[manley86]

In my free time I have been playing with cad and have come up with a possible design for a benchtop machining centre tough enough to do some proper work.
Specs would be:
16c auto collet spindle
20mm thk linear rail on all axis
16mm ballscrews on all rail
Plate aluminium construction dowelled at all joints - base 12mm - rear backplate 16mm thick tailstock 30mm headstock 40mm
Live Tool ATC
400W mitsubishi Servo spindle
200W mitsubishi servo's on other axis
Running mach 3




I really want to combine the awesome looking machine blight has made and a machine on the mach3 forum:
http://www.machsupport.com/forum/ind...c,11422.0.html

Can anyone see any immediate flaws in the design? Sizes are about 450mm x 700mm x 450mm so its quite small. I already have servo's, ballscrews, drivers, linear rail and some chunks of aluminium so it would make a good summer project.

I would love it to do polygon turning also but so far have not found a solution to where the tool would be mounted. Youtube polygon turning if you have a spare half an hour!

Rob.

[The Blight]

Thanks mate!

Noticed the linear rails on the headstock, and the shaft in the middle of the ATC. So this is going to be a 3 axis lathe, with live tooling?

I have one recommendation. Go with steel plates for the frame. I'm replacing the back and bottom plate on mine.

Awesome design! Looking forward to this build.

[wildwestpat]

Hi Rob

The spindle support bearings look as if they are too close together to provide much rigidity. I am assuming that the bearings are a pair of angular contact bearings and a single row ball race from the aspect ratios in your sectional sketch. The arrangements for pre loading the bearings is not clear but I assume with all those parts you have arranged for this.

The advice to use steel is good but weight for weight the aluminimum woud be OK but steel would be better if you are limited by thickness.

Hope to see the finished machine posted here soon. - Regards Pat

[manley86]

The main bearing is a double row angular contact bearing which I hope will do the job - how good is a double row angular contact against a matched pair? any difference?. I will move the deep groove ball bearing further back by extending the bearing housing/collet closer further outwards so that the drawtube and its locking screws slightly protrudes from the outer box. I reckon i can get another 2" between the bearings. That front bearing is over 5" in diameter I haven't sorted the preload spacers out yet as im not totally sure what im going to do there.
A 5c adapter can be bolted to the nose for smaller stuff.

The yellow ring contains an oil seal and is held in position with 4 spigots containing springs which locate into the front bearing cap but are free to slide allowing the plate to press against the inside of the headstock plate to stop swarf getting in. The headstock can move approx 1.125" in either direction from home.

The collet closer uses air to open but springs to keep the collet closed - I remember someone on here posting the recommended spring force.

The reason it's aluminium is I can get bits cheap. The base and rear plate will have aluminium plate bolted to the underside/back as a stiffener.

Im going to make the ATC a bit beefier and fit some larger bevel gears inside but yes, it should have live tooling - i still need to work out how much torque the gears are likely to receive on 1/4" tooling. It will accept 1/2" indexable lathe tools.

I enjoy the hardware part but heaven save you guys when i come to wire this badboy up :devious:

Im really hoping that this will be able to do some paid work by the end and earn its keep. It should be quite capable. Im slowly thinking that I dont have anywhere to put a polygon turning head which is a pity, would've liked to have been able to do hexagons/squares of any length with a decent finish.

Rob.

[wildwestpat]

Hi Rob

When you look at bearings you are looking into a can of worms and not all the worms are equal!

Bearings are made to various tollerances and these are sold as branded products. The generic unbranded bearings are an unknown quantity BUT I have had some good experience of double row unbranded A/C bearings for ball screw support. These were both cheap and exhibit no measurable axial movement but I would hesitate to use then as spindle bearings.

The ridgidity of the spindle is determined by the bearings obviously and the front bearings will carry the major part of the radial thrust imposed by cutting but the back bearing contributes to the spindle ridgidity. I find it helps to think about waht happens when the cutting forces are applied and how they try and move the bearings.

The conventional designs would use one or more A/C bearings at each end of the spindle and apply a preload to provide the required load on the bearing under all conditions (the forces on the spindle could act in several directions depending on the machining operations). The use of double row A/C bearings avoids the pree loading conundrum as this has been set during manufacture. If you are going to use branded bearings from a reputable supplier then you can ask about the bearing selectionand tolerances etc. However if you are going to use generic gearings you run some risks that the bearings may be poor. To put this into context I have purchased unbranded generic 10 mm and 12 mm bore double row A/C bearings and measured the axial movement under moderate axial load. I found that most were OK and showed no axial movement indicating the pre load was satisfactory as set during manufacture but one in ten exhibited some measureable movement. The bearings were run for several minutes at 2,000 rpm to throw any excess grease clear of the race ways BUT the bearings were not mounted. The force fitting would alter the pre load a bit by expanding the inner and compressing the outer. Obviously using single row bearings alows the pre load to be set after any movements due to the 'force' fit in the respective housings. The manufacturer's literature gives advice as will their applications engineers.

Good luck with the build - regards - Pat