Rigidity of DB200 Way Bars
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I am not an engineer so I need to ask a specific question concerning way bar flex. What would be the increase in rigidity (flex) if the way bar diameter was increased from .472” to .500”. ?I understand from past discussions that material hardness has little to do with flexing whereas diameter does. I am considering trying to increase diameter asas it will not require a modification to the base casting. I will however need to modify the cross slide hole size or build a new cross slide to suit and tail stock. Any input would be welcome.
Dick |
开云体育Hello Dick: The formula for deflection uses the square of the cross section. So 0.5 squared = 0.25 and 0.472 squared = 0.222. The difference is 0.027 or about 10% stiffer, for the same material. Others have increased the bar diameter and bored the carriage out. I would suggest turning the ends down to 12mm so you don't have to modify the base. Deflection
= wL3/24AD2 w = load L = Length ( cubed ) A = Sectional area of beam D = Depth of beam ( squared ) This
formula is for 16Kpsi steel with a Young's Modulus of about
200 GPa, while for carbide is it about 600 GPA. Now if you went with carbide bars the material stiffness is much higher. At work we did this for the grinding mandrills for 3mm holes. It might take some fancy tricks to mount 12mm carbide bars, but I think it could be done. McMaster has 10mm x 100mm bars for $42 each, so someone should have 12mm x 300mm available.
Carl. (
Retired tool design engineer ) On 4/15/2020 10:09 AM, OldToolmaker via
groups.io wrote:
I am not an engineer so I need to ask a specific question concerning way bar flex. What would be the increase in rigidity (flex) if the way bar diameter was increased from .472” to .500”. ?I understand from past discussions that material hardness has little to do with flexing whereas diameter does. I am considering trying to increase diameter asas it will not require a modification to the base casting. I will however need to modify the cross slide hole size or build a new cross slide to suit and tail stock. Any input would be welcome. |
开云体育That's right, unless you change the material to tungsten or something stiffer, all steels will have about the same deflection.If you change diameter to 1/2" from 12mm, the stiffness will increase about 20%. But perhaps there are other ways to reduce flex in the way bars, what are you trying to do or achieve? Do you need the full length of the way bars for the cross slide travel? Roman On 2020-04-15 10:09 a.m., OldToolmaker
via groups.io wrote:
I am not an engineer so I need to ask a specific question concerning way bar flex. What would be the increase in rigidity (flex) if the way bar diameter was increased from .472” to .500”. ?I understand from past discussions that material hardness has little to do with flexing whereas diameter does. I am considering trying to increase diameter asas it will not require a modification to the base casting. I will however need to modify the cross slide hole size or build a new cross slide to suit and tail stock. Any input would be welcome. |
开云体育Carl,Just a little correction, you forgot that there is also diameter (squared) involved in the sectional area of the beam (A) so it is power of four relationship, so stiffness would be 26% greater (I said earlier about 20% - rounded ;-) ). Roman On 2020-04-15 11:29 a.m., Carl wrote:
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Carl,
I know the Unimat is a light duty machine with limitations. ?I am just wanting to remove some of the deflection of the ways when the cross slide is in mid range of the travel when it is most likely to flex when under load. This is really not a big deal to me but I am always looking for ways to improve the machine. ?I like the idea of substituting carbide for steel without the need to adjust any other features. Working with carbide however in the hobby shop is a little hard to do. I do have diamond grinding equipment for cutoff and end grinding but putting a cross hole through carbide would be an issue for me. I would have to find another means of clamping the bars. Dick |
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Dick,
To increase stiffness of way bars, you can use two pieces of 1/2" flat bar (1/2" is about the maximum thickness that would fit between the bed and the way bars) and clamp them together, "sandwiching" way bars between them. Ideally, if they can be contoured (round slot or v-slot) to "hug" the way bars, that would be even better. This would increase tremendously the stiffness of clamped portion of way bars and reduce deflection of the whole way bar(s). See attached picture for what I mean, it shows 0.5" x 2.5" flat bar, 4" long. You can have two shorter sets of flat bars, on either side of the cross slide, to have the same effect, longer the flat bar the better. Of course, it reduces cross slide travel, but one can have several sets of these flat bars to suit the task at hand. Roman |
开云体育Hi Dick: I know
a cross hole would be a real challenge, so I think a eye bolt
around a turned down end section to pull the bars into the bed
"Vs" might work. But
then the other question, related to the size of the machine,
is the balance of Power / Size / Accuracy / Etc. To increase
any one factor may compromise another. I've had my Unimat for 45 years and haven't made any modifications to it. I did put it on a 20mm ground steel plate and added a mounting flange for the milling post, but I still have the same way shafts, etc. It is a small machine and needs a gentle hand, but with care I have drilled 3/4" holes in aluminum plate. Not as fast as a 5hp radial drill, but it did do the job. So please enjoy your Unimat. Thanks,
Carl. On 4/15/2020 3:45 PM, OldToolmaker via
groups.io wrote:
Carl, |
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John,?
A few years ago I made a full set of way bars from Thompson linear way material. I never used them and still have them. The smaller diameter 8 mm bars were made longer to facilitate mounting a dial travel indicator. It might be time to mount them for use now while I am doing these other things. Dick |
OldToolmaker
Carl
Roman Gargulak
Dave W
Lionel