Opinions & Advice on A rigid Epoxy/Granite/Concrete

**Saintgp** · 10-26-2020, 04:07 PM

Hi Guys

I have been a member of this forum for some time now and loving it. Long Live the CNC Brotherhood.
I’m based in South Africa and have been a Hobbyist CNC machinist for about 15 years now. I have 2 small 3 axis mills and a small 3 axis Plasma table (800mm x 800mm), all running Mach3 with LPT port break out boards. I now I need more, I need to feed this Beast(passion).

I’m planning to build a machine with a cutting area of 1500mm x 1200mm x 300mm to cut aluminum and some steels. The machine needs to be extremely rigid/stiff and have the commercial quality tolerances (fingers crossed). I have done some renderings in fusion and this is what I have come up with.

Specs are as follows (so far):
Y – 1200mm
X – 1500mm
Z – 300mm
Frame Epoxy/Granite/Concrete
Nema 34 1600oz (12nm) closed loop hybrid steppers with 60v psu
RM1605 ball screws
HGR20 Linear Rails
2.2kw Water cooled spindle with VFD, ER20 collet
100mm 4th axis with a 4 jaw chuck

I have purchased all the above items already. Accept the epoxy/granite.

The base and Y-axis uprights are 200mm thick (RED)and hoping to be cast as a sigle piece.
The X-axis gantry is 100mm thick (GREEN)

Advice/Opinions needed:
1> Concrete will be the cheapest and easiest to cast. Will it have the rigidity and with stand the stress loads?
2> What type of Epoxy can be used for the frame? Where in South Africa can I find it at a good price?
3> What speeds can I expect from the motor and ball screw combination?
4> Seeing as the motors are 12nm can it be geared up for more speed? Maybe 2:1
5> Have 3 bearing blocks on each rail. Will it have any rigidity and load bearing benefits?
6> Is having the 4th axis in the y-axis upright a good or bad idea?

All advice and opinions will be highly valued.

**awerby** · 10-26-2020, 05:50 PM

Did you mean to attach a drawing to this post? It's hard to do more than hazard wild guesses unless we know how you plan to use these various components and materials. Was the concrete in place of the epoxy-granite or an alternative? Which hybrid steppers did you buy? How fast did the manufacturer say they'd go? Holding torque is not a good indicator of performance, but it's rare to see steppers geared up for speed. What do you mean by "having the 4th axis in the y-axis upright"? That spindle sounds like it might work for aluminum and softer materials, but not for machining steel except for with very small cutters.

**Saintgp** · 10-26-2020, 08:17 PM

sorry about that. i did attached the pics but they seem to have disappeared.

the motor is rated at 2000rpm

it is either epoxy granite or concrete or even a mix of both if that works

the spindle i a Chinese 2.2kw 24000rpm hobby model. my thinking is if i have enough speed and rigidity i would be able to shallow cuts in steel. am i right or wrong

**pippin88** · 10-26-2020, 10:24 PM

Gantry is way too thin.

Gantry should be as deep as it is tall. Gravity is not the major force here.

Torsion (twisting) load on the gantry from machining is much more important than gravity.

**awerby** · 10-27-2020, 06:40 PM

That 24000 rpm spindle would be able to make very shallow surface cuts in steel using a very small cutter. So engraving-type operations would probably work okay. But it would be pretty useless for most of what we think of as machining in that material. For that you'd need a much slower spindle with lots of low-speed torque.

I'm not sure what you mean by a mix of epoxy-granite and concrete. These are entirely separate materials which don't mix together well. Were you talking about using e-g for some parts and concrete for others?

**Saintgp** · 10-27-2020, 10:33 PM

Originally Posted by awerby

That 24000 rpm spindle would be able to make very shallow surface cuts in steel using a very small cutter. So engraving-type operations would probably work okay. But it would be pretty useless for most of what we think of as machining in that material. For that you'd need a much slower spindle with lots of low-speed torque.

I'm not sure what you mean by a mix of epoxy-granite and concrete. These are entirely separate materials which don't mix together well. Were you talking about using e-g for some parts and concrete for others?

i would like to drill and profile some mild steel ( not very big parts 150mm x 150mm)

i was under the impression that if i could get the feed rate up high enough that would compensate for the high rpm. is this correct thinking?

yes E/G or similar type of material that can be formed/poured and should be able to be a DIY project

**Saintgp** · 10-27-2020, 10:40 PM

Originally Posted by pippin88

Gantry is way too thin.

Gantry should be as deep as it is tall. Gravity is not the major force here.

Torsion (twisting) load on the gantry from machining is much more important than gravity.

i did not think of that. making the gantry square (height and width equal) will this not make the gantry twice as heavy and take up some of the cutting area?

i redesigned the gantry. let me know if this is moving in the right direction or not.

**pippin88** · 10-28-2020, 01:07 AM

Gantry can hang out the back. Doesn't have to be centred on bearings. No loss of cutting area.

Drilling steel is not a good idea on a standard 24k spindle. They don't have the bearings for it.

If it was easy to cut steel with a cheap 24k spindle everyone would be doing it.

**awerby** · 10-28-2020, 04:27 PM

[QUOTE=Saintgp;2410242]i would like to drill and profile some mild steel ( not very big parts 150mm x 150mm)

i was under the impression that if i could get the feed rate up high enough that would compensate for the high rpm. is this correct thinking?
/QUOTE]

There has been a shift towards High Speed Machining in production shops lately; it sounds like that's what you're thinking about. The idea is that with a CNC machining center one can take shallower cuts at higher feedrates and RPMs and save time compared to heavier cuts at slower speeds. But the spindles on these machines don't run at 24k; they're more like 8k RPM. They have beefier bearings and a lot more torque than the little 2.2kw spindle you're talking about. And those machines are a lot more massive than yours, which will be marginal at best for cutting steel.

**Saintgp** · 10-28-2020, 05:29 PM

yes that is exactly what i was think about. shallow cuts a high feed rates.

i have seen people using DMM servo motors 0.75KW on mini mills cutting steel. do you think that a this coupled to a bt30 spindle would be good upgrade?

**mactec54** · 10-29-2020, 03:35 PM

Originally Posted by Saintgp

yes that is exactly what i was think about. shallow cuts a high feed rates.

i have seen people using DMM servo motors 0.75KW on mini mills cutting steel. do you think that a this coupled to a bt30 spindle would be good upgrade?

You want more than 750W if you want to drive a BT30 spindle there 1.8Kw would be better the BT 30 spindle would be a good up grade for cutting steel, you could mount a spindle on each side of the Gantry Back side for steel and the High speed spindle on the front

**Saintgp** · 11-24-2020, 08:20 AM

I feel like a kid on Christmas morning all my new toys
(see pics)

Now to finalize the design and get started

This is the low viscosity resin that I could find in South Africa. Do you think it will work?

What do you think of this as aggregate? I have been using this for sandblasting for some time now. I have also been reading a lot of the E/G forums and have come to the conclusion that using 1 size of aggregate will yield the same strength as using multiple size aggregates. it may use a bit more epoxy though.

Also thinking of adding 10mm mild steel plates into the middle of the casting. will this has any significant strength advantages?