easily rotate the tool post, which is necessary anytime I change the angle of the compound slide. Really, the biggest downside is now I want tool holders for each tool bit I use, and that adds up quickly at $20 a pop.

By the way, I like to center my tool bits by turning the end of a piece of rod to a point. It's easily to tell if the created cone shape comes to a point or is actually flat tipped.








Mill: Solid Column Upgrade

The tilting column's mechanism sucks. It's a pain adjusting with any precision since you need to use a big wrench and high torque. It's also hangs off the back of the table which greatly reduces it's rigidity. Finally, it can easily go out of X alignment without you knowing it. For those reasons I upgraded to the Little Machine Shop solid column:
http://www.littlemachineshop.com/products/product_view.php?ProductID=4483&category=

It has a column with thicker walls than stock with bolts via four bolts to the bed. It's much superior. Yo have to use shims to adjust both X and Y alignment. You can view the two adjustment separately, and simply sum the shim on the shared corner. You should always have at least one corner without shims on it when you're done. When installing the solid column kit I upgraded the mounting bolts to grade 12.9 M8x40 socket head cap screws which are 5mm longer, and made sure to use a torque wrench to tighten the bolts to make my readings consistent between adjustments, using 20 ft/lbs with lubricated threads.



 In the above picture you can just see the end of an aluminum foil shim sticking out between the base and column.

Mill: DRO

Please read the complete article:
http://benchtopmachineshop.blogspot.com/2017/04/mill-touchdro.html


One of the biggest problems with the X2 is the amount of backlash in all the axis. Instead of trying to lessen the backlash I simply installed digital scales with remote displays on the X and Y axis (Z is being installed this weekend) to act as a poor man'd DRO. Since they read the table's position directly, you can pretty much ignore the backlash present. The X scale is mounted on the back of the table with a protective aluminum shield over it. If you're using the tilting column then you'll lose Y travel unless you offset the scale's pickup to one side. I originally did that by extending the aluminum channel past the table and mounting the end of the scale in it.




However, that problem goes away with the solid column upgrade. The Y scale in mounted to the base on the left of the mill. The base's sides have a slight angle to them, but that doesn't matter when mounting the scale so long as it remains parallel to Y axis. I've found the table itself shields the scale sufficiently.




This is absolutely, positively the first upgrade which should be done to the mill. I don't really understand how someone can use an X2 without it.

Lathe: Carriage Lock and Way Protector

I have a Grizzly 7x12 mini lathe, which is just a repainted Sieg C2 lathe. If you're not looking for super accuracy you can start working with it right out of the box. It's biggest drawback is lack of a carriage lock. Locking the half huts helps, but there's still a lot of play, and it will adversely affect any facing or parting operations. I think one of the first things to do is add one. There are plenty of designs out there, but I think the best ones clamp the carriage down onto the ways.

The one I made bolts to the mounting holes for the follower rest on the left of the carriage. You can't see it, but the center socket cap screw threads into a T shaped piece which rides under the ways. Aside from the screw there is also a locating pin between the two pieces to keep it from spinning when tightening or loosening. One half turn of the screw goes from free to locked. If it gets too annoying on the left of the carriage I'll drill and thread holes to move it to the right, but I hate threading so it'll need to get fairly annoying. You can also see the added way protector, which is just the bellows ways cover from a X2 mini mill cut to size. 



UPDATE: I added a lever to make locking it easier. I also glued a magnet in place to hold the lever in place and accessible until needed.


Mill: Alignment and Tramming

Same as my lathe, my mill Grizzly's version of the Sieg X2 mini mill. Unlike the lathe, the mill needs some work right out of the crate to be usable. At a minimum the column needs to be trammed to do work of any precision at all.

To really set up the mill properly the head needs to be aligned to the column first. The method is described nicely here:
http://www.homemodelenginemachinist.com/f28/mini-mill-spindle-column-alignment-5337/

I would add that it's imperative the four socket head cap screws be completely tightened before taking a Y axis reading because the screws' tighten will affect alignment. Also, make sure to take another set of Y readings after finishing adjusting in the X axis. Also, tighten the screws slowly when locking in X and do each in turn in very small increments. Even then the head will rotate slightly (about 0.0015 to 0.0025 in my experience), so account for that.

Let me also add something on using Rollie's Dad's Method. It's a very elegant way to do the alignment since it averages out any runout present. In case the linked didn't make it clear how to use RDM you take a reading away from the head and adjust the dial on your indicator until there's equal movement to either side of zero as you turn the rod through a complete rotation (and turn the rod using the drawbar at the top of the head and not turning the rod itself). Once you have that turn it until the needle is on zero and move the head close to the indicator and take another reading and see where the zero now lies. The difference in distance between the two zeros is your misalignment. If it's properly aligned then the zero at both locations will be the same. When using RDM I like to mark the two areas I'm taking readings, and then mark where zero is at each area.

Once the head is aligned you need to align the column to the table. With the tilting column of the X2 you use the tilt mechanism to align in the X axis and have to shim to align in the Y. For shims I buy feeler gauge sets and use those. They're cheap and accurate. If I need something below 0.0015" (which is the lowest feeler gauge sets seem to go) I use aluminum foil. Standard aluminum foil is 0.0006" while heavy duty aluminum foil is 0.0008".

Benchtop Machine Tools

I own a mini mill and a mini lathe. Why? Simply because I don't have room for anything more, and probably won't have room for a long, long time. That's ok though, since with some modifications both mill and lathe are capable of doing good work. Not very fast, but still capable. A machinist friend told me anyone can produce good work on a good machine, but a good machinist can produce good work even on a bad machine.

Since they're going to be with me for some time, the decision to upgrade them and make them easier to use was an easy one to reach. 

I may add more to this post later.

May 2013


December 2014