Surface Grinder CNC Notes

Controlling the Z axis

Option A - Stepper motor

Using a stepper motor with 1.8 deg steps will give a resolution of 200 steps in full step mode per rotation. This would require a 46:1 reduction to achieve the desired 0.5um resolution on a 5mm lead screw.  Cons: discreet steps, not sure if this will be an issue, holding torque goes down with reducing step size.

Option B - Servo

With a servo motor using dual loop position feedback control will make more sense since steps are no longer discrete. The dunker motor I have had lying around for years, seems like a good fit. 23:1 gear ratio with a 100ppr encoder, without using the linear encoder as additional feedback this will give a resolution of 9200 steps or 0.543 um. Quick estimation with Bresenham algorithm, gave the following approximations for 0,001um increments. But i would like to hit them exactly...well in theory at least.

1	0,000543
2	0,001087	0,001
3	0,00163
4	0,002174	0,002
5	0,002717
6	0,003261	0,003
7	0,003804	0,004
8	0,004348
9	0,004891	0,005
10	0,005435
11	0,005978	0,006
12	0,006522
13	0,007065	0,007
14	0,007609
15	0,008152	0,008
16	0,008696
17	0,009239	0,009
18	0,009783	0,01
19	0,010326
20	0,01087	0,011
21	0,011413
22	0,011957	0,012
23	0,0125

This made me research the possibility of using dual loop feedback, seems common in commercial machines.

LinuxCNC supports it out of the box

http://wiki.linuxcnc.org/cgi-bin/wiki.pl?Combining_Two_Feedback_Devices_On_One_Axis

http://linuxcnc.org/docs/2.7/html/man/man9/offset.9.html

This great explanation https://granitedevices.com/wiki/Dual-loop_feedback_position_control

Gave me another idea, just use Elm Chan SMC3 Velocity control mode with LinuxCNC, feeding position from Heidenhain encoder to LinuxCNC, which controls the SMC3 servo in velocity mode.
http://elm-chan.org/works/smc/report_e.html

Eagle Surface Grinder MK3 Rebuild - Progress

This will be the last post for 2018. It has been an exciting year with life happening.

I finally managed to get some time to start the reassembling the Eagle Surface Grinder. All the surfaces have now been precision ground. All that is left to do is machine the oil grooves, match the dovetail surfaces, scrape the oil pockets, add the news spindle and ball screws, write some macros for the grbl g-code controller to behave like a surface grinder and the machine is ready for action.

Spindle and Drive Motor

After ordering and installing the new balls as described in a previous post, I am not satisfied with the resulting stiffness and pre-load design of the original spindle. The thinking now is to use a self contained spindle and machine an adapter sleeve. The new spindle axis diameter is 20mm compared to 25mm and the bearings considerably smaller, but for my needs this will be more accurate, even if I can only run 7" wheels. The motor in the base will drive a flat belt transmitting power to a love-joy style coupling in the "head stock" similar to old lathes with addition of the coupling if that makes any sense. Initially I wanted to go with poly V-Belts, but when I saw the very modern Schaublin 102 N-VM-CF still uses crown pulleys and flat belts. I started investigating the advantages of flat belts. The main advantages I could find was, better efficiency, less vibration, and higher speeds. not sure if any of this holds true, but worth a try. Here I will need to design the housing at the back of the spindle housing, which will hold the crown pulley and shaft coupling, for driving the spindle. A direct drive would be more efficient, economical with less vibration, but that would have the motor extend  at the back of the spindle housing, similar to the later eagle model shown below, in a small shop not an option.

Knee Oil groves

On the MK3 model the only the table sports oil groove. As discussed in a previous post, I have decided to go with the zig-zag oil grove pattern, it is more time consuming to machine, but should give better results. Below some images on the layout and machining process. The fixed dovetail had a ridge, where it meets the flat surface on the knee, this had me confused for some time as to why I do not get full bearing on the entire flat surface. a few head-scratches later I used a carbide ball nose end mill to machine some clearance in the corner, all this with a portable hand drill. The only straight edge I could find to fit the angled recess was a carpenters knife blade, this worked great!

Ball Screws /Servo/Stepper Motor

The ACME lead screw needs to be replaced, it has a lead of 10 TPI. I do not feel like cranking the hand wheel so installing a feed motor is a given. Sourcing an ACME 1-1/4" x 10TPI  or similar in my part of the world is not feasible. So I opted to go full CNC and use 5mm pitch ball screws.  After doing some calculations on THK ball screw specifications with a combined load of 100kg., a 20mm diameter screw will work within load ratings, if a maximum sliding speed of 38mm/s is not exceeded. For a 5mm pitch screw this will give maximum motor speed of 456 rpm. And 8NM motor would be required for https://www.nidec.com/en-EU/technology/calc/torque/ballscrew/ to drive this load.

Engraving a cylinder with graduations to be used as machine tool / lathe dial.

Using gcmc to generate the gcode, this becomes a trivial exercise in more modern programming techniques.

feedrate(400.0mm);

sf = 10.0mm;    /* Scaling factor */
diameter = 80.0mm;      /* Scaling factor */

n = 0;

for (c=0deg; c<360deg ; c=c+3.6deg ) {
// message(">>>> ", c, " ",c%36);

 if ( ( c % 36 ) == 0deg || ( c % 36 ) == 36deg) {
 message("--- ", n);
 vl  = typeset(to_string(n), FONT_HSANS_1);
 vl = scale(vl, [sf / (2.0*pi()), sf]);
 n = n + 10;
 goto([diameter/2,-,0.0mm,-,-,c]);
 move([-,-,14.0mm,-,-,-]);

 }
 elif ( ( c % 36 ) == 18deg ) {
 message("--  ", c);
 goto([diameter/2,-,0.0mm,-,-,c]);
 move([-,-,10.0mm,-,-,-]);
 }
 elif ( ( c % 36 ) != 0deg || ( c % 36 ) != 18deg  ) {
 message("-   ", c);
 goto([diameter/2,-,0.0mm,-,-,c]);
 move([-,-,7.0mm,-,-,-]);
 }
}

Interesting site with lathe dial reductions to suit different lead screws.
http://www.modelengineeringwebsite.com/Mini_handwheel_1.html

Eagle Surface Grinder Model 3 Rebuild - Spindle Refurbish

My MK3 is almost as described by Tony on lathes.co.uk/eagle
with one difference, at the back there are two deep grove ball bearings.


He writes:
Sized imperially, the ball races consisted of two opposed angular contact ball bearings behind the wheel with a single deep groove ball bearing at the drive pulley (rear) end. Interestingly, if the angular contact bearings are dismantled (they push apart) an owner reports that they can be revived by polishing the races lightly with diamond paste, fitting new balls and lubricating with ordinary lithium grease.

The spindle of my variant runs in two angular contact bearings, each pair in back to back (DB) arrangement. The preload adjustment is not clear, need to do more research here. The bearings are of imperial dimensions ID 25.4 OD 63.5 ( 1" x 2.5" ). 

The spindle assembly is pulled out to the front, there is a hole in the middle to restrain the shaft from rotating.

Once out, the bearings pull apart and the balls measure in at 10.30 mm since this is an imperial sized bearing my thinking is this was probably a 13/32" ball. (10,31875mm ). Initial measurements of the races ID and OD confirm this.

I could not find a suitable new replacement for the Bearings.
The replacement is still manufactured under the following names from R&M, SKF and some other brands.

RMS8    MJ1 - Deep Grove
AMS8    MJT1 - Angular Contact




Since the bearings show significant wear, there are two options under consideration.

1) Use 7006 bearings 30x55mm and machine new sleeves to fit the 1" shaft and 2.5" outer seat.

2) Do what  Tony describes. Order new balls, polish the races and see what I get.

Eagle Surface Grinder Model 3 Rebuild - Planning

The surface grinder is made up of the following precision guided parts, a vertical column, a knee, a cross slide saddle, and a horizontal table.

The machine is build in a typical column knee setup as seen in horizontal milling machines.
The column has two parallel and one perpendicular planes.
The knee has two perpendicular planes.
The cross slide has two perpendicular and two parallel planes
The horizontal table has on plane

Starting at the biggest surface first and then working my way back seems to make the most sense. This includes all surfaces on a specific component. ordering the components by guide ways surface area yields this sequence.

• Vertical Column 2 x 800x50mm flat+ 2 x 800x30mm Dovetails
• Knee  2x300x50mm flat + 2x300x50 dovetail + 2x400x50 flat + 1x400x50 dovetail
• Table 2x 800x50mm flat + 2x 800x50mm dovetail
• Cross slide 2x 200x30 flat + 1x200x30mm dovetail + 2x400x30mm flat + 1x400x30mm dovetail
• Gib's with two surfaces 60Deg, 3 x

Planned steps and sequence:

• The table was surface ground to be used as a template.
• Build a holder for all three angular flat gib's 
• Scrape in all flat and angled surfaces of the gib's so they can be used for templates later.
• Build indicator jig for measurement, geometry and dimensions to provide 3 point contact to measure all surfaces in need of scraping
• Scrape Column front side flat, using the feed table, this will preserve the original machined surface on the center of the column. indicator jig should provide 3 points of contact on the original machined surface and two on the dovetail surface, to measure dovetails.
• pre scrape knee vertical slides with straight edge or gib as template.
• align vertical and scrape horizontal slides of knee on surface plate
• pre scrape knee right side dovetail using the knee's gib as template.
• Fit knee on column and scrape column dovetails to match,
• finish scrape knee vertical slides.
• pre scrape cross slide flat ways and align on surface plate
• pre scrape dovetail ways using the gib's as templates
• scrape knee side on cross slide using the knee as template, align to column.
• scrape table side of cross slide using the table as template align to column and knee
• use the knee with planing attachment on the feed table to machine a recessing center column, 
• would give a nice sharper like finish, maybe when I have energy left at that point.

Eagle Surface Grinder Model 3 Rebuild - Assessment

Before rebuilding my machine I like to plan the entire rebuild, and simulate different aproaches in my head, to make sure I do not start at the tail end of the project. Here it helps to write down my thought process to keep track.

Firstly I do an assessment of the wear surfaces to establish where what needs to be done.

On the surface grinders the feed (x) axis does the most traveling, for standard flat grinding job, the feed axis will travel the length of the material x 2 for each in-feed (y) advance, and the down feed (z) axis will travel one depth of cut for complete feed , in-feed cycle.  Assuming maximum material to be removed on a surface grinder is less then 1mm, and the in-feed is 0,5 and down feed is 0,1. And stroke length is 150mm and table length is 300mm. Then the total distance traveled for full table work piece to remove 1mm material is:

feed (x) ( 150mm/ 0,5 ) x (300mm+300mm) = 180 000mm = 180m
in-feed( y) 10 x ( 150mm + 150mm) =  3000mm = 3m
down feed (z) = 10x 0,1 = 1mm

With this in mind you will have to spin the feed hand wheel 190 times, and if a feed cycle takes 10seconds it will take half an hour to complete the job ( hand wheel is 300mm diameter) most parts are not that big, but still a long time to stand there.

Perhaps the reason for full automatic grinders, and why I consider automating the machine.

Anyways, so I expect to see most wear on the feed axis, then in-feed, and then down-feed.

Below the measurement, the maximum wear on the feed ways is 0.582 rear and 0.809 front.

The down-feed guide ways on the column, are worn on the bottom side to about 0,03mm. Due to the cantilever effect of the knee, I expect the dovetail guides on the back to show similar wear in the center portion.