Need comments on design concept please

[Jester966]

I'm in the design phase of building a bench mill of similar size to the X3, and I'd like to be able to machine plastics, aluminum, and steel. The plan for the Z-axis (at the moment) is to support the head with eight SR15 bearing blocks on four rails, as shown in the pic below.

Here is a basic model to show the idea:


Is the 15mm rail and SR15 blocks large enough for this size mill? (I've already bought them, so I hope so) Do I really need four rails and eight blocks, or would two rails on the front of the column with two blocks per rail be enough?

The column is built from 1" and 1/2" 1020 steel. Will it be rigid enough to mill steel? Any other comments or ideas on the general design concept?

Thanks

[austin.mn]

if you already have the rails, use them. you may not need to have the extra rails and blocks, but it isn't going to hurt any. as long as you get all the rails parallel to each other. i don't know what you have for machinery, but i would suggest that you mill the plates, square, parallel and flat before you mount the rails. it will make aligning your head that much easier if your axis moves in a straight line from the get go. i like the counterweighted head, it should help you get smoother movement from your z.

i would think that the sr15 rails you have will be quite sufficient, especially if you use all four. keep us posted on your progress

[philbur]

I see two issues.

Your open column design is seriously un-stiff in torsion about the z axis and in simple bending in the x axis. This will be especially true with a bolted structure. You need either two separate closed box section columns or at least to buttress your existing design. At least include it as a possible future modification. It's not possible to say if your current design is stiff enough without doing some comparative calculations. With out these the next best approach to avoid disappointment is to over-design.

The other point is the space between the two columns for the belt drive. A 1hp plus drive needs a minimum pulley size of 50 mm and preferably 70 mm. For a two to one ratio this gives you a large pulley size of 100 to 140 mm. You need to include sufficient space for the belt. A toothed belt might allow higher power transmission at smaller diameters. In addition the unsupported belt length is quite long so at high rpm it may flap around a bit.

Just some thoughts.

Good Luck.

Phil

Originally Posted by Jester966 I'm in the design phase of building a bench mill of similar size to the X3, and I'd like to be able to machine plastics, aluminum, and steel. The plan for the Z-axis (at the moment) is to support the head with eight SR15 bearing blocks on four rails, as shown in the pic below. Here is a basic model to show the idea: Is the 15mm rail and SR15 blocks large enough for this size mill? (I've already bought them, so I hope so) Do I really need four rails and eight blocks, or would two rails on the front of the column with two blocks per rail be enough? The column is built from 1" and 1/2" 1020 steel. Will it be rigid enough to mill steel? Any other comments or ideas on the general design concept? Thanks

[Jester966]

Thanks for the tips.

I'll have access to a large machine shop (with both manual and CNC machines) in the summer, so I should be able to machine most, if not all, of the structural components I need. Until then, I want to work out any kinks in the design, and collect all the parts that I can't make so I can put dimensions on everything.

Do you think it would be better to just mount the motor on the front with the spindle head as on other bench mills? That would allow me to have a more solid column, with the braces between the front and back rails going around the column rather than through it. The only concern I had with that idea is can the rails and blocks support all that weight on the front with no counterbalance on the back?

If I can find someone with a welder, I may weld most of the column rather than use bolts for everything, and that should give me a lot more rigidity.

[10co]

hello all of you,

please excuse me for my english (I'm french).

I think 4 rails is too much you will have many problems to have all of them very parrallel. As I made A routeur whith 2 rails per axis, I can tell you that you must have the body surface in contact with block with very good accuracy less than 0,02 mm if you used size 15 rail grade with null gap (no backslash).
have a look on the mecanical specifications of this rail (you can have them on the web. hepco product as example), and you will see these rail are though enough, and see the dimensional recommendations.

it's better to have 4 blocks with the maximum of distance beetween them than 8 very close each other.

take care of welding deformations, you must machine the parts after welding.

this is just my point of view, based on my studies, professionnal expérience and home made CNC machine. nobody have the true !


about rigidity, some CAD sofware (solidworks) have a simple add on to have a idea of the deformation of parts, even if this is not very accurate it can give some comparison beetween different structure. I think the rigidity is one important point but also the ability of the body to reduce vibration seems to be important (but I haven't any advince except to have a machine as heavy as possible ! ! ! ). I can't tell more, my router haven't yet machine any part .

I hope this can help you.

I also use this post to give a great thank you to all the people on this forum, beccause I learn so much with reading all of you (sometime difficult with some technical words !)

best regards.

[philbur]

The twin column with the motor at the back for counter balance is a neat concept. If you go with a single enclosed column with the motor at the front you may as well buy an X3 (or whatever) and save yourself several hundred manhours. A large machine shop will surely have welding equipment. Welding will give you a stiffer structure but you will need post weld heat treatment before machinng. However you look at it it's quite a machining exercise to achieve the necessary tolerances.

Regards
Phil

Originally Posted by Jester966 Thanks for the tips. I'll have access to a large machine shop (with both manual and CNC machines) in the summer, so I should be able to machine most, if not all, of the structural components I need. Until then, I want to work out any kinks in the design, and collect all the parts that I can't make so I can put dimensions on everything. Do you think it would be better to just mount the motor on the front with the spindle head as on other bench mills? That would allow me to have a more solid column, with the braces between the front and back rails going around the column rather than through it. The only concern I had with that idea is can the rails and blocks support all that weight on the front with no counterbalance on the back? If I can find someone with a welder, I may weld most of the column rather than use bolts for everything, and that should give me a lot more rigidity.